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Wetting of ferrofluids: Phenomena

and control
Mika Latikka a,⁎, Matilda Backholm a , Jaakko V.I. Timonen a , Robin H.A. Ras a,b

Ferrofluids are liquids exhibiting remarkably strong response to

magnetic fields, which leads to fascinating properties useful in https://doi.org/10.1016/j.cocis.2018.04.003
various applications. Understanding the wetting properties and 1359-0294/© 2018 Elsevier Ltd. All rights reserved.
spreading of ferrofluids is important for their use in microfluidics
and magnetic actuation. However, this is challenging as magneti-
cally induced deformation of the ferrofluid surface can affect 1. Introduction
contact angles, which are commonly used to characterize wetting
properties in other systems. In addition, interaction of the magnetic Ferrofluids are colloidal suspensions of small (~3–15 nm
nanoparticles and solid surface at nanoscale can have surprising diameter [1]) superparamagnetic nanoparticles in a liquid
effects on ferrofluid spreading. In this review we discuss these issues carrier medium. They combine liquid properties with a strong
with focus on interpretation of ferrofluid contact angles. We review magnetic response, and thus exhibit fascinating phenomena,
recent literature examining ferrofluid wetting phenomena and
such as field-induced pattern formation on the ferrofluid
outline novel wetting related ferrofluid applications. To better
surface [2], self-assembly of droplets [3] and magnetic
understand wetting of ferrofluids, more careful experimental work
micro-convection [4] (Fig 1). The superparamagnetic nanopar-
is needed.
ticles used in ferrofluids usually consist of ferri- or ferromag-
netic metals or metal oxides [5•]. Due to the small nanoparticle
Address size, Brownian motion is enough to keep the particles from
a
Department of Applied Physics, Aalto University School of Science, settling in gravitational or magnetic fields [2]. The small size
P.O. Box 15100, FI-00076 Aalto, Espoo, Finland also renders the nanoparticles superparamagnetic: each parti-
b
Department of Bioproducts and Biosystems, Aalto University cle acts as a single magnetic domain (although surface effects
School of Chemical Engineering, P.O. Box 16100, FI-00076 Aalto, and structural defects can influence this [6]), which can flip its
Espoo, Finland magnetization direction due to thermal agitation. Due to this
Neél relaxation together with rotational Brownian relaxation,
ferrofluids show no remanent magnetization in room temper-
⁎
Corresponding author. ature once the external magnetic field is removed [5•]. This lack
(mika.latikka@aalto.fi) of magnetic hysteresis is also exhibited by paramagnetic
materials, but otherwise superparamagnetic particles resemble
Keywords: ferri- or ferromagnetic materials with high susceptibility and
Ferrofluid nonlinear response to magnetic fields [1].
Wetting
Superparamagnetic nanoparticles can be fabricated using
Magnetic field
Contact angle
either a top-down approach, such as grinding of larger
Magnetic nanoparticles particles [7], or a bottom-up approach, i.e. chemical
synthesis. The latter is nowadays more widely used,
especially chemical co-precipitation involving Fe3+ and Fe2+
Current Opinion in Colloid & Interface Science (2018) 36, 118–129
salts in water [5•], [8]. To prevent aggregation caused by
For a complete overview see the issue and the Editorial
attractive forces between the particles, such as van der
Article History: Waals and magnetic dipole interactions [9], the nanoparti-
Received 16 January 2018 cles need to be stabilized. This is done by introducing
Received in revised form 22 April 2018 interparticle repulsion by charging the particles (electro-
Accepted 25 April 2018 static stabilization) or by coating them with capping agents
Available online 01 May 2018 (steric stabilization) [5•]. These capping agents are usually

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
119 Wetting of ferrofluids: Phenomena and control

Fig. 1 Phenomena exhibited by ferrofluids under magnetic field. a) Field-induced surface pattern formation. b) Self-assembly of
ferrofluid droplets on a superhydrophobic surface. c) Snapshots of micro-convective mixing of ferrofluid and water in a microfluidics
channel in different magnetic flux densities B. c) adapted from [4] with permission from Elsevier.

