Effective ultrasonication process for better colloidal dispersion of nanofluid I.M. Mahbubul a , R. Saidur a,⇑ , M.A. Amalina a , E.B. Elcioglu b,c , T. Okutucu-Ozyurt b a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Mechanical Engineering, Middle East Technical University, Dumlupinar Bulvari, No. 1, 06800 Ankara, Turkey c Eskisehir Osmangazi University, Sivrihisar Vocational School, Mechanics Programme, Eskisehir Cad No. 140, Sivrihisar, Eskisehir, Turkey a r t i c l e i n f o Article history: Received 26 September 2014 Received in revised form 5 January 2015 Accepted 5 January 2015 Available online 12 January 2015 Keywords: Nanofluid Ultrasonica tion duration Sonicator amplitude Microstructure Particle size distribution Zeta potential a b s t r a c t Impr oving disper sion stability of nanofl uids through ultrason icatio n has been shown to be effective. Determining specific conditions of ultrasonication for a certain nanofluid is necessary. For this purpose, nanofluids of varying nanoparticle concentrations were prepared and studied to find out a suitable and rather mono-dispersed concen tration (i.e., 0.5 vol.%, determined through transmission electron micro s- copy (TEM) analyses). This study aims to report applicable ultrasonication conditions for the dispersion of Al 2 O 3 nanoparticles within H 2 O through the two-step production method. The prepared samples were ultra sonicated via an ultra sonic horn for 1–5 h at two differ ent amplitu des (25% and 50%) . The micro- structure, particle size di stribution (PSD), and zeta potentials were analyzed to investigate the disp ersion chara cteristics. Better particle dispersi on, small er aggr egate sizes, and highe r zeta poten tials were obse rved at 3 and 5 h of ultra sonic ation durat ion for the 50% and 25% of sonic ator powe r amplitud es, respectively. 2015 Elsevier B.V. All rights reserved. 1. Introduction Stab ility is a critic al and nece ssary cond itio n for most of the materials used in industry, since it implies a fairly predictable and controllable condition of their behavior. In this regard, nanofluids are desired to have thermodynamic, kinetic, chemical, and disper- sion stab iliti es [1] . Sin ce nan ofluid s ha ve be en con sid ere d as advan- tag eo us in heat tra nsf er applications due to the ir impro ved thermophysical properties, their stability in heat transfer experi- ments needs to be investigated. Due to the inter-particle adhesion forces, nanoparticles become agglomerated and their settlement canbe obser veddue to thegravi ty for ces. In ord er tostart wi th a sta - bleandusableconditionof nanofluid s, it is de sir ed to have an agg re- gate - and sed imen t-fre e structure whe re all the nano parti cles contribute to the dispersion, which will give the maximum benefit fro m the nan op art icles, in ter ms of the ir thermophys ica l properties [2] . In this regard, a nanofluid with the stable dispersion can be defi ned in whi ch the nano particles are mon o-di spersed. Due to the presence of nanoparticle aggregates, the dispersion stability may decay with time [1]. Elcioglu and Okutucu-Ozyurt [2] indicate the requirement of performing stability measurements in a frequent and periodic manner. To increase the stable lifetime of nanofluids, ultrasonication has been widely utilized, and has been accepted as an essential step in the production of nanofluids through two-step method[3]. However, no standard has been established to prepare nanofluids especially on how long should a nanofluid have to be homoge nize d, how much soni cato r power amp litud e is need ed, and what typ e ordurationsofpulsemod e shouldbeused.Ne ver the - less , the Nati onal In stitu te of Stan dard s and Tech nolo gy (NIS T, Gai- thers burg , MD) with the Center for the Envi ronmenta l Implications of Na not ech nol ogy (CE INT of Duke Unive rsi ty) ha s sta rte d to develop some standardized and validated protocols for the disper- sion of nano part icles [4] . Useof co oli ng ba th, pulse mo de opera tio n, and cylin dric al shap ed flat-b ottom beak ers are some pro pose d guid elin es. The y urgedthat, the opti mal ultra soni catio n parameters should be determined by considering different parameters of the ult ras oun d process. It could be no ted tha t ult rasonicatio n is a co m- plic ated physico chemical process, which can brea k down the aggl ome ratio n as wel l as crea te furth er aggr egat ion,and man y othe r effects toge ther with chemical reac tion s [4] . The re are con tra di cto ry res ult s among the resear che rs ab out the effect of ultrasonication duration on colloidal dispersion of nano- part icles. Some researchers pointed out that, high er ultrasoni ca- tio n dur ati on is bet ter fo r pro pe r dis pe rsi on of nan op art icl es. Among them, Yang et al. [5]studied the effect of ultrasonication on agglomeration size for nanotube-in-oil dispersions. They char- act eri ze d the sampl es by TE M, and found tha t the clu ste r size http://dx.doi.org/10.1016/j.ultsonch.2015.01.005 1350-4177/2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +60 3 7967 7611; fax: +60 3 7967 5317. E-mail addresses: [email protected], [email protected](R. Saidur). Ultrasonics Sonochemistry 26 (2015) 361–369 Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Effective ultrasonication process for better colloidal dispersion
