Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluids containing Fe2O3 and CuO nanoparticles

Abstract

In this paper, the effects of alignment, pH, surfactant and solvent on heat transfer nanofluids
containing Fe2O3 and CuO nanoparticles are studied and analyzed. The microscope images show that
Fe2O3 could form some kind of alignment spontaneously in water even without external magnetic
field. With the addition of external magnetic field, the alignment is strengthened. In water, the
magnetic particle agglomeration to larger size occurs easily, which makes the directional alignment
much faster and easier. Ethylene glycol solvent and chemical surfactant sodium dodecyl benzene
sulfonate, NaDDBS could separate the Fe2O3 and CuO nanoparticles well in the fluids and avoid
possible aggregation. Therefore, magnetic alignments are hard to observe. The measured thermal
conductivities of each individual sample coincide with the microscope images and assumptions. In
addition, pH values of Fe2O3 and CuO nanoparticles are measured and it has been determined that at
those pH values, thermal conductivities of those nanoparticles would not be influenced according to
the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The highlight of this paper is that our
microscope images could well explain most of the literature data and conclusions and may open
new door to better understanding fundamental nature of nanofluids

Full text

Page 1

Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluidsEffects of alignment, pH, surfactant, and solvent on heat transfer nanofluids
containing Fe2O3 and CuO nanoparticlescontaining Fe2O3 and CuO nanoparticles
Hammad Younes, Greg Christensen, Xinning Luan, Haiping Hong, and Pauline Smith

Citation: J. Appl. Phys. 111111, 064308 (2012); doi: 10.1063/1.3694676
View online: http://dx.doi.org/10.1063/1.3694676
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i6
Published by the American Institute of Physics.

