Nanofluid with tunable thermal properties
John Philip, P. D. Shima, and Baldev Raj
Citation: Applied Physics Letters 92, 043108 (2008); doi: 10.1063/1.2838304
View online: http://dx.doi.org/10.1063/1.2838304
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APPLIED PHYSICS LETTERS 92, 043108 共2008兲
Nanofluid with tunable thermal properties
John Philip,a兲 P. D. Shima, and Baldev Raj
Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102,
Tamilnadu, India
共Received 27 November 2007; accepted 3 January 2008; published online 29 January 2008兲
We experimentally demonstrate the tunable thermal property of a magnetically polarizable nanofluid
that consists of a colloidal suspension of magnetite nanoparticles with average diameter of 6.7 nm.
Controlling the linear aggregation length from nano- to micron scales, the thermal conductivity 共TC兲
of the nanofluid has been enhanced up to 216%, using 4.5 vol % of nanoparticles. Repeated
magnetic cycling shows that the TC enhancement is reversible. It has been confirmed that the large
enhancement in TC is due to the efficient transport of heat through percolating nanoparticle paths.
Our findings offer promising applications in “smart” cooling devices. © 2008 American Institute of
Physics. 关DOI: 10.1063/1.2838304兴
Magnetic nanofluid is a unique material that has both the
liquid and magnetic properties.1 Such fluids have been found
to have several fascinating applications such as magnetooptical wavelength filter,2,3 optical modulators,4 nonlinear
optical materials,5 tunable optical fiber filter,6 optical
grating,7 and optical switches.8 Many of the physical properties of these fluids can be tuned by varying the magnetic
field.9–11 In addition, they have been a wonderful model system for fundamental studies.12 Very recently, it has been
demonstrated that manipulation of nanoparticles with carbon
nanotubes can result in enhanced thermal conductivity
共TC兲.13,14
In this paper, we demonstrate yet another fascinating application of magnetic nanofluid for thermal management.
Nanofluids are considered to be the future coolants for electronic devices and engines.15–17 Several nanofluids with enhanced TC have been tailored during the last several
years.18–21 The effective medium theory fails to explain the
large enhancement of k in nanofluids and the temperature
dependence of k. The possible mechanisms responsible for
the enhancement of TC in nanofluids still remain as a
puzzle.16 Two hotly debated mechanisms for heat transport in
nanofluids are the Brownian motion induced convection and
local percolation behavior.22–26
We used a stable colloidal suspension of magnetite nanoparticles of average diameter 6.7 nm, coated with Oleic acid
and dispersed in hexadecane.27 The size distribution of the
nanoparticles measured using dynamic light scattering experiment is shown in the inset 共a兲 of Fig. 1. The transmission
electron microscopy 共TEM兲 image of the nanoparticles is
shown in the inset 共b兲 of Fig. 1. Measurement details are
discussed in an earlier paper.21
Figure 1 shows the TC ratio 共k / k f 兲 of the nanofluid 共NF兲
as a function of volume fraction of Fe3O4 nanoparticles,
where k and k f are the TC of NF and the base fluid, respectively. No enhancement in TC is observed up to a volume
fraction of 1.5% of Fe3O4 nanoparticle. At 2.6 vol %, the
increases in k / k f was marginal. The highest value of k / k f
observed was 1.23 at 7.1 vol % that corresponds to an enhancement in TC of 23%. The data fitted with Maxwell
a兲
Author to whom correspondence should be addressed. Electronic mail:
philip@igcar.gov.in.
model28 show good agreement especially at higher volume
fractions. Slight deviation from the Maxwell fit is observed
at lower volume fraction 共below 4.5%兲. The observation of
TC enhancement at low particle loading 共⬍2 vol % 兲 in presence of magnetic field 共no enhancement without field兲 shows
that aggregation is the main cause of TC enhancement within
Maxwell’s limit. The enhancement above 1.76 vol % could
be due to small clusters 共dimmers or trimmers兲 formed in the
NF due to magnetic dipolar attractions. It appears that the
wetting of nanoparticles is enhanced due to the organic
sheath, which in turn leads to a lower interfacial thermal
resistance. We find that nanofluids with very small particles
共⬃5 nm兲 are also prone to settling when particles are
uncoated.
Figure 2 shows the k / k f of the NF as a function of applied magnetic field, during rise and decay, in 2.6 vol % of
Fe3O4 nanoparticles. It can be seen that the enhancement
starts above 20 G. Further increase in magnetic field leads to
drastic enhancement in the TC. The maximum enhancement
of TC 共128%; k / kf = 2.23兲 is observed at a magnetic field of
94.5 G, above which the TC value starts to decrease slightly.
While lowering the magnetic field, the TC value shows a
small hysterisis but comes back to the original value when
the magnetic field is turned off. After repeating the magnetic
cycles several times, the TC comes back to the original
FIG. 1. The TC ratio 共k / k f 兲 of the NF as a function of volume fraction of
Fe3O4 nanoparticles. The size distribution of Fe3O4 nanoparticles 共a兲 and the
TEM image of nanoparticles 共b兲 are shown in the inset.
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© 2008 American Institute of Physics
On: Mon, 22 Dec 2014 12:22:33
043108-2
Appl. Phys. Lett. 92, 043108 共2008兲
Philip, Shima, and Raj
FIG. 2. The TC ratio 共k / k f 兲 of the NF as a function of increasing and
decreasing applied magnetic fields in 2.6 vol % of Fe3O4 nanoparticles.
FIG. 3. 共Color online兲 The TC ratio 共k / k f 兲 and the enhancement as a func-
value. The above experiments have been repeated in a varition applied magnetic field in the NF with 4.5 vol % of Fe3O4 nanoparticles.
