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Efficient transfer of heat is a major challenge in a plethora of industrial systems and devices. Lower device temperatures can improve energy efficiency and reduce premature device failure. We report the development of a single channel magnetofluidic cooling (MFC) device with high passive cooling performance. MFC is based on thermo-magnetofluidic (TMF) convection, i.e., spontaneous ferrofluid motion due to the gradients of external magnetic field and temperature. Our novel Cu-silicone hybrid design exhibited the highest heat load temperature drop of 183 °C, which is ~3 times higher cooling than previous reports in the literature. Experimental studies of TMF flow are challenging due to the opaque nature of the ferrofluid. Hence we developed a novel TMF setup and quantified the temperature and velocity profiles. We also developed a simulation model to describe the MFC process; the results are in good agreement with our experimental findings. The high cooling performance of our device was found to be due to high vorticity and mixing. Our MFC device is useful for transferring waste heat load from a variety of systems. It is a passive, green, self-regulating, noise, and vibration-free cooling technology. National Research Foundation (NRF) This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus of Research Excellence and Technological Enterprise (CREATE) program.

Wireless and programmable manipulation of droplets is a challenge. We addressed this challenge by a combination of magnetic fluids and hybrid magnetic fields. We investigated the remote, wireless and programmable manipulation of ferrofluid droplets in a capillary microfluidic platform by a combination of uniform and non-uniform magnetic fields. The time-dependent motion of droplets under the influence of magnetic field was studied. Actuation and inter-droplet spacing of the droplets could be controlled by tuning the magnetic field strength. The influence of viscosity on the inter-droplet spacing was investigated. The time-dependent merging of (a) ferrofluid-ferrofluid and (b) ferrofluid-rhodamine droplets was demonstrated. Simulation and experimental results are in good agreement. The present work can be used for magnetically controlled, remote, wireless and programmable droplet actuation and merging relevant to biomedical assay, cell manipulation, tissue culture, drug efficacy and synthesis of magnet-polymer composite particles. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Efficiently removing waste heat by novel cooling devices can reduce material failure and enhance service life. Magneto-fluidic cooling, which is based on the thermomagnetic convection of a ferrofluid, offers a passive, self-regulating approach for the removal of waste heat. The performance of a novel multi-torus magnetic cooling device for kilowatt level cooling was investigated. The performance was determined for a range of heat load power. Heat load cooling increased from 148 °C to 214 °C when heat load power was increased from 0.5 kW to 1 kW, respectively, demonstrating the self-regulating nature of the device. The heat load cooling performance was assessed for various magnet positions. The temperature profile of the ferrofluid along the axial and radial directions of the flow channel revealed an asymmetric temperature distribution. The transient effect of periodic magnetic field switching on the heat load temperