Study on Ultrasonic by Mahbubul
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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.
⇑ 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
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decreased with increasing sonication time/energy. Amrollahi et al.
[6] studied the effects of ultrasonication parameters on the settling
time, and TEM microstructure of carbon nanotube (CNT)–ethylene
glycol (EG) nanofluids. Their results showed that, for lower ultra-
sonication times, the settling in the most concentrated nanofluid
(2.5 vol.%) was less than that of the most diluted nanofluid
(0.5 vol.%), and for the longer ultrasonication durations, the phe-
nomenon was reversed. The precipitation measured by human
eye is not a precise method even though the author claimed that
the precision was ±10 min. Again, the author carried measure-
ments with TEM only after three durations as 15 min, 5 h, and
20 h of ultrasonication and for only 2.5 vol.% concentration of par-
ticles. Ruan and Jacobi [7] applied 5, 40, 140, 520, and 1355 min of
ultrasonication duration to homogenize multi-walled carbon
nanotube (MWCNT) in EG. The nanofluids were prepared by using
both continuous and pulsed mode of ultrasonication. Microstruc-
ture, agglomerate size, nanotube length, and aspect ratio were
determined through TEM to study the effect of ultrasonication.
They observed that, average cluster size, length, and aspect ratio
of nanotubes decreased with increasing sonication time or energy.
Most other researchers report that, there are specific optimal
ultrasonication durations available based on different conditions/
properties of nanofluids, e.g., particle concentration and type,
and amount of and type of base fluid [8]. Chen et al. [9] ultraso-
nicated TiO2-EG suspension up to 40 h to find out the optimum
sonication duration. Their characterization with light scattering
for agglomeration size showed that, 20 h of homogenization gave
the best result that was 140 nm size and for longer durations no
further size reduction was achieved. Garg et al. [10] investigated
the effect of sonication time on the dispersion behaviors of
nanofluids. They prepared four samples of 1 wt.% MWCNT in
DIW with GA as additives and subjected the samples to ultrason-
ication for 20, 40, 60, and 80 min. They performed analyses with
TEM and found that the optimum ultrasonication time for
homogenization was 40 min, using a 130 W and 20 kHz ultraso-
nicator. Zhu et al. [11] determined the influence of ultrasonica-
tion time on average cluster size. They analyzed thedispersions of CaCO3–water, which were ultrasonicated for 1–
45 min and found that, the cluster size rapidly decreased within
20 min of ultrasonication, after that, it was slightly increased
with ultrasonication duration. As their primary substance was
in paste form, therefore, most of the aggregates were soft and
they were broken up rapidly within 20 min. Nguyen et al. [12]
studied the effect of ultrasonication duration, power, and pulse
mode on de-agglomeration of alumina nanoparticles dispersed
in water, where the maximum input power of the machine
was 400 W with a frequency of 20 kHz. They used 10%, 30%,
and 60% of vibration amplitude with different pulse modes and
optimal break-up of agglomeration were found for 30% ampli-
tude. In the case of 60% amplitude, the cluster size again
increased after 300 s of ultrasonication. Hence, the authors pointout that, higher power of ultrasonication could result in re-
agglomeration of the particles. Nevertheless, for 10% and 30%
amplitudes, the aggregate sizes were continuously decreased
by the increase of sonication time. They used different modes
of pulse as continuous and pulsed with long and short durations;
however, no difference and similar outcomes were observed.
Chakraborty et al. [13] analyzed the influence of ultrasonication
durations on TiO2 nanofluid. They added 0.1, 0.2, and 0.4 wt.% of
silver (Ag) nanoparticles and ultrasonicated for 10, 20, and
30 min of durations. They observed the settling time and report
that for lower concentration of particles, ultrasonication did not
have a significant role. Kole and Dey [14] ultrasonicated ZnO
nanoparticles in EG up to 100 h and characterized the PSD and
microstructure. They reported that, the lowest cluster size wasobtained for 60 h of sonication and after that, cluster size again
increased. Mahbubul et al. [15] investigated the effect of the
ultrasonication duration (0–180 min, 50% amplitude, 2 s ON
and 2 s OFF pulses) on the colloidal structure of 0.5 vol.% alu-
mina–water nanofluid. The authors observed a decrease in the
aggregate size for ultrasonication up to 90 min. For longer dura-
tions (i.e., for 120 and 150 min) particles formed aggregates,
again. Further ultrasonication until 180 min yielded more homo-
geneous dispersion of nanoparticles.
