Energy Conversion and Management 149 (2017) 660–674

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Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

Review

Advanced applications of tunable ferrofluids in energy systems and energy MARK

harvesters: A critical review
⁎
M.A. Khairula, Elham Doroodchib, Reza Azizianc, Behdad Moghtaderia,
a
Priority Research Centre for Frontier Energy Technologies and Utilisation, Chemical Engineering, Faculty of Engineering and Built Environment, The University of
Newcastle, Callaghan, NSW 2308, Australia
b
Center for Advanced Particle Processing, Chemical Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308,
Australia
c
Advanced Thermal Solution Inc., 89-27 Access Road, Norwood 02062, MA, USA

A R T I C L E I N F O A B S T R A C T

Keywords: Ferrofluids or Magnetic nanofluids (MNFs) are the suspensions of magnetic nanoparticles and non-magnetic base
Magnetic nanofluid fluid. The heat transfer performance of a magnetic nano-suspension is influenced by the strength and orientation
Ferrofluid of an applied magnetic field. The main attraction of these types of nanofluids is that they not only enhance the
Electromagnetic energy harvester fluids’ thermophysical properties but also possess both magnetic characteristics like the other magnetic materials
Stability
and flow ability similar to any other fluids. Such an exclusive feature enables to control heat transfer, fluid flow
Heat transfer enhancement
Electromechanical systems (MEMS)
and movement of the nanoparticles by using the external magnetic fields. This review paper summarises the
recent investigations of magnetic nanofluids with the aim of identifying the effects of major parameters on the
performance of heat transfer. In addition, this study also acknowledged the novel application of ferrofluids in the
electromagnetic energy harvesters, and its challenges as well as the potentiality in the future research.

1. Introduction A unique class of nano-suspensions named magnetic nanofluids

demonstrate both fluid and magnetic properties. Magnetic nanofluids
Currently, cooling is one of the most important scientific challenges have drawn considerable attention due to the possibility of tuning their
in production related industries, such as transportation, manufacturing heat transfer and flow properties through the application of an external
and microelectronics. Technological advancements have led to in- magnetic field [8,9]. The nanoparticles can be either ferromagnetic
creased thermal loads and thus, improvements to cooling systems have materials, such as cobalt or iron, or ferrimagnetic materials such as
become a necessity. Maximising the surface area of heat exchanger magnetite. Magnetite nanoparticles are not susceptible to oxidation and
systems is the conventional approach to enhance heat dissipation. therefore are a far better alternative to iron or cobalt nanoparticles
However, this method needs an unwanted rise in the size of thermal which tend to lose their magnetic characteristics over time because of
management systems, thus, there is an urgency for novel coolants with oxidation [10]. Magnetite-based nanofluids with particle sizes less than
enhanced performance [1]. One such coolant is the innovative concept 10 nm, known as ferrofluids, were initially introduced by Stephen
of ‘nanofluids’, which are a mixture of metallic/nonmetallic nano- Pappell in the 1960s (at NASA) as an advanced method for controlling
particles in a base fluid. The term nanofluid was originally introduced fluid in space [11]. He concluded that magnetite nanofluids have a
by Choi at Argonne National Laboratory [2]. A substantial improve- wide range of applications from lubricating rotary shafts to biomedi-
ment in liquid thermal conductivity, specific heat and viscosity are the cine. These types of nanofluids have improved thermal properties (such
unique features of nanofluids. The relatively large overall surface area as heat capacity, thermal conductivity, and viscosity), as well as mag-
of nanoparticles not only improves heat transfer capabilities but also netic properties, both of those tunable characteristics help to control
increases the stability of the suspension by alleviating particle settling the heat transfer and the movement of the particle by applying the
phenomenon. There are also several potential benefits from nano-sus- magnetic fields. As a result, they are believed to be one of the promising
pension testing, specifically: better long-standing stability compared to fluids in different engineering fields such as bioengineering, thermal
the millimetre or even micrometre sized particle suspensions and lower engineering, electronics, and energy harvests [12,13].
erosion and pressure drop, especially in micro-channels [3]. Nanofluids However, when subjected to an external magnetic field, the thermal
are very potential fluids for heat transfer application [4–7]. conductivity of magnetite nanofluid can be raised to levels much higher

⁎
Corresponding author.
E-mail address: Behdad.Moghtaderi@newcastle.edu.au (B. Moghtaderi).

http://dx.doi.org/10.1016/j.enconman.2017.07.064
Received 3 May 2017; Received in revised form 17 July 2017; Accepted 29 July 2017
0196-8904/ © 2017 Elsevier Ltd. All rights reserved.
M.A. Khairul et al. Energy Conversion and Management 149 (2017) 660–674

Nomenclature T temperature (K)

V velocity (m/s)
A constant
Cp specific heat capacity (J/kg K) Greek symbols
d sample size (m)
→ φ particles volume fraction
f vector sum of the gravitational body force (N/m3)
g gravitational acceleration (m/s2) ρ density (kg/m3)
g∗ effective acceleration (m/s2) Δρ density difference (kg/m3)
Gr total potential energy (J) μ viscosity (N s/m2)
h liquid height (m) μ0 vacuum permeability (H/m)
H magnetic field (A/m) β relative coefficient
kB Boltzmann constant (J/K) γ system dependent coefficient
K thermal conductivity (W/m K)
M0 magnetisation (A/m) Subscripts
Pr Prandtl number
ΔP pressure drop (Pa) f base fluid
R radius of the container (m) L liquid
R′ radius of the spherical particle (m) m magnetic
Ra Rayleigh number p particle
Re Reynolds number T thermal