of nanofluid
I.M. Mahbubul a, R. Saidur a,⇑, M.A. Amalina a, E.B. Elcioglu b,c, T. Okutucu-Ozyurt b
a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Mechanical Engineering, Middle East Technical University, Dumlupinar Bulvari, No. 1, 06800 Ankara, Turkeyc Eskisehir Osmangazi University, Sivrihisar Vocational School, Mechanics Programme, Eskisehir Cad No. 140, Sivrihisar, Eskisehir, Turkey
a r t i c l e i n f o
Article history:
Received 26 September 2014
Received in revised form 5 January 2015
Accepted 5 January 2015
Available online 12 January 2015
Keywords:
Nanofluid
Ultrasonication duration
Sonicator amplitude
Microstructure
Particle size distribution
Zeta potential
a b s t r a c t
Improving dispersion stability of nanofluids through ultrasonication has been shown to be effective.
Determining specific conditions of ultrasonication for a certain nanofluid is necessary. For this purpose,
nanofluids of varying nanoparticle concentrations were prepared and studied to find out a suitable and
rather mono-dispersed concentration (i.e., 0.5 vol.%, determined through transmission electron micros-
copy (TEM) analyses). This study aims to report applicable ultrasonication conditions for the dispersion
of Al2O3 nanoparticles within H2O through the two-step production method. The prepared samples were
ultrasonicated via an ultrasonic horn for 1–5 h at two different amplitudes (25% and 50%). The micro-
structure, particle size distribution (PSD), and zeta potentials were analyzed to investigate the dispersion
characteristics. Better particle dispersion, smaller aggregate sizes, and higher zeta potentials were
observed at 3 and 5 h of ultrasonication duration for the 50% and 25% of sonicator power amplitudes,
respectively.
2015 Elsevier B.V. All rights reserved.
1. Introduction
Stability is a critical and necessary condition for most of the
materials used in industry, since it implies a fairly predictable and
controllable condition of their behavior. In this regard, nanofluids
are desired to have thermodynamic, kinetic, chemical, and disper-
sion stabilities [1]. Since nanofluids have been considered as advan-
tageous in heat transfer applications due to their improved
thermophysical properties, their stability in heat transfer experi-
ments needs to be investigated. Due to the inter-particle adhesion
forces, nanoparticles become agglomerated and their settlement
canbe observeddue to thegravity forces. In order to start with a sta-
bleand usableconditionof nanofluids, it is desired to have an aggre-
gate- and sediment-free structure where all the nanoparticles
contribute to the dispersion, which will give the maximum benefit
from the nanoparticles, in terms of their thermophysical properties
[2]. In this regard, a nanofluid with the stable dispersion can be
defined in which the nanoparticles are mono-dispersed. Due to the
presence of nanoparticle aggregates, the dispersion stability may
decay with time [1]. Elcioglu and Okutucu-Ozyurt [2] indicate the
requirement of performing stability measurements in a frequent
and periodic manner. To increase the stable lifetime of nanofluids,
ultrasonication has been widely utilized, and has been accepted as
an essential step in the production of nanofluids through two-step
method [3]. However, no standard has been established to prepare
nanofluids especially on how long should a nanofluid have to be
homogenized, how much sonicator power amplitude is needed,
and what type or durations ofpulsemode shouldbe used. Neverthe-
less, the National Institute of Standards and Technology (NIST, Gai-
thersburg, MD) with the Center for the Environmental Implications
of Nanotechnology (CEINT of Duke University) has started to
develop some standardized and validated protocols for the disper-
sion of nanoparticles [4]. Useof cooling bath, pulse mode operation,
and cylindrical shaped flat-bottom beakers are some proposed
guidelines. They urgedthat, the optimal ultrasonication parameters
should be determined by considering different parameters of the
ultrasound process. It could be noted that ultrasonication is a com-
plicated physicochemical process, which can break down the
agglomeration as well as create further aggregation,and many other
effects together with chemical reactions [4].