surfactants, especially in case of ferrofluids with organic and liquid-air interfaces [15•]. The smaller the angle, the
carrier liquid. For a more thorough introduction to the better the liquid wets the surface. The contact angle value
properties of ferrofluids we direct the reader to an excellent typically depends on the length scale it is measured on (Fig
recent review by Torres-Díaz and Rinaldi [10••]. 2a-c) [16–19]. The microscopic, actual contact angle
Stable, magnetically controllable liquids are useful in a existing locally at each point on the contact line would be
range of mechanical to biomedical applications. Ferrofluids the ideal quantity for measuring wetting properties, but it is
are widely used as liquid seals and lubricants held in place by unfortunately very difficult to probe experimentally (Fig
magnetic fields, while actuation with dynamic magnetic 2b). Usually a macroscopic apparent contact angle (mea-
fields allow building of ferrofluid based pumps, valves and sured at length scales over tens of micrometres) is used
tunable optical systems [5•,10••]. Their anisotropic heat instead, which represents an average value of the micro-
transfer capabilities find use as heat transfer fluids and scopic local contact angles (Fig 2a). Care must be taken
magnetic buoyancy can be exploited in separation processes when measuring the apparent contact angle, because body
[5•,10••]. Recently ferrofluids have been increasingly inves- forces (for example gravity) not related to wetting proper-
tigated for microfluidic [11•–13•] and biomedical [14•] ties can deform the droplet profile. Because of this, the
applications. In many of these cases, the ferrofluid wetting apparent contact angle must be determined using appropri-
properties need to be carefully understood and tuned in ate magnification. Finally, all three phases (solid, liquid and
order to ensure reliable function of these systems. In this gas) are very close to one another near the contact line at
review we discuss the wetting properties of ferrofluids, how the nanoscale. This gives rise to a disjoining pressure, which
they can be controlled with external magnetic fields and determines the curvature of the liquid surface near
some interesting applications from the past few years. (b100 nm) the solid surface (Fig 2c) [16–18]. The exact
surface geometry is again difficult to probe experimentally
due to the small scale.
2. Wetting of ferrofluids The apparent contact angle can be interpreted to reflect
the average molecular wetting properties, that is the
2.1. Contact angles interfacial tensions between solid, liquid and surrounding
fluid (immiscible liquid or gas), which is why it is used for
Wetting characterization relies heavily on the concept of wetting characterization. This angle is rarely unique, but
contact angle, which is the angle between the solid-liquid can instead have a range of metastable values, for example

Fig. 2 Contact angles in wetting characterization. a) Apparent contact angle at the macroscale. b) Actual contact angle at the microscale.
c) Transition to a thin film at the nanoscale due to disjoining pressure. d) Advancing contact angle. e) Receding contact angle.

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
 M Latikka et al. 120

due to surface inhomogeneities. The highest metastable material toward higher field strength. For example, a sessile
value is called advancing contact angle, which appears when ferrofluid droplet flattens when a permanent magnet with a
the wetting front is slowly advancing on previously dry strong field gradient is placed underneath the surface
surface (Fig 2d). Similarly, the lowest metastable value, supporting the droplet. This force can also be used, for
receding contact angle, is seen when the wetting front is example, to move a ferrofluid droplet or to manipulate
slowly receding from previously wetted surface (Fig 2e). This non-magnetic objects immersed in ferrofluid [21].
range of metastable states, called contact angle hysteresis, The steady-state flow of inviscid, incompressible
is related to the mobility of the contact line. If the contact ferrofluid can be described by the ferrohydrodynamic
line is strongly pinned, the droplet can be deformed with Bernoulli equation [2]:
large forces before it starts to move. Droplet mobility can
1
also be assessed by measuring the roll-off angle (the angle of p þ ρv 2 þ ρgh−pm ¼ const ð3Þ
inclination needed to unpin the droplet from the surface). It 2
is determined by depositing a droplet on a surface and tilting Z  
H
∂ðMυÞ
it until the droplet starts to move. p ¼ p þ μ0 dH ð4Þ
Because of the contact angle hysteresis, a difference in 0 ∂υ H;T
contact angles does not automatically imply a difference in
Z H
wetting properties. For the same reason, reporting a single
pm ¼ μ0 MdH ð5Þ
value of contact angle (often called a static contact angle) 0
gives only limited information about wetting properties of
the system. Furthermore, the macroscopic droplet profile where p⁎ is the composite pressure in the magnetic fluid, p is
depends also on body forces acting on the liquid, such as the thermodynamic pressure, pm is the fluid-magnetic
gravity, which can affect the measurement of apparent pressure, v is velocity, g is the acceleration of gravity and
contact angles. h is the elevation from a chosen reference level. In addition,
the following boundary condition must be met [2]:
2.2. Ferrofluid shape in magnetic field p þ pn ¼ p0 þ pc ð6Þ

Arguably the most interesting quality of ferrofluids is the μ0 2

pn ¼ M ð7Þ
possibility to control the liquid flow and shape of the liquid 2 n
surface with a magnetic field. This arises from the
interactions between magnetic field and the dipole mo- pc ¼ Kγ ð8Þ
ments of each nanoparticle. The related forces can be where pn is the magnetic normal traction due to field
described with a magnetic stress tensor T [2]: continuity requirements at the interface, p0 is the pressure
(Z ) outside the magnetic fluid, pc is the capillary pressure, Mn is
 