It can be inferred from the literature that the studies about the
effect of ultrasonication on the aggregation tendency of nanoparti-
cles within nanofluids are still immature. Some of the researchers
recommend higher sonication time for better dispersion, while
some other researchers claim that the agglomeration could be min-
imized after certain duration of ultrasonication. Nevertheless,
there is no specific or common duration of ultrasonication sug-
gested by the researchers that could be followed for better disper-
sion. Moreover, most of the literature studied only the sonication
period and most are concerned with CNT nanofluids. Hence, the
present study aims to evaluate the effective ultrasonication condi-
tions (sonicator amplitudes and sonication duration) on dispersion
characteristics to prepare an alumina–water nanofluid through
two-step method. The research is the extension of our previous
study [15] that was fixed with 50% amplitudes and until 180 min
of duration only. Here a prolonged ultrasonication duration until
5 h is considered for analysis. Moreover, two different amplitudes
as 25% and 50% of sonicator power were used for the analysis with
the hope that this study will give more guidelines for the research-
ers regarding ultrasound sonication.
2. Experimental method
2.1. Nanofluid preparation
The Al2O3 nanoparticles in powder form (manufactured by
Sigma–Aldrich, USA) with the manufacturer defined average parti-
cle size of 13 nm and a purity of 99.5% was dispersed in distilled
water, to prepare the nanofluids. The nanofluids were preparedvia the two-step method, i.e., the nanoparticles were primarily
arranged and then mixed with the base fluid using ultrasound
[16]. Four volume concentrations (0.01, 0.1, 0.5, and 1 vol.%) of
Al2O3–water nanofluids have been prepared using 50% ultrasonica-
tion amplitude with 2 s ON and 2 s OFF pulses for 1 h of ultrason-
ication. Then the microstructures of these four samples were
analyzed by a TEM (Model LIBRA 120, Zeiss, Germany). The TEM
results are provided in Fig. 1.
Based on the TEM analyses, the dispersion characteristics of the
samples with varying nanoparticle concentrations can be observed
in Fig. 1. It is revealed from the TEM micrographs that, the particles
were in a rather involved and overlapping condition for 1 vol.%
nanofluid compared to the 0.01, 0.1 and 0.5 vol.% samples. Such
an observation of the sample microstructure can give preliminaryconclusions on the nanoparticle-clustering tendency, which is
inevitable in the long term. In order not to render the possible
improvements in thermophysical properties coming with the
increased nanoparticle concentration, 0.5 vol.% nanofluid is
selected for further investigation as it appeared to be the prefera-
ble one among the concentrations studied, in terms of the nanopar-
ticle dispersion. The sample of 1 vol.% was found to be the most
concentrated nanofluid. However, 0.01 vol.% was observed to have
the most diluted concentration. Hence, 0.5 vol.% of Al2O3–H2O
nanofluids have been further investigated for the effective ultra-
sonication parameters.
First, the nanoparticles were suspended in the base fluid, and a
very narrow (3 mm diameter) glass tube was used to stir the mix-
ture for 1 min to enable the nanoparticles to be mixed with thebase fluid completely. Then, the nanofluids were ultrasonicated
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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.
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for TEM analyses were prepared right after the preparation of the
nanofluid in a transparent and thin film of Formvar with an evap-
orated layer of carbon on the 300-mesh copper grid. Then the film
was dried for at least 2 days inside a desiccator at room tempera-
ture (25 C) and magnifications of as 6300 and 12,500 times were
used to capture the images at 500 and 200 nm scale, respectively.