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Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluids
containing Fe2O3and CuO nanoparticles
Hammad Younes,1Greg Christensen,1Xinning Luan,1Haiping Hong,1,a)and Pauline Smith2
1Department of Material and Metallurgical Engineering, South Dakota School of Mines and Technology,
Rapid City, South Dakota 57701, USA
2U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA
(Received 22 December 2011; accepted 18 February 2012; published online 20 March 2012)
In this paper, the effects of alignment, pH, surfactant and solvent on heat transfer nanofluids
containing Fe2O3and CuO nanoparticles are studied and analyzed. The microscope images show that
Fe2O3could form some kind of alignment spontaneously in water even without external magnetic
field. With the addition of external magnetic field, the alignment is strengthened. In water, the
magnetic particle agglomeration to larger size occurs easily, which makes the directional alignment
much faster and easier. Ethylene glycol solvent and chemical surfactant sodium dodecyl benzene
sulfonate, NaDDBS could separate the Fe2O3and CuO nanoparticles well in the fluids and avoid
possible aggregation. Therefore, magnetic alignments are hard to observe. The measured thermal
conductivities of each individual sample coincide with the microscope images and assumptions. In
addition, pH values of Fe2O3and CuO nanoparticles are measured and it has been determined that at
those pH values, thermal conductivities of those nanoparticles would not be influenced according to
the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The highlight of this paper is that our
microscope images could well explain most of the literature data and conclusions and may open
new door to better understanding fundamental nature of nanofluids. V
Physics. [http://dx.doi.org/10.1063/1.3694676]
C 2012 American Institute of
I. INTRODUCTION
Nanofluids is a relatively new expression that is used to
describe a fluid in which nanometer-sized particles are sus-
pended. Oxides, metals, nitrides and nonmetals like carbon
nanotubes can be used as nanoparticles in nanofluids, while
water, ethylene glycol, oils, and polymer solutions can be used
as base fluids. Although many factors such as pH, particle
size, and zeta potential can affect the thermal conductivity of
the nanofluids, they still show promise in improving thermal
conductivity (TC) values1–12and could be useful for a variety
of heat transfer related applications including coolants and
lubricants. However, nanofluids utilizing a simple composite
structure do not enhance the TC effectively. For instance,
nanofluids with low percentage metal oxide loading show no
significant improvements in TC, while loading of 4?5 vol. %
metal oxide only results in about 10?20% TC increase.
Several efforts have been conducted related to metal
(Fe, Cu) and metal oxide (Fe2O3, CuO) nanoparticles in the
heat transfer nanofluids. Hong et al.13reported that nano-
fluids containing Fe showed a more rapid increase of the
thermal conductivity than that of nanofluids containing Cu as
the volume fraction of the nanoparticles increase. Also,
Hong and Yang et al.14reported that thermal conductivity
of Fe nanofluid is increased nonlinearly up to 18% as the
volume fraction of particles is increased up to 0.55 vol. %.
Zhu et al.15concluded that the abnormal thermal conductiv-
ities of Fe3O4nanofluids are attributed to the observed nano-
particle clustering and alignment. Philip et al.16observed a
dramatic enhancement of thermal conductivity in a nanofluid
containing magnetic particle of Fe3O4(6.7 nm diameter)
under the influence of an applied magnetic field. The maxi-
mum enhancement in thermal conductivity observed was
300% (k/kf¼ 4.0) at a particle loading of 6.3 vol. %. Shima
et al.17found that the thermal conductivity, viscosity, and
size distribution are found to be time independent in Fe3O4
while these features (properties) are time dependent in CuO
nanofluid. Abareshi et al.18reported that with the presence
of tetramethyl ammonium hydroxide as a dispersant, the TC
of nanofluids containing magnetite Fe3O4does not show a
significant increase. Altan et al.19reported that the thermal
conductivities of nonpolar solvents such as hexane and hep-
tane were found to increase linearly with the concentration
of magnetic nanoparticles. However, oil-borne and water-
borne nanoparticles were found to give significantly different
enhancements.
Unfortunately, the published results are contradictory
and do not provide reasonable explanations and conclusions
to the systems they studied. For example, in Hong’s pa-
per,13,14no good explanations are provided to explain the
difference of thermal conductivities. In Zhu’s paper,15the
evidence for alignment is observed by the TEM image. How-
ever, it is well known that a TEM image is taken in the solid
sample. The image does not reflect the nature of real time
fluid and particle movement. In Shima’s paper,17pH values
of Fe2O3and CuO fluids are not reported. According to
Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, flu-
id’s stability is influenced largely by the pH value. Also, in
Abareshi’s paper,18the author does not understand well why
their results are lower than that of other references.13–15In
a)Electronic mail: haiping.hong@sdsmt.edu.
0021-8979/2012/111(6)/064308/7/$30.00
V
C 2012 American Institute of Physics111, 064308-1
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Page 3