than any other nanofluid. In one such example, Philip et al. [14] have with ferrofluid.
shown remarkable enhancements in thermal conductivity of up to Recently magnetite nanofluids have come to the attention of the
300% for a magnetite based nanofluid. This high rise in thermal con- research community after showing high enhancement in heat transfer
ductivity is associated with the effective heat conduction through the by applying external magnetic field as well as their usage in the elec-
chain-like structures induced in the magnetite nanofluid. The ad- tromagnetic energy harvesters to generate more power with a small
vantage of using a magnetically polarisable nanofluid, such as magne- vibration with compare to the traditional harvesters. The objective of
tite nanofluid, is that the size, shape and form of aggregates can be this paper is to concentrate on the recent investigations of magnetic
precisely controlled by the external magnetic field. More importantly, nanofluids in the application of heat transfer and energy harvesting. It
unlike other nanofluids, the aggregation observed in magnetite nano- is anticipated that this review paper will help to provide a clear idea
fluids is perfectly reversible due to the super-paramagnetic nature of about the current status of ferrofluid, and also specify the re-
particles [15]. This tunable nature offers great opportunities for resol- commendations for the future study.
ving the inherent problems associated with conventional nanofluids,
such as lower heat transfer capacity, clogging and blockage of the flow 2. Preparation and characterisation of magnetic nanofluids
passage. The ferrofluid with magnetic fields in the application of heat
transfer is considered as the compound heat transfer method [16]. The The magnetic nanofluids are prepared by dispersing of super-para-
magnetic nanofluids offer the following advantages compared to the magnetic nanoparticles into a non-magnetic base fluid such as water,
nonmagnetic nanofluids [17]; hydrocarbon oil and so on [25]. Generally, the preparation of ferro-
fluids consist of two steps: (a) the magnetic nanoparticles preparation,
(a) the temperature gradient and non-uniform magnetic field are in- (b) the dispersion of the synthesised magnetic nanoparticles in different
duced by using a magnetic field, which may initiate a flow in the polar/non-polar carrier liquids. In the first step, the nano-sized mag-
fluid. Such phenomenon is called thermomagnetic convection [18] netic particles can be produced by various processes such as ball milling
and it is readily handled; [26], sonochemically synthesised [27], sol-gel method [28,29], reverse
(b) the thermomagnetic convection is higher compare to the gravita- micelle technique [30], and thermal decomposition [31]. Moreover, the
tional convection; metal oxide magnetic nanoparticles are synthesised by micro-emulsion,
(c) the possibility of changing thermal properties of ferrofluids by ap- chemical co-precipitation and phase transfer methods [25,29,32–34].
plying external magnets/solenoids [19,20] In the second step, the magnetic nanoparticles are coated by various
methods such as co-precipitation [29], core-shell [35], and then dis-
Furthermore, a ferrofluid is a suspension of solid-liquid, made-up persed in the base fluids.
from nano-sized permanent magnetic dipoles [21]. The magnetic na- Currently some researches have completed experiments to synthe-
nofluids are potential fluids for vibrational energy harvesting applica- sise metal and metal oxide magnetic nanoparticles with a specific size
tion due to their fluidity and magnetic properties in which they act as a distribution [32]. Metallic nanoparticles were prepared using different
soft ferromagnetic substance [22,23]. Energy harvesting is an alteration techniques such as the thermal decomposition of metal carbonyl com-
of the environmental energy to the electrical energy at a small scale and plexes, the simple reduction of metal salts, thermolysis of metal-
was originally introduced in 1966 [24]. The energy sources for energy polymer complexes, submerged arc nanoparticle synthesis system, and
harvesters are freely available in the environment. Examples of energy gas phase reduction of metal complexes [32,36]. Among the metal
sources include vibration, wind energy, wave energy, and thermal oxide nanoparticles, Fe3O4, γ -Fe2O3 and spinel type ferrites (MFe2O4,
temperature gradients. Nowadays energy harvesting has become a hot with M = Mn, Co, Zn, Ni, etc.) are most commonly used nanoparticles
topic for research because of its exceptional advantages. The cost of the because of their chemical stability. The most efficient method for the
battery supply, in particular, the maintenance cost to replace the dis- ferrite nanoparticles and subsequent magnetic fluid preparation is
charged batteries, make the energy harvesters a very attractive option. chemical precipitation [37]. The preparation of magnetic nanofluids
The conventional energy harvesting techniques only capable to gen- consist the following steps;
erate a few micro-watts, whereas vibration energy harvesters demon-
strate high performance in less vibration by replacing the solid magnets • co-precipitation (≈80 °C) of magnetite from aqueous solutions of
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Fe3+ and Fe2+ ions in solutions of sodium hydroxide (NaOH), am- • repeated flocculation and redispersion of magnetic nanoparticles to
monium hydroxide (NH4OH) or potassium hydroxide (KOH) [38]; eliminate the free surfactant known as advanced purification
and ammonium hydroxide has better crystallinity, smaller size and • mono-layer stabilised magnetic nanoparticles
higher magnetic saturation among them [39] • dispersion in a polar solvent (stabilisation with a secondary sur-
• subdomain magnetite particles factant, e.g. dodecylbenzenesulphonic acid, physically adsorbed to
• steric stabilisation (chemisorption of lauric acid, myristic acid or the first layer)
oleic acid; 80–82 °C)
• phase separation, and magnetic decantation as well as repeated The details of the multi-step procedures applied to prepare the
washing magnetite nanofluids at the University of Newcastle described in Fig. 1
• mono-layer covered magnetite nanoparticles + free surfactant [34], which were refined and optimised for the synthesis of magnetic
• extraction of mono-layer covered magnetite nanoparticles (acetone fluids for various applications, such as vibration based energy har-
addition; flocculation) vesters and as a highly efficient heat transfer fluids. The magnetic na-
noparticles and nanofluids are characterised using different techniques
Cobalt ferrite and Fe2O3 nanoparticles were prepared using NaOH and methods, as shown in Table 1.
instead of NH4OH.
Finally, the dispersion of lauric acid, myristic acid or oleic acid
3. Stability of magnetic nanofluids
mono-layer coated synthesised magnetic nanoparticle in non-polar
carriers (such as kerosene, toluene, cyclohexane, transformer oil, etc)
A nanofluids’ stability relies on a number of aspects [53]: (a) na-
are demonstrated as follows;
nofluids are multiphase dispersion systems with high surface energies

• the dispersion process of nanoparticles in non-polar carriers carried

and hence, are thermodynamically unstable, (b) nanoparticles dis-
persed in the nanofluids have strong Brownian motions. The nano-
out at the temperature of 120–130 °C [40]
• magnetic decantation/filtration; and repeated flocculation and re-
particles’ movement can offset their sedimentation due to gravity, (c)
the dispersed nanoparticles in the fluids may settle out with time be-
dispersion of magnetic nanoparticles for the elimination of free
cause of nanoparticle aggregation, which is initiated by van der Waals
surfactant called advanced purification process
forces, (d) no chemical reactions among the suspended nanoparticles
nor between the base fluid and nanoparticles are expected. Conse-
whereas, the dispersion method of magnetic nanofluids in organic
quently, aggregation and sedimentation are the two critical phenomena
polar carriers such as alcohol, diester, ketone, amine, different mixtures
relating to the stability of a nanofluid.
of mineral and synthetic oils involves;
The sedimentation of the particle in the magnetic nanofluid may
occur due to the effects of magnetic field, gravitational field and/or
• primary magnetic fluid on light hydrocarbon carrier magnetic field gradient, because of the external magnetic field is

Fig. 1. Methodology of ferrofluids production [34].

Deoxygenated Water FeCl2 4H2O FeCl3 6H2O
28 ml 1.72 g 4.70 g

Mix

Poly sodium salt Stir for 10 min till

2.0 g foaming

Ammonium hydroxide
Stir for 30 min
10 ml (at different flow rate)

Add acetone to precipitate the particles

Wash the samples with acetone and

separate them using a permanent magnet

Heat it up to remove the excess acetone

and nanoparticles dispersed in DI-water

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Table 1
Physicochemical characterisation of magnetic nanoparticles.

Characterisation Method and technique Measured parameter

Geometry Transmission Electron Microscopy (HRTEM) and Field Emission Scanning Size and the shape of magnetic particles
Electron Microscopy (FESEM) [41–43]
Dynamic Light Scattering (DLS), Quasielastic Light Scattering (QELS) [43,44] Mean particle size and size distributions
X-ray Diffraction and Mössbauer Spectroscopy [45]

Structure X-ray diffraction, thermal analysis, and Mössbauer and infrared spectroscopy Nanoparticle structure
[44]
Thermogravimetric and differential thermogravimetric analysis (TGA), The structures of composite particles made of iron oxide and organic
Differential Scanning Calorimetry (DSC), Fourier Transform Infrared molecules The nature of the interactions between the iron oxide core and
spectroscopy (FTIR) and Statistic Secondary Ion Mass Spectra (SSIMS) [45–47] the associated molecular structures within the composite
Atomic Force Microscopy (AFM) and Chemical Force Microscopy (CFM) [48] The morphological changes occurring on the surface of iron oxide
nanoparticles upon exposure to a coating material