The inset of the figure shows the schematic of the mechanism of heat transety of NF samples with aqueous and nonaqueous carrier fluport from a cylindrical device immersed in NF without and with magnetic
ids and the results obtained were similar.
field.
Figure 3 shows the TC ratio 共k / k f 兲 and the corresponding enhancement as a function of applied magnetic field in
the NF with 4.5 vol % of Fe3O4. The variation of k / k f at
ues of enhancement for the upper HS and parallel modes are
three different magnetic cycles 共rise and decay兲 shows that
150% and 225%, respectively. Strikingly, the observed enthe enhancement is reversible with a slight hysterisis. The
hancement of 216% is close to the predicted value for paralmaximum enhancement was 216% at an applied magnetic
lel mode conduction.
field of 101 Gauss. The reasons for the drastic enhancement
In ferrofluids, nanoparticles are usually smaller than doof TC with magnetic field is discussed in an earlier
main
size and are free to rotate independently from each
publication.21 As we increase the magnetic field in a magother but they can be aligned by an external magnetic field.
netically polarizable nanofluid, the nanoparticles align in the
After turning off the field, the dipole moments can relax in
direction of magnetic field when the magnetic dipolar intertwo ways, i.e., Brownian motion and Neel rotation. Brownaction energy Ud共ij兲 dominates over the thermal energy kBT,
ian motion and Neel rotation are relaxation mechanisms due
where kB is the Boltzmann constant and T is the temperature.
to particle and spin rotation, respectively.1 Brownian relaxWhen the dipolar interaction energy becomes sufficiently
ation is achieved via bulk rotation diffusion of particles in
strong, the magnetic particles form chainlike structure.
the fluid. The relaxation time for Brownian motions given by
Here, each magnetite particles are single domain super␶B = 3V⬘␩ / kBT, where V⬘ is the hydrodynamic volume of the
paramagnetic with a magnetic moment m.1 Without any exparticle and ␩ is the dynamic viscosity. Neel relaxation is
ternal magnetic field, the magnetic moments of the scatterers
attributed to the rotation of the moment in the particle with a
are oriented in random direction. With the increase in magrelaxation time given by ␶N = ␶0 exp共KV⬘ / kBT兲, where K is
netic field, the moments of the magnetic particles start to
the anisotropy constant and ␶0 is typically of the order of a
align themselves along the direction of the magnetic field.
few nanoseconds. For a particle of 10 nm size, the value of
As nanoparticles start to form chains, the convection ve␶N and ␶B are 10−9 and 7.6⫻ 10−7 s, respectively. However,
locity 共v = 冑18kBT / ␲␳d3兲 drops drastically due to the cube
the value of ␶N increases sharply with the size of the particle
dependence of particle size. Therefore, the Brownian motion
due to the exponential dependence on V⬘. Typically, the TC
is severely arrested as the chain length increases. As the conmeasurement requires 30 s, which is much larger than ␶B of
vection velocity decreases with increasing magnetic field, the
primary nanoparticles. However, when the clusters of nanoobserved enhancement in TC cannot be due to the microconparticles are formed, the relaxation times can be much larger
vection mechanism. In a NF, series and parallel modes of
than the measurement times, which is the reason for the obthermal conduction through the base fluid and the nanoparserved hystersis at higher applied magnetic field.
ticles can be visualized within the mean-field models. The
The observed reversible tunable thermal property of NF
parallel mode has the geometric configuration that allows the
may
find many technological applications for this fluid in
most efficient means of heat propagation.25,26 Extremely
nanoelectromechanical system 共NEMS兲 and microelectromelarge TC can be expected from such parallel modes. In the
chanical system 共MEMS兲 based devices. For example, delower Hashin–Shtrikman 共HS兲 limit,29 nanoparticles are well
pending upon the cooling requirement, the current or magsuspended and conduction is through series modes whereas
netic field can be precisely programmed to obtain the desired
in the upper HS limit, the conduction path is through dislevel of TC enhancement or cooling. The mechanism of heat
persed particles. The clustering of nanoparticles can dramatitransport from a cylindrical device immersed in such a NF
cally broaden the TC. In the limit 共␾␬ p / k f 兲 Ⰷ 1, the predicted
coolant without and with magnetic field is depicted in the
values of k / k f for the upper HS and parallel modes are
inset of Fig. 3. When the field is off, the nanoparticles be共2␾ / 3兲␬ p / k f and 共␾␬ p / k f 兲, respectively. These values are
have as a normal fluid with random arrangement of particles.
comparable to k / k f = 共␾ / 3兲␬ p / k f predicted by the aggregaWhen the field is turned on, the parallel mode conduction
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leads to drastic enhancement of TC. The slight decrease in
tion model.26 Using the experimental data, the expected valOn: Mon, 22 Dec 2014 12:22:33
043108-3
TC observed above the peak magnetic field strength is probably due to “clumping” of chains and the subsequent distortion in the nematiclike order. The time dependence TC measurements under magnetic field shows a little influence at
magnetic field strength below the peak enhancement. Above
the peak enhancement magnetic field, TC decreases slightly
with time due to clumping of chains.
In summary, we present the details of a magnetically
polarizable NF with tunable thermal properties. We demonstrate that by controlling the linear aggregation length from
nano- to micron scales, the TC of the NF can be tuned from
a low to very high value. Further, it has been found that
under repeated magnetic cycling, the TC is reversible.
Our findings offer promising applications in efficient thermal
management.
We thank Dr. P. R. Vasudeva Rao and Dr. T. Jayakumar
for the support and encouragement. J.P. is grateful to Dr. J.
Eapen of Los Alamos National Laboratory for stimulating
discussions on percolation theory and fruitful comments on
our results.
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