UNIVERSITY OF CALGARY Molecular dynamics study of nanoscale heat transfer at liquid-solid interfaces (LSIs) by Khaled M. Issa A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILISOPHY DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING CALGARY, ALBERTA September, 2012 c Khaled M. Issa 2012
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UNIVERSITY OF CALGARY
Molecular dynamics study of nanoscale heat transfer
at liquid-solid interfaces (LSIs)
by
Khaled M. Issa
A THESIS
SUBMITTED TO THE FACULTY OF GRADUATE STUDIES
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILISOPHY
DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
4.1 Simulation results. The LSI thermal resistanceRLSI has units of (×10−9Km2/W).The % difference in interfacial resistance is based on S1. . . . . . . . . . . . 30
3.1 Separation distance and force vectors between atoms i and j. . . . . . . . . . 173.2 Velocity-Verlet algorithm steps. Cells with darker shades indicate current
3.3 MD system with periodic boundary conditions, and minimum imaging, shownhere in two dimensions. Simulation cells B-I are images of cell A. Each cellis of size L× L. Cutoff circles of radius rc are shown by darker shades, withthe one at the H-G border representing minimum image convention. Arrowsshow atoms crossing cell boundaries. . . . . . . . . . . . . . . . . . . . . . . 21
3.4 General setup for the direct method. Arrows indicate the direction of heat flowout of the heat source (red) and into the heat sink (blue). Periodic boundaryconditions are imposed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1 Simulation cell for S1. The heat source is placed at the solid film mid-plane,with the heat sink at the outermost liquid layers. . . . . . . . . . . . . . . . 28
4.2 Top and side views of the solid surface nanopatterning examined: (a) S2, (b)S3, (c) S4, and (d) S5. The patterned unit cell layer is shown in dark gray. . 30
4.3 Temperature profile for S4, the temperatures used in calculating RLSI areshown with darker shades. The liquid temperatures represent those of the IAand EA liquid. In the absence of nanopatterning (S1), only the EA liquid andthe outermost solid film layer contribute to ∆Tav. Inset shows temperatureprofile over entire domain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.4 Liquid density profile for S1, and corresponding surface minimum energy contour 314.5 Liquid density profile for S2, and corresponding surface minimum energy contour 324.6 Liquid density profile for S3, and corresponding surface minimum energy contour 324.7 Liquid density profile for S4, and corresponding surface minimum energy contour 324.8 Liquid density profile for S5, and corresponding surface minimum energy contour 334.9 Vibrational density of states (VDOS): (a) S1, (b) S2, (c) S3, (d) S4, and (e)
C4 in order of increasing pressure. . . . . . . . . . . . . . . . . . . . . . . . . 425.4 Liquid density profiles: (a) W surface, (b) NW surface. Insets show the %
relative increase in adsorbed liquid density peak based on C1 . . . . . . . . . 435.5 Averaged VDOS of LSI atoms for cases C1-C4: (a) W, and (b) NW. SM:
solid mid-plane, SO: solid outer surface, AL: adsorbed liquid. Insets show theVDOS of the adsorbed liquid, and the solid surface superimposed for C1-C4. 44
vii
5.6 Correlation between relative increase in adsorbed liquid density for the NWsurface, and the corresponding relative drop in RLSI. All computations arebased on C1 for the NW surface. . . . . . . . . . . . . . . . . . . . . . . . . 45
6.1 Honeycomb structure of a graphite sheet. Chiral vectors Cnm, and transla-tional vectors Tnm are shown for armchair (5, 5), and zig-zag (8, 0) CNTs. . . 47
6.2 Simulation cell for (5,5) CNT immersed in liquid. Darker region in the liquiddenotes the thermostatted region. The simulation cell has a depth of Lz = 250A. 49
6.3 Temperature history of the CNT (TCNT), and surrounding liquid (TL) in theNVE ensemble following equilibration at 120K. . . . . . . . . . . . . . . . . . 51
6.4 (a) Temperature profile of CNT cooling in liquid after sudden heating to 300K,(b) exponential fit of ∆T (t). . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.5 (a) Close correspondence between total liquid and adsorbed liquid temper-ature (TAds) (b) Radial density distribution starting from the CNT surface.The shaded region denotes where TAds was measured. . . . . . . . . . . . . . 51
6.6 Equilibrium VDOS of CNT (green) and adsorbed liquid (blue) at 120K. Insetis a zoom on 0− 15THz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.7 Time history of CNT temperature (TCNT). The temperature of the adsorbedliquid (TAds) is unaffected by the low TCNT as a result of the high RLSI. . . . 53
6.8 VDOS plots and their corresponding averaging periods following the CNTquench to 1K. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
r, rij position vector, separation distance vector
R Kapitza resistance
ix
t, δt time, time step
T ,∆T temperature, temperature drop
T translational vector
u potential energy
U total potential energy
v velocity vector
V volume
Greek symbols:
ε Lennard-Jones energy parameter
θ bond angle
κ wave vector
λ wavelength
µ distribution function
ρ density
σ Lennard-Jones size parameter
τ relaxation time
φ angle
Φ dihedral
χ velocity scaling factor
ω frequency
Subscripts:
eq equilibrium
l liquid
ll liquid-liquid
x
ls liquid-solid
LSI liquid-solid interface
ne nonequilibrium
R relative
s solid
sp spectral
ss solid-solid
Prefixes:
f femto (×10−15)
n nano (×10−9)
p pico (×10−12)
T tera (×1012)
Abbreviations:
AL adsorbed liquid
CNT carbon nanotube
EA externally adsorbed
IA internally adsorbed
NVE microcanonical ensemble
NVT canonical ensemble
NW nonwetting
SM solid mid-plane
SO solid outer plane
VDOS vibrational density of states
W wetting
xi
Operators:
∇ gradient
xii
Chapter 1
Introduction
1.1 Importance of nanoscale heat transfer at LSIs
With the emergence of Nanotechnology, production and processing of structures that vary
over length scales of several nanometers, has been made standard by recent improvements
in manufacturing, and microanalysis [1]. These nanostructures may interact with a liquid
medium as part of their operation. In many instances, the nature of this interaction is ther-
mal, and dominated by heat transfer between the two phases. Examples of such occurrences
are wide ranging, as depicted in Fig. 1.1. Thermal management is perhaps one of the fields
most influenced by heat transfer at liquid-solid interfaces (LSIs), specifically in relation to
cooling future generation electronic devices. The projected miniaturization of these devices,
coupled with a demand for higher performance [2], renders gas cooling techniques insufficient.
A high heat flux must be dissipated efficiently to protect the reliability, integrity, and per-
formance of the nanoscale device. Liquid-based cooling methods are a promising technique
in dissipating excess heat in high power devices [3–5]. In some liquid-based cooling systems,
the liquid is confined in nanoscale channels that are attached to the heat source, giving rise
to heat transfer across LSIs. Thermal energy transport at LSIs is also a key factor in un-
derstanding heat transfer in nanoparticle suspensions, also termed nanofluids [6]. While the
issue of thermal conductivity improvements in such suspensions remains a contested subject,
a more definitive conclusion will require a broad understanding of LSI heat transfer. Another
field that is affected by LSI heat transfer is the thermal therapy of cancers, based on the
selective heating of magnetic nanoparticles [7]. Hence, the subject of nanoscale heat transfer
at LSIs is of both: theoretical, and industrial significance.
1
Figure 1.1: LSIs encountered in various nanotechnologies.
1.2 Heat conduction at the nanoscale
Heat transfer by conduction is the flow of energy within a medium as a result of a temperature
difference. How well a material allows this form of energy flow is dictated by its thermal
conductivity (k). From Fourier’s law, k can be seen as a constant of proportionality between
the temperature gradient (∇T ) within the material, and the corresponding heat flux (q):
q = −k∇T (1.1)
By combining Eq. 1.1 with an energy balance on a material control volume, the heat diffusion
equation can be derived [8]. In an isotropic material, where the thermal conductivity reduces
to a scalar (k→ k), the heat diffusion equation becomes:
k∇2T = ρC∂T
∂t(1.2)
where ρ is the mass density (kg/m3), C is the specific heat (J/kgK), and t is time. With
suitable initial and boundary conditions, the heat diffusion equation can be solved for the
spatiotemporal temperature profile. In the presence of a LSI, conservation of energy neces-
sitates that the heat flux be continuous at the boundary. Such an assumption is also often
imposed on the temperature. This, however, has no concrete physical basis, especially fol-
lowing the pioneering work by Kapitza [9] on the thermal resistance at liquid helium-metal
2
interfaces, termed the Kapitza resistance (RLSI). The Kapitza resistance can be estimated
using:
RLSI =∆TLSIq
(1.3)
where ∆TLSI is the temperature drop at the interface. When expressed in terms of the
Kapitza length (LLSI = kRLSI), it provides a measure of the bulk material over which an
equivalent temperature drop can be attained. From the macroscopic perspective, the tem-
perature drop at the LSI can be negligible, with LLSI ranging from an order of a molecular
size (for a wetting surface), to tens of molecular sizes (non-wetting surface) [10]. However,
for structures at the nanoscale, this can have a significant impact on the thermal energy
transport across the LSI.
One of the major limitations in applying Eq. (1.2) to nanoscale heat transfer at the LSI,
comes from the assumption that k is independent of the length, and timescales, involved. In
other words, the thermal conductivity of the material is unaffected by the size of the domain,
or the rate at which heat is added. At the nanoscale, this becomes problematic from both
the solid’s, and the liquid’s perspective. In crystalline solids that are dielectric, and when
photon’s contribution to heat transfer are negligible [11], thermal transport is dominated
by crystal lattice vibrations, or phonons. The highly efficient heat transfer in such solids is
brought about by the ability of phonons to travel ballistically over relatively long distances
(phonon mean free path), before being scattered by other phonons, structural defects, or
boundaries. The mean free path of phonons in solids can cover a range of 10− 100nm [12].
The length scales associated with a nanostructure that is present at the LSI, are therefore
comparable to these thermal energy carriers. In such scenarios, heat conduction cannot be
described solely by the heat diffusion-based Eq. 1.2. Furthermore, the phonon spectrum
is sensitive to the local temperature, and can be affected during a nanoscale heat transfer
process. Consequently, the assumption of constant thermal conductivity, k, can be argued
at the nanoscale.