Altan’s paper,19it is demonstrated that the current mathe-
matical models underestimate the enhancement of the ther-
mal conductivity obtained experimentally, which could be
attributed to the fact that these models do not take the inter-
action between the base fluid and the particle surface or the
corresponding particle size into account.
Therefore, more detailed work needs to be done to clar-
ify and better understand the nature of these interesting and
challenging results. For example, how alignment, solvent,
and surfactant influence the dispersion (morphology) and
thermal conductivity of nanofluids? How does pH value
influence the stability, dispersion, and TC of nanofluids?
What is the difference between Fe2O3and CuO nanopar-
ticles? How does chemical surfactant react with Fe2O3and
CuO nanoparticles?
In this paper, the effects of alignment, pH, solvent, and
chemical surfactant on heat transfer nanofluids containing
Fe2O3and CuO were investigated. The reason for choosing
Fe2O3and CuO is that Fe2O3is a magnetically sensitive parti-
cle and CuO is not a magnetically sensitive particle. Under-
standing these effects and behavior of such nanofluids with
different magnetic properties and at various pH values would
be quite valuable in analysis and synthesis of novel nanofluids.
II. EXPERIMENTAL
Nanosize metal oxides Fe2O3(20?60 nm APS powder),
CuO (30?50 nm APS powder), ethylene glycol (99%), and
chemicalsurfactantsodium
(NaDDBS) were purchased from Sigma Aldrich. Sonication
was performed using a Branson Digital Sonifier, model 450.
Thermal conductivity data was obtained by the Hot
DiskTMthermal constants analyzer,20using the following pa-
rameters: measurement depth of 6 mm, room temperature,
power of 0.012 W, measurement time of 15 s, sensor radius of
3.189 mm, temperature coefficient of resistance of 0.0471/K,
disk type Kapton, and temperature drift rec yes. The magnetic
field intensity was recorded by F.W. Bell Gaussmeter model
5060. Microscope image was taken by Leica Z16 APO, Zoom
microscope.
The pH values were measured by pH Mettler Toledo
model SevenEAsy S20. Viscosity and rheology data were
recorded by Brookfield LV DV-IIþProv Viscometer and
Bohlin CVO rheometer, a gap of 150 lm was experimentally
determined. Zeta potentials and average particle sizes were
measured by Zetasizer Nano ZS and MPT-2 Autotitrator.
Transmission electron microscopy (TEM) images were
acquired with a JEOL JEM-2100 LaB6 transmission electron
microscope.
Fe2O3and CuO particles were dispersed in deionized (DI)
water and ethylene glycol with and without chemical surfac-
tant (dispersant) by sonication. Stability of fluids was observed
by the naked eye and recorded by a digital camera. Adjustment
of pH value was reached through addition of drops of acid
(HCl or HSCH2CO2H) and base (NaOH or CH3NH2).
dodecylbenzene sulfonate
III. RESULTS AND DISCUSSION
Transmission electron microscope (TEM) micrographs
of (a) Fe2O3and (b) CuO nanoparticles prepared by rapid
drying of a diluted sample are shown in Fig. 1(a) and Fig.
1(b). It is apparent that Fe2O3particles form large clusters
consisting of small primary particles and these particles are
spherical or near spherical. The average diameter of individ-
ual Fe2O3particles is around 20?30 nm. The particle diame-
ter measurements from the TEM results are consistent with
the data provided by vendor (20?60 nm). Also, CuO par-
ticles show less clusters and aggregations and each particle
could be observed clearly and discretely. The average diame-
ter of individual CuOparticles is around 10?30 nm. The par-
ticle diameter measurements from the TEM results are
consistent with the data provided by vendor (30?50 nm).
Normally, TEM needs the dry sample, no water and solvents
are allowed. Therefore, the nanoparticles aggregate on the
TEM images.
Figure 2 shows the microscope image of nanoparticles
dispersed in water [(a): CuO H¼0 kG, (b): Fe2O3H¼0 kG,
(c): Fe2O3H¼0.12 kG]. In Fig. 2(a), CuO nanoparticles are
randomly dispersed (distributed) in water and any kind of
alignment could hardly be seen. In fact, CuO is not a mag-
netically sensitive particle. In Fig. 2(b), surprisingly, without
magnetic field, Fe2O3nanoparticles still show some kind of
alignment. In Fig. 2(c), with relatively strong magnetic field
(0.12 kG), Fe2O3nanoparticles are perfectly aligned under
the direction of the magnetic field and tend to aggregate into
chains (agglomerate). The perfect alignment image is in
agreement with previous results and literature report.16,21,22
FIG. 1. TEM micrograph of Fe2O3and CuO nanoparticles prepared by rapid
drying of a diluted sample. (a) Fe2O3, (b) CuO.
064308-2 Younes et al. J. Appl. Phys. 111, 064308 (2012)
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Page 4

We want to attribute that the slight alignment of Fe2O3
is due to the magnetic field generated by the earth’s poles
(Earth’s geomagnetic field)23and local weak magnetic field
induced by building, facilities, equipments, etc. In order to
verify this assumption, a compass was used and adjusted to
locate the North Pole and the direction of Earth’s magnetic
field. At the same time, nanofluids containing Fe2O3particles
were also put under the microscope lens. It is interesting to
find that the alignment direction of Fe2O3nanoparticles is
the same as that of Earth’s magnetic field, both observed on
the microscope screen and by naked eye. The results indicate
that our assumption is correct.
It is interesting to explore what would happen if water is
replaced by other organic solvents with different viscosity
and polarity, such as ethylene glycol. Figure 3 shows the
microscope image of nanoparticles dispersing in ethylene
FIG. 2. (Color online) Microscope image
of nanoparticles dispersing in water under
different conditions [(a): CuO H¼0 kG,
(b): Fe2O3H¼0 kG, (c): Fe2O3H¼0.12
kG].
FIG. 3. (Color online) Microscope image
of nanoparticles dispersing in ethylene
glycol under different conditions [(a):
CuO H¼0 kG, (b): Fe2O3H¼0 kG, (c):
Fe2O3H¼0.12 kG].
064308-3 Younes et al. J. Appl. Phys. 111, 064308 (2012)
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Page 5