Colloidal Stability Dynamic Light Scattering (DLS) [49,50] The colloidal stability is based on the hydrodynamic particle size
Turbidity measurements [51,52] Stability of magnetic nanoparticles by measuring the aggregation kinetics

directly associated with the size distribution of magnetic nanoparticles of the existence of London-van der Waals and magnetic forces, con-
[54]. The sedimentation of the particle has a significant impact on the tributing to the irreversible aggregation of nanoparticles. Thus, mag-
stability of magnetic nanofluids. The stability against the particle se- netic nanofluids introduce the repulsive forces between the magnetic
dimentation can assure when the thermal energy of the nanoparticles nanoparticles to repel the dipole-dipole magnetic interactions and
goes higher than the gravitational and magnetic energies, accordingly. London-van der Waals force. The repulsive force between the nano-
The maximum size of the nanoparticle was calculated by using the particles can be attained by using a polymer surfactant as a coating
Odenbach [54] formulas; d < (6kB T / μ0 M0 πH )1/3 in the presence of around the particles, which could produce an entropic repulsion, and/
magnetic field and d < (kB T /Δρghπ )1/3 under the gravitational field, or by introducing a coulombian repulsion from the variation of the
where kB, T ,μ0 , M0, H ,Δρ, g and d denote the Boltzmann constant, tem- nanoparticle surface [55,56]. Generally, the dispersion process of the
perature, vacuum permeability, the spontaneous magnetisation of the magnetic nanofluids is carried out by ultrasonic homogenization
magnetic material, magnetic field, density difference between nano- method in the presence of a surfactant.
particle and the base fluid, gravitational acceleration, and size of the Finely divided iron is very sensitive in the presence of water or
sample, respectively. In principle, the particle aggregation rises the humid air and oxidising agents. Therefore, protection of magnetic na-
active diameter, hence a destabilisation of the suspension occurs due to noparticles is the prior requirement to obtain chemically and physically
the sedimentation. In this case, the maximum diameter (d) of the par- stable colloidal systems, and such protection may be attained by surface
ticle was estimated as, d < (144kB T / μ0 M02)1/3 corresponding to the coating around the nanoparticle as shown in Fig. 2 [57]. A couple of
maximum interaction of energy [54]. methods have been suggested to attain stable nanofluids, such as che-
The use of super-paramagnetic particles in magnetic nano-suspen- mical or physical treatment. These treatments may involve the mod-
sion is not necessarily definitive of the stability of magnetic nanofluids ification of the dispersed nanoparticle surface, the addition of an extra
[55]. Magnetic nanoparticles in a base fluid will not be stable because surfactant, or enforcing strong forces on the agglomerated

Fig. 2. Schematic representation of the stabilisation of magnetic nanoparticles by surface coating (a) inorganic material, (b) organic material, (c) encapsulation into nanospheres, (d)
nanocapsules [57].

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nanoparticles. The active surface agent was applied for the modification significant impact on the heat transfer and flow features of nanosus-
of hydrophobic materials, thus allow dispersion in an aqueous solution pensions [72]. Generally, nanofluids containing smaller particles are
[58]. The chemical treatment changes the suspension stability through offering more improvement of heat transfer with compare to the bigger
the resultant surface potential, as well as surface charge states [59]. particles [73]. The Brownian motion can enhance the heat transfer in
Otherwise, sedimentation, aggregation, and clogging may occur and two different ways, firstly, the motion of the nanoparticles help to
hamper the thermal properties of the nanofluids. It is well known that transport the heat (diffusion of nanoparticles) through direct con-
the theory of aggregation and clustering is one of the primary causes of tribution and the indirect contribution because of the micro-convection
stability and unexpected increase of nanosuspensions thermal con- is acting on the surrounding of the individual nanoparticles [71,74].
ductivity [60]. Philip et al. [14] and Evans et al. [60] showed that the Some authors [68,75] were recommended that the thermal con-
greater aspect ratio structure of the fractal-like aggregates is a funda- ductivity is controlled by Brownian motion at nano and micro levels.
mental factor that allowed for enhanced heat flow over large distances. The thermal conductivity and particle motion rise with the increment of
Normally, nanoparticles need to fulfill the two principles in order to temperature [68]. Besides, the motion of nanoparticles due to the
gain a high-quality suspension, the diffusion principle as well as the Brownian motion was noticeably slow in heat transfer, because a par-
zeta potential principle [61]. The stability of a colloidal suspension is ticle needs to travel massive distances to achieve a perfect place and
highly influenced by its zeta potential value or surface charge [62]. It is this distance may be a shorter one. Consequently, the random motion
well known that the dividing line between stable and unstable sus- could not be a significant phenomenon in heat transfer.
pensions is generally around ± 30 mV and nanoparticles with a zeta Current analyses refused the hypothesis of Brownian motion, re-
potential more negative or positive than ± 30 mV accordingly are porting that the enhancement of thermal conductivity of magnetic na-
considered physically stable [53,63]. In addition, zeta potential of a nofluids can be described from the theory of particle clustering [14,20].
colloidal suspension is highly dependent on the pH value of the solution Philip et al. [20] presented that the micro-convection model overvalued
[7]. the thermal conductivity, thus the micro-convection of the fluid
In a stationary state, a nanoparticles’ sedimentation velocity in a medium around randomly moving nanoparticles did not have any effect
nano-suspension complies with Stokes Law [64]: on nanofluids thermal conductivity. The diffusion of magnetic nano-
2 particles exhibits a vital role at low nanoparticle volume concentrations
2R′ (φ < 2 ), which can be described by the effective medium (Maxwell)
V= (ρ −ρ ) g theory instead of the mechanism related to the Brownian motion in-
9μ p L (1)
duced hydrodynamics. The improvement in thermal conductivity of
where V is the sedimentation velocity of nanoparticles, R′ is the radius magnetic nanofluids at the high-volume fraction (φ > 2 ) was associated
of spherical particle, μ is the viscosity of the liquid medium, ρp and ρL with the presence of dimmers or trimmers in the field. These experi-
are the density of particle and the liquid medium, respectively and g is mental results had a reasonable agreement with the Maxwell-Gannet
the acceleration of gravity. Eq. (1) exposes a balance of the viscous model, especially at a greater nanoparticle volume concentrations [20].
drag, buoyancy force and gravity, those are actively influencing the Tsai et al. [76] examined the effect of viscosity on the thermal con-
dispersed nanoparticles. The following actions might be helpful to re- ductivity of magnetic nanofluids. They concluded that the experimental
duce the sedimentation speed of nanoparticles in nano-suspension, and value of the nanofluids thermal conductivity progressively matches the
henceforward to produce an improvement in nanofluids stability: (a) value of thermal conductivity predicted by using the Maxwell equation.
reducing nanoparticles size/radius, (b) increasing the viscosity of the In addition, diffusion of magnetic nanoparticles could be one of the
fluid medium and (c) minimising the difference between nanoparticle potential reasons for thermal conductivity improvement.
and base fluid density. It is obvious that the reduction in particle size
should significantly reduce the nanoparticles sedimentation speed, as a 4.2. Liquid layering on the nanoparticle-liquid interface
result enhance the nanofluids’ stability, since V is proportional to the
square of R. According to the colloid chemistry theory, when the na- Previous studies clearly showed that the liquid molecules may form
noparticle size reduces to a critical size, there is no sedimentation due layers around the nanoparticle due to the strong force of the nano-
to the Brownian motion of the particle [65]. However, nanoparticles particle and the atomic structure of liquid layer is more arranged
have the possibility to aggregate due to its higher surface energy. compared with the bulk liquid. Since phonon transfers in crystalline
Therefore, the preparation of a stable nanofluid is associated with using solid is very effective, such ordered layer in the liquid shows higher
the smaller nanoparticles to prevent the aggregation process con- thermal conductivity and as a result, the thermal conductivity of na-
currently [65]. nofluids was enhanced [77]. Keblinski et al. [78] suggested that the
resultant higher effective volume of the particle-layered-liquid struc-
4. Mechanism of heat transfer enhancement using magnetic ture can improve the value of thermal conductivity (refer to Fig. 3). Yu
nanofluids et al. [79] explained the existence of liquid molecules near to a solid
surface. Ren et al. [80] confirmed the thermal conductivity increment
Numerous methods have been identified on the mechanism of the with this liquid layer, maximum 165% enhancement in thermal con-
heat transfer enhancement in the different studies. The thermal prop- ductivity was found for 3 nm liquid layer compared to 1 nm. Yu et al.
erties especially the thermal conductivity of magnetic nanofluids was [81] also clarified the significance of the solid-liquid interfacial layers
the main focus of some investigations, but the mechanisms to justify the in the enhancement of nanoparticle (< 10 nm) thermal conductivity.
experimental data in the presence and absence of external magnetic
field are still desirable. The Brownian motion [66–69], liquid layering 4.3. Effects of nanoparticles clustering
on the particle-liquid interface, and the effects of nanoparticles clus-
tering is the much debated among all of the proposed heat transfer Aggregation of nanoparticles is considered as one of the main me-
mechanisms [14,69]. chanisms of heat transfer enhancement using nanofluids, and nowadays
it receives a wide interest of research community [71,82,83]. The
4.1. Brownian motion particle clusters may involve the fastest heat transport along the long
distance, because of the high heat conductivity of the solid particle
Brownian motion can be expressed as a random motion induces compare to the base fluid [84,85].
from the collision of the base fluid molecules, consequently particle Bishop et al. [86] showed that the magnetic nanofluids have the
experiences a random walk motion [70,71]. This parameter has a magnetic interaction to introduce a self-assembled aggregation even