3
When in contact with a solid surface, the structure of a liquid can exhibit a higher degree
of order in comparison to the liquid bulk. The density distribution is also affected, with a
profile that displays an oscillatory behavior at the LSI. This occurs as a result of the strong
interaction between the liquid and solid atoms. The oscillation in density, or layering, can
extend over several atomic distances, depending on the interaction strength with the solid
atoms [13, 14]. The structural variations in the liquid at the LSI have important effects
on properties such as: liquid-solid phase transition [15–17], flow and tribology [18–21],and
viscosity increase due to confinement and solidification [22]. The lack of a highly ordered
atomic structure in the bulk of confined liquids prevents the superior form of thermal energy
transport witnessed in crystalline solids, and results in an effective mean free path of the
order of an atom size [23]. The ordering of the liquid at the LSI, on the other hand, is
believed to facilitate ballistic heat transport, enhancing by that its thermal conductivity
[24]. This is thought to be one of the factors for the improved thermal conductivity in
nanofluids [25]. These effects on the physical properties of liquids at the nanoscale LSI are
ignored in continuum based modeling. The mentioned modifications to the liquid structure
and distribution at a nanoscale LSI, are affected not only by the strength of interaction
between the liquid and solid on an atomic level, but also by the atomic surface structure of
the solid itself. In continuum based approaches, the lack of a proper treatment of the above
factors in modeling nanoscale heat transfer across a LSI, severely limits its applicability.
Theoretical models based on phonon physics have been proposed to attempt alleviating
the deficiencies in continuum modeling. Such models include the Acoustic Mismatch Model
(AMM), and the Diffusive Mismatch Model (DMM) [26,27]. In the AMM, the transmission
and reflection of thermal phonons is governed by the acoustic impedance (which is the
product of density and speed of sound ρc) of the materials at the interface. This is usually
many orders of magnitude larger in a solid than in a liquid. As a result of this acoustic
mismatch, a sizable fraction of the phonons impinging upon a LSI from both sides is impeded,
4
resulting in a temperature discontinuity at the LSI. The DMM, is stochastic in nature, and
assumes all phonons to be diffusively scattered with a probability that is dependent on the
bulk density mismatch between the solid and the liquid. The AMM is known to over predict
the LSI thermal resistance, and thus, provide an upper bound on RLSI. The DMM, on the
other hand, provides a lower bound on the LSI thermal resistance. Their lack of accuracy
stems mainly from the assumption that the bulk material properties are applicable at the
nanoscale LSI, in addition to neglecting the atomic details of the liquid and solid structures.
While these models suffer from some of the disadvantages of continuum modeling, they still
provide better insight into the parameters that can be controlled to manipulate the LSI
thermal resistance.
For a proper modeling of thermal energy transport at a nanoscale LSI, an atomic resolu-
tion is necessary. This is more the case, considering the fact that experimental work at these
scales is still in its infancy [1]. Thus, computational modeling is a vital tool for the proper
understanding of this phenomenon, and the overall advancement in the field of nanoscale
heat transfer. One such powerful computational tool is Molecular Dynamics (MD), which
has been invaluable in shedding light on the physics behind nanoscale heat transfer. This
is the primary tool used in the thesis to examine various aspects of LSI heat transfer at
the nanoscale. In Chapter 2, a literature review of MD modeling of LSI heat transfer is
summarized, based on which the objectives of the thesis are laid out. An introduction to
MD is given in Chapter 3, along with the simulation strategies employed throughout the
thesis. Chapters 4-5 contain the first two studies in the thesis which examine the effects of
nanopatterning, and system pressure on RLSI, respectively. In Chapter 6, a frequency based
analysis of RLSI in Carbon Nanotubes (CNTs) is investigated. Important derivations, and
supporting documents, can be found in the Appendices.
5
Chapter 2
Literature Review
In one of the earliest studies on nanoscale LSI heat transfer [28], MD simulations were carried
out to measure the LSI thermal resistance. The system consisted of a saturated vapor region
sandwiched between two liquid regions that were in contact with the solid walls. Fluid atoms
interacted via the Lennard-Jones (LJ) potential with the parameters of Argon (Sec. B.1).
The solid walls were modeled using three layers of a face-centered-cubic (fcc) (111) surface
of harmonic particles. The solid atomic mass, spacing, and spring constant, corresponded
to those of Platinum (Pt), which in this case was assumed to be electrically insulating.
The temperature of the solid surfaces was controlled through the use of external phantom
molecules [29]. The interaction strength parameter between the liquid and solid atoms was
varied to study the effects of the degree of surface wettability. The wall temperatures were
maintained at 100K and 120K, respectively. The importance of this study, is that in almost
all of the following work in the literature, a similar computational setup was used. The study
reported a temperature jump at the LSI, which decreased with increasing surface wettability.
Consequently, LSI thermal resistance was found to drastically drop with stronger liquid-solid
interaction. A similar model was later incorporated [30], excluding the vapor region. The
study attempted to explain the temperature jump in terms of an intermolecular energy
transfer model that they developed. It was proposed that the temperature jump was related
to an energy exchange at the LSI that was opposing the ’macroscopic’ heat flux direction.
Through experiments, and MD simulations, the work in [31] investigated the interfacial
thermal resistance of individual Carbon Nanotubes (CNTs) immersed in a hydrocarbon
liquid. They discovered that despite the high thermal conductivity of CNTs, the interfacial
thermal resistance was remarkably excessive with most of the temperature drop occurring
6
at the interface between the heated CNT and the liquid. They attributed this trend to the
coupling of only a small number of low frequency vibrational modes between the CNT and
the surrounding liquid. This in turn required that the high frequency vibrational modes
of the CNT be first transferred to the low frequency modes, via phonon-phonon coupling,
before being exchanged with the liquid matrix. The proposed explanation was later further
supported by MD, in which shorter CNTs displayed higher interfacial resistance. This was
attributed to shorter CNTs possessing less of the low frequency modes, resulting in a weaker
coupling to the liquid [32].
Two different regimes in LSI Kapitza resistance were reported in [33], based on the
wetting/nonwetting characteristics of the solid surface. Liquid and solid atoms were modeled
using the LJ potential, but with the solid-solid interaction being ten times larger than that
within the liquid. The liquid-solid interaction strength was varied to study its effect on
the Kapitza resistance. In addition to confirming the sizable drop in the Kapitza resistance
with increased wettability, the study pointed out a switch in the functional dependence of
the Kapitza resistance on the liquid-solid interaction strength. In the wetting case, the LSI
thermal resistance was inversely proportional to the interaction strength parameter. Whereas
in the non-wetting case, there was an exponential drop in the LSI thermal resistance as the
interaction strength was increased. One of the interesting findings of the study was that
the liquid layering at the solid surface was found to exhibit no noticeable effect on the
thermal transport properties. This aspect was later investigated [23], and it was concluded
that the liquid layering at the LSI had a surprisingly small effect on the thermal transport
characteristics of the liquid, with a thermal conductivity that was indistinguishable from
that of the bulk. This was found to be true regardless of whether the solid surface was
wetting or non-wetting, and for both cases where the heat traveled normal, and parallel
to the layering. To explain the findings, three main factors were suggested as follows: (i)
although the liquid at the LSI was more ordered relative to the bulk, it still lacked the degree
7
of order witnessed in solids, (ii) the ordering extended only over several atomic distances in
the liquid, and (iii) the liquid atoms interaction with solid atoms can result in the scattering
of collective motions of the layered liquid. Thus, it was concluded that while layering can
significantly affect the liquid mechanical properties, that was not the case when it comes to
thermal characteristics. Contrary to a gas-solid interface, for which the degree of layering at
the solid surface enhances thermal transport [34]. The study did, however, acknowledge that
the findings can be different for more complex liquids, or for other computational setups.
The effect of nanostructural patterning on LSI heat transfer was first reported in [35], in
which the effect of the solid wall structure on the transfer of momentum and thermal energy
at the LSI was investigated. Three different crystal planes of an fcc structure were used in
shearing a confined liquid (both atomic and linear molecular). Two types of energy transfers
were studied: parallel, and normal to the wall velocity. For the parallel component, the
dictating factor was the atomic roughness of the solid wall (or the unevenness of the potential
surface). As for the normal component, which was responsible for the heat transport at the
LSI, the key factor was the number density of the solid surface plane interacting with the
liquid at the LSI.
An investigation of the relationship between velocity slip and thermal slip in a nanochan-
nel flow was carried out in [10]. Two types of liquids were tested, simple monatomic and
polymeric liquids. For both cases, all atomic interactions were governed by the LJ poten-
tial. In the case of the polymeric liquid, FENE springs [36] were used to connect adjacent
atoms. The liquids were heated in two forms: (i) by shearing, and (ii) by adding thermal
energy to the liquid in the absence of shear. The interaction between the liquid and solid
surfaces was found to strongly influence both: the velocity, and thermal discontinuities. The
thermal discontinuity was found to exist even in the absence of a velocity slip at the LSI.
The polymeric liquid witnessed a higher velocity slip which was coupled by an increase in
the Kapitza length by up to a factor of two. The main conclusion was that the LSI heat
8
transfer was strongly coupled to LSI momentum transfer.
The effect that different nanometer structures had on the energy exchange at the LSI was
investigated in [37]. Three different rectangular structures were patterned on the wall, with
the same number of atoms each. Atoms for the liquid and solid were modeled using the LJ
potential. A direct correlation between the density of particles that are adsorbed along the
nanostructures, and the effectiveness of energy exchange was concluded. The performance
of the various nanostructures was, however, also dependent on other factors such as the fluid
density, and the temperature gradient.
While all previous studies focused on systems that involved periodic boundary conditions,
the work in [38] explored the LSI thermal transport in a triangular heat pipe by extending
the system in [28] to include side walls. It was reported that for a weak interaction strength
between the liquid and the solid atoms, the lateral walls played a minor role, and the LSI
thermal resistance was dominated by the evaporation and condensation zones at the lower
and upper walls, respectively. The further increase in the interaction potential resulted
in the increase of the LSI thermal resistance along the lateral walls. This also acted to
transfer the liquid from the condensed region to the evaporation region. At its initiation,
this phenomenon was found to provide an optimal LSI thermal transport, since cooler liquid
was reversely transported to the evaporative region.
The work reported in [39] proposed a solid wall model that is more efficient computation-
ally for the purpose of LSI thermal transport studies. This was accomplished by excluding
interatomic interactions within the solid surface layers. Instead, each wall atom was tethered
to an equilibrium position by harmonic springs. The temperature control of the solid surface
was maintained by scaling the atomic velocities on a per layer basis. The effectiveness of the
wall model was demonstrated by simulating the effects of liquid-solid interaction strength
on the Kapitza resistance. Also, the effect of the solid wall stiffness on the temperature
jump at the LSI was shown, with the stiffer walls resulting in a higher temperature jump.
9
These effects were later investigated by the authors in more detail, in an attempt to extract
empirical models [40].
The effect of the nanostructural clearances along a nanochannel wall on the LSI thermal
resistance was studied in [41]. The SPC/E water model [42] was used for the liquid part,
while the solid was modeled using the LJ potential, with parameters representative of iron.
It was reported that the LSI thermal resistance passed through a minimum value at which
the residence time of the water molecules at the LSI was found to be maximum. Thus,
facilitating by that optimum thermal energy transport.