H
∂ðMυÞ 1 2 the fluid magnetization component normal to the fluid
T ij ¼ − μ0 dH þ μ0 H δij þ Bi H j ð1Þ surface and K is the sum of principal surface curvatures.
0 ∂υ H;T 2
As a simple example, let us consider a deep pool of
where H is the magnetic field intensity, μ0 is the vacuum ferrofluid that is partly exposed to a local vertical uniform
permeability, M is the fluid magnetization, υ is the specific magnetic field created by an electromagnet. Far away from
volume (υ = ρ−1, ρ is the density), T is the temperature, δij is the magnet the field is zero and the Bernoulli equation on
the Kronecker delta function and B is the magnetic the fluid surface simplifies to p∗1 + ρgh1 = const. On the fluid
induction. Here H and B refer to local total field quantities, surface at the axis of the magnet, the equation is p∗2 + ρgh2-
not to applied external fields. The subject is complicated − μ0∫H0 MdH = const. The surface is assumed flat. Boundary
and only the most important results can be shown here. For a conditions from Eq. (6) give p∗1 = p0 and p2 ¼ p0 − μ20 M2 (M =
more complete description and derivations see Mn in a field normal to the ferrofluid surface). The ferrofluid
Ferrohydrodynamics by Rosensweig [2] or a more recent in magnetic field rises compared to the ferrofluid in zero
treatment by Stierstadt and Liu [20]. field [2]:
Magnetic body-force density can be calculated from Eq.  Z H 
(1) as: 1 μ0 2
Δh ¼ h2 −h1 ¼ μ MdH þ M ð9Þ
ρg 0 0 2
( )
Z H 
∂ðMυÞ Similarly a ferrofluid droplet in uniform magnetic field
f m ¼ ∇∙T ¼ −∇ μ0 dH þ μ0 M∇H ð2Þ
0 ∂υ H;T elongates along the field direction. This deformation
increases the surface area, and thus also the energy due to
The integral term vanishes for dilute ferrofluids, but for interfacial tension Eγ = ∫AγdA, which was neglected from the
concentrated ferrofluids it cannot always be neglected, previous example. The equilibrium droplet shape is then
because particle density can affect dipolar interactions determined by balancing the gravitational, magnetic and
between the nanoparticles. The last term proportional to surface energies. In case of small droplets gravity can be
field gradient ∇H describes a force pulling the magnetic ignored and the relative strengths of uniform magnetic field

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
121 Wetting of ferrofluids: Phenomena and control

and capillarity can be conveniently described with a Experimental studies with ferrofluids are especially lacking
magnetic Bond number Nm = μ0H2R/γ, where R is the drop and usually the interfacial tensions are assumed to be
diameter and γ is the surface tension [22]. independent of magnetic fields. However, magnetic fields
Fig 3 a shows a silicone oil-based ferrofluid droplet can have a large effect on tension values, for example in
suspended in glycerol in uniform magnetic fields from 6.032 case of magnetically responsive surfactants [25].
to 162.33 kA/m [23]. Calculating the exact shape is not easy Afkhami et al. compared simulated ellipsoidal shape of a
even in case of a uniform magnetic field, as droplet ferrofluid droplet suspended in immiscible fluid to experimental
magnetization affects the total magnetic field H [24]. An and analytical results [23]. While the agreement was good in
example of a magnetized sphere in uniform magnetic field is weak uniform magnetic fields, there were apparent deviations
given in Fig. 3b. A change in droplet shape further alters the between numerical and experimental droplet shapes in fields
magnetic field geometry, making the calculation of the above 12 kA/m. Authors were able to solve this discrepancy by
magnetic energy Em = ∫V∫B0 H · dBdV and the equilibrium allowing the interfacial tension of the droplet to change with
droplet shape difficult [2]. the magnetic field in the simulations. This apparent change of
several mN/m in interfacial tension was speculated to arise
from rearrangement of nanoparticles within the fluid and at the
2.3. Effects of magnetic field on interfacial tension interface. However, Rowghanian et al. were able to reproduce
the experimental data in their theoretical description with a
Droplet shapes and wetting phenomena are governed by single interfacial tension value [24]. According to them the
minimization of surface energies within given boundary discrepancy between the theoretical prediction and experi-
conditions. Unfortunately, interfacial tensions under mag- mental results in [23] was due to the ellipsoidal shape
netic fields have not been thoroughly investigated. approximation of a ferrofluid droplet.

Fig. 3 Shape of a ferrofluid droplet in an external uniform magnetic field. a) Sideview photographs of a ferrofluid droplet
suspended in glycerol under vertical uniform fields with strengths from 6.032 to 162.33 kA/m (field values increase from left to right).
Reproduced with permission from [23]. b) A magnetized sphere in a uniform magnetic field showing the total magnetic field H. (c–f)
Schematic of a sessile ferrofluid droplet in an external uniform magnetic field Hext of increasing field strength: c) No field. d) Field
elongates the droplet while the contact line remains pinned. Ferrofluid flows away from the contact line due to mass conservation
and θ decreases. e) θ reaches receding value and the contact line starts to move. f) Almost vertical droplet edges pull θ close to 90°
angle.

