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

362 I.M. Mahbubul et al. / Ultrasonics Sonochemistry 26 (2015) 361–369



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.

I.M. Mahbubul et al. / Ultrasonics Sonochemistry 26 (2015) 361–369 363

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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


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.




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


6 3 0 0

m

a g n i f i c a t i o n s

1 2 5 0 0


a c

b d

500 nm 500 nm

200 nm200 nm



6 3 0 0


1 2 5 0 0


a c

b d

500 nm 500 nm

200 nm200 nm



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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


6 3 0 0


1 2 5 0 0


a c

b d

500 nm 500 nm

200 nm200 nm



6 3 0 0


1 2 5

0 0


a c

b d

500 nm 500 nm

200 nm200 nm





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

% i n

c l a s s

2 h o u r s

% i n

c l a s s

% i n

c l a s s

3 h o u r s

% i n

c l a s s

% i n

c l a s s

4 h o u r s

% i n

c l a s s

% i n

c l a s s

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’’.

References

[1] H. Zhu, S. Liu, L. Xu, C. Zhang, Preparation, characterization and thermal

properties of nanofluids, in: D.M. Sabatini (Ed.), Leading Edge Nanotechnology

Research Developments, Nova Science Publishers, Inc., New York, 2007.

[2] E.B. Elcioglu, T. Okutucu-Ozyurt, An experimental study on the dispersion

stability of alumina–water nanofluids via particle size distribution and zeta

potential measurements, in: International Conference on Thermophysical and

Mechanical Properties of Advanced Materials, Cesme - Izmir, Turkey, 2014.

[3] I.M. Mahbubul, R. Saidur, M.A. Amalina, Latest developments on the viscosity

of nanofluids, Int. J. Heat Mass Transf. 55 (4) (2012) 877–888 .

[4] J. Taurozzi, V. Hackley, M. Wiesner, Preparation of nanoparticle dispersions

from powdered material using ultrasonic disruption, in: NanoEHS Protocols,

National Institute of Standards and Technology, Gaithersburg, MD, 2012.

[5] Y. Yang, E.A. Grulke, Z.G. Zhang, G. Wu, Thermal and rheological properties of

carbon nanotube-in-oil dispersions, J. Appl. Phys. 99 (2006) 114307 .

[6] A. Amrollahi, A. Hamidi, A. Rashidi, The effects of temperature, volume fraction

and vibration time on the thermo-physical properties of a carbon nanotube

suspension (carbon nanofluid), Nanotechnology 19 (31) (2008) 315701.

[7] B. Ruan, A.M. Jacobi, Ultrasonication effects on thermal and rheological

properties of carbon nanotube suspensions, Nanoscale Res. Lett. 7 (1) (2012)

127.

[8] M.E. Kabir, M.C. Saha, S. Jeelani, Effect of ultrasound sonication in carbon

nanofibers/polyurethane foam composite, Mater. Sci. Eng. A, Struct. Mater. 459

(1–2) (2007) 111–116.

[9] H. Chen, Y. Ding, C. Tan, Rheological behaviour of nanofluids, New J. Phys. 9

(10) (2007) 367.

[10] P. Garg, J.L. Alvarado, C. Marsh, T.A. Carlson, D.A. Kessler, K. Annamalai, An

experimental study on the effect of ultrasonication on viscosity and heat

transfer performance of multi-wall carbon nanotube-based aqueous

nanofluids, Int. J. Heat Mass Transf. 52 (21–22) (2009) 5090–5101.

[11] H. Zhu, C. Li, D. Wu, C. Zhang, Y. Yin, Preparation, characterization, viscosity

and thermal conductivity of CaCO3 aqueous nanofluids, Sci. China Technol. Sci.53 (2) (2010) 360–368.

[12] V.S. Nguyen, D. Rouxel, R. Hadji, B. Vincent, Y. Fort, Effect of ultrasonication

and dispersion stability on the cluster size of alumina nanoscale particles in

aqueous solutions, Ultrason. Sonochem. 18 (1) (2011) 382–388.

[13] S. Chakraborty, J. Mukherjee, M. Manna, P. Ghosh, S. Das, M.B. Denys, Effect of

Ag nanoparticle addition and ultrasonic treatment on a stable TiO2 nanofluid,

Ultrason. Sonochem. 19 (5) (2012) 1044–1050.

[14] M. Kole, T.K. Dey, Effect of prolonged ultrasonication on the thermal

conductivity of ZnO–ethylene glycol nanofluids, Thermochim. Acta 535

(2012) 58–65.

[15] I.M. Mahbubul, T.H. Chong, S.S. Khaleduzzaman, I.M. Shahrul, R. Saidur, B.D.

Long, M.A. Amalina, Effect of ultrasonication duration on colloidal structure

and viscosity of alumina–water nanofluid, Ind. Eng. Chem. Res. 53 (16) (2014)

6677–6684.

[16] E. Goharshadi, Y. Ding, M. Jorabchi, P. Nancarrow, Ultrasound-assisted green

synthesis of nanocrystalline ZnO in the ionic liquid [hmim][NTf2], Ultrason.

Sonochem. 16 (1) (2009) 120–123.

[17] S.J. Chung, J.P. Leonard, I. Nettleship, J.K. Lee, Y. Soong, D.V. Martello, M.K.

Chyu, Characterization of ZnO nanoparticle suspension in water: effectivenessof ultrasonic dispersion, Powder Technol. 194 (1–2) (2009) 75–80.

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6

A b s o l u t e z e t a p o t e n t i a l , m V

Ultrasonication duration, h

50%

25%

Excellent stability

Physical stability

Fig. 10. Absolute zeta potentials of Al2O3–water nanofluid after varying ultrason-

ication durations, at 25% and 50% amplitudes.


http://refhub.elsevier.com/S1350-4177(15)00006-1/h0005












































































































[18] C.-K. Lam, K.-T. Lau, H.-Y. Cheung, H.-Y. Ling, Effect of ultrasound sonication in

nanoclay clusters of nanoclay/epoxy composites, Mater. Lett. 59 (11) (2005)

1369–1372.

[19] A. Ghadimi, R. Saidur, H.S.C. Metselaar, A review of nanofluid stability

properties and characterization in stationary conditions, Int. J. Heat Mass

Transf. 54 (17–18) (2011) 4051–4068.

[20] K. Kwak, C. Kim, Viscosity and thermal conductivity of copper oxide

nanofluid dispersed in ethylene glycol, Korea–Aust. Rheol. J. 17 (2) (2005)

35–40.

[21] N. Mandzy, E. Grulke, T. Druffel, Breakage of TiO2 agglomerates in

electrostatically stabilized aqueous dispersions, Powder Technol. 160 (2)

(2005) 121–126.

[22] R.H. Müller, Zetapotential und Partikelladung in der Laborpraxis, first ed.,

Wissenschaftliche Verlagsgesellschaft, Stuttgart, Germany, 1996.

[23] J. Lee, K. Hwang, S. Jang, B. Lee, J. Kim, S. Choi, C. Choi, Effective viscosities and

thermal conductivities of aqueous nanofluids containing low volume

concentrations of Al2O3 nanoparticles, Int. J. Heat Mass Transf. 51 (11–12)

(2008) 2651–2656.












































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