Building on the concept of the acoustic mismatch between the solid and the liquid at the
LSI, the study in [43] demonstrated that it was possible to reduce the LSI thermal resistance
by: increasing the fluid pressure (through a temperature increase in a fixed volume), and
enabling additional liquid layering through increased surface hydrophilicity. The SPC/E
water model was used for the liquid, while for the solid wall, silicon atoms were utilized that
were tethered, and interacted via an LJ potential.
The transient behavior of nanoscale LSI heat transfer was reported in [44], in which
an argon-iron system was modeled. The LJ potential was used for the liquid argon, and
the embedded atom model (EAM) [45] for the solid iron. It was demonstrated that the
LSI thermal resistance is time dependent, increasing as the liquid equilibrated towards the
increased solid temperature. The system was also found to equilibrate at a much slower rate
compared to analytical predictions based on continuum modeling of heat conduction.
The LSI thermal transport in a water-silica (SiO2) system was modeled in [46]. The
water molecules, in addition to forming an adsorbed layer at the solid surface, also pene-
trated the silica cells (modeled using an LJ type potential) for a depth of approximately
two molecular diameters. This resulted in significantly lowering the LSI thermal resistance,
relative to silicon membranes where no such water penetration is present. The successive
absorption/adsorption water layers was believed to have promoted the ballistic transport of
10
phonons across the LSI.
The thermal boundary resistance of single-walled carbon nanotubes (CNTs) in both
solid, and liquid argon matrices, was investigated in [47]. The aim was also to extend the
previous work in [31] to longer CNTs. It was shown that the interfacial thermal resistance
remained constant for nanotube lengths over a 20 − 500A(1A = 10−10m) range. The LSI
thermal resistance was higher for the liquid matrix compared to the solid one. In support of
the findings in [31], resonant coupling between the low frequency CNT phonon modes and
the matrices was observed. However, these low frequency modes carried only very little of
the total energy. The CNT heat dissipation was, therefore, strongly retarded by the slow
intermode energy transfer from higher to lower frequencies within the CNT.
The mechanism of friction induced fluid heating in a nanoscale Poiseuille flow was exam-
ined in [48]. Two liquids, argon and helium, and two different solid wall materials, silver and
aluminum, were tested. The heat generation in the liquid originated from friction both: at
the LSI, and within the liquid itself. The latter was found to be the dominating contributor
at high flow driving forces. At a low driving force, the stick condition applied at the wall and
the liquid heating was caused by the friction within the liquid. The LSI thermal resistance
affected the steady state liquid temperature, and consequently, the temperature jump at the
solid wall. In the cases where different materials were used for the top and bottom walls, it
was shown that the liquid temperature profile lost its symmetry about the channel center.
Additionally, the wall temperature itself was found to affect the source of friction induced
heating. When the liquid-solid binding energy was lower than the wall temperature, slip
occurred at relatively smaller driving forces, causing the heating to be influenced more by
friction at the LSI.
With previous LSI heat transfer studies focused on controlling the solid temperature, the
work in [49] demonstrated the existence of a temperature jump within the solid surface that
is adjacent to the temperature controlled liquid. The heating and cooling was carried out
11
through liquid water surrounding two silicon walls. This ’solid-side’ temperature jump, in
conjunction with the Kapitza resistance, contributed to the overall hindrance to nanoscale
LSI thermal transport. In contrast to the case of the Kapitza resistance, increasing the
temperature of the interfacial fluid resulted in a higher solid-side thermal resistance.
The study in [50] reported LSI thermal conductance results in functionalized hydrophobic
and hydrophilic silica-water interfaces. It showed an increase in the hydrophilic surface
conductance with temperature. More importantly, a rectifying effect was demonstrated
when reversing the direction of heat flow in the silica slab. This rectification was further
studied with the use of self-assembled monolayers (SAMs) at the silica surface [51]. It was
attributed to the hydrogen bonding in water being highly sensitive to temperature. When
the heat flows from the water side towards the SAMs, the higher temperature results in a
break in the hydrogen bonding, affecting by that the local hydrogen bond network, and the
vibrational characteristics of water. Consequently, causing a relatively lower conductance
of heat in this direction. A more recent study [52] showed the possibility of reducing the
Kapitza resistance across graphene sheet-oil interfaces by functionalizing the graphene sheet
with alkanes that possessed vibrational modes, which coupled well with the surrounding oil
matrix.
In [53], MD simulations were performed for the LSI thermal conductance across water-
SAMs interfaces with varying chemistries. The results were found to be in excellent agree-
ment with experimentally obtained data [54]. The interfaces ranged from the highly hy-
drophobic to the highly hydrophilic. The accuracy of the MD simulations suggested the
possibility of utilizing LSI thermal transport measurements in predicting LSI environments,
and bonding strength.
The work in [55] focused on the effects of water nanoconfinement between two quartz
surfaces on the LSI thermal conductance. An optimum thickness for the water layer was ob-
served, which resulted in the freezing of the water molecules at such an extreme confinement.
12
This was coupled with a good match in vibrational states between the water molecules and
the quartz hydrophilic surface.
The effect of argon liquid layer thickness on the LSI thermal conductivity was investigates
in [56]. The study showed that the thermal conductivity of nanoconfined liquid argon was
higher for a larger film thickness, and a stronger liquid-solid interaction strength. In contrast
to the findings in [23], the liquid layering at the LSI was found to affect the thermal conduc-
tivity. The degree of layering was dependent on the temperature in addition to liquid-solid
interaction strength. Interestingly, the study reported a drop in the thermal conductivity
of the film for stronger liquid-solid interactions due to higher depletion in the second liquid
layer. This was argued to affect the frequency of migration of higher temperature liquid
atoms.
The study in [57] investigated the effect of the magnitude of the driving force on the LSI
thermal resistance in nanochannel driven flows. Liquid argon flows in both copper, and silver
solid channels were tested. It was shown that the LSI thermal resistance had an optimum
value as a function of the driving force. The initial increase in the LSI thermal resistance
was attributed to reduction in the vibrational mobility of the liquid atoms due to a stick
boundary condition, and a stiffening effect from the applied force. Once the force is high
enough to induce slip at the LSI, the friction heating of the liquid atoms at the LSI enhanced
their ability to transfer energy to the walls, resulting in a drop in the LSI thermal resistance.
Thus, the liquid-solid interaction strength was concluded to be an important factor for this
phenomenon to exist.
The thermal conductivity of adsorbed liquid argon layers was compared to that of the
bulk liquid in [24]. An improvement of up to a factor of 2 was reported. This was attributed
mostly to the relatively higher density of the liquid at the LSI, with a less pronounced
influence of the increased structural order of the liquid. The same group later examined
the effects of thickness of the thermal conductivity of a liquid confined between two solid
13
slabs [58]. It was shown that the thermal conductivity improved as the thickness was reduced,
with a maximum enhancement in the case of a liquid monolayer. This was demonstrated to
be caused by a relatively better vibrational matching between the liquid and solid atoms at
the LSI.
The work in [59] examined the effects of surface functionalization on the LSI thermal
resistance of water-silica interfaces. First, they demonstrated a significant improvement in
the LSI thermal conductance with increased surface hydrophilicity, via (-OH) terminated
headgroups. For the second part, geometric functionalization was explored, in which silica
nanopillars were added to the surface. This was also found to improve the LSI thermal con-
ductance at the base surface. This rise in conductance plateaued as the nanopillar heights
were increased. This behavior was attributed to the variations in vibrational coupling be-
tween the two phases at the LSI.
A similar, but more detailed, analysis was conducted in [60] for water-SAM interfaces.
The study demonstrated a nonlinear drop in LSI thermal resistance when hydrophobic (-
CF3) headgroups were replaced by hydrophilic ones (-OH). Surface roughness was controlled
by using SAMs with varying chain lengths, and was shown to improve the LSI thermal
conductance. Furthermore, a thermal rectification effect was reported in which the LSI
thermal resistance was lower when heat flow was directed from the SAMs to liquid water.
Over the past years, a substantial amount of knowledge has been accumulated on the
subject of LSI thermal resistance, using MD simulations. The work in this thesis attempts
to contribute to these findings, by exploring new areas in this exciting field. This is broken
down into three main parts, as follows:
1. The issue of surface geometric functionalization is considered via nanopat-
terning in Chapter 4. In this regard, the studies summarized above lacked a
systematic approach in analyzing the important factors contributing to ther-
mal energy exchange between liquid and solid atoms at the LSI. Therefore,
14
this part of the thesis examines how the level of nanopatterning affects both:
the interaction potential between the liquid and solid atoms, in addition to
the solid surface vibrational characteristics and its coupling to that of the
adsorbed liquid.
2. The effects of pressure on LSI thermal resistance have not being explored yet.
For nanoscale heat transfer at solid-solid interfaces, pressure has been shown
to have a contrasting effect, depending on the strength of bonding between
the two solids [61]. In Chapter 5, this issue is examined for both: wetting,
and nonwetting surfaces. Apart from it being a possible avenue for tuning the
thermal resistance at LSIs, a good understanding of pressure effects on LSI
thermal resistance is important for reliable MD modeling. This is given that
pressure levels can vary during a MD simulation as a result of energy addition,
or extraction, in a constant volume system.
3. There is strong evidence that the vibrational coupling between a solid and
liquid at the LSI is limited to a relatively low frequency range. Under such
a scenario of frequency dependent thermal energy exchange, the efficiency of
heat conduction between low, and high frequency modes within the solid, be-
comes a crucial factor. Such is the case in CNTs, where it has been shown
that the high LSI thermal resistance is not merely a result of weak interaction
strength with the liquid, but rather as a result of the impeded inner low-to-
high frequency energy exchange within the CNT itself. In Chapter 6, a novel
technique is devised to provide a clearer view for the frequency dependence of
energy exchange between a CNT, and its surrounding liquid. This is carried
out with the aim of locating the origin of the inadequate internal energy ex-
change in CNTs, with potentially significant implications on their LSI thermal
resistance.
15
Chapter 3
Molecular dynamics: An introduction
3.1 Overview
Molecular dynamics (MD) is a computational tool that can predict the behavior of materials
at the atomic level. It is a discrete method in which the MD particles represent atoms
constituting the examined material. The two main components in MD modeling are the
atomic structure, and the interatomic potential function (u). The functional form of u can
be fitted to match experimental data, or quantum mechanics calculations [62]. Ideally a
potential energy function must combine simplicity, accuracy, efficiency, and transferability.