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though in the absence of an external magnetic field. The structures of

aggregations of magnetic nanoparticles control the heat conduction in
the fluid. Without the application of the external magnetic field, the
nanoparticles distribution in the nanofluid is disordered and the
thermal conductivity is isotropic [16]. Magnetic nanoparticle tries to
align its magnetic moments toward the direction of the local magnetic
field because of the external magnetic field or the neighbouring nano-
particles [54,87]. This allows magnetic nanoparticles to form micron-
sized, one-dimensional chains, rings, two-dimensional aggregates or
even three-dimensional super lattices. The mechanisms of the aggregate
formation, manipulation of nanofluids and distribution morphology in
the presence of external magnetic fields are required to understand the
thermal conduction mechanism of magnetic nanofluids.

5. An overview of thermal properties of magnetic nanofluids

Investigations on magnetic nanofluids without the application of

external magnet exhibit that thermophysical properties of nanofluids
are affected by different factors such as nanoparticle size, types and
Fig. 3. Thermal conductivity improvement due to the formation of highly conductive intensities of magnetic fields, nanoparticle volume concentration, base
liquid structure at liquid/particle interface for various layer thickness (h) as a function of fluid properties, temperature, the chemical composition of the nano-
particle diameter (d) [78]. particle, the coating around the nanoparticles etc. The properties of
magnetic nanofluids and their effects on the volume fraction of

Table 2
Summary of most common thermal conductivity models of magnetic nanofluids.

Model Expression Description

Maxwell-Gannet model [91] Kp + 2Kf + 2φ (Kp − Kf ) Spherical particles with low volume
Kp + 2Kf − φ (Kp − Kf ) fraction φ
Modified Maxwell-Gannet model [92] Ki Kp + 2Kf + 2φi (Kp − Kf ) Spherical particles with volume
=
Kf Kp + 2Kf − φi (Kp − Kf ) fraction φi; i = x ,y,z
Microconvection model [93] Kp + 2Kf + 2φ (Kp − Kf ) A is constant, Re and Pr represents
(1 + A Reγ Pr 0.003φ) ⎡ ⎤
Reynolds and Prandtl numbers,
⎢
⎣ Kp + 2Kf − φ (Kp − Kf ) ⎥
⎦
correspondingly, and γ denotes system
dependent coefficient
Bruggeman model [94] K 1⎡ Kp Kf Spherical-sized particles with high
= (3φ−1) + (2−3φ )⎤ + Δ volume fraction of the particle
Kf 4⎢ Kf ⎥ 4
⎣ ⎦
2
Kp Kp
with Δ = (3φ−1)2 ⎛ ⎞ + 2(2 + 9φ−9φ2) ⎛ ⎞
⎜ ⎟ ⎜ ⎟

⎝ Kf ⎠ ⎝ Kf ⎠
Jeffrey model [95] 3κ3 9κ3 3κ 4 High order terms represent pair
K
Kf
= 1 + 3φ + φ2 ⎡3κ 2 +
⎣ 4
+
16 ( α+2
2α + 3 )+ 64
+ ··· ⎤
⎦ interaction of randomly dispersed
α−1 Kp particles
with κ = and α =
α+2 Kf

Rayleigh model [96] K Kp Kf Suspensions of spherical particles with

= Kf + 3φ
Kf 2Kf + Kp φ [1 + 3.939φ2 (Kp Kf ) / 4Kf + 3Kp] Kp Kf a regular particle distribution
Murshed model [97] Qp ω (Kp − ωKf )(2γ13 − γ 3 + 1)(Kp + 2ωKf ) γ13 [Qp γ 3 (ω − 1) + 1] ⎫ Murshed model considers the effects of
K = ⎧K f nanolayer, size and movements of the
⎨ γ13 (Kp + 2ωKf ) − (Kp − ωKf ) Qp (γ13 − γ 3 + 1) ⎬
⎩ ⎭ particle, and surface chemistry of
3Λ2 9Λ3 ⎛ K cp + 2Kf 3Λ4 ⎤ ⎫ nanoparticles. Cp − p and Cp − f represent
+ ⎧Qp6 γ 6Kf ⎡3Λ2 + + ⎜
⎞+
⎟

⎨ ⎢ 4 16 2K cp + 3Kf
⎝ ⎠ 26 ⎥ ⎬ the specific heat capacity at constant
⎩ ⎣ ⎦⎭
pressure for nanoparticles and base
⎧1 ⎡ 3κB T (1 − 1.5γ 3Qp) GT ⎤ ⎫
+ ρ Cp − cp ds ⎢ +
6πrp ds ⎥ ⎬
fluid, respectively. Gr denotes the total
⎨ 2 cp 3
2πcp γ 3r p
⎩ ⎣ ⎦⎭ potential energy between two
t t K cp − Kf interacting nanoparticles.
with γ = 1 + ,γ =1+ , Λ= ,
rp 1 2rp K cp + 2Kf

2(Kp − Klr ) + γ 3 (Kp + 2Klr )

K Cp = K lr ,
(Klr − Kp) + γ 3 (2Klr + Kp)

ρcp and Cp − cp are density and specific heat of complex particle,

1 1
respectively, and they are given by ρcp = ρ + ⎛1− 3 ⎞
γ3 p ⎝ γ ⎠

{ 3ρp
pb3
(2t 2 + 2brp t + b2r p2)−
3ρf
pb3
[(2 + 2b + b2) + brp (brp + 2bt + t )] }
1 1
Cp − cp = Cp − p + ⎛1− 3 ⎞
γ3 ⎝ γ ⎠
2 2
⎧ 3Cp − p 2 3Cp − f ⎫
(2t + 2b′rp t + b′ r p2)− ′ ′ ′ ′ ′
3 [(2 + 2b + b ) + b rp (b rp + 2b t + t )] ⎬
⎨ pb′3 pb′
⎩ ⎭
p Cp − p ρ
where p = 3r p2 + 3rp t + t 2; b = ln ⎛ ⎞; b′ = ln ⎛ ⎞⎜ ⎟ ⎜ ⎟
ρ
⎝ f⎠ ⎝ Cp − f ⎠

