State of stick-slip motion in confined films “A Liquid or not a liquid” Present student: I-Cheng Chen Date: Dec. 2, 2011 Course: CHEN 633.

State of stick-slip motion in confined

films “A Liquid or not a liquid”

Present student: I-Cheng Chen

Date: Dec. 2, 2011Course: CHEN 633

Motivation

Stick-slip motion is everyday phenomena:creaking door, brake sound, earthquakes…

In class, confined structures shows density distribution and viscosity elevation….What’s happening with stick-slip motion in confined area?

Apply to computer hardware, miniature moter, and aerospace components, lubricants, low-friction surfaces…..

Friction

Normal frictionoccur with surface being damaged

Boundary friction (boundary lubrication)Occur restricted to thin region. Damage also occur during sliding.

Interfacial friction(interfacial sliding) ,1990Molecularly thin region, uniform gap thickness and well defined contact area. No presence of damage

Surface Friction Theory

Amontons’ law (presence of damage on surface) F= u * L u: friction coefficient L: normal loads

Bowden and Taber (adhesive contacts dominate) F= Sc * A ; Sc: critical shear stress A: molecular contact area

Dynamic condition

Static condition

Hertz and JKR theory(1971)Adhesive force between surfaces

Apparatus

Surface force apparatus (SFA)Taber, Winterton, Israelachvili developed 1969,1972

SCIENCE, VOL 240,1988

Material

Surface: Mica (discussed here) Material between gap: dry air, vapor, organic

compound, electrolyte solution…etc.

Shear properties of thin film

Experimentallya)effective viscosity rise 5-7 order than bulk value.b)Newtonian viscosity breaks down.c) Some Properties are quantized with the number of layers(layers ~<10)

Theoretical simulationa)Monte Carlo simulation with Lennard-Jones liquids in 6-10 molecular diameter filmsd

Sliding traces of liquid or solid

For liquid(Couette flow) when sliding stopped

F=K x ; S=F/A

Main feature between Solid and liquid1) Slope of solid curve2) Decay after the sliding stopped3) Magnitude of the shear stress (Solid>liqud)

Types of surface deformation

Spherical molecules used as lubricant oil like cylcohexane and octamethylcyclotetrasiloxane(OMCTS)

Octane and tetradecane as linear chain molecules

Easy to observe layer transition

Longer time needed to reachsteady state liquid-like (d) solid like(f)

Spherical molecule did not exist liquid-like behavior in comparable thin films

Traces of friction force

1.∆F increase as number of layers falls;quantization property(for spherical)2. With increasing velocity frequency increases ∆F fallsuntil critical velocity reach∆F*frequency=velocity

Inset:

No stick-slip regime

Above critical velocity F is independent of v

Spherical molecules: S is constantat different layers; S=F/A

Chain molecules: S is proportional to loads S=C*L ;C is different from the well-known frictioncoefficient.C is related to the surface and liquid molecule property.Smoother surface with smaller C

Stick-slip regime

Sensitive to sliding velocity and immediate previous sliding spherical molecule persisted to high velocity chain molecule is not ; shear induced Molecular ordering is sensitive to shear rate

Relaxation phenomenaFor chain molecule like tetradecane or 2-methyloctadecane, when the surface stop sliding, molecules in the gap starts to relax and change theconfiguration.

Tetradecane C14H30

2-methyloctadecane C19H40

Regimes for relaxation conformation

Sliding regime v>vc; chain molecules most shear aligned

Resting regimemolecules relaxes to a more solid-like state.Latency time: time needed for molecules to fully frozen.

Sticking regimeWhen exceed latency time, solidlike structure reformed

Slipping regime reorder back to liquidlike

Latency time:1. branched chain molecule >chain > spherical 2. increased with applied load 3. no spikes when stop time is shorter than latency time

Slip time vs. latency time(spring (film property)constant)

Relaxation behavior

Higher velocity lower the subsequent spike height (same pressure)

Latency time increases

with pressure.

Effect of waterFriction is low when covered with water on mica surfaces. Repulsion hydration forces between them. Humidity control is important in this kind of experiment

Effect of contaminants and impurity

Brief conclusion

Properties of molecularly thin film a) structure of the molecule b) structure and commensurability of the surface c) surface-liquid interaction potential d) pressure(load) between surfaces e) direction of shear f) shear rate g)history

Two confining surfaces are needed for a liquid to becomesolid-like

Cobblestone modelProposed by Taber 1978To explain the two solid hydrocarbon surfaces sliding each in the absence of wear.

Pushing a cart over a road of cobblestone. Cart wheel: liquid molecule Cobblestone: atoms of the surfaceSo wheels have to roll before cart move.

At rest(static), wheels find grooves to sit in potential energy minimum.Dynamic: Lateral force to raise the wheel and then move (adhesion)

Cobblestone model

∆d= move distance ; ∆ D=normal distance; Fad=adhesion forceF= Shear force 2rsA= surface energy

Typically 2rs is 5x10-2 Jm-2, and if ∆d, ∆D ~1A. ɛ ~0.1 (10% surface energy needs to overcome)

S=F/A=5X107 Nm-2 experimental value 2x107 Nm-2 for cyclohexane

Stick-Slip models

Surface topology model Surface topography and system mechanical property dominate in this model. Irregular stick-slip spike

Distance-dependent model

Rate and state model( velocity dependent model)mainly for lubricated surfaces

Distance dependent model

Propose at 1950s Stuck surfaces creep over a

characteristic distance ,D0,before friction drops

Often used in dry surfaces. Mayalso apply to polymer surfaces

Velocity dependent model

Mainly for lubricated surfaces Solid-like to liquid-like transition

Velocity dependent model

Underdamped regime( inertia-dominated)High spring constant and low mass, where mechanical response time is longer than slip time

Velocity dependent model

Overdamped regime( friction-dominated) low spring constantslip response time is longer than mechnical time, so we can observe doubleexponential decay

Starting spikes and stopping spikes

Two surfacesdid not stop instantly

There is some timev<vc, even very short,

so the molecule starts to relax.(not seen in branchedmolecule because of longrelaxation time.)

Take home conclusions

Stick-slip motion in confined area is like freezing-melting transition.

Molecular thin films can be in a state of neither solid nor liquid. Properties can be changed continuously from liquid-like to solid-like, unlike the bulk first order transition.( Two confining surfaces are needed)