Generally, the total potential energy (U) of a system of MD particles can be expressed as a
sum of terms involving single, pairs, triplets, and so forth, of atoms [63]:
U =∑i
u1(ri) +∑i
∑j>i
u2(ri, rj) +∑i
∑j>i
∑k>j>i
u3(ri, rj, rk) + ... (3.1)
The summation limits prevent the inclusion of the interaction potential between a set of
atoms more than once. The first term in the potential energy function, u1(ri), can be used
to incorporate the effect of an external field acting on the system. All the remaining terms
represent particle-particle interactions. The inclusion of more than two-body interactions is
computationally demanding, and when possible is either ignored, or its effects are incorpo-
rated into u2. In that case, and in the absence of an external potential field, the potential
function reduces to a two-body, or pair potential. The total pair potential depends only on
the distance of separation between two particles (rij), and U is now1:
U =∑i
∑j>i
u2(ri, rj) =∑i
∑j>i
u(|ri − rj|) =∑i
∑j>i
u(rij) (3.2)
1The 2 subscript in u2 is dropped for clarity.
16
The motion of each atom in MD is governed by Newton’s second law, which is given by:
d2ridt2
=Fi
mi
(3.3)
where t is time, mi is the mass of atom i, ri is its position vector, and Fi is the total force
vector acting on the atom. In the case of the simple pair potential, Fi is calculated as follows:
Fi = −∑i6=j
Fij = −∑i6=j
∂u(rij)
∂rij= −
∑i6=j
∂u(rij)
∂rij
∂rij∂rij
= −∑i6=j
∂u(rij)
∂rijrij (3.4)
where Fij is the force acting on atom i by atom j, rij = ri − rj is the separation distance
vector, with rij as its unit vector. A depiction of these vectors is shown in Fig. 3.1.
Figure 3.1: Separation distance and force vectors between atoms i and j.
3.2 Equations of motion for MD systems
Following the calculation of forces, Eq. 3.3 must be solved to advance the trajectory of the
system in time. One of the most commonly used methods in solving the atomic equations
of motion is the finite-difference Verlet algorithm [64]:
rt+δt = 2rt − rt−δt + δt2at (3.5)
vt+δt = 12δt [rt+δt − rt−δt] (3.6)
17
where δt is the time step, v, and a = F/m, are the velocity and acceleration vectors,
respectively. One drawback of the Verlet algorithm is that velocities do not appear in Eq.
3.5, and are only calculated following the atomic positions update. To add, or remove, energy
(kinetic) from a MD system, it is important that velocities appear in Eq. 3.5. This issue
is discussed further in Sec. 3.5. Furthermore, Eq. 3.6 has errors of O(δt2), compared to
O(δt4) for Eq. 3.5. To resolve the above shortcomings, the velocity-Verlet algorithm was
proposed [65], which is carried out in the following order:
rt+δt = rt + δt vt + 12δt2at (3.7)
vt+ 12δt = vt + 1
2δt at (3.8)
vt+δt = vt+ 12δt + 1
2δt at+δt (3.9)
The first step Eq. 3.7 updates the positions of the atoms to t + δt using the positions,
velocities, and accelerations at t. The velocity update takes place in two steps, and starts
with calculating the velocity at t+ 12δt using the acceleration at t. Then there is a ’hidden’
step between Eq. 3.8 and Eq. 3.9 in which the forces at t + δt are calculated based on the
updated positions. These forces are then used to complete the velocity update to t+δt. The
steps for the velocity-verlet algorithm are depicted in Fig. 3.2. The derivation of Eqs. 3.7-3.9
is given in Appendix A. The velocity-Verlet algorithm is mathematically equivalent to the
original Verlet algorithm, and is the algorithm choice in this reported work. The choice for
a time step δt is problem dependent. A larger time step would require less computational
time, but it should always be limited to ensure that the system dynamics with the highest
frequencies are sufficiently captured. An additional good measure of the suitability of the
time step is how strongly the total energy of the system is conserved during a run.
18
Figure 3.2: Velocity-Verlet algorithm steps. Cells with darker shades indicate current vari-able time step. Solid arrows are for updates, and broken arrows indicate the variables used.
3.3 Thermodynamic properties
In MD simulations, thermodynamic properties are calculated from the dynamics of atoms
in the system, in what could be viewed as a ’bottom up’ approach. The total energy of the
system, E, is the sum of the total potential energy U , and kinetic energy K:
〈E〉 = 〈U〉+ 〈K〉 = 〈N−1∑i
N∑j>i
u(rij)〉+ 12〈N∑i
miv2i 〉 (3.10)
where the brackets 〈...〉 denote ensemble averaging, N is the total number of atoms in the
system, and the pair potential assumption is implied (Eq. 3.2). In heat transfer studies,
two important thermodynamic parameters are the temperature (T ), and pressure (P ), of
the system. For an atomic system these are calculated using:
T =2〈K〉3Nkb
=〈∑N
i miv2i 〉
3Nkb(3.11)
P =NkbT
V+
1
3V〈N−1∑i
N∑j>i
rij · Fij〉 (3.12)
where kb is the Boltzmann constant. Both expressions can be derived from the generalized
equipartition theorem [63]. A quick check of Eq. 3.11 shows that 〈K〉 = (3N)12kbT , i.e.
each degree of freedom contributes on average an equal amount of 12kbT , in accordance with
the equipartition principle. The two terms in the pressure expression Eq. 3.12 represent
19
contributions from the system kinetic and potential energies, respectively. For an ideal gas
the second term, which is referred to as the virial, vanishes. An extensive derivation of Eq.
3.12 can be found in [66].
3.4 Simulation strategies
The proper initialization of a MD simulation is essential in avoiding instabilities. This is best
accomplished by ensuring that atoms start at their equilibrium solid state positions, where
they witness no forces. Liquids are initialized in a similar manner, but with the initial solid
structures melting into a fluid phase. To set the simulation in motion, atoms are assigned
random velocities from the equilibrium Maxwell-Boltzmann distribution given by:
µ(vα,i) =
(mi
2πkBT
) 12
exp
[−1
2miv
2α,i
kBT
](3.13)
where α = x, y, z. This velocity assignment is done under the condition that there is zero
net momentum:∑N
i=1mivα,i = 0, in all three dimensions.
The most computationally demanding element of any MD simulation is the calculation
of the atomic forces, as given for example in Eq. 3.4 for a pair potential. When the potential
function is short-ranged, as is the case for the Lennard-Jones (LJ) potential (see Appendix
B), the majority of the contribution to the potential energy, and forces, comes from nearby
atoms. If only atoms within a cutoff sphere of radius rc are considered (see Fig. 3.3), the
computations can be reduced by a factor of 43πr3c/V , where V is the system volume. In fact,
at rc = 2.5σ (σ is the LJ size parameter) , the potential is only 1.63% of the well depth. It
can also be shown that for this choice of rc, the total potential from rc to∞ is only ≈ 2% of
the total potential from σ to rc. Hence, given the computational savings, this is the common
practice in MD modeling. To avoid the discontinuity in the truncated potential at r = rc,
the potential can be adjusted by removing its constant term at that separation distance:
20
u(rij) = u(rij)− u(rc), for rij ≤ rc.
The size of systems accessible to conventional MD simulations are (for the time being)
limited to a few hundred nanometers. This limitation can be overcome by employing periodic
boundary conditions as depicted in Fig. 3.3. The central cell A is infinitely replicated to
produce an unbound two-dimensional space. Each atom and its infinite images have exactly
the same dynamics. When an atom crosses a boundary, as shown at the top of cell A, its
image from cell H renters cell A, which ensures the conservation of the number of atoms.
Atoms at boundaries interact with images of the atoms in the adjacent cells, which is known
as the minimum image convention. An example of that is shown by the atom at the left
boundary of cell H.
Figure 3.3: MD system with periodic boundary conditions, and minimum imaging, shownhere in two dimensions. Simulation cells B-I are images of cell A. Each cell is of size L× L.Cutoff circles of radius rc are shown by darker shades, with the one at the H-G borderrepresenting minimum image convention. Arrows show atoms crossing cell boundaries.
21
3.5 Thermostatting, heating, and cooling
MD simulations in the microcanonical ensemble (NV E) involve a fixed volume (V ), and a
constant number of atoms (N), and total energy (E). An alternative to the NV E ensemble
is to control the system temperature by adding/removing energy from the system, in what
is termed the canonical (NV T ) ensemble. The temperature control can be performed by
adjusting the system kinetic energy via velocity scaling. If the current ’instantaneous’ tem-
perature (as opposed to the actual ensemble averaged temperature) of the system is T1, and
the aim is to adjust the system temperature to T2, then from Eq. 3.11, the temperature
ratio can be expressed as:
T2T1
=
∑Ni (χvxi)
2 + (χvyi)2 + (χv2zi)∑N
i v2xi + v2yi + v2zi
orT2T1
= χ2 (3.14)
Therefore, the scaling factor is χ =√T2/T1. Although this form of direct velocity scaling
does achieve the required temperature T2, it lacks physical soundness since it prevents fluc-
tuations in the system temperature, that are present in the NVT ensemble. The Berendsen
thermostat [67] is an alternative method that allows such temperature fluctuations. This
is done by relaxing the system temperature towards the target temperature, To over a pe-
riod of time, τ , according to: T2 = T1 + (δt/τ)(To − T1), where T2 is now an intermediate
temperature. Using the result from Eq. 3.14:
χ2T1 = T1 +δt
τ(To − T1) or χ =
√1 +
δt
τ
[ToT1− 1
](3.15)
Thus, τ can be seen as a coupling parameter to a heat bath, the value of which should be
chosen to allow adequate temperature control without overly suppressing the fluctuations.
The Berendsen thermostat does not sample the NVT ensemble, but rather (depending on the
choice of τ) an ensemble that lies between those of the NVE and the NVT [68]. Nevertheless,
22
because of its simple approach and effectiveness, the Berendsen thermostat is frequently used
to initialize the system temperature. This is the thermostat of choice in this reported work.
In MD modeling of thermal conductivity, one of two methods can be employed: equilib-
rium MD, as in the Green-Kubo method, or nonequilibrium MD, as in the direct method.
Each of these methods have their advantages and drawbacks [69]. In the Green-Kubo
method, the thermal conductivity is deduced from integrating the autocorrelation of the
thermal current, for a system in equilibrium. The direct method, on the other hand, mim-
ics an experimental setup numerically by imposing a temperature gradient (or a heat flux)
across the system. The thermal conductivity is then computed using the resulting heat flux
(or temperature gradient) in Fourier’s law. Relative to the direct method, the Green-Kubo
method requires longer simulation times. Although the Green-Kubo method can be applied
at the LSI [70], the smaller number of atoms present at the LSI region only exacerbates the
computational requirements. This renders the direct method a more convenient approach
towards MD studies of LSI thermal conductance, as has been the case in the overwhelming
majority of reported studies.