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alignment was one of the main reasons for the irregular improvement of
thermal conductivity. They conclude that the Fe3O4 nanofluid with 4%
volume fraction of nanoparticle shows 38% enhancement in thermal
conductivity. Jiang and Wang [99] synthesised a magnetic nanofluid by
one step phase transfer, showed that the particle coating on the surface
of the nanoparticle was an important parameter in the cluster forma-
tion. They concluded that the thermal conductivity of magnetic nano-
fluid can be influenced by using a stabiliser, hence it controls the
structure of the aggregates. Besides, the thermal conductivity decreases
in case of well-dispersed composites. There is no recognisable standard
for the superlative mix of combining methods. The techniques are (a)
the surfactant or activator addition, (b) pH control (surface chemical
effect), and (c) ultrasonic vibration [100]. According to the literature,
three effective techniques are applied to achieve suspension stability
against nanoparticles sedimentation. Some researchers used all of these
tactics to achieve better stability [101–103] but others applied just one
[104] or two techniques [105–107] with satisfaction. Wang et al. [108]
experimentally examined the effect of nanoparticle size (4–44 nm) on
the thermal conductivities and exhibited that the magnetic nanofluids
showed greater thermal conductivities with compare to the heat
transfer oils, and the enhancement of thermal conductivity was found
with reducing the size of the nanoparticle. The improvement in thermal
Fig. 4. Schematic diagram of (a) electromagnet and (b) solenoid. conductivity has showed up to 26.4% at 4 vol% of Fe3O4 nanofluids
with 4 nm size of the particle. Additionally, the values of viscosity for
all nanofluids were significantly lower than that of the carrier fluid.
nanoparticle and temperature have been discussed in most of the stu-
Hong et al. [109] investigated the effect of nanoparticles clustering for
dies. Syam Sundar et al. [88] studied the viscosity and effective thermal
a glycol based magnetic nanofluids in the absence of magnetic field,
conductivity of Fe3O4/water nanofluids and showed that the nanofluid
and it was found that thermal conductivity decreased with sonication
demonstrates Newtonian behaviour, and the thermal conductivity in-
time. Thus, the size of the clusters formed by the nanoparticles had a
creases with the rise of nanoparticle volume fraction as well as tem-
major influence on the thermal conductivity. The non-linearity was
perature. Pastoriza-Gallego et al. [89] concluded that a linear im-
attributed to the rapid clustering of nanoparticles in the condensed
provement of the thermal conductivity with rising the particle volume
nanofluids. Moreover, the carrier fluid effect on the ferrofluids prop-
concentration and the nanofluids was nearly temperature independent.
erties was demonstrated in some analyses. Chen et al. [110] suggested
Magnetic nanofluids were prepared by dispersing magnetite nano-
that the aggregation of nanoparticles was one of the main reasons for
particle in the water using a surfactant called tetramethyl ammonium
the enhancement in viscosity of nanofluids beyond the classical Eines-
hydroxide, and the highest thermal conductivity enhancement was
tien equation. They proposed their model by adding an aggregation
found 11.5% with 3% volume concentration of nanoparticle at 40 °C
effect to the Krieger and Dougherty [111] model, finding good agree-
[90]. Many studies have been focused on the thermal conductivity of
ment between their experimental data and the proposed model.
nanofluids and proposed various correlations to calculate its value,
Table 2 shows the frequently used models for the determination of the
thermal conductivity of magnetic nanofluids. 5.2. Effects of magnetic field

Applying a magnetic field can affect the thermophysical properties

5.1. Effects of nanoparticle size of magnetic nanofluids, as a result, some researchers were analysed the
properties of magnetic nanofluids in the presence of magnetic field. The
A number of researchers have been investigated the effect of mag- effect of external magnetic fields and its directions and strengths on the
netic nanoparticle cluster and size on the thermal properties of nano- thermophysical properties of nanofluids have also been assessed.
fluids. Zhu et al. [98] stated that the clustering, as well as nanoparticle Results revealed that the thermophysical properties of magnetic

Fig. 5. Relative viscosity as a function of perpendicular external magnetic field strength (a) Fe3O4/water, (b) Fe/water [113].

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nanofluids are easily changeable by varying the strength and direction concluded that a significant enhancement of thermal conductivity with
of the magnetic field. Krichler and Odenbach [112] used an advanced the existence of the magnetic field parallel to the temperature gradient
measuring device (a hot plate technique with a non-stationary mode) because of the development of chain-like magnetic nanoparticle ag-
for measuring the thermal conductivity of ferrofluid by changing the gregates. Fu and Gao [91] introduced the two-step homogenisation
direction and strength of the magnetic fields. The thermal conductivity method for studying the anisotropic thermal conductivity of magnetic
improved with the increment of the magnetic field strength for parallel nanofluids by considering the effect of physical anisotropy as well as
alignment of the magnetic field and heat flux. On the other hand, the the magnetic field. The result showed that the aspect ratio of the chain-
relation between the thermal conductivity and magnetic field strength like aggregates of magnetic nanoparticle exposed a vital role in the
was totally opposite in the case of perpendicular alignment. Therefore, improvement of anisotropic thermal conductivity. Philip et al. [20]
the strength, parameters, and orientation of the external magnetic field found the highest thermal conductivity enhancement about 300% and
can also affect the properties of magnetic nanofluids because of the 216% under the influence of external magnetic fields for Fe3O4-kero-
structure formation of the magnetic particles in the magnetic nanofluid sene and Fe3O4-hexadecane nanofluids, accordingly. Such a large im-
depends on the types of magnetic fields. Azizian et al. [84] used elec- provement was found due to the formation of chain-like structures of
tromagnets and solenoids to apply the parallel and perpendicular magnetic nanoparticles. Parekh and Lee [119] also reported approx-
magnetic fields to the temperature gradient, accordingly (as shown in imate 30% of thermal conductivity increment at 4.7% of volume con-
Fig. 4); and illustrated the influence of nanoparticle aggregation on the centration of magnetite nanofluids in the presence of a magnetic field
thermal conductivity of nanofluids. They concluded that the maximum and this may happen due to the continuous development of three-di-
enhancement of thermal conductivity of Fe3O4/water nanofluid is mensional zipper-like structures of nanoparticle inside the magnetic
167% under external magnetic field configuration. fluid.
Li et al. [113] shows the enhancement in relative viscosity of aqu- Furthermore, the nanoparticle size, magnetic field as well as chain-
eous Fe and Fe3O4 nanofluids by applying the magnetic field, as shown like structure of magnetic nanoparticle are the important parameters to
in Fig. 5. The higher viscosity of the magnetite nanofluid under an change the thermal properties of magnetic nanofluids in particle ap-
externally applied uniform magnetic field caused a reduction in fluid plication. It is very important to consider the viscosity and thermal
velocity. conductivity of magnetic nanofluids at the same time. Because, Prasher
Shima and Philip [114] examined the effect of magnetic field on the et al. [120] have shown that if the increase in viscosity becomes more
thermal properties of magnetic nanofluids and showed that the thermal than four times that of a comparable increase in thermal conductivity of
properties can be easily adjusted from very low to high values by nanofluid, then the use of nanofluid is not economically viable. Venerus
changing the strength and orientation of the magnetic field. Baby and et al. [121] also ran a benchmark study on the viscosity behaviour of
Ramaprabhu [115] investigated the effect of the application of magnets ten different nanofluids. They concluded that all of the nanofluids used
on the thermal conductivity of the magnetic nanofluid. It was con- in their study clearly failed the qualifications of practical nanofluids as
firmed that a small augmentation in thermal conductivity in the ab- proposed by Prasher et al. [120].
sence of magnetic field, whereas a significant rise in the thermal con-
ductivity was noticed in the presence of magnetic field. They specified 6. Convective heat transfer and pressure drop characteristics of
that the aligned nature of Fe2O3/MWNT toward the direction of the magnetic nanofluids
magnetic field may one of the possible reasons for the improvement of
thermal conductivity under the application of magnetic field. Parekh Nanofluids are dilute suspensions of functionalised nanoparticles
and Lee [116] concluded that the enhancement in thermal conductivity and their objective is to enhance the heat transfer performance of
of Mn-Zn and magnetic nanofluids exhibit 45% and 17%, respectively coolants/fluids, and nowadays has evolved into a promising nano-
without the existence of the magnetic field. However, the magnetite technological area [122,123]. Although thermal conductivity of mag-
nanofluid demonstrated a further development in thermal conductivity netic nanofluids in the absence and presence of magnetic field has been
in the presence of a transverse magnetic field, although no variation the subject of many past studies, relatively little effort has been focused
was detected for Mn-Zn ferrite. Gavili et al. [8] showed that the en- on the convective heat transfer of magnetic nanofluids. It has been
hancement of thermal conductivity was not significant in the absence of proven that the thermal conductivity of magnetic nanofluids increases
magnetic field. However, a maximum 200% improvement in thermal under applied magnetic field parallel to the temperature gradient. If
conductivity of ferrofluid was observed under the effect of the magnetic this is the case, enhancement in heat transfer coefficient is expected. A
field. Mehrali et al. [117] investigated the thermophysical and mag- review of studies related to the convective heat transfer of magnetic
netic properties of hybrid graphene/Fe3O4 ferro-nanofluid in the pre- nanofluid in the absence and presence of an external magnetic field is
sence of a magnetic field, and thermal conductivity has shown an de- provided in this section.
velopment of 11%. Chiu et al. [118] analysed the influence of magnetic One of the main attractions of magnetic nanofluids application is
field and temperature on the specific heat of a water based magnetic the induction of a body force due to the application of magnetic fields
nanofluid. The specific heat did not depend on the temperature under which allows the magnetic nanofluids to control the fluid motion. The
the lower magnetic field strength, but it decreased noticeably with the thermogravitation phenomenon may define as the natural movement of
rise of temperature in the presence of higher magnetic field strength. the fluid because of the buoyancy force, where the reasons for the fluid
This may be due to the reduction in the degrees of freedom of the ferrite motion are the presence of temperature gradient and density difference.
nanoparticle in the magnetic nanofluid under the higher magnetic field Moreover, these types of magnetic nanofluids are temperature sensitive,
strength. and its magnetisation reduces with the rise of temperature. The tem-
perature gradient involves a non-equilibrium state in the magnetisation
5.3. Effects of chain-like structures of the magnetic nanoparticle of the fluid under the magnetic field. As a result, a net magnetic driving
force induces, which may initiate a flow in the fluid. Such phenomenon
Studies have been carried out on the thermophysical properties of is called thermomagnetic convection.
magnetic nanofluids, and their structural changes with the application When the approximation of equilibrium magnetisation with the
of the external magnets, such as chain-like structures formation, have magnetic susceptibility is only depend on the local external magnetic
been suggested as one of the efficient factors to enhance the heat field as well as the density then the gravitational and magnetic forces
transfer. Nkurikiyimfura et al. [93] studied the influence of chain-like are potential. The Kelvin body force produces a static pressure field in
aggregation of magnetic nanoparticle on the thermal conductivity of the fluid flow, which is symmetric about the external magnetic field
magnetic nanosuspensions in the presence of the magnetic fields. They creating an irrotational force field. This type of symmetric field does not