In order to determine the effective ultrasonication parameters such
as the optimum concentration from 0.01 vol.% to 1 vol.%, the 6300-
time magnification was used (image scale was 500 nm) as shown
in Fig. 1. A Zetasizer (Model 3000HS, Malvern, UK) was used to
check the average aggregate size and zeta potential after the ultra-
sonication of each sample. The analyses with Zetasizer were con-
ducted at 25 C after 24 h from the nanofluid preparation. The
zeta potentials were analyzed without changing the pH of the
mixtures.
3. Result and discussion
To study the effect of ultrasonication on the colloidal dispersion
of Al2O3–water nanofluids, first, the microstructure of Al2O3 nano-
particles was observed, prior to their mixing with water. After the
suspension of Al2O3 nanoparticles in water, the microstructure wasanalyzed once more with TEM. The microstructure of the as
received Al2O3 nanoparticles imaged by FESEM is shown in Fig. 2.
The presence of spherical/cylindrical nanoparticles and their loose
formed clusters can be observed in the Fig. 2.
The microstructure of Al2O3–water nanofluid samples after
each ultrasonication at two different amplitudes was analyzed
through TEM analyses. The TEM images of Al2O3–water nanofluid
after 1 h ultrasonication with 2 different amplitudes (25% and
50%) is presented in Fig. 3. In Fig. 3, it is shown that, 1 h ultrason-
ication was not enough to establish a good dispersion of nanopar-
ticles. The left-side figures (Fig. 3(a)–(b)) are the micrographs of
25% amplitude, which shows that the aggregates were still present,
while the right-side figures (Fig. 3(c–d)), the micrographs of 50%
amplitude showing a better colloidal dispersion compared to that
for 25% amplitude. Fig. 3 states that, better dispersion of nanopar-
ticles was obtained for higher power (amplitude) of sonicator even
for the same duration. Lam et al. [18] reported that, for lack of
enough ultrasonic energy, nanoparticles were not likely to be able
to escape from the clusters, and a considerable amount of aggrega-
tion would be observed. The higher aggregation seen in the case of
nanofluids prepared by 1 h of ultrasonication with 25% amplitude
is in agreement with the above statement.
Fig. 4 showsthe microstructureof Al2O3 nanoparticlesinwaterafter
2 h of ultrasonication with 25% and 50% amplitude. Fig. 4(a)–(b) (the
left-side figures) are the micrographs of 25% amplitude at 6300 and
12500 magnifications, in 500 and 200 nm scales, respectively. Simi-
larly, the right-side figures (Fig. 4(c)–(d)) are micrographs of 50%
amplitude. It is clear from Fig. 4 that, the nanoparticles were well dis-
persed and almost similar types of dispersions havebeen observed for
the nanofluids prepared by 2 h of ultrasonication with 25% and 50%
amplitude. Nevertheless, few small overlaps have been observed,
which are the nano-clusters formed among the particles. Such nano-
clusterscouldnotbe fully brokendown, even after prolongedultrason-
ication. It is impossible to get theinitialsize of particles after dispersed
into fluid. Ghadimi et al. [19] reported that, the cluster of nanofluids
wouldbeat least threetimeshigherthantheaverageparticle diameter.
The microstructures of Al2O3–water nanofluid prepared by 3 h
of ultrasonication with 2 different amplitudes (25% and 50%) are
shown in Fig. 5. The left-side figures (Fig. 5(a)–(b)) are the micro-
graphs of 25% amplitude, while the right-side figures (Fig. 5(c)–
(d)) are the micrographs of 50% amplitude. More spreading (means
better dispersion with smaller cluster size) of nanoparticles is seen
from Fig. 5. There are only few empty areas are visible in the micro-
graph. Even though, there is no large agglomeration was observed
but there are small nano-clusters of particles are existed. Either the
agglomeration of nanoparticles did not have enough energy to
completely breakdown the clusters or the nanoparticles have
received over energy and started to re-agglomerate. Nevertheless,
it is impossible to completely breakdown the clusters of particles
[19]. It is reported in literature [12,18,20] that higher power of
ultrasonication could re-agglomerate the particles as the collision
of each particle will increase and they will tangle up. A compara-
tive higher dispersion of particles is observed for the nanofluidsprepared by 50% amplitude in comparison to 25% one. This
25 % amplitude 50 % amplitude
6 3 0 0
m a g n i f i c a t i o n s
1 2 5 0 0
m a g n i f i c a t i o n s
a c
b d
500 nm 500 nm
200 nm200 nm
Fig. 3. TEM images showing the microstructure of Al2O3–water nanofluid samples after 1 h of ultrasonication.