In this work, the direct method is used in Chapters 4, and 5, to model the LSI Kapitza
resistance. This is accomplished by applying a heat flux (q) to the system, then using the
temperature drop across the interface (∆TLSI) to estimate the Kapitza resistance: RLSI =
∆TLSI/q. There are advantages to applying a heat flux as opposed to imposing a tempera-
ture gradient via temperature control. The latter does not conserve the initial total energy
of the equilibrated system, and is more susceptible to momentum drifts. Additionally, the
calculation of heat flux is far more demanding compared to temperature, and longer simu-
lation times would be needed to ensure sufficient convergence in the measured flux. On the
other hand, the application of a heat flux can be carried out in the NVE ensemble, where
total energy is strongly conserved. This is accomplished by moving a predetermined amount
of kinetic energy (−∆K) from atoms in the heat sink, and delivering that same energy to
23
atoms in the heat source (+∆K). When repeated every time step, this induces a heat flux
in the system, as is depicted in Fig. 3.4 for a solid film. In this scenario, the heat flux is
given by: q = ∆K/(2δtA), where A is the cross-sectional area normal to the flux, and the
division by 2 is to account for the fact that the heat flux travels along ±x.
Figure 3.4: General setup for the direct method. Arrows indicate the direction of heat flowout of the heat source (red) and into the heat sink (blue). Periodic boundary conditions areimposed.
The heat addition/subtraction can be carried out without affecting the net linear mo-
mentum of the atoms in the heat source/sink as follows [71]:
KR =1
2
N∑i
miv2i −
1
2
N∑i
miv2g (3.16)
χ =
√KR ±∆K
KR
(3.17)
vi,new = vg + χ (vi − vg) (3.18)
where vg =∑N
i mivi is the net momentum of the atoms in the heat source/sink, and KR
is the relative kinetic energy, from which the energy associated with the net momentum is
deducted. In Eq. 3.18, vg is added back after scaling is applied to the net velocities.
A different technique is used in Chapter 6, which is based on the ’relaxation’ method.
This involves applying a heat pulse to the solid by direct velocity scaling. The solid is then
allowed to equilibrate thermally with the surrounding liquid, without any further tempera-
ture controls. Such a method is suitable for LSI heat transfer studies that involve nanofibers,
24
and/or nanoparticles, in liquid suspensions. This is in order to avoid the excessive perturba-
tions to the dynamics of the system, when a heat flux or a temperature control is constantly
imposed on the entire solid.
3.6 LAMMPS, WestGrid, VMD, and Octave
Prior to utilizing any MD softwares, codes were developed personally, compiled in Fortran,
and validated from literature. This was carried out for LJ systems [63], SPC/E water [42],
in addition to carbon nanotubes, and graphene sheets [72]. The aim of this process was to
experience firsthand the various computational aspects of MD. One of these aspects is the
statistical nature of the measured thermodynamic variables. By opting for larger systems,
variable statistics can be considerably improved. This was the motivation to take advan-
tage of the parallel computing resources, provided by the WestGrid clusters. LAMMPS [73]
(Large-scale Atomic Molecular Massively Parallel Simulator) is one of the MD softwares
installed on WestGrid. It is extensively used in research involving nanoscale heat transfer,
as it is well suited for these types of MD simulations. For Chapters 4-5, the initial atomic
structures were created using personally developed codes. In Chapter 6, the CNT structure
was created through the Nanotube Builder module in VMD [74] (Visual Molecular Dynam-
ics). In both cases, after importing the structures into VMD, atomic structural data files
appropriate for LAMMPS were produced using the Tk Console in VMD.
The calculation of the VDOS from atomic velocities was carried out in a post-processing
fashion. A program was written that extracts velocities from a LAMMPS output file, and
simultaneously computes the velocity-autocorrelation function. This is then imported into
Octave, which contains a biult-in FFT routine, out of which the VDOS plots are produced.
An example of the LAMMPS input script used in Chapter 4 is given in Appendix E, along
with a copy of the velocity-autocorrelation function program.
25
Chapter 4
Lowering LSI thermal resistance with nanopatterned surfaces
4.1 Introduction & Objectives
The ability to fabricate surfaces with nanoscale features, facilitates a means of controlling
the behavior of a liquid in contact with the solid surface [75]. For instance, the wetting
characteristics of a surface can be lowered to enable self-cleaning [76], and reduce friction
drag [77]. Or, on the other hand, the wetting can be enhanced to improve multiphase
flow [78]. Investigations of the effect of surface nanopatterning on wetting [79], and flow
behavior [80] have been reported in recent years. When it comes to analyzing the effects of
surface nanopatterning on LSI heat transfer, factors other than the wetting properties of the
surface need to be examined. Specifically, it is important to understand how modifications
to the surface topology affect its vibrational properties, i.e. its vibrational density of states
(VDOS) (Appendix C). In that regard, previous studies have controlled the surface topology
via self-assembled monolayers (SAMs) [60], or by the addition of nanopillars of the same
material to the surface [37, 41, 59]. While all these studies displayed that surface topology
can affect the LSI thermal conductance, with the exception of [59], none of them examined
the vibrational behavior of the surface. In [59], both the height, and cross-sectional size of the
nanopillars were varied. The improvements in RLSI were attributed to a better vibrational
matching between the two phases, in addition to liquid confinement effects.
In this study, nanopatterning is introduced by periodically removing atoms from the solid
film surface. This is at a relatively smaller scale than has been previously reported. Such
atomic scale surface manipulations can be accomplished via techniques such as scanning
probe lithography (SPL) [81]. With this form of patterning, the connectivity of the modified
atomic layers, apart from the removed portions, is retained. The aim is to examine how
26
nanopatterning at this level can be utilized to lower RLSI. The results for the nanopattered
surfaces are compared to a crystalline smooth surface. The cases that are studied allow a
systematic investigation of how the degree of patterning affects liquid adsorption, and the
vibrational properties of the solid film surface. The combined effect of these two factors
directly influences the liquid-solid vibrational coupling at the LSI, and consequentially, RLSI.
4.2 Simulation details
MD simulations are conducted to model the effects of solid surface nanopatterning on the
LSI thermal resistance. Liquid-liquid, and liquid-solid, interactions are governed by the
Lennard-Jones (LJ) potential (see Appendix B). For the liquid, the interaction parameters
of argon are used with: σll = 3.40 A, and εll = 0.998 kJ/mol. The solid atoms interact via
the Morse potential, with D = 32.0kJ/mol, α = 1.37A−1
, and a nearest neighbor distance of
ro = 2.89A. The interaction between the solid film atoms is limited to the bonded nearest
neighbors in order to prevent the disintegration of the patterned structures. The mass of
the solid atoms is set to ms = 1.6ml, with ml = 0.04kg/mol. The liquid-solid interaction
strength is taken equal to that within the liquid, εls = εll, which results in a wetting surface. A
relatively strong εls is chosen to prevent a wetting-dewetting transition that can be induced
by changes to the surface structure [82]. A size parameter of σls = 0.8σll is used. The
chosen solid film properties are representative of cubic metals [83], and produce vibrational
characteristics that are in good agreement with fcc metals such as Cu.
The simulation cell consists of a solid film surrounded on both sides by liquid, as shown
in Fig. 4.1. The solid film is constructed from a face-centered cubic (fcc) lattice, with a unit
cell length of ao = 4.09A. The simulation cell is 54ao along the z-axis, with two-thirds of the
domain occupied by the liquid. The cross sectional area is 16ao × 16ao in the x-y plane. An
additional half unit cell layer of atoms is added to one of the sides of the solid film to ensure
symmetry of the solid atomic structure that is in contact with the liquid. In all cases there
27
are 12,584 liquid atoms with a bulk density of ≈ 1330kg/m3, which ensures a liquid state at
the simulated temperature, and pressure. Five cases are examined in total (S1-S5), which
includes the case without nanopatterning (S1). In the remaining four cases (S2-S5), the solid
surface at the LSI is patterned by periodically removing atoms in unit cell increments. The
patterning periodicity (pi) is two-dimensional, uniform (px = py), and is equivalent in the
number of unit cells extracted in the x-y plane. Thus, resulting in a checkerboard structure
as shown in Fig. 4.2, where a total of half the atoms in the outermost unit cell are extracted.
The nanopatterning periodicities in unit cells are: px × py =8 × 8 (S2), 4 × 4 (S3), 2 × 2
(S4), and 1 × 1 (S5). Consequent to this choice of patterning, there are a total of 17,920
and 18,944 atoms for the cases with, and without patterning, respectively. The resulting
surface cavities allow a relatively equivalent number of interstitial liquid atoms, which aids
in maintaining the bulk liquid density, and pressure, at comparable levels.
Figure 4.1: Simulation cell for S1. The heat source is placed at the solid film mid-plane,with the heat sink at the outermost liquid layers.
Periodic boundary conditions are used in all directions throughout the simulation cell.
The equations of motion are integrated using the velocity Verlet algorithm [63], with a
timestep of δt = 1.0fs, which was found to provide sufficient energy conservation. A cutoff
radius of rc = 2.5σll is employed for the LJ interactions. The system temperature is first
equilibrated at To = 140K for a period of 1ns using the Berendsen thermostat [67], with
28
a time constant of 0.5ps. Following the equilibration period, at each time step an assigned
amount of energy, ∆Ek = 0.01kJ/mol, is transferred from the liquid atoms in the heat sink
to the solid film mid-plane, as depicted in Fig. 4.1. This is accomplished using an algorithm
that conserves the linear momentum of the system [71]. Therefore, the calculation of the
heat flux along the z-axis is given by: q = ∆Ek/(2δtA) = 207MW/m2, where A is the cross
sectional area in the x-y plane. The simulation is then run without any thermostatting for
10ns, with the last 5ns used for data gathering. This ensures a sufficient convergence of the
temperature profiles in the system.
4.3 Results & Discussion
Temperature data is recorded over bins of size ao/2 ≈ 2.05A along the z-axis. In the cases
with nanopatterned surfaces (S2-S5), the adsorbed liquid is analyzed in two parts: the inter-
nally adsorbed (IA) liquid which resides within the formed surface cavities, and the externally
adsorbed (EA) liquid which adheres to the outermost solid surface. From the final tempera-
ture profiles, the temperature drop across the LSI is used to estimate RLSI = ∆Tav/q, where
∆Tav is taken as the difference between the average temperature of the adsorbed liquid, and
that of the exposed solid film planes (Fig. 4.3). The simulation results for the LSI thermal
resistances are given in Table 4.1. There is a steady drop in RLSI that is associated with
the decrease in the periodicity of nanopatterning, pi. To help explain the trends in RLSI,
the liquid density, and the liquid-solid interaction potential energy distributions, are first
examined. Furthermore, the vibrational properties of the solid film surface, and their level
of matching with the IA and EA liquid, are investigated for all the surfaces. This part of
the study is carried out using separate simulations in the NVE ensemble, at a temperature
of 140K.
29
Figure 4.2: Top and side views of the solid surface nanopatterning examined: (a) S2, (b) S3,(c) S4, and (d) S5. The patterned unit cell layer is shown in dark gray.
Table 4.1: Simulation results. The LSI thermal resistance RLSI has units of (×10−9Km2/W).The % difference in interfacial resistance is based on S1.