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change the velocity profile and the convection inside the fluid could not the application of magnetic field, known as the magneto-viscous effect.
rise [124]. The non-potentiality in the bulk forces is coming into the Recently, some of the studies considered the effects of thermal prop-
scenario if a fluid experiences a non-uniformity of the magnetisation erties on the enhancement of heat transfer as well as pressure drop of
and density because of their dependence on the volume fraction of magnetic nanofluids in the absence and presence of the external mag-
nanoparticle or temperature [125]. Afterwards, the condition is fulfilled netic field, shown in Table 3.
for non-isothermal systems with an asymmetric temperature distribu- Goharkhah et al. [128] studied the convective heat transfer and
tion about the applied magnetic field. The resultant Kelvin body force hydrodynamic features of magnetite nanofluid under the influence of
induces a field force, which contributes to the self-organised adjective magnetic fields. The improvement of heat transfer in the presence of the
motion of the magnetic nanofluid across isotherms. The condition for alternating magnetic field was compared with the heat transfer at the
the free convection to develop is estimated by the following Eq. (2) constant magnetic field as well as in the absence of magnetic field
[125]: configurations in Fig. 6a. In the absence of magnetic field, a maximum
16.4% enhancement was found in magnetite nanofluid compare to DI-
→ → → χ (H )
∇ × f = ∇ T × ⎡βT ρo→g ± μo βm o ∇H 2⎤ ≠ 0 water, although, it was enhanced up to 37.3% and 24.8% in the pre-
⎢ 2 ⎥ (2)
⎣ ⎦ sence of alternating and constant magnetic fields, correspondingly.
→ Fig. 6b shows the influence of magnetic field on the pressure drop of the
where f represents the vector sum of the gravitational body force and
magnetic nanofluid at different nanoparticle volume concentrations.
Kelvin body force; βT denotes the relative volumetric expansion coef-
The pressure drop rises with the increase of ferrofluid volume con-
ficient, the ± sign means the anti-parallel and parallel alignment of the
centration and magnetic field because of the increase in fluid viscosity
magnetic field gradient with respect to the gravitational force and βm is
as well as the tendency of chain alignment of magnetic particle in base
the relative pyromagnetic coefficient of the fluid.
fluid [128].
The intensity of the heat transfer is calculated by the Rayleigh
Mehrali et al. [117] analysed the hybrid graphene/Fe3O4 ferro-na-
number Ra, which is the combination of thermogravitational and
nofluid flow under the influence of a magnetic field. Fig. 7a shows that
thermomagnetic parts as follows;
the total entropy generation reduces as the rise of fluid velocities and
ρcp l 4 dT the effect of frictional entropy generations were low (the maximum
Ra = RaT + Ram = ⎛β ρg + μo βm M dH ⎞
ηK dz ⎝ T dz ⎠ (3) value of frictional entropy generations were < 1 in all cases). The effect
of magnetic field arrangements on the thermal efficiency index is also
where RaT and Ram represent the thermal and magnetic Rayleigh shown in Fig. 7b. The maximum thermal efficiency index was only 1 in
number, accordingly. The efficiency of a device by applying the mag- the absence of the magnetic field. After applying a magnetic field, this
netic nanofluid gives the magnetic field, the temperature distributions amount increased between 1.1 and 1.7. Therefore, use of hybrid mag-
fields as well as the tunable thermal properties of ferrofluid and the netic nanofluids could be beneficial for improvement of heat transfer
pyromagnetic coefficient. The pyromagnetic coefficient of a magnetic coefficient under the effect of magnetic fields.
nanofluid is expressed as follows [17]; Li and Xuan [135] studied the heat transfer coefficient of magnetic
∂M ⎞ dm ∂M nanofluids under the application of the external magnetic field in the
β = βT M + ⎛ + laminar flow regime. The maximum Nusselt number enhancement of
⎝ ∂m ⎠ dT ∂T (4)
40.6% under the application of an external magnetic field compared to
1 dρ
where, thermal expansion coefficient, βT = − ρ dT ,
and ρ denote the the result in the absence of an external magnetic field.
density of magnetic nanofluid. Almost all of the studies have been From the above literature review, it is expected that the thermo-
discussed the influence of temperature on the magnetic nanofluids, magnetic convection and magneto-viscous effect should be assessed
which involves the dependency of magnetisation on the fluid tem- together and not separately in the future studies. Therefore, this may
perature, thus the existence of thermomagnetic phenomenon allow to understand the mechanism of the occurrence of thermo-
[18,126,127]. It is considered that the thermal properties of ferrofluids magnetic convection. Moreover, the uniform magnetic fields have been
are independent as well as constant under the external magnetic field studied in the most of the current literature. Although the application of
[18]. Though some of the inquiries have been concluded that the ex- uniform magnetic field is essential and beneficial to get a clear idea
ternal magnetic fields can influence the thermal properties of ferro- about the mechanism of thermomagnetic convection, using electro-
fluids, thus has a significant impact on heat transfer performance of magnets and Helmholtz coils to provide uniform magnetic fields looks
nanofluids. The magnetic nanofluid varies the value of viscosity under uneconomical because of the consumption of the electrical energy.