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 M Latikka et al. 122

Kalikmanov proposed a statistical theory, where field, the contact angle was observed to decrease. However,
ferrofluid carrier liquid molecules and magnetic dipoles are as pointed out in the Section 2.1, a change in apparent
treated separately [26]. It was shown that direct magnetic contact angle does not necessarily mean that the wetting
contribution to the surface tension was negligible but that properties have changed. The contact angle could have
dipole interactions can cause a non-uniform nanoparticle remained within the contact angle hysteresis range,
distribution, and as a result surface tension increases weakly reflecting only a change in droplet shape. In more recent
with magnetic field strength. A more complex model using experiments, flattening of a ferrofluid droplet accompanied
surface tension tensor and surface magnetization was by a decrease in contact angle from approximately 70° to
recently developed by Zhukov [27]. The model predicts a 50° was observed in the field created by a permanent
relative change of 0.5% in the interfacial tension tangential magnet under the substrate [36]. Due to the field gradient,
component in 14 kA/m magnetic field parallel to the the droplet experiences a downwards force and spreads on
interface of water and a suspension with 15 vol-% of the surface, as described in Section 2.2. The magnet was then
magnetic nanoparticles in dodecane. According to the moved laterally and the ferrofluid droplet followed with
results, tangential magnetic field increases the interfacial advancing and receding contact angles of 64° and 46°,
tension, whereas normal field decreases it. Slight increase in respectively. Interestingly the reported contact angle in
ferrofluid surface tension in external magnetic field has also absence of a magnetic field was higher than the advancing
been experimentally observed [28]. Field-induced change in contact angle in a magnetic field, even though the advancing
interfacial tensions affects also the contact angles [29]. contact angle is supposed to be the highest metastable angle.
This could be due to insufficient magnification while measuring
the contact angles or indeed suggests an actual change in
2.4. Effects of magnetic field on contact angles intrinsic wetting properties due to the magnetic field.
Recently Rigoni et al. investigated ferrofluid droplet
Care must be taken when assessing wetting properties under shape and contact angle dependence on field parameters
external fields using contact angle goniometry. While and nanoparticle concentration [37•]. Depending on the
magnetic fields can affect the interfacial tensions, and magnetization and field gradient the droplets were observed
thus contact angles, this effect can be difficult to observe to either elongate or flatten in magnetic field created by a
because of the deformation of the whole ferrofluid surface. permanent magnet. The contact angle (approximately 110°)
As pointed out in Section 2.1, contact angles reflect the hardly varied while the droplets were flattened by increas-
forces acting on the liquid interface. These forces vary ing the magnetic field. For elongated droplets the contact
depending on the length scale under investigation, and not angle on the other hand decreased to 60° as the droplet
all of them are related to the wetting properties of the height increased in the field. However, during droplet
system. In other words, an external field can distort the deformation the contact line remained pinned, and thus
interface and the apparent contact angle without affecting the change in contact angle probably reflects the change in
any intrinsic wetting properties. droplet shape rather than wetting properties.
The change in apparent contact angle due to external field Droplet shape hysteresis was recently experimentally
has been studied extensively in the case of electric fields. This studied with ferrofluid (Fe3O4 nanoparticles with mean
phenomenon is called electrowetting and it was originally diameter of 10 nm in light hydrocarbon carrier liquid) and
interpreted to originate from a change in solid-liquid interfacial magnetic paint (Fe3O4-coated flake pigments with mean
tension due to the electric field [30]. However, the decrease in diameter of approximately 35 μm in water) [38]. In the
apparent contact angle and spreading of the liquid can be experiments the magnetic field was first increased and then
explained, arguably better, with electrostatic pressure that decreased by controlling the separation between the droplet
deforms the profile of the liquid surface [31,32]. As a and a permanent magnet underneath it. The ferrofluid
consequence, at the microscopic scale the contact angle equals droplet was observed to elongate when the field was
Young contact angle, which has since been shown also increased from 0 to 20 mT. On the other hand, the magnetic
experimentally [33]. Similarly, it has been long hypothesized paint droplet flattened when the field was changed from 0 to
that while the apparent contact angle changes with magnetic 330 mT. This conflicting behavior was explained to originate
field, the microscopic contact angle remains the same [34]. from different magnetic properties of the carrier fluids,
There are also some experimental results that support this view even though both liquids contained large amounts of ferro-
[22]. It is important to notice that the magnetic field induced or superparamagnetic particles, as evident from their strong
curvature of a ferrofluid surface can be much higher compared response to magnetic fields. A more likely explanation is
to gravity, since magnetic forces on ferrofluid can be several that the microparticles in the magnetic paint sedimented
orders of magnitude stronger than gravitational force. This because of the magnetic field. Curiously, the contact angle
means that higher magnification might be needed when of the ferrofluid droplet first decreased from 45° to 25° and
measuring ferrofluid contact angles in magnetic fields than in then increased to 38° as a function of the magnetic field.
mere gravitational field. The initial decrease can be easily understood due to mass
The apparent effect of magnetic field on ferrofluid conservation: fluid is flowing from the contact line to the
contact angles was reported already in 1983 [35]. When a elongating peak, resulting in a decrease in contact angle.
sessile ferrofluid droplet was elongated with a magnetic The following increase in angle is accompanied by decreasing

