Emma Thomée, PhD candidate at Elvesys email@example.com Elvesys SAS, 172 Rue de Charonne 75011 Paris
1. Introduction into magnetic fluids and microfluidics
Magnetic manipulation of micro-fluids is an attractive concept. Due to the non-invasive nature of magnetic fields, magnetic particles or magnetic fluids can be manipulated inside a microfluidic channel by external magnets that are not in direct contact with the fluid. Ferrofluids make up a specific class of magnetic fluids. Ferrofluids are stable colloid suspensions of magnetic nanoparticles in a nonmagnetic carrier fluid, and they exhibit both magnetic and fluidic properties.
External magnetic fields can be applied to control their fluid motion and their fluidic properties are retained even under the influence of strong magnetic fields. Ferrofluids can move just as single component fluids thorough microchannels of microfluidic devices. This review will present an overview of the use of ferrofluids in microfluidic devices for pumping, mixing, droplet generation, sorting and separation, valves and seals and digital microfluidics.
2. Ferrofluids for microfluidics
A magnetic fluid or ferrofluid, constitutes particles of magnetic materials such as magnetite, maghemite or cobalt ferrite dispersed in a basefluid, such as water or organic solvent. The size of the particles is typically 5-20 nm, and the number of particles are in the order of 1023 particles per cubic meter . If the particles are sufficiently small, about 15 nm or less, the ferrofluid behave as a homogenous continuum. This means that the particles are kept suspended by Brownian motion and do not settle out due to gravity or a magnetic field. In order to maintain the spacing between adjacent particles, the nanoparticles are often coated with a surfactant layer, as they otherwise tend to adhere together as a result of magnetic forces and van der Waals forces. Ferrofluids must be synthesized as they do not appear in nature.
There are two methods to prepare ferrofluids:
- One-step method: nanoparticles are fabricated in base fluids directly
- Two-step method: nanoparticles are first fabricated and then separated and redispersed in base fluid.
Figure 1: Ferrofluid on a reflective glass plate under the influence of a strong magnetic field (Wikipedia)
Ferrofluids exhibit superparamagnetic properties. Superparamagnetism means that the particles are magnetized when a magnetic field is present, but when the magnetic field is removed they behave like nonmagnetic materials as they have no magnetic memory.
Thermomagnetic convection in ferrofluids
The magnetic particles in a ferrofluid are small enough to contain a single magnetic domain, and the net magnetization is hence temperature dependent. Below a critical temperature, referred to as the Curie temperature (TC), the magnetic moments of the ferrofluid are aligned in a one direction resulting in a net magnetization. As the temperature exceeds the Curie temperature the magnetic moments become randomly oriented and the ferrofluid loses its net magnetization. An applied thermal gradient thus results in a non-uniform magnetization and the ferrofluid will experience a body force, the Kelvin body force, inducing a fluid flow along the thermal gradient. This thermomagnetic convection can drive the ferrofluid without a pump .
Heat transfer of ferrofluids
The thermal conductivity of magnetic nanoparticles is typically order-of-magnitude higher than the basefluid. The addition of nanoparticles therefore remarkably increases the thermal conductivity of the basefluid. This is usually quantitatively demonstrated as enhancement in thermal conductivity or enhanced convective heat transfer coefficient. Further enhancements in the heat transfer coefficient of ferrofluids have been observed when magnetic fields are applied. This is thought to be due to alignment of magnetic nanoparticles in the direction of the applied magnetic field, forming chainlike structures that provide a heat conductive pathway . These results have been observed under both forced and free convective heat transfer conditions. Ferrofluids have therefore been used as heat transfer fluids, e.g. for cooling of micro devices .
3. Ferrofluids in microfluidic systems
An external magnetic field acting on ferrofluids in microfluidic devices can be generated by permanent magnets or electromagnets. They can be positioned outside the microfluidic device, or be integrated the into the microfluidic device. Integrated magnets have the advantage of close proximity with the fluid, thus requiring lower field strength. However, that requires more complex fabrication steps that may require clean room facilities.
Hatch et al.  presented a ferrofluidic pump where plugs of ferrofluid were used to pump non-ferrofluids through microchannels. The ferrofluid and the non-ferrofluid must be immiscible. Their device consisted of a circular pump with two ferrofluidic plugs and two external magnets that controlled the movement of the plugs. One plug was held in place between the inlet and the outlet, acting as a closed valve. The other plug was dragged around the circle by a rotating magnet, serving as a piston, pushing and pulling the liquid in an out of the circular channel.
Figure 2. Principal sketch of a circular ferrofluidic pump. The black structures represent the ferrofluid plugs and two magnets (M) were used to move these (figure from Hatch et al. ).