Table 3
Recent development on magnetic nanofluid as a heat transfer fluid.

Author Types of magnetic field Result

Mehrali et al. [117] Constant magnetic field The local convective heat transfer coefficient increased up to 4% The thermal efficiency index was
only 1 in the absence of the magnetic field and it was increased between 1.1 and 1.7 after applying a
magnetic field. In addition, the total entropy generation rate was reduced up to 41% compared to
distilled water
Goharkhah et al. [128] Oscillating and constant magnetic field The heat transfer coefficients were enhanced by 37.3% and 24.9% under alternating and constant
magnetic field, accordingly. The rise in pressure drop under the alternating magnetic field was not
significant compared to heat transfer enhancement
Goharkhah et al. [129] Constant and alternating magnetic field The average convective heat transfer grows up to 18.9% and 31.4% by application of constant and
alternating magnetic field
Yarahmadi et al. [130] Constant magnetic field with different The local convective heat transfer coefficient enhanced by 19.8% with Re = 465 and the
magnetic field arrangements concentration of 5%
Azizian et al. [131] Constant magnetic field with different The local heat transfer coefficient of magnetite nanofluids enhanced noticeably up to 300%, whereas,
arrangements of magnets only 7.5% increment in the pressure drop under applied magnetic field
Ghofrani et al. [132] Alternating magnetic field A maximum of 27.6% enhancement in the convection heat transfer was observed
Sundar et al. [133] Without magnetic field The heat transfer coefficient and friction factor were improved 30.96% and 10.01%, respectively
Lajvardi et al. [134] Constant magnetic field The use of magnetic particles dispersed in distilled water cannot improve the convective heat transfer
in the absence of magnetic field

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Fig. 6. The Fe3O4/water nanofluid under different magnetic field (a) heat transfer enhancement versus Reynolds number, (b) pressure drop versus Reynolds number [128].

Fig. 7. The rGO-Fe3O4 nanofluid for different arrangements of magnetic bars (a) total entropy generation rate versus velocity, (b) thermal efficiency versus velocity [117].

Thus, the phenomenon of thermomagnetic convection can utilise in a lifetime, large size, difficulty in replacement and containing dangerous
more economical and practical way by applying the magnetic fields and hazardous substances. An energy harvester can be utilised to im-
using the permanent magnets. Such magnetic fields are more accessible prove the capability and lifespan of these devices by swapping the
as well as energy efficient. In addition, the pressure drop induced usage of the batteries [139,140]. Energy harvesting devices can use to
during coolant flow is a significant parameter to determine the effi- get vital information on structural and operational conditions by en-
ciency of nanosuspensions. Different researchers have concluded that gaging them in remote areas [141]. Hence, an energy harvester can be
nanofluids have a higher pressure drop than base fluids and as a result a used to charge or replace the existing battery and enhance the lifetime
notable rise in pumping power [136]. Hence, the heat transfer en- of the system [137,142,143].
hancement and pressure drop should be evaluated at the same cir- Energy harvesting devices are classified based on the form of energy
cumstances to characterise the magnetic nanofluids. they use to scavenge power. For example, piezoelectric harvesters
scavenge mechanical energy and change it into electrical energy.
7. Ferrofluid based energy harvester Primarily there are four types of ambient energy available in the en-
vironment: thermal energy (temperature variations), radiant energy
Over the current years, interest has raised in the development of (RF, sun, infrared), mechanical energy (deformations, vibrations) and
micro-electromechanical systems (MEMS) and miniaturised system. In chemical energy (biochemistry, chemistry). These sources are char-
this approach, there has been great effort to decrease the energy con- acterised by different power densities as shown in Fig. 8 [144]. Fig. 8
sumption of these devices from being in the order mW to the order μW demonstrates that the accessible power ranges from 10 μW to 100 μW
[137]. Applications such as sensors in buildings, medical implants, and is a good order of magnitude for a 1 cm2 or a 1 cm3 energy har-
wireless sensors networks, sensors used in military applications, mon- vester. Apparently, 10–100 μW is a small amount of power; yet it can be
itoring of the environment, sensors that may provide different in- adequate for various types of applications, in particular, wireless sensor
formation about the maintenance requirements of various industrial networks.
equipment and sensors for structural monitoring are just a few of the The mechanical vibrations are the most dominant and attractive
many examples [137,138]. Currently, batteries are used to supply the ambient energy source because of its abundance [145,146]. Unlike
power to such systems, but some limitations of this are their: limited solar cells, a vibration energy harvester can supply the power during

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the advantage of being protected from the outside environment. In