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
123 Wetting of ferrofluids: Phenomena and control

contact area, and arises when the droplet surface away from The nanoparticles themselves can also affect the spreading
the peak becomes more and more vertical, thus pulling the of ferrofluid in a complex way: the particles tend to adsorb on
contact angle closer to 90° angle. Again, it can be argued that the solid surface and near the contact line, typically enhancing
the change in contact angle due to magnetic field does not spreading, as discussed by Wasan et al. in their review [42]. This
reflect a change in liquid-solid wetting properties, but happens phenomenon can have counter-intuitive effects. For example,
within the range of normal contact angle hysteresis and is due to an increase in nanoparticle concentration increases the
changing droplet shape. Similar schematic example of a viscosity of the fluid, which should decrease the wetting
ferrofluid droplet in an increasing uniform field is shown in velocity. On the contrary, however, the velocity was observed
Fig. 3: first the liquid flows away from contact line to the axis of to increase due to ordering of nanoparticles near the wetting
the droplet, reducing contact angle (c and d). Contact line front. The arising microstructures created a disjoining pressure
starts to move when the angle is equal to receding contact angle gradient on the film near the contact line, which enhanced the
(e). Finally, the angle increases again as the droplet surface wetting process.
becomes more vertical due to the normal stresses induced by The adsorption of colloidal nanoparticles on surfaces can be
the magnetic field (f). caused by London-van der Waals and electrical double layer
Magnetically induced change in ferrofluid contact angle was forces, described by DLVO theory, or solvation, hydrophobic and
recently investigated also numerically using the Young-Laplace steric forces [43]. Ordering of magnetic nanoparticles on
equation and by taking into account the normal stresses created surfaces has been investigated both theoretically [44] and
by the magnetic field [39]. Downward magnetic force flattened experimentally. Magalhães et al. explored nematic assembly of
the droplet and decreased the contact angle, whereas upward ionic ferrofluids on untreated and PTFE-coated glass by
magnetic force elongated the droplet and increased the contact measuring birefringence of the fluid near the surface [45,46].
angle. It is important to notice that these numerical results On the other hand, smectic-like ordering was found in ferrofluid
correspond to approximate equilibrium contact angles, and near SiO2 surface using in-situ neutron reflectometry [47]. The
cannot be directly compared to experimental results, where nanoparticles formed a wetting double-layer on the hydrophilic
contact angle hysteresis plays a role. Complex dependency of surface, which could be extended to 15 colloidal layers with a
contact angles on magnetic field was found for a cylindrical moderate uniform magnetic field perpendicular to the surface.
sessile nanodroplet of Ising fluid using density functional theory A field parallel to the surface, however, lead to only
[40]. On one hand magnetization increased the attraction short-ranged and perturbed ordering. Similar results were
between magnetic fluid molecules, which lead to an increase in recently reported by Theis-Bröhl et al. using magnetite
contact angle. On the other hand, in a non-uniform field nanoparticles stabilized by carboxylic acid (Fig. 4) [48]. They
created by a permanent magnet under the droplet, the fluid found that carboxylic acid covers the SiO2 surface eliminating
was pulled towards the solid surface deforming the droplet and most of the water from the wetting layer created by the
decreasing the contact angle. Even though the model relied on nanoparticles. Under shear this layer stayed adsorbed on the
Ising interactions instead of dipole interactions, it agreed surface while a depletion layer was formed between the
qualitatively with experiments performed with ferrofluids. wetting layer and flowing bulk ferrofluid. With a magnetic
field parallel to surface, slow reorientation of the particles was
observed in the wetting layer along the field.
2.5. Nanoparticle adhesion As with other nanofluids, ferrofluids show complex,
time-dependent wetting properties even without the appli-
Ferrofluids are complex liquids consisting of carrier fluid and cation of an external field. When a magnetic field is applied,
dispersed colloidal nanoparticles. Both components play an a variety of additional phenomena emerges.
important role in the wetting properties of these fluids. The
carrier liquid typically contains capping agents used to
stabilize the nanoparticles, such as surfactants, which can 3. Field-induced control
dramatically alter the surface tension and wetting proper-
ties of the ferrofluid compared to a pure carrier liquid. 3.1. Spreading of ferrofluids
Surfactants not only change the surface tension of the
ferrofluid, but can affect the wetting properties also by While it can be argued that magnetic fields only slightly change
adsorbing to solid-liquid interface. This surfactant layer can intrinsic wetting properties between ferrofluid and solid, they
remain on the surface when the ferrofluid retracts from the can alter the force balance on the contact line, affecting
substrate, changing its surface free energy. As a result, the ferrofluid spreading. A classic example is a ferrofluid film rising
wetting properties can be different depending on whether against gravity on a vertical current carrying wire due to the
the surface has been previously wetted by the ferrofluid or magnetic field induced around the conductor [49]. When the
not, which needs to be taken into account when investigat- current is increased over a certain threshold value, the
ing ferrofluid wetting. It is also important to remember that ferrofluid film height is suddenly increased. This has been
surfactant diffusion to the interfaces is a dynamical process, treated as analogous to the wetting transition caused by van der
and changing the interfacial area can also change the Waals forces [50]. The same phenomenon with different
interfacial tension [41]. Because of this, dynamic measure- boundary conditions has since been studied extensively using
ments or long equilibration times are often needed. more sophisticated numerical models, also very recently