glycol [(a): CuO H¼0 kG, (b): Fe2O3H¼0 kG, (c): Fe2O3
H¼0.12 kG]. It is interesting to find that there is no align-
ment in all three microscope images. From images, ethylene
glycol could separate the nanoparticles quite well. The possi-
ble explanation for no evidence of alignment is that regard-
ing the ethylene glycol media, the higher viscosity of
ethylene glycol (around 16 cp) and separated nanoparticles
can be a factor for less magnetic alignment.
Figure 4 shows microscope image of nanoparticles,
sodium dodecyl benzene sulfonate, NaDDBS (chemical
surfactant) dispersing in water under different conditions
[(a): Fe2O3 H¼0 kG, (b): CuO H¼0 kG, C: Fe2O3
H¼0.12 kG]. In Figs. 4(a) and 4(b), without external mag-
netic field, Fe2O3and CuO nanoparticles are both randomly
dispersed in water. The effect of earth poles (Earth’s geo-
magnetic field) on Fe2O3is not observed clearly. Also, in
Fig. 4(c), Fe2O3nanoparticles show very slight alignment
under quite strong external magnetic field (0.12 kG). The ex-
planation is that if the additives or the media prevent
agglomeration of magnetic particles (like chemical surfac-
tants), the magnetic alignment will be difficult. When com-
pared to water, magnetic particle agglomeration to larger
size occurs easily, which makes the directional alignment
much faster and easier.
It should be emphasized here that the concentrations of
nanoparticles for microscope images and TC measurement
are different. In order to get the best visual effect, the con-
centrations for microscope images are quite dilute. Table I
shows the thermal conductivities (TC) of nanoparticles
(Fe2O3and CuO) dispersing in water, in ethylene glycol, and
in water with NaDDBS as surfactant. It is clearly shown that
TC of Fe2O3in water (0.73 W/mK) is higher than that of
CuO in water (0.69 W/mK) and the better aligned Fe2O3in
water under relative strong magnetic field (0.12 kG) show
highest TC value (1.12 W/mK). The TC of the nanofluids is
nonisotropic. The detail discussion and data analysis could
be found in author’s previous publications.9,22
In ethylene glycol, TC values of all three samples are
about the same (0.28?0.29 W/mK), indicating the fact that
no alignment, no significant TC changes. In water with
NaDDBS surfactant without magnetic filed, TC of Fe2O3
and CuO show slight decrease from 0.73 to 0.72 W/mK and
0.69 to 0.67 W/mK, respectively. The decrease is probably
due to the high interfacial resistance. With the external mag-
netic field, the TC show slight enhancement (0.75 W/mK)
because of the minor alignment. It is clearly seen that all TC
data could be explained reasonably and correlated to the
microscope images.
Here, we want to emphasize that we are very confident
to our measured data. We joined the international benchmark
efforts led by Buongiomo,6and our measured data match the
group’s data perfectly.
In a previous study,24it is found that pH does have
effect on the thermal conductivity of nanofluids. In order to
make sure in our experiments that alignment of nanoparticles
dominate the TC enhancement rather than pH effect, in
another word, pH value does not affect TC results. We have
measured the pH value of Fe2O3and CuO in water as well as
their zeta potentials and average particle size versus different
pH values. Table II lists the pH value of Fe2O3and CuO
FIG. 4. (Color online) Microscope image
of nanoparticles, NaDDBS (chemical sur-
factant) dispersing in water under differ-
ent conditions [(a): Fe2O3H¼0 kG, (b):
CuO H¼0 kG, (c): Fe2O3H¼0.12 kG].
064308-4 Younes et al.J. Appl. Phys. 111, 064308 (2012)
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Page 6