Figure 4.3: Temperature profile for S4, the temperatures used in calculating RLSI are shownwith darker shades. The liquid temperatures represent those of the IA and EA liquid. Inthe absence of nanopatterning (S1), only the EA liquid and the outermost solid film layercontribute to ∆Tav. Inset shows temperature profile over entire domain.
Figure 4.4: Liquid density profile for S1, and corresponding surface minimum energy contour.
31
Figure 4.5: Liquid density profile for S2, and corresponding surface minimum energy contour.
Figure 4.6: Liquid density profile for S3, and corresponding surface minimum energy contour.
Figure 4.7: Liquid density profile for S4, and corresponding surface minimum energy contour.
32
Figure 4.8: Liquid density profile for S5, and corresponding surface minimum energy contour.
The liquid density profiles are shown in Figs. 4.4-4.8. A higher resolution of 1A is
employed for the liquid density bins in order to accurately capture the locations of the
density peaks for the IA, and EA liquid. The nanopatterning employed exposes half the
surface area of the lower planes in the solid film, which is consistent with the reduction in
the density of the IA liquid by a factor of ≈ 0.5, relative to the EA liquid density in S1.
The EA liquid in S2-S5 includes the liquid adsorbed to the outer solid film surface, and
the liquid that resides in that same bin over the cavities. This explains the slight reduction
in the EA liquid density from S2-S5, as the area of the cavities is reduced. Although the
density of the IA liquid is comparable among S2-S5, the distribution of the liquid atoms
within the formed cavities is strongly influenced by the nature of patterning. The accessible
area of these cavities in the x-y plane, dictates how the IA liquid atoms are distributed.
For example, in S5, this accessible area is ≈ 3.30A × 3.30A, or ≈ 1.2σls × 1.2σls. This in
turn permits the access of a single liquid atom per cavity at a given time. In addition to
affecting the distribution of the liquid atoms, the nanopatterning influences the adsorption
characteristics of the surfaces. To further illustrate this point, the surface minimum potential
energy contours are also presented in Figs. 4.4-4.8. The contours are constructed over an
area of 8ao × 8ao about the center of the surfaces, using a 250 × 250 grid, which gives a
resolution of ≈ 0.13A in the x-y plane. At each point, the scanning begins at the solid film
33
surface using a liquid atom, and moves along the z-axis in 0.05A increments until the edge of
the liquid-solid interaction zone (at rc from the solid surface). The minimum potential energy
value is then recorded and projected onto the corresponding x-y coordinates. As expected,
for S1 the contour is uniform. When nanopatterning is introduced to the surface, regions of
lowered potential minima appear in the cavities, with the highest intensity resulting from
S5. Hence, with nanopatterning the interaction of the IA liquid with the solid film surface
becomes more localized.
In order to examine the liquid-solid vibrational coupling, the vibrational density of states
(VDOS) are computed by taking the Fourier transform of the velocity autocorrelation func-
tion (Appendix D). For the solid film, this was conducted for all atoms in the middle (SM),
and patterned (SO), planes. Whereas for the liquid, the VDOS is calculated for the IA and
EA liquid layers. As can be seen in Figs. 4.4-4.8, the VDOS for SM (VDOSSM), which is
far removed from the surface, remains essentially unaffected for S1-S5. Compared to SM,
VDOSSO is shifted to lower frequencies. Such a shift is expected as a result of the lower
coordination number of the surface atoms [84, 85]. This effect is enhanced from S2 to S5,
with each patterned surface possessing a lower average coordination number for the surface
atoms. Hence, the most drastic modification for VDOSSO appears for S5, with a pronounced
peak in the lower frequency range (≈ 1.25THz). The more noticeable shift in VDOSSO for S5
is consistent with the vibrational properties of adatoms on metallic surfaces [86,87]. An im-
portant feature in VDOSIA, and VDOSEA, is the zero frequency value, which is proportional
to the liquid self-diffusion coefficient [88]. For VDOSEA, there is an upward shift in the zero
frequency value for S5, compared to S2-S4. This results from the reduction in the EA liquid
adsorption for this surface as indicated in the density profile plot of S5. More noticeable
differences appear in VDOSIA, with the value for S5 being significantly lower. This can be
interpreted using the minimum potential energy contours from Fig. 4.8, where the potential
minima are highly localized in S5. A similar trend has been reported for flows in rough
34
nanochannels [89], for which the residence time of the adsorbed liquid correlated with the
degree of solid wall roughness. There is also a slight shift in VDOSIA to higher frequencies,
when compared to VDOSEA, as a result of the stronger vibrational coupling to the solid film.
This is the most apparent in S5, which develops an additional peak that coincides closely
with that of VDOSSO.
There is a correlation between the trends in RLSI, and the degree of vibrational cou-
pling between the adsorbed liquid, and the solid film surface. To elucidate this relation-
ship, the following quantity is computed separately for the IA and EA liquid: OVDOS,i =∫ +∞−∞ [VDOSi (ω)] [VDOSSO (ω)] dω, where: i = IA, EA, and ω is the frequency. For each sur-
face, the average: 12
[OVDOS,IA +OVDOS,EA] is then normalized by the maximum value, which
belongs to S5. When plotted along with the normalized RLSI, a strong inverse relationship
exists between the two parameters, as shown in Fig. 4.10.
4.4 Conclusion
It is shown through MD simulations that solid surface nanopatterning can notably influence
the LSI thermal resistance. Through nanopatterning, the wetting characteristics of the
solid surface, and its vibrational properties can be tailored to provide a stronger vibrational
coupling with the internally adsorbed liquid. The VDOS of the solid film surface are shifted
to lower frequencies due to the drop in the surface atoms coordination number. When
combined with a highly localized interaction potential minima, as was the case for S5, a
clear drop in RLSI is witnessed. This drop correlates directly with the degree of vibrational
coupling that is computed from the VDOS profiles of the two phases at the LSI. It is hoped
that the findings in this study provide some insight for designing surface nanoscale geometries
with the goal of improved thermal management.
35
Figure 4.9: Vibrational density of states (VDOS): (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5.SM: solid film mid-plane, SO: solid film patterned layers, IA: internally adsorbed liquid, EA:externally adsorbed liquid.
36
Figure 4.10: Correlation between normalized RLSI and CVDOS for surfaces S1-S5.
37
Chapter 5
Pressure effects on liquid-solid interfacial thermal resistance
5.1 Introduction & Objectives
The transmission of thermal energy carriers, or phonons, across interfaces is impeded due
to the vibrational mismatch between the materials in contact. In the case of LSIs, there
is a considerable disparity in the phonon spectrum between the two phases, resulting in
a relatively high RLSI. Recently, the possibility of pressure tuning the interfacial thermal
resistance at solid-solid interfaces has been investigated experimentally [90], and computa-
tionally [61], through MD simulations. It was found that the interfacial thermal resistance
dropped with increasing pressure when the bonding between the two solids was weaker than
that in the bulk. This was attributed to a rise in the interfacial bonding stiffness, resulting
in an enhanced phonon transmission at the interface [91]. In the case where interfacial bond-
ing is stronger, relative to the bulk, MD results indicated that the conductance is relatively
unchanged, with a slight drop at higher pressures. This was explained by the changes to the
VDOS of the two strongly bonded solids at high pressures, where the overlap was somewhat
reduced due to a disproportionate shift to higher frequencies among the two solids.
The effect of pressure on RLSI has not been fully investigated yet, with the focus mainly
on surface functionalization [59,60,92], and liquid confinement effects [58]. In this work, MD
simulations are conducted to examine the effects of pressure on the RLSI. Analogous to the
case of solid-solid interfaces, this is carried out for a wetting (W), and a nonwetting (NW)
solid surface. This is achieved by controlling the liquid-solid interaction strength.
38
5.2 Simulation Details
The simulation setup is similar to that from Sec. 4.2, and consists of a solid film sandwiched
between two liquid regions, as shown in Fig. 5.1. Here, all atoms interact via the Lennard-
Jones (LJ) potential. For all interactions, σss,ll,ls = 3.4A. The interaction strength for the
liquid atoms is taken to be that of argon εll = 0.998kJ/mol. A stronger interaction is chosen
for the solid atoms with εss = 10εll, which provides sufficient cohesiveness for the simulated
conditions. For the wetting case, εls = 1.0, whereas for the nonwetting case εls = 0.1. The
solid film is constructed from a face-centered cubic (fcc) lattice with a lattice constant of
ao = 1.56σ. The simulation cell has dimensions of Lx = Ly = 12ao, in the x-y plane. For
all cases studied, the solid film has a thickness of 12ao along the z-axis. The total Lz of
the simulation domain is adjusted, for a fixed number of liquid atoms, in order to vary the
system pressure. Four cases (C1-C4) are examined for the wetting (W), and nonwetting
(NW) surfaces, respectively. In the initial case (C1), Lz = 33ao, which is subsequently
reduced in increments of 2.526A to steadily increase the system pressure. There are a total
of 15,200 atoms in the system.
Figure 5.1: Pressure effect MD simulation cell.
Periodic boundary conditions are used in all three dimensions. The equations of motion
are integrated using the velocity Verlet algorithm, with a timestep of δt = 1fs. For computa-
tional efficiency, a cutoff radius of 2.5σ is used. The system is first equilibrated at 130K for
39
a period of 1.0ns using the Berendsen thermostat, with a time constant of 0.5ps. After the
equilibration period, a small amount of energy ∆E = 4.90× 10−3kJ/mol, is removed at each
time step from the liquid atoms in the heat sink, and added to the midplane of the solid film,
as depicted in Fig. 5.1. This process is equivalent to inducing a heat flux along the z-axis
with a magnitude of: q = ∆E/(2δtA) = 100MW/m2, where A is the cross-sectional area in
the x-y plane. The energy exchange is run for an additional 15ns, with the last 10ns used for
temperature averaging. This provides sufficient convergence in the developed temperature
profiles.
5.3 Results & Discussion
Data for the temperature profiles along the z-axis is recorded in bins of size ∆z = 0.5ao. The
temperature drop ∆T across the LSI is calculated as shown in Fig. 5.2, and is the average
temperature difference between the outermost solid film plane, and the adsorbed liquid. As is
expected, the temperature profile in the liquid region displays a higher temperature gradient
due to the lower thermal conductivity compared to the solid film.
Figure 5.2: Temperature profile for case C4-NW.
The results for RLSI are given in Fig. 5.3, for both the W and NW surfaces, as a function
40
of pressure. There is a stark difference in how pressure affects the heat transfer across the
LSI for a W, and a NW surface. The pressure effect is negligible for the W surface, with all
resistances ≈ 20 × 10−9m2K/W, as the pressure is increased from ≈ 35MPa to ≈ 60MPa.