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 M Latikka et al. 124

Fig. 4 Models of wetting layer structures of aqueous ferrofluid on a silicon substrate deduced from neutron reflectometry
measurements. Ferrofluid consists of asymmetric magnetite nanoparticles (diameter 11 nm) stabilized with oleic acid double layer.
Layer #1 contains mostly oleic acid adsorbed on the surface, followed by densely packed layer (#2) of nanoparticles and a layer (#3)
containing ligands and some core material. Layer #4 is a less densely packed transition region between the wetting layer and bulk
ferrofluid. In the static case without magnetic field (left) the particles have no preferred orientation and they can be modelled as
spherical. However, in a magnetic field applied parallel to the surface the particle asymmetry needs to be taken into account in the
model, as the field orients the nanoparticles and decreases the thickness of layer #2 (middle). The wetting layer persists under shear
(right) but a depletion layer with low nanoparticle concentration forms between the wetting layer and the bulk ferrofluid.
Reproduced from [48] with permission of The Royal Society of Chemistry.

[51,52]. Similar magnetic field induced changes in spreading rotating ferrofluid droplets in external magnetic field
have also been investigated experimentally on vertical walls undergo different kinds of instabilities depending on their
[53], nanochannels [54], porous media [55], and in the cases of wetting state [58]. Magnetic capillary origami, that is an
thin films [56,57] and droplets [35–38]. elastic membrane wrapped around a ferrofluid droplet, can
Ferrofluid spreading depends strongly on the geometry of be controlled with a magnetic field and display an
the magnetic field. For example, Rosensweig et al. investigated overturning instability when a critical magnetic field value
a vertical wall partially immersed in a pool of ferrofluid in is reached [59]. Similar magnetic control schemes can be
uniform magnetic field [53]. When the field was horizontal and applied to wetting phenomena.
parallel to the wall, it did not affect ferrofluid spreading, Perhaps the most extreme example of wetting control is
whereas a horizontal field perpendicular to the wall reduced to prevent wetting altogether. With ferrofluid this can be
the meniscus height. On the other hand, the vertical field realized by suspending a droplet in air with a
enhanced spreading and increased meniscus height. A radial computer-controlled magnetic field [60]. Wetting states of
magnetic field on a horizontal substrate can be used to induce sessile droplets can be similarly manipulated with implica-
spreading of a ferrofluid film analogously to centrifugal force tions to droplet transport applications. Cheng et al.
used in spin coating [57]. Contrary to spin coating, the spreading controlled mobility of a ferrofluid droplet on a
pattern could be tuned by using a perpendicular field to alter superhydrophobic iron surface by magnetizing and
the film shape before turning on the radial field. Wetting demagnetizing the surface [61]. In a demagnetized state
dynamics of a ferrofluid thin film was also recently investigated the droplet moved easily with a roll-off angle of 7°, but
in more detail using image analysing interferometry [56]. When remained pinned on a magnetized surface. The authors
the film was exposed to a non-uniform magnetic field created speculated that this is due to a transition from a
by a cylindrical permanent magnet, an interesting transient Cassie-Baxter to a Wenzel state (the former is a wetting
increase in the ferrofluid surface curvature near the contact state where some air remains trapped between the liquid
line region was observed during the first 2–3 s, followed by a and parts of the solid, the latter describes a wetting state
small increase in the adsorbed film thickness and spreading of where the liquid has completely penetrated the microscopic
the entire film. The advancement of the film could be changed surface structure [15•]). They later demonstrated that a
significantly by varying the magnetic force. ferrofluid droplet can be reversibly switched between
Cassie-Baxter and Wenzel states and showed qualitatively
that a larger magnetic force is needed for Wenzel to
3.2. Ferrofluids in controlling and characterizing Cassie-Baxter transition [62]. Magnetically induced pressure
wetting required for the Wenzel transition has been measured by
Al-Azawi et al. for both static and laterally moving ferrofluid
Interplay between capillary, magnetic and other forces can droplets on micropillared surfaces (Fig. 6a) [63•]. Moving
lead to complex and interesting phenomena. For example, droplets were observed to collapse more easily to the