There are contradictory results among the researchers about the
effect of ultrasonication duration on colloidal dispersion of nano-
particles. Some researchers pointed out that, higher ultrasonica-
tion duration is better for proper dispersion of nanoparticles.
Among them, Yang et al. [5] studied the effect of ultrasonication
on agglomeration size for nanotube-in-oil dispersions. They char-
acterized the samples by TEM, and found that the cluster size
http://dx.doi.org/10.1016/j.ultsonch.2015.01.005
1350-4177/ 2015 Elsevier B.V. All rights reserved.
for of 30, 60, 90, 120, and 150 min using an ultrasonic horn (Model
505, Fisher Scientific, USA). This type of ultrasonication is called as
‘‘direct sonication’’, according to the CEINT/NIST Protocol on nano-
particle dispersion preparation using ultrasonication [4]. As indi-cated in the Protocol, direct sonication is recommended over
indirect sonication applied via ultrasonic baths, for the purpose
of dispersing dry powders, as carried out in the current study.
The capacity of the machine is designed as 20 kHz operating fre-
quency and a maximum power of 500 W. During the ultrasonica-
tion, 25% and 50% amplitudes, and 2 s ON and 2 s OFF pulses
were applied. Such an approach is generally recommended, since
operating in pulsed mode retards the rate of the temperature
increase of the ultrasonicated material; hence minimizing
unwanted results and allowing better temperature control com-
pared to continuous mode operation [4]. Ultrasonication could
affect the total volume and the concentration of nanofluids as
the agitation increases the temperature by 10 C/min initially
[17]. For this reason, a digital refrigerated circulator bath (ModelC-DRC 8, CPT Inc., South Korea) was connected with a recursion
beaker, and the nanofluids were prepared inside this beaker at
15 C to avoid vaporization.
It is noteworthy that, for the setting of the above-mentioned
durations; the total elapsed durationsof sonication were the double
periods (as for the settingof 2 s ONand2 s OFF pulses, homogenizer
machine counted only the ON/running periods). Therefore, for the
effective ultrasonication periods of 30, 60, 90, 120, and 150 min,
total ultrasonication durations were taken 1, 2, 3, 4, and5 h, respec-
tively. As the homogenizer unit was run/operate until 1, 2, 3, 4, and
5 h of periods, the authors would like to address the sonication
durations as1, 2,3, 4,and5 h inthisandotherSections ofthis study.
Moreover, in our previous study [15], the ultrasonication durations
0–180 min were based on the total elapsed time where effective
sonication times were set to be 0–90 min.
2.2. Colloidal dispersion inspection
The microstructure and composition of the nanoparticles were
characterized using field emission scanning electron microscopy(FESEM) (Model AURIGA, Zeiss, Germany). At first, as received
nanoparticles were characterized with FESEM at 1 kV accelerating
voltage. A 10,000-time magnification was used to capture the
images at 1 lm scale (see Fig. 2). The TEM of 120 kV acceleration
voltage capacity was used to capture the microstructure of the
nanofluid for the analysis of the colloidal dispersion. The samples
a c
b d
500 nm
500 nm
500 nm
500 nm
Fig. 1. TEM images showing the microstructure of 1 h ultrasonicated Al2O3–water nanofluids of (a) 0.01, (b) 0.1, (c) 0.5, and (d) 1 vol.% concentrations.
Fig. 2. The FESEM images of Al2O3 nanoparticles at 1 lm scale.