(0.4 vol. %) in water, ethylene glycol and in water with
chemical surfactant. It is found that the pH value of these
nanoparticles in different media shows slight difference. pH
of Fe2O3is around 4 (acidic) and pH of CuO is around 9
(basic).
Figure 5 shows Fe2O3and CuO zeta potentials and aver-
age particle size versus different pH values. (a) Fe2O3, (b)
CuO. pH value for Fe2O3in water is 4.88 without the surfac-
tant and 3.97 with surfactant. In these pH points, the particle
size distributions are far from maximum particle size at the
isoelectrical point (IEP), therefore, according to DLVO
theory,25the particles are stable and less aggregation and
coagulation. Figure 6 shows the dispersion behavior of 3 wt.
% Fe2O3nanofluids at different pH values (4, 6, 8, 11) [(a):
0 h, (b): 2 h]. Initially, after appropriate sonication, all fluids
containing Fe2O3nanoparticles with different pH (4, 6, 8,
11) show excellent dispersion and homogeneity. After 2 h,
the fluids with pH 4, 8, 11 maintain good dispersion and
homogeneity. At the same time, precipitations are clearly
observed with fluid at pH 6. The dispersion results further
supported our explanation: nanoparticle size increases
significantly around the IEP. Therefore, these nanoparticles
become unstable, aggregate, form clusters, and finally pre-
cipitate. Also Thermal conductivity data are measured with
different pH, as reported earlier, only the sample near IEP
show significant TC enhancement, as shown in Table III.
Therefore, in our microscope samples, the thermal conduc-
tivity data would not be influenced by the measured pH
value.
With the above discussions of microscope images, TC
results and pH values, we feel that it is comfortable to
explain most of the previously published results. Hong
et al.13,14reported that nanofluids containing Fe show a more
rapid increase of the thermal conductivity than nanofluids
containing Cu as the volume fraction of the nanoparticles
increases. Obviously, the explanation is that Fe nanoparticles
are magnetically sensitive and they will automatically form
some kind of weak alignment under the earth’s poles
(Earth’s geomagnetic field). Therefore, the TC value shows
abnormal enhancement. Abareshi et al.18reported that with
the presence of tetramethyl ammonium hydroxide as a
dispersant, the TC of nanofluids containing magnetite Fe3O4
does not see significant increase. The authors concluded in
their paper: “It is clear that the (TC) data observed by other
researchers (Hong et al.) are quite different from those of the
present study. The main reason for this contrast is not clear.
It may be caused by several factors such as difference in the
particle size, particle preparation, even the measurement
technique.” According to what we have observed and under-
stood, the possible reason is that dispersant prevent the align-
ment of Fe3O4. Therefore, no significant TC enhancement is
obtained by the authors.
Sinha et al.26reported that in ethylene glycol carrier
fluid, copper nanofluids are even superior in thermal conduc-
tivity compared to iron nanofluids. However, no pH values
of these nanoparticles are recorded. As we learn from above
discussion, appropriate pH value influences the thermal con-
ductivity of nanofluids, especially near the isoelectric point
(IEP). In addition, ethylene glycol media separate the nano-
particles well so that iron could not from some kind of align-
ment. Also Pastoriza-Gallego et al.27reported thermal
conductivities of both types of nanoparticles (Fe3O4and a-
Fe2O3) in EG have been determined experimentally as a
function of temperature and concentration (up to 6.9% vol-
ume fraction). The enhancement of the thermal conductivity
increases with the concentration almost linearly, reaching
the highest values of 11% and 15% for magnetite and hema-
tite nanofluids, respectively. Also, the experimental results
show that both nanoparticles in this base fluid present no sig-
nificant aggregation. These results coincide with our meas-
ured microscope images. Since nanoparticles do not show
TABLE I. Thermal conductivities (TC) of nanoparticles (Fe2O3and CuO,
0.4 vol%) dispersing in water, in ethylene glycol, and in water with
NaDDBS as surfactant.
Fe2O3
CuO
Fe2O3under magnetic
field (0.12 kG)
In water
In ethylene glycol
In water with surfactant
0.73
0.29
0.72
0.69
0.29
0.67
1.12
0.28
0.75
TABLE II. pH of Fe2O3, CuO in different mediums.
WaterWater with Surfactant Ethylene Glycol
Fe2O3
CuO
4.88
9.46
3.97
9.23
5.33
8.40
FIG. 5. (Color online) Fe2O3and CuO zeta potentials and average particle
size vs different pH values. (a) Fe2O3, (b) CuO.
064308-5Younes et al. J. Appl. Phys. 111, 064308 (2012)
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Page 7