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125 Wetting of ferrofluids: Phenomena and control

Wenzel state than static droplets. A wetting transition can the ferrofluid acts as a lubricant, but here it also covers the
also be induced with a weak dynamic magnetic field when droplet or the particle, allowing it to be pulled with a
the field frequency is near the resonant frequency of the permanent magnet. Recently also an array of ZnO nanorods
droplet [64]. Oscillating sessile ferrofluid droplets have also infused with ferrofluid was used in a similar way [69•].
been recently used for microrheological measurements [65]. Capillary pressure keeps the ferrofluid between the rods and
Ferrofluids can also be used as a surface or a template to the surface remains smooth. However, the ferrofluid
control wetting properties of other liquids. Peyman et al. self-assembles in microstructures on the surface when a
recently introduced a ferrofluid-based ice-resistant surface, perpendicular magnetic field is applied. This changes the
which could delay ice formation orders of magnitude more surface wetting properties, increasing the water contact
than state-of-the-art counterparts (Fig. 5) [66••]. They used angle from approximately 30° to almost 90°. The authors
silicon wafers coated with a 300 μm thick layer of oil-based claim that the magnetic field gradient leads to different
ferrofluid, which was exposed to a magnetic field created by contact angles on opposite sides of the droplet, which can be
a permanent magnet underneath the substrate. The mag- used to drive the droplets with velocities up to 0.9 m/s.
netic field pulls the ferrofluid against the substrate, creating Magnetically induced roughness was used to change wetting
a magnetic buoyancy effect which forces any water droplets properties also on an elastomer surface embedded with iron
to float near the ferrofluid surface [2]. This prevents water microparticles [70•]. The contact angle changed from
from touching the solid surface and impedes heterogeneous approximately 100° to 165° when a 250 mT magnetic field
ice nucleation. On these surfaces ice formation temperature was applied, while sliding angle decreased to 10°. Without
is lowered to −34 °C, approximately 10 °C lower than on magnetic field the droplets remained pinned on the surface
superhydrophobic or slippery liquid-infused porous surfaces even when it was turned upside down. More information
(SLIPS) [67]. Furthermore, the ice adhesion force is about liquid transport on surfaces responsive for external
extremely low, making any formed ice easy to remove. A fields, including magnetic fields, can be found in a recent
similar scheme was used by Khalil et al. to manipulate review by Li et al. [71•]. Ferrofluid has also recently been
non-magnetic droplets and solid particles on ferrofluid indirectly used to control wetting of water droplets. Huang
infused micro-pillared surfaces [68]. As with other SLIPS et al. used magnetic disks of 150 μm diameter to create an

Fig. 5 Ice formation on different water-repellent surfaces. a) On superhydrophobic (left) and liquid-infused surfaces (center)
water can get in contact with the solid surface, which promotes heterogeneous ice nucleation. However, on magnetic slippery surface
(MAGSS, right) the magnetic buoyancy effect prevents the water-solid contact. b) MAGSS outperforms other ice-repellent surfaces
with 2–3 orders of magnitude higher ice nucleation delay time τav. c) Ice adhesion strength on MAGSS is five orders of magnitude
lower than on other ice-repellent surfaces.
Adapted from [66••] under CC BY.

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
 M Latikka et al. 126

array of ferrofluid spikes, which were used as a master for potential [74]. The droplet was displaced from its potential
molding arrays of polymer microcones [72]. By controlling energy minimum at the axis of a cylindrical permanent
the magnetic field direction, orientation of the spikes could magnet and allowed to oscillate freely on the surface.
be controlled and the resulting surface showed anisotropic Dissipative forces relating to contact angle hysteresis and
wetting properties with 40% higher retention force for viscous dissipation in the droplet were calculated by
droplets moving against the direction of cone inclination measuring the damping rate of the oscillations. Investigating
than along it. mobility directly from a moving droplet overcomes many
Ferrofluid can also be used for droplet manipulation problems related to indirect methods such as contact angle
without a solid substrate. Yang and Li used a thin ferrofluid goniometry, which suffers optical inaccuracies when mea-
film floating on a water surface to move and coalesce water suring superhydrophobic surfaces [75]. The same method
droplets placed on the film [73]. In addition to controlling was later used to measure energy dissipation on
the droplets on the film, they could be pulled to the ceiling micropillared surfaces and its dependency on solid fraction
or the bottom of the container. The ferrofluid film on the and field strength (Fig. 6b) [63•]. Droplet adhesion forces on
surface of the droplet prevents the droplet from mixing with fibers have also been characterized with ferrofluids by
the surrounding water, unless a strong enough magnetic placing droplets on the fibers and bringing a permanent
field is used to break the film, making a controlled release of magnet incrementally closer until the droplet detaches [76].
the contents of the droplet possible. The fibers were attached to a balance, which was used to
Highly concentrated ferrofluids show spectacular insta- measure the maximum magnetic force acting on the droplet
bilities and strong response to magnetic field, but also dilute before detachment.
magnetic liquids can be useful. A minute amount of
superparamagnetic nanoparticles can render a water droplet
controllable with magnetic fields without significantly 3.3. Droplet actuation
altering other physical properties of water. This approach
was used to measure the wetting properties of Wetting plays a critical role in many applications of
superhydrophobic surfaces by observing the motion of a ferrofluids. This is obvious for example in magnetic droplet
water-like ferrofluid droplet in a parabolic magnetic actuation, where droplet mobility depends on the surface