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indicates that using 25% amplitudes of sonicator power, even after
3 h of ultrasonication, nanoparticles do not get enough energy to
completely be dispersed into water.
The micrographs taken by TEM for the nanofluids prepared by
4 h of ultrasonication with 25% and 50% amplitudes have shown
in Fig. 6. Fig. 6(a)–(b) (the left-side figures) are the micrograph of
25% amplitude at 6300 and 12500 magnifications, respectively
in 500 and 200 nm scales, respectively. Similarly, the right-side fig-
ures (Fig. 6(c)–(d)) are standing for micrograph of 50% amplitude.
More spreading of nanoparticles is seen in the Fig. 6. No significantempty areas are visible in the micrographs taken for the nanofluid
prepared by 50% amplitude. However, still a fewempty areas could
be seen for the nanofluids ultrasonicated with 25% amplitude.
Moreover, some clusters of particles were present. Therefore, it
could be expected that, further longer ultrasonication with 25%
amplitude could break down the remaining aggregates.
Fig. 7 shows the microstructures of Al2O3–water nanofluid after
5 h of ultrasonication. The left-side figures (Fig. 7(a)–(b)) are the
micrographs of 25% amplitude while the right-side figures
(Fig. 7(c)–(d)) are the micrographs of 50% amplitude. More spread-
ing of nanoparticles is seen in the figure for 5 h of ultrasonicationand almost similar trend was observed for the applied power of
25 % amplitude 50 % amplitude
6 3 0 0
m
a g n i f i c a t i o n s
1 2 5 0 0
m a g n i f i c a t i o n s
a c
b d
500 nm 500 nm
200 nm200 nm
Fig. 4. TEM images showing the microstructure of Al2O3–water nanofluid samples after 2 h of ultrasonication.
25 % amplitude 50 % amplitude
6 3 0 0
m a g n i f i c a t i o n s
1 2 5 0 0
m a g n i f i c a t i o n s
a c
b d
500 nm 500 nm
200 nm200 nm
Fig. 5. TEM images showing the microstructure of Al2O3–water nanofluid samples after 3 h of ultrasonication.
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25% and 50% sonicator amplitude. However, there are minor over-
laps of nanoparticles but no empty areas can be seen in Fig. 7 for
50% amplitude. A higher particle dispersion but with few empty
areas and minor overlapping of particles were observed in
Fig. 7(a)–(b) for 25% amplitude. Therefore, nanofluids prepared
by 25% amplitudes did not face enough ultrasound energy yet to
have mono-dispersed condition. The images of Fig. 7 are darker
black color, especially in Fig. 7(c)–(d) for 50% amplitude. This could
be due to the erosion of the sonicator tip. Mandzy et al. [21]
reported that, erosion of an ultrasonic tip could be contaminated
with the fluid as a result of longer ultrasonication duration.
The effects of1, 2, 3,4, and 5 h ofultrasonicationat 25% and 50%
amplitude on the PSD are reported in Fig. 8. Considering the initial
particle size (13 nm), the aggregated state of the nanoparticles can
be observed through the PSD results presented in Fig. 8. The aggre-
gation is also evident in the FESEM image shown in Fig. 2. Accord-
ing to the distributions in Fig. 8, the largest particle size detected
by the PSD device is approximately 200–250 nm. However, the
25 % amplitude 50 % amplitude
6 3 0 0
m a g n i f i c a t i o n s
1 2 5 0 0
m a g n i f i c a t i o n s
a c
b d
500 nm 500 nm
200 nm200 nm
Fig. 6. TEM images showing the microstructure of Al2O3–water nanofluid samples after 4 h of ultrasonication.