aggregation and alignment in the EG system, therefore,
the thermal conductivities of nanofluids containing these
particles follow the classic effective medium theory and
increase linearly, no abnormal enhancement is observed.
It would be interesting to investigate further to better
understand the discussed results. For example, high polar
solvents such as dimethylformamide (DMF), tetrahydrofuran
(THF) and nonpolar solvents such as hexane and heptane
could be used to determine how polarity influences the align-
ment and thermal conductivity of nanofluids. Also, it is pos-
sible to use different chemical surfactants with negative
charge such as cetyl trimethylammonium bromide CTAB to
see how to influence dispersion and thermal conductivity. In
addition, metallic nanoparticles such as Fe, Cu will be used
to replace metal oxide Fe2O3and CuO to see the difference.
The research may possibly open new doors to exploration of
fundamental nature of nanofluids.
In conclusion, effects of alignment, pH, surfactant and
solvent on heat transfer nanofluids containing Fe2O3 and
CuO nanoparticles are studied and analyzed. From the micro-
scope images, Fe2O3could form some kind of alignment
spontaneously in water even without an external magnetic
field. With the addition of external magnetic field, the align-
ment is strengthened. In water, the magnetic particle agglom-
eration to larger size occurs easily, which makes the
directional alignment much faster and easier. Ethylene glycol
solvent and chemical surfactant sodium dodecyl benzene sul-
fonate, NaDDBS could separate the Fe2O3and CuO nanopar-
ticles well in the fluids and avoid them forming the possible
aggregation. Therefore, magnetic alignments are hard to
observe. The measured thermal conductivities of each indi-
vidual sample coincide with the microscope images and
assumptions. In addition, pH values of Fe2O3and CuO nano-
fluids are measured and it has been determined that at those
pH values, thermal conductivities of those nanofluids
wouldn’t be influenced according to the DLVO theory. The
highlight of this paper is that our microscope images could
well explain most of the literature data and conclusions and
may open new door to better understanding fundamental na-
ture of nanofluids.
ACKNOWLEDGMENTS
The financial support of Army Research Laboratory
(Cooperative agreement W911NF-08–2-0022) and NASA
EPSCoR (award No. NNX09AU83 A) are acknowledged.
The authors would like to thank Sungho Jin, Department
of Mechanical & Aerospace Engineering, University of
California San Diego and Edward Duke, Department of
Geology and Geological Engineering, SD School of Mines
and Technology, for their stimulating comments. Also thank
Carine Tchamakam Wamkam of Material and Metallurgical
Engineering Department, SD School of Mines for meas-
uring the zeta potential and particle size distribution of
nanoparticles.
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FIG. 6. (Color online) Dispersion behav-
ior of 3 wt. % Fe2O3nanofluids at differ-
ent pH values (4, 6, 8, 11) [(a): 0 hr, (b):
2 h].
TABLE III. TC of 3wt% Fe2O3aqueous solutions at different pH.
pH46811
TC (W/mK)0.720.820.740.73
Note: Density of Fe2O3: 5.24 g/ml at 25?C. TC of water: 0.60 W/mK.
064308-6 Younes et al.J. Appl. Phys. 111, 064308 (2012)
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