The magnetocaloric effect resulting in thermomagnetic convection, as described earlier in this review, can be used to pump fluid in a microchip. Love et al.  reported a magnetocaloric pump which used only a thermal field and magnetic field to drive fluid flow in a microchannel. They used a plug of ferrofluid with low Curie temperature which was attracted to a region by a magnetic field. The ferrofluid plug was then subjected to heating and as its temperature approaches the Curie temperature, the magnetic attraction of the fluid weakened and cooler fluid was then attracted to the magnetic field and displaced the heated fluid. This eventually will establish a flow and the pump was used to push non-ferrofluids through the microsystem. The principle is demonstrated in figure 3.
Figure 3. The field effect acting on the ferrofluid in a magnetocaloric pump (figure from Love et. al. )
Mixing is a key concept in microfluidics and rapid mixing is essential for many biological and chemical assays. Several strategies, both passive and active, have been suggested to improve mixing efficiency. Active mixers generally provide better mixing efficiency than passive mixers, but usually also require more energy and have higher fabrication costs. Another issue with active mixers is the Joule heating that may damage biological samples. Tsai et al.  developed a simple low-cost y-shaped micromixer to study the mixing between a water-based ferrofluid and water. They placed a permanent magnet right below the microchannel, and varied the volumetric flowrate and channel width to obtain an improved mixing efficiency. They found that the mixing efficiency of ferrofluid and water under the influence of a permanent magnet could reach over 90%, while the mixing efficiency in the same microchannel by only diffusion was always below 15%. This type of magnetic mixer has the advantage of not consuming energy and not producing heat. Other types of magnetic micromixers have also been suggested, such as microscopic stirrer bars  which are rotated by conventional stirrer plates or magnetophoresis of magnetic particles .
Figure 4. Diffusion between DI water and water-based ferrofluid (left) without magnet and (right) with magnet (figures from Tsai et al )
Droplets of ferrofluid can be generated in microfluidic systems by conventional methods of droplet formation at a T-junction or a flow-focusing junction. The ferrofluid droplets can then be manipulated with external magnetic fields in an immiscible fluid or on flat surfaces or substrates. Tan and Nguyen  studied the formation of ferrofluid droplets at a microfluidic T-junction in the presence of a magnetic field generated by a small circular permanent magnet. They found that in the absence of a magnetic field the droplet sized varied linearly with the flow rate of the continuous phase. In the presence of a magnetic field, the droplet size could be manipulated with magnetic field strength, the magnetization of the ferrofluid and the position of the magnet. Placing the magnet upstream of the T-junction lead to the formation of bigger droplet as the magnetic force pulled the emerging droplet back, delaying the droplet breakup. Placing the magnet downstream had the opposite effect, of accelerating the droplet breakup.
Valves and seals
Hartshorne et al.  demonstrated the use of ferrofluidic plugs about 10 mm in length as a valve- and seal component in microfluidic devices fabricated in glass. They investigated the performance of ferrofluids as a plug in three different microfluidic configurations; a plug seal in a straight channel and as a Y-valve and as a well valve. They used permanent magnets to maneuver the ferrofluids. The valves could open and close well against a low pressure differential. At higher pressures they observed some loss of small portions of ferrofluid that was carried downstream with the fluid.
Figure 5. Principal sketch of ferrofluidic valves and seals (Figure from Hartshorne et al. )
Sorting and separation
Magnetic digital microfluidics
In digital microfluidics, droplets of fluid can be manipulated on a flat open surface with no confinements. Discrete droplets can act as individual reaction chambers for chemical or biological reactions or be used for transport of reagents. In magnetic digital microfluidics, droplets containing magnetic particles, such as ferrofluids or liquid marbles, are manipulated with permanent magnets or electromagnets. Nguyen et al.  showed that a sessile ferrofluid droplet could be deformed by a permanent magnet, changing its apparent contact angle. The apparent contact angle was seen to decrease with an increasing magnetic field. They called this phenomenon magnetowetting. The magnet was placed under a planar homogeneous surface and if the magnetic force was strong enough to overcome the resistant friction force and the capillary force, the droplet could slide along a flat surface with the same velocity as the magnet. A general problem with ferrofluidic droplets is however that they are not compatible with the environment of the aqueous buffer in many bioanalytical assays , as the buffer destabilizes the colloid suspension.
Figure 6. Illustration of magnetowetting (Figure from Kitanovski A., Tušek J., Tomc U., Plaznik U., Ožbolt M., Poredoš A. (2015) Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes. In: Magnetocaloric Energy Conversion. Green Energy and Technology. Springer, Cham)
4. Conclusions and outlook on magnetic fluids and microfluidics
A selection of works where magnetic fluids have been used in microfluidic systems are presented in this review. Some of the advantages of magnetic manipulation compared to electric manipulation include the ability to control both flow and transfer properties (such as heat transfer) as well as not being influenced by pH, surface charges or ionic concentration. However, micro-magnetofluidics remains mainly at a proof-of-concept stage. There are several challenges that must be overcome before the techniques can be practical. General problems with ferrofluids are fouling and clogging of microchannels as a result of the surfactants that are used to improve the stability of the ferrofluid. In addition, the nanoscopic properties of ferrofluids are not yet fully understood and must be further studied to really exploit their unique properties.
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