For the NW case, RLSI is highly sensitive to the system pressure, and decreases by a factor of
≈ 2, as the pressure is raised from ≈ 34MPa to ≈ 64MPa. To explain the stronger response
to pressure for RLSI of the NW surface, two important factors are analyzed: (i) adsorbed
liquid density, and (ii) vibrational density of states (VDOS) of the atoms at the LSI. The
liquid density profiles are averaged over bins of size ∆z = 0.1ao which sufficiently captures
the liquid layering at the solid film surface. The VDOS is computed by taking the Fourier
transform of the velocity autocorrelation function. This was conducted for all the liquid and
solid atoms used in calculating ∆T , in addition to the solid mid-plane. The calculation of
the VDOS is carried out in the NVE ensemble at 130K.
The adsorbed liquid density profiles are given in Fig. 5.4. When compared to the NW
surface, the stronger interaction between solid and liquid atoms for the W surface results in
a considerably higher liquid density peak (at ≈ 35A) next to the solid film. The pressure
increase has a minor effect on the adsorbed liquid density peak for the W surface. In the
NW case, the increase in the pressure results in a hike in the density peak by up to ≈ 50%,
from C1 to C4. This in turn permits more liquid atoms at the LSI, which enhances the
thermal energy exchange between the liquid and solid atoms. Consequently, this causes a
more significant drop in RLSI for the NW surface as the pressure is increased.
The vibrational properties of the LSI atoms were found to be relatively unaffected by
the pressure increase (from C1-C4) for the W, and NW surfaces, respectively. The averaged
VDOS of the LSI atoms (AL: adsorbed liquid, SO: solid outer surface) are shown in Fig. 5.5,
along with that of the solid film mid-plane (SM). For both types of surfaces, the VDOS of
the SO and SM are similar, with the SO occupying a lower frequency range as a result of the
lower coordination number of the atoms at the film surface. The frequency range occupied
41
Figure 5.3: Pressure effect on RLSI for wetting (W), and nonwetting (NW) surfaces. C1-C4in order of increasing pressure.
by the solid atoms (up to 6.0THz) is, as anticipated, higher than that of the adsorbed liquid
(mostly below 2.0THz). The liquid adsorbed to the W surface vibrates at higher frequencies,
compared to the NW liquid. This is a consequence of the stronger interaction between the
liquid, and the W surface atoms. This improves the AL vibrational coupling with the SO
atoms, and combined with the relatively higher AL density for the W surface, results in a
much lower RLSI (see Fig. 5.3) when compared to the NW surface. An important aspect
of the VDOS in liquids is the zero frequency value, which is directly proportional to the
adsorbed liquids self-diffusion coefficient [88]. The strong interaction between the W surface
and the AL considerably lowers its self-diffusion coefficient [93] relative to the NW surface.
This can be deduced from the AL zero frequency values in Fig. 5.5 (a) and (b), respectively.
In the case of the W surface, within the pressure range studied here, there is little effect
on the adsorbed liquid density, or on the VDOS of the LSI atoms. Hence, RLSI appears
impervious to system pressure. Similarly for the NW surface, there are no noticeable shifts
in the VDOS as a result of system pressure increase. In that case, the significant reduction
in RLSI has to be related to the increase in the AL density. This is demonstrated in Fig. 5.6,
by plotting the relative increase in the adsorbed liquid density peak, along with the relative
42
Figure 5.4: Liquid density profiles: (a) W surface, (b) NW surface. Insets show the %relative increase in adsorbed liquid density peak based on C1
43
Figure 5.5: Averaged VDOS of LSI atoms for cases C1-C4: (a) W, and (b) NW. SM: solidmid-plane, SO: solid outer surface, AL: adsorbed liquid. Insets show the VDOS of theadsorbed liquid, and the solid surface superimposed for C1-C4.
44
Figure 5.6: Correlation between relative increase in adsorbed liquid density for the NWsurface, and the corresponding relative drop in RLSI. All computations are based on C1 forthe NW surface.
drop in RLSI, based on C1 values. There is a clear correlation between the two parameters.
A similar trend of the effect of adsorbed liquid density on RLSI was recently reported for a
nonwetting carbon nanotube [94].
5.4 Conclusion
Through MD simulations, it was shown that the interfacial thermal resistance across a LSI is
appreciably more sensitive to the system pressure when the solid surface is nonwetting. For
the W surface, the pressure appears to have a limited effect on the adsorbed liquid density,
and the vibrational characteristics of the atoms at the LSI. The latter also applies to the
NW surface. On the other hand, the adsorbed liquid density for the NW surface experiences
a significant relative increase with system pressure. This facilitates a noticeable reduction
in RLSI as more liquid atoms exchange thermal energy with the solid atoms at the LSI. The
results imply the possibility of tuning RLSI for NW surfaces through controlling the system
pressure. It also shows that when studying LSI heat transfer involving wetting surfaces,
pressure is not expected to be a key parameter.
45
Chapter 6
Spectral analysis of LSI heat transfer in carbon nanotubes (CNTs)
6.1 Introduction & Objectives
Since the discovery of carbon nanotubes (CNTs) [95], there has been a steady increase in their
projected microscale, and nanoscale applications [96]. While their discovery might be fairly
recent, the treatment of various aspects of CNTs in the literature is exhaustive [97,98]. This
is primarily driven by their superior mechanical, electrical, and thermal properties [97, 99].
A review of synthesis procedures can be found in [98]. Among the attractive applications of
CNTs is employing them as additives for enhancing the thermal conductivity of polymers
[100], organic liquids [101], and water [102], for thermal management applications. Thermal
conductivities of up to 3000W/mK have been measured [103] for CNTs. However, when
submerged in liquids, CNTs suffer from high LSI thermal resistance [31]. A comprehensive
understanding of thermal interactions of CNTs in liquid media is imperative for many of
their expected applications in thermal management.
On the theoretical front, CNTs provide an ideal model for MD studies of thermal energy
transport, with heat conduction in CNTs being primarily due to phonons [104]. In itself, a
CNT is a molecular scale carbon fiber that possesses a well defined vibrational spectrum.
Thus, enabling the mode coupling studies that explained the roots of their low LSI thermal
conductance. When visualizing a CNT, it can be thought of as a rolled up graphite sheet
with a honeycomb pattern, as can be seen in Figure 6.1. The chiral vector, C, defines the
roll up direction in terms of the basis vectors a1 and a2:
Cnm = na1 +ma2, n ≥ m (6.1)
When m = 0, the edges of the CNT take on a zig-zag form, whereas with m = n, the CNT
46
Figure 6.1: Honeycomb structure of a graphite sheet. Chiral vectors Cnm, and translationalvectors Tnm are shown for armchair (5, 5), and zig-zag (8, 0) CNTs.
edges resemble an armchair. For all other chiralities the CNT is termed simply as chiral.
The translational vector, Tnm points along the CNT axis, and is normal to Cnm. The length
of the chiral vector represents the circumferential length of the CNT, and the CNT radius
rCNT is given by:
rCNT =|Cnm|
2π=a√m2 +mn+ n2
2π(6.2)
where a = 1.42√
3A, is the lattice constant of the graphite sheet.
In addition to the vibrational mismatch between a CNT and its liquid surroundings,
there is evidence that the weak interfacial thermal conductance in CNTs is induced by
the the slow transfer of thermal energy between high, and low frequency modes within
the nanotube [32, 47]. This was primarily based on the spectral temperature analysis which
displays a disparity between the energy of modes having frequencies comparable to the liquid,
and those that are higher. The spectral temperature (Tsp) for a frequency range between ω1,
47
and ω2 is given by:
Tsp(t) = Teq
(1
ω2 − ω1
)∫ ω2
ω1
VDOSNE(ω, t)
VDOSEQ(ω)dω (6.3)
where VDOSEQ and VDOSNE are the vibrational density of states at equilibrium and during
thermal energy exchange, respectively. The temperature Teq is the average equilibration
temperature on which VDOSEQ is based. It is believed that the rate of thermal energy
exchange between low and high frequency modes is the limiting factor in LSI heat transfer
for CNTs. However, the causes of this type of obstruction to heat transfer in CNTs remains
unclear. Methods of improving RLSI in CNTs have primarily focused on utilizing chemical
functionalization [40, 105], which provides a vibrational bridge between the CNT and the
surrounding liquid. By understanding the nature of the inter-modal energy exchange in
CNTs, new avenues can be explored to improve RLSI .
6.2 Methodology
MD simulations are conducted to provide a detailed analysis of the frequency dependent
energy exchange in CNTs, that are interacting with a liquid medium. Interactions between
the carbon atoms in the CNT are modeled using the REBO potential [106] (see Appendix
B). The REBO potential is one of the most commonly used potentials in MD of CNTs, and
is capable of capturing important features in the phonon spectrum of the CNT [107]. Liquid,
and liquid-solid interactions are modeled with the LJ potential. Argon parameters are used
for the liquid with σll = 3.4A, and εll = 0.998kJ/mol. For the liquid-solid interactions:
σls = 3.38A, and εls = 0.48kJ/mol [47]. The simulation cell consists of an armchair CNT
with a chirality of (5,5), immersed in liquid, as shown in Fig. 6.2. The CNT has a radius
of ≈3.39A. The simulation cell has a cross section of Lx × Ly = 50A × 50A, and a depth
that is equal to the CNT length of 250A. The CNT contains 2040 carbon atoms, while the
liquid consists of 10,935 atoms which results in a bulk liquid density of 1200kg/m3. Periodic
48
boundary conditions are used in all dimensions, with a time step of 0.5fs. A cutoff radius of
3σll was utilized.
Figure 6.2: Simulation cell for (5,5) CNT immersed in liquid. Darker region in the liquiddenotes the thermostatted region. The simulation cell has a depth of Lz = 250A.
The simulation procedure is based on the thermal relaxation method [108]. Generally, this
involves applying a heat pulse to the solid, followed by an equilibration period during which
the solid is cooled by the liquid. Using the decay of the temperature difference, an exponential
fit can provide an estimate of RLSI based on the lumped capacitance assumption [8]:
∆T (t) = ∆Toe−[
1mlCl
+ 1msCs
]AsR
−1LSIt (6.4)
where ∆T is the time dependent temperature difference between the CNT and the liquid,
∆To is the initial temperature difference, ml and ms are the total mass of the liquid and
the solid, respectively, whereas Cl and Cs are their corresponding heat capacities. For the
system under consideration Cl = 312J/kgK, and Cs = 1000J/kgK [47]. The effective surface
area of the CNT is taken to be: As = πLCNT(dCNT + σls), where LCNT, and dCNT, are the
49
CNT length, and diameter, respectively. Using the relaxation time (τ) from the exponential
fit, the LSI thermal resistance is finally obtained through:
RLSI = τ
[1
mlCl
+1
msCs
]As (6.5)
6.3 Results & Discussion
In this study, the relaxation method is used first to validate the simulation model. The
system is first equilibrated at 120K using the Berendsen thermostat [67], with a time period
of 0.25ps. Due to the high RLSI in this system, separate thermostats are applied to the CNT,
and the liquid, during which momentum conservation is enforced. This circumvents the
need for unnecessarily long equilibration times, and was found to provide a well equilibrated
system, as is shown in Fig. 6.3. Following equilibration, the CNT temperature is suddenly
raised to 300K by velocity scaling, and is held at that value for 10ps. All temperature
controls are then turned off, and the CNT is allowed to cool as shown in Fig. 6.4(a). The
exponential fit to the temperature decay is given in Fig. 6.4(b), and it produced a time
constant of τ ≈ 417ps. This results in a Kapitza resistance of RLSI = 0.971× 10−6m2K/W,
or alternatively a conductance of 1.03MW/m2K, which is in good agreement with values
reported in the literature [47].