25 % amplitude 50 % amplitude
6 3 0 0
m a g n i f i c a t i o n s
1 2 5
0 0
m a g n i f i c a t i o n s
a c
b d
500 nm 500 nm
200 nm200 nm
Fig. 7. TEM images showing the microstructure of Al2O3–water nanofluid samples after 5 h of ultrasonication.
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frequency of such large particles is very low compared to that of
smaller particles within the base liquid. Based on the analyses per-
formed for each case in Fig. 8, the range for the particle size has
been obtained between 42 nm and 300 nm, approximately,
depending on the ultrasonication duration and amplitude. Having
quantitatively realized the nanoparticle aggregation through PSD
analyses, reduction in the average particle size can be observed
for increasing ultrasonication durations from 1 h to 5 h. The peaks
for the average particle size reduce for almost all cases presented.
In addition to the averaging of the aggregate sizes, their distri-
bution characteristic is of great importance, as well. It is realized
from Fig. 8 that the PSDs of the samples ultrasonicated at 25%
amplitude are mostly narrower than those for 50% amplitude, forthe same ultrasonication duration. This result becomes more
pronounced for longer ultrasonications. For a given ultrasonication
duration, the only variable in the comparison for the character of
PSD’s is the ultrasonication amplitude. Hence, it can be concluded
from Fig. 8 that, higher amplitude results in a more effective ultra-
sonication yielding smaller particles for the same ultrasonication
duration. However, for the PSD-sensitive and especially narrow
PSD requiring applications, smaller amplitudes may be preferred,
considering the advantages and drawbacks of having a slightly lar-
ger but narrower PSD.
The average aggregate size variation with ultrasonication dura-
tion at different amplitudes is provided in Fig. 9. As illustrated in
Fig. 9, the average cluster size decreased with increasing ultrason-
ication duration. As the ultrasonication duration increases, the
total amount of ultrasonication energy that the sample is subjected
to increases, according to the relation E = P t , where E , P , and t
stand for the total amount of energy delivered to the suspension,
the applied power, and the total amount of time [4]. Having quan-
titatively realized the nanoparticle aggregation through PSD anal-
yses, reduction in the average particle size can be observed for
increasing ultrasonication durations from 1 h to 5 h. In addition,
for higher amplitudes, lower the aggregate sizes were observed.
However, after 5 h of ultrasonication, the cluster size was almost
same for the nanofluids prepared by 25% and 50% amplitudes. This
phenomenon was attributed to the possibility that the lowest
attainable cluster size was reached after 5 h and further ultrason-
ication might not decrease the cluster size. Such as criteria have
been reported in literature [9,17].
Zeta potential was measured for each sample to quantify the
stability of the nanofluid. The zeta potential of the 0.5 vol.% of
Al2O3–water nanofluid have been investigated for 1, 2, 3, 4, and
5 h of ultrasonication durations and with 25% and 50% sonicator
amplitudes. The results are illustrated in Fig. 10, together with
the limits of excellent and physical stability [22]. As it is apparent
in Fig. 10, the zeta potential of the sample is always lying on the
maximum limit of the physical stability and is approaching the
excellent stability. In this study, the highest zeta potential value
58.4 mV was observed for 3 h of ultrasonication with 50% ampli-tude of power and further sonication until 5 h could not increase
the value. In the case of 25% amplitude, the zeta potential value
was slowly increased until 5 h of sonication and the highest value
was 57.5 mV at this ultrasonication period. The nanoparticles are
usually tend to agglomerate over time because of inter-particle
adhesion forces. The ultrasonication techniques affect the surface
and structure of nanoparticles (act as repulsive forces) and prevent
the agglomeration of particles to achieve stable nanofluids [19]. At
25% amplitude 50% amplitude
1 h o u r
% i n
c l a s s
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c l a s s
2 h o u r s
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5 h o u r s
% i n
c l a s s
% i n
c l a s s
a f
b g
c h
d i
e j
Fig. 8. PSD results (based on intensity) of the Al2O3 nanoparticles at different
ultrasonication durations with different power amplitudes.