addition, it is reliable and may reduce the mechanical damping because
there is no mechanical contact between the parts as well as the separate
voltage source does not necessary [162].
Sazonov et al. [163] have described a self-powered wireless system
that utilises the vibrations of a bridge induced by passing traffic. The
vibrations are converted into the electrical energy by using the elec-
tromagnetic generator, which can produce power up to 12.5 mW with a
vibrational frequency of 3.1 Hz. Sterken et al. [164] studied on the
electrostatic micro generator and concluded that this system was
compatible up to the power of 50 µW for 0.1 cm2 surface area. Dallago
et al. [165] have introduced an active electronic interface for an energy
Fig. 8. Power densities of ambient sources before conversion [144]. harvester containing an electromagnetic transducer. This transducer
can deliver a voltage of 3.25 V at resonance frequency of 10.4 Hz.
Furthermore, electromagnetic materials are large in size and are com-
the day as well as the night, whether light is present or not. Here, the
plex to integrate with MEMs [166]. Bayrashev et al. [167] and
focus is on the vibration which can convert into electric power. Some
Staley & Flatau [168] focused on energy harvesting from magnetos-
vibration sources are available in the environment including electric
trictive materials because these types of materials were employed to
motor rotation, wind energy, vehicle motion, wave energy, seismic vi-
build sensors and actuators as they were capable of altering magnetic
brations and human movement all of which vary widely in both am-
energy into kinetic energy. The magnetostrictive materials are flexible,
plitude and frequency. Vibrational energy is found in most of the built
suitable for the high frequency of vibration as well as overcome the
environment [147]. The sources of vibration vary with the variation of
drawbacks of the other vibration sources. Wang & Yuan [166] described
dominant frequency and amplitude [148]. Roundy et al. [149] com-
magnetostrictive materials which were used to harvest energy and
pleted a number of measures of vibration sources [149] and concluded
supply power to the wireless sensors in a health monitoring application.
that the amplitude and frequency of the vibration sources differs from
El-Hami et al. [143] described a vibration based electromechanical
12 ms−2 to 0.2 ms−2 at 200 Hz and 100 Hz, respectively. Most of the
energy harvester that contained a pair of magnets and cantilever beam.
sources were measured to a fundamental frequency in the range of
Nowadays, electromagnetic generator utilises the sloshing of ferro-
50–200 Hz. Table 4 shows a market survey of mechanical vibration
fluid columns to harvest vibration energy. A ferrofluid is strongly
based energy harvesting products [150].
magnetised under the influence of a magnetic field. Ferrofluid is a
An inertial mass is responsible for initiating the movement due to
stable suspension with a mixture of magnetic nanoparticles in a con-
the vibration of a device. This movement can be transformed into the
ventional carrier fluid such as water or oil. Generally, the nanoparticles
electrical energy by applying three types of mechanisms: electro-
are ferri- or ferromagnetic particles with the diameter of < 10 nm and
magnetic (inductive), piezoelectric and electrostatic (capacitive) [151].
coated with a surfactant layer of 1–2 nm. The difference between
Electrostatic and piezoelectric energy harvesters have several short-
magnetorheological fluid and ferrofluid is the size of the particles. Bibo
comings. Electrostatic generators require an additional power source to
et al. [21] studied the use of ferrofluids in an electromagnetic micro-
charge the capacitor with an initial voltage in order to initiate the al-
power generator and determined the influence of the ferrofluid quan-
teration process. Another disadvantage is the difficulty of manu-
tity, magnetic field strength and acceleration level on the output vol-
facturing these converters because capacitor electrodes must not come
tage of the harvester. They concluded that the harvester output voltage
into contact with each other. Piezoelectric energy harvesters are not
increased with an increase in acceleration level, but the height of the
efficient with microelectronics and are more compatible for moderately
fluid column had a small effect on the output voltage. Oh et al. [146]
high vibrational frequencies. Among the other vibrational energy har-
analysed the effect of ferrofluid sloshing movement on energy harvester
vesting techniques, an electromagnetic energy harvester is able to drive
to generate an electromotive force (EMF), and the experimental set-up
low impedance load with a high current level [139]. Moreover, vibra-
used in this study is shown in Fig. 9. The influence of ferrofluid volume
tion energy harvesters are usually assembled on resonant structures
and different strength of permanent magnets on the characteristics of
with rigid suspensions such as membranes [152], cantilevers [153] or
induced electromagnetic force were also examined. Results showed
springs [154]. Their construction processes are complicated because of
that, output power (μW) increased with an increase in vibration speed
the low resonant frequency and it is difficult to fabricate with micro-
(RPM) and a decrease in magnetic flux densities (mT). Alazmi et al.
fabrication process. Furthermore, a rigid suspension may tend to failure
[169] investigated the sloshing motion of ferrofluids, and concluded
or breakage in the presence of strong vibration and in the long-run.
that the sloshing motion of the magnetised nanofluids creates a time
Hence, an energy harvester with sturdy suspension structure and low
dependent magnetic flux, which can generate an EMF in a coil. Seol
resonant frequency is highly desired.
et al. [170] studied the ferrofluid based triboelectric-electromagnetic
The basic principles of electromagnetic energy harvesters were in-
hybrid generator for vibration energy harvesting. The result showed
troduced almost two hundred years ago. The Lorentz force caused by a
magnetic field on moving charged carriers is initially introduced by
Hans Christian Ørsted in 1920 [155,156]. Michael Faraday [155,157] Table 4
invented the electric motor, and describe the transformation of me- Market survey of vibration based energy harvesting products [150].
chanical to electrical energy and vice versa [158]. The mechanism of
Product Max. harvested power Frequency (Hz)
the energy harvester depends on the variation of the flux linkage in the
(mW)
coil of the energy harvester because of the natural vibration. According
to the Faraday’s law of induction, a voltage is found and partly energy is Volture Piezo Energy Harvester- 20 50–150
supplied to the connected load and some loss of energy due to the re- PEH20W (Mide)
PMG27 Microgenerator 4 17.2
sistance of the coil. Using a permanent magnet is one efficient way to
(Perpetuum)
induce an electromagnetic induction for energy harvesting [139]. Since VEH-APA400M-MD (Cedrat) 95 110
the late 1990s, many researchers [159–161] have acknowledged the VEH360 (Ferro Solutions) 10.8 60
different approaches used to generate power from electromagnetic re- Energy Harvesting Shoe (Scientific 800 mW of power/shoe
sources. The electromagnetic generator is an enclosed system and has Research Institute) at a place of 2 steps/s

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M.A. Khairul et al. Energy Conversion and Management 149 (2017) 660–674

fluids (e.g. ethylene glycol, water, and mineral oils, etc.), although
some doubts have been raised recently about these exaggerated claims.
The overall effectiveness of nanofluids in heat transfer applications can
be best evaluated if the enhancements in both thermal conductivity and
viscosity are considered at the same time. In comparison, relatively
little work has been done on the factors affecting nanofluid stability,
specifically the effect of zeta potential and particle size distribution on
nanofluid viscosity and thermal conductivity. Stability is an important
concern in nanofluids research as attaining the desired stability of nano-
suspensions remains a big challenge.
The development of magnetic nanofluids as a heat transfer fluid is
still challengeable by many aspects ranging from the production and
characterisation of nanofluids to the practical applications through the
understanding of the mechanisms responsible for the observed heat
transfer improvement. Future investigations should emphasise the
preparation of ferrofluids and influence of different parameters on the
size of the final synthesised nanoparticles. Another important aspect is
Fig. 9. Experimental setup for measuring induced electromotive force of electromagnetic the control of particle morphology during the preparation of the na-
energy harvester [146].
noparticles, because the cluster formation may be affected by the par-
ticle morphology. The control of the particle morphology and particle
that the hybrid energy harvester was advantageous for harvesting size may enable to prepare a unique ferrofluid with desired thermal
subtle and irregular vibrations in the extremely low threshold ampli- properties as well as long-term stability, and further on the develop-
tude and a wide operating frequency range. Wang et al. [171,172] ment of devices using magnetic nanofluid for practical applications.
examined ferrofluid liquid springs for vibration energy harvesting. The The ferrofluid based electromagnetic energy harvesters implement
results showed that the ferrofluid liquid spring can attain relatively low ferrofluids, which can easily change shape and respond to very small
resonant frequency when engaging small volume of liquid. It is also vibrations. The further research will be needed to examine the science
compatible in the presence of high input acceleration. The modal fre- underpinning the behaviour of tunable magnetic nanofluids with a view
quencies of the fluid column are determined by the following equation towards creating a step-change improvement in the performance of
[21,173]: electromagnetic energy harvesters. Most of the researches have been
done on conventional electromagnetic energy harvester using solid
2
1 g ∗kmn ⎛ σkmn ⎞ kmn h ⎞ magnets, and very few studies have been performed on ferrofluid based
fmn = ⎜1 + ⎟ tanh ⎛
2π R ⎝ ∗
ρg R2 ⎠ ⎝ R ⎠ (5) energy harvesters. The effect of fluid quantity, the shape, and size of the
fluid container, magnetic field induction, vibration speed on EMF and
χ dB (z ) power output of ferrofluid based energy harvester have been studied.
where, the effective acceleration, g ∗ = g− ρμ B (z ) , and g re-
o dz z=h But, the consequence of changing the physicochemical properties (such
presents the gravitational acceleration, m & n are the different oscilla-
as pH, the weight concentration of nanoparticles, and the size of na-
tion nodes, h is the liquid height, R is the radius of the container, μo is
noparticles) of ferrofluids on the output power of the energy harvester
the magnetic permeability of vacuum. From Eq. (5), it is clear that the
will need to be examined and analysed. Then, the feasibility of the
ferrofluids properties (like density and surface tension) have an im-
proposed method will need to be determined and the characteristics of
portant impact on the modal frequencies as well as the electromagnetic
electromagnetic force should be explained through the use of the ex-
force, accordingly.
perimental result.
A large number of studies have been done on conventional elec-
tromagnetic energy harvester, where solid magnets were used to gen-
Acknowledgments
erate electromotive force. Dissimilar to the solid magnets, ferrofluids
can easily change its shape and injected into hard to access locations
The authors gratefully acknowledge the financial support provided
due to its fluidity. Moreover, ferrofluid based energy harvester can also
by the University of Newcastle (Australia), Granite Power Pty Ltd and
respond at infinite closely spaced modal frequencies which may en-
the Australian Research Council through the ARC-Linkage grant
hance the performance of energy harvester under the influence of non-
LP100200871, for the present study.
stationary and random excitations. Therefore, a broad study is needed
on ferrofluid based electromagnetic energy harvester in the future to
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