During the CNT cooling process, there is little variation in the temperature of the liquid
spatially. This is a consequence of the high RLSI for the CNT, relative to the thermal conduc-
tivity of liquid argon (≈ 0.1W/mK [109]), which was the basis for the lumped capacitance
approximation discussed earlier. To put this into perspective, the Kapitza length in this case
would be LLSI = 97.1nm, which exceeds the radial distance covered by the simulation cell.
To further illustrate this point, the temperature of the adsorbed liquid (TAds) within 4A of
the CNT surface was measured and plotted against the entire liquid temperature, as shown
in Fig. 6.5. There is almost no deviation between TAds, and TL, throughout the simulation
time, regardless of the fact that TCNT starts at 300K.
50
Figure 6.3: Temperature history of the CNT (TCNT), and surrounding liquid (TL) in theNVE ensemble following equilibration at 120K.
Figure 6.4: (a) Temperature profile of CNT cooling in liquid after sudden heating to 300K,(b) exponential fit of ∆T (t).
Figure 6.5: (a) Close correspondence between total liquid and adsorbed liquid temperature(TAds) (b) Radial density distribution starting from the CNT surface. The shaded regiondenotes where TAds was measured.
51
The spectral temperature analysis is primarily based on the disruptions caused to the
VDOS of a heated CNT, relative to a CNT in thermal equilibrium. An example of the VDOS
of the CNT, and the liquid, in an NVE ensemble at 120K is shown in Fig. 6.6. The inherent
CNT peak at a frequency ≈ 55THz [110] can be seen. Additionally, the liquid VDOS extends
to only ≈ 2THz, with a peak at 1THz [47]. If the liquid surrounding the CNT is linked to
an infinite heat bath by thermostatting an outer ring area (Fig. 6.2), then it would be
fair to assume that the adsorbed liquid temperature will remain relatively unchanged when
TCNT is increased, or decreased. To provide a clearer picture of the frequency dependence
of energy exchange at the LSI of the CNT, an alternative would be to quench the CNT
to a lower temperature. By that, the thermal noise in the CNT can be greatly reduced,
and the evolution of the VDOS would be statistically easier to track. A similar approach
has been previously utilized to study heat pulse propagation in CNTs [111]. To examine
this hypothesis, following the equilibration period described earlier, the temperature of the
CNT was quenched to TCNT = 1K, while the liquid outside a radius of 22A was constantly
thermostatted at 120K, via velocity scaling. As can be seen in Fig. 6.7, the adsorbed liquid
temperature is maintained at 120K throughout the CNT thermal equilibration process.
Following the 10ps quenching period, the velocities of the CNT atoms were recorded
every time step (0.5fs) for a total period of ≈ 5.0 × 105 time steps (≈ 250ps). From these
velocities, 10 sets of VDOS curves were computed by averaging the velocity autocorrelation
function over periods of 25ps. The VDOS plots and their corresponding time frames are
shown in Fig. 6.8. In the period following the quench (0 − 25ps), the density of states are
overwhelmingly confined in the low frequency range. This is further confirmation of the low
frequency coupling between the CNT and the liquid. With each time period, the VDOS
begin to extend to higher frequencies, and the initial confinement to the low frequency range
diminishes. This trend validates the view that thermal energy in the high frequency range
communicates with the liquid via the low frequency zone [32].
52
Figure 6.6: Equilibrium VDOS of CNT (green) and adsorbed liquid (blue) at 120K. Inset isa zoom on 0− 15THz
Figure 6.7: Time history of CNT temperature (TCNT). The temperature of the adsorbedliquid (TAds) is unaffected by the low TCNT as a result of the high RLSI.
53
Figure 6.8: VDOS plots and their corresponding averaging periods following the CNT quenchto 1K.
As the CNT is being heated by the liquid, its VDOS is attempting to regain its equilibrium
distribution prior to the quench, which is given in Fig. 6.6. To illustrate this point, the
CNT VDOS plots in Fig. 6.8 are projected onto the equilibrium VDOS, as shown in Fig.
6.9. To facilitate a clearer view, VDOSNE is set equal to VDOSEQ at all points where:
VDOSNE ≥ VDOSEQ. There is an interesting trend that signals the presence of a ’bottle-
neck’ type behavior during the evolution of VDOSNE. The recovery of VDOSNE is fully
completed in the range 0 − 15THz within 50ps of CNT heating by the liquid. Beyond this
frequency range, the response is significantly slower, with the frequency range of 20−30THz
still lagging up to 200ps later. It is important to note that the response in the vicinity of
≈ 10THz is not linked to the liquid, given that the VDOS overlap with the liquid region is
limited to only ≤ 2THz. Instead, this phenomenon can be explained by exploring the shape
of the VDOS, and consequently, the dispersion relation in the frequency range 10− 20THz.
As can be seen in Fig. 6.6, within this region, there is a sharp drop in the VDOS, which
54
signals a relatively depleted region in the phonon dispersion of the CNT [112]. In analogy to
phononic crystals [113], when thermal energy enters the CNT at relatively low frequencies,
this ’pseudo’ phonon band gap impedes the energy exchange between the low and high
frequency modes. Hence, limiting by that the ability of the CNT to exchange heat efficiently
with the surrounding liquid, and consequently resulting in a high RLSI. Moreover, in a recent
study [55], a steady-state spectral temperature profile for a heated graphene sheet displayed
a sharp drop in spectral temperature that was also in the frequency range 10− 20THz. This
is additional supporting evidence for the above hypothesis.
6.4 Conclusion
By quenching the CNT to a low temperature, a vivid picture emerged of the frequency de-
pendence of thermal energy exchange at the LSI. There is an obvious propagation of thermal
energy in frequency space. From the analysis of the dynamics of this energy propagation, an
energy exchange bottle-neck appears in the frequency range of 10− 20THz. As a result, the
low-to-high frequency heat conduction dominates the thermal interaction at the LSI of the
CNT. This frequency range coincides with a depleted region in the VDOS of the CNT, and
agrees well with recent findings in the literature. This can provide new avenues of improving
RLSI in CNTs via defect engineering [114] or isotope doping [115].
55
Figure 6.9: Projection of CNT VDOSNE onto VDOSEQ. Also shown is the overlap with theliquid VDOS.
56
Chapter 7
Contributions and future research direction
7.1 Summary of contribution and findings
Following the introduction in Chapter 1, a detailed literature review was provided in Chapter
2. At the end of Chapter 2, the objectives of the thesis were given, which aimed to: (i)
provide a systematic examination of the factors affecting the LSI thermal resistance in the
presence of surface nanopatterning, (ii) explore the potential for pressure-tuning the LSI
thermal resistance for wetting and nonwetting surfaces, and (iii) demonstrate the frequency
dependence of thermal energy exchange at LSIs involving CNTs. In Chapter 3, the molecular
dynamics method was introduced, along with the techniques relevant for studying nanoscale
heat transfer. In Chapters 4-6 the above listed objectives were carried out, respectively. In
this chapter, a summary of the contributions, and recommendations for future research are
given as follows:
• Surface nanopatterning and heat transfer at LSIs : It was demonstrated in
Chapter 4 that modifying the nanoscale topology of a solid surface can have
a significant effect on lowering the Kapitza resistance. This was attributed to
how surface nanopatterning affects the interaction strength with the adsorbed
liquid, as well as the vibratonal properties of the patterned surface. In this
study the nanopatterning was limited to unit cell depth, in order to focus
more on the role of vibrational coupling between the solid and liquid atoms
at the LSI. The depth of the patterning, which also translates to the height
of surface nanopillars, could directly affect the thermal conductivity of the
solid surface itself. Increasing the depth of patterning for case S5 would result
in a larger jump in the liquid-exposed surface area relative to S2. However,
57
the hindrance to phonon transport within the solid surface is expected to be
higher in S5, due to the increased nanoporosity of the surface [116, 117]. For
future work, the interplay between these factors can be further investigated.
Another potentially important factor is the geometry of the surface pillars.
Consideration has mostly been given to nanopillars of rectangular shape. The
effect of other geometries, such as pyramidal and hemispherical, would provide
more insight into the role that surface geometric functionalization plays in LSI
heat conduction.
• Pressure effects on LSI thermal resistance: In Chapter 5, the effects of system
pressure on the LSI thermal resistance were found to be significantly more
pronounced for nonwetting surfaces. For all the cases considered, the phonon
coupling at the interface was relatively unaffected by the increase in pressure.
The relative increase in adsorbed liquid density for the nonwetting surface was
shown to be the major factor in lowering the Kapitza resistance. The outcome
demonstrates the possibility for pressure tunable LSI thermal resistance for
nonwetting surfaces. A future extension of this study can examine this pressure
effect for surfaces of intermediate wetting properties. This would provide a
more detailed picture of how the wetting characteristics of the solid surface
affects the relative increase in adsorbed liquid density, and consequently the
range of tuning the LSI thermal resistance.
• Frequency dependence of heat conduction at LSIs involving CNTs : The find-
ings in Chapter 6 provided a strong confirmation of the low frequency coupling
between CNTs, and the surrounding liquid medium. The high Kapitza resis-
tance was also linked to the poor exchange of thermal energy between high,
and low, frequency modes withing the CNT. Hence, one way to improve the
LSI thermal conduction for CNTs is to promote the high-to-low frequency en-
58
ergy exchange. This is directly linked to altering the dispersion relations in
the CNT. A further investigation into this phenomena should focus on two
important avenues: (i) isotope doping, and (ii) CNT diameter. Both of these
parameters can influence the CNT dispersion relations, and thermal conductiv-
ity. For purposes of thermal management, a balance must be struck between
the improvement in LSI thermal conduction, and the possible reduction in
the CNT thermal conductivity as a result of modifications to the dispersion
relations.
The study of nanoscale heat transfer using molecular dynamics remains a highly active
field of research. It is hoped that the above findings contribute to our understanding of
thermal energy exchange at LSIs. The recommendations for future studies aim at extending
these findings to continue exploring this phenomena. This is crucial for the proper thermal
design of nanodevices, which continue to play an expanding role in various sectors and