100
110
120
130
140
0 1 2 3 4 5 6
A v e r a g e c l u s t e r s i z e , n m
Ultrasonication duration, h
25%
50%
Fig. 9. Average cluster sizes of Al2O3 nanoparticles after varying ultrasonicationdurations, at 25% and 50% amplitudes.
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higher ultrasonication, nanoparticles overcome the adhesion forces
and higher zeta potential was observed. As a result of the zeta
potential analyses, it could be predicted that, for 50% and 25%amplitude, 3 h and 5 h ultrasonication was effective on establish-
ing a stable dispersion, respectively. It could be concluded in terms
of the ultrasonic power–ultrasonic energy relation such that the
ultrasonic energies applied to the samples were effective at these
amplitudes (50% and 25%) for 3 h and 5 h durations, respectively.
Almost similar types of trends were also observed in TEM micro-
structures of Fig. 3–7, where nanoparticles ultrasonicated with
25% amplitudes were not properly homogenized possibly due to
the lack of sufficient sonication power. It can be predicted that
for longer ultrasonication durations with 25% amplitudes, the zeta
potential value can increase and may shift to the excellent stability
range. Kwak and Kim [20] reported the highest absolute zeta
potential value about 50 mV for CuO-EG nanofluid after 9 h of son-
ication, whereas Lee et al. [23] found about 34.5 mV for Al2O3–water nanofluid with 5 h of sonication. Hence, the electro-dynamic
stability of the prepared samples in the current study can be con-
sidered as outstanding.
4. Conclusions
In this study, it is aimed to prepare a stable nanofluid via two-
step method, and to investigate the effects of the ultrasonication
process on the colloidal dispersion. Varieties of indicators, such
as TEM, FESEM, PSD, and zeta potential analyses were utilized for
the investigation. The dispersion of Al2O3–water nanofluid of
0.5 vol.% concentration has been studied for 1–5 h of ultrasonica-
tion durations, with 25% and 50% amplitudes. The FESEM micro-
structure images show that nanoparticles were in the aggregatedform before being suspended in water. A 1 h of ultrasonication is
not sufficient for proper dispersion of nanoparticles and better dis-
persion was observed for nanofluid prepared by 3 h of ultrasoni-
cation with 50% amplitude of sonicator power. However, further
ultrasonication after 3 h showed more spreading of nanoparticles,
but a fewnano-clusters were present. The higher dispersion of par-
ticles was observed after 5 h of ultrasonication in the case of 25%
amplitude. However, there were some aggregations; therefore,
we conclude that further ultrasonication may disperse the particles
better. Erosion of the sonicator tip was observed at 5 h of ultrason-
ication, especially for the operation with 50% amplitude of power.
The PSD analysis showed that, the cluster size was decreased with
increasing ultrasonication duration, which initially decreased rap-
idly. In addition, for the higher amplitudes, lower aggregate sizeswere observed. The zeta potential was also increased with
ultrasonication time. The sample ultrasonicated at 50% amplitude
had slightly greater zeta potential than that at 25% amplitude, for
all ultrasonication durations studied. The highest zeta potential
value 58.4 mV was observed for 3 h of ultrasonication with 50%
amplitude of power and further sonication until 5 h could not
increase the value. In the case of 25% amplitude, the zeta potential
value was slowly increased until 5 h of sonication and the highest
value was 57.5 mV at this ultrasonication period. Therefore, it
could be predicted that, with 50% amplitude, the nanoparticles
received highest ultrasound energy at 3 h of duration. However,
in the case of 25% amplitude, the ultrasound energy was effective
until 5 h period; even further ultrasonication could increase the
charge. In brief, better particle dispersion, lower aggregate size,
and higher zeta potential were obtained with the 50% amplitude
of sonicator power. The optimum duration was found to be 3
and 5 h, respectively for 50% and 25% amplitudes.
Acknowledgement
The authors are thankful to University of Malaya for financial
support under the ‘‘High Impact Research MoE Grant: UM.C/625/
1/HIR/MoE/ENG/40 (D000040-16001) from the Ministry of Educa-tion Malaysia’’.
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