Fig. 6 Applications based on ferrofluid droplet actuation: (a–b) Wetting characterization and (c–e) Boolean algebra. a)
Magnetically induced wetting transition of a moving ferrofluid droplet on a micropillared superhydrophobic surface. b) Oscillation of a
ferrofluid droplet in a magnetic potential used for wetting characterization. c) Schematic of a fluidic chip used in synchronous control
of ferrofluid droplets. d) Schematic of an OR/AND logic gate showing possible trajectories of droplets coming from inlets A and B.
Numbers 1–4 denote the potential wells activated by angular orientations of a rotating in-plane field Bi. e) Experimental results with
droplets coming from inlet B (top) or A (middle) or both (bottom). a) and b) reprinted from [63•], c)–e) adapted from [82•] with
permission from Springer Nature.

Current Opinion in Colloid & Interface Science (2018) 36, 118–129 www.sciencedirect.com
127 Wetting of ferrofluids: Phenomena and control

wetting properties. Mats et al. investigated magnetic Magnetic fields induce body forces and surface stresses on
actuation of aqueous droplets with micrometer-sized mag- ferrofluid droplets, which can cause either droplet flatten-
netic particles on three different surfaces: PTFE, ing or elongation depending on the field geometry. These
superhydrophobic Colocasia leaf and glass slide coated with deformations also affect apparent contact angles, even if
a commercial superhydrophobic spray solution [77]. The intrinsic wetting properties remain unchanged. In addition,
commercial spray proved to be best suited for magnetic there is evidence that magnetic fields can slightly affect
actuation, while PTFE showed higher friction and the leaf interfacial tensions of ferrofluids via nanoparticle dipole
was damaged by the magnetic microparticles. It is notewor- interactions and changing particle distribution within the
thy that the static contact angles on PTFE were almost fluid.
independent of the microparticle concentration, but de- When investigating ferrofluid wetting properties using
creased over 20° on the superhydrophobic surfaces as contact angle goniometry, all these different contributions
concentration was increased. This was attributed to micro- must be carefully considered. High enough magnification
particles becoming associated with the surface and covering must be used to distinguish field-induced curvature of a
the micro- and nanostructures necessary for ferrofluid surface from the contact angle determined by the
superhydrophobicity. Furthermore, an applied magnetic wetting properties. Both advancing and receding contact
field reduced the static contact angle another 20° on the angles should be measured to avoid confusing a change in
Colocasia leaf, while the effect was insignificant on PTFE intrinsic wetting properties with a flow-induced change in
and glass coated with superhydrophobic spray. This is the contact angle. Previous studies have often reported only
probably because Colocasia has microstructures at the a single static contact angle value, which is not enough to
same scale as the magnetic particles, unlike the two other accurately describe the wetting system.
surfaces. This goes to show that a high static water contact To better understand and control ferrofluid wetting,
angle does not guarantee that the surface is suitable for further systematic, high quality experimental studies are
actuation of aqueous magnetic droplets. needed. This will facilitate development of magnetic
Ferrofluids have been increasingly studied and used in droplet transport and other applications relying on ferrofluid
continuous-flow microfluidics applications [11•], [12]. Here actuation.
magnetic fields allow control over droplet generation,
coalescence and separation, as has been recently investi-
gated both experimentally and numerically [78–80]. On the Acknowledgements
other hand, digital microfluidics and manipulation of
individual droplets has so far been dominated by electric
This work was supported by the European Research Council ERC-
actuation. Magnetic actuation in this context has mostly 2016-CoG (grant agreement No 725513) and the Academy of Finland
been realized using large magnetic particles [13•,14•]. (Centres of Excellence Programme (2014−2019), (grant agreement
However, progress is also being made in digital microfluidics No 272361), and Academy Postdoctoral Researcher, grant agree-
using ferrofluids. Chakrabarty et al. presented a numerical ment No 309237).
analysis of magnetic manipulation of a ferrofluid droplet
using a micro-coil array, which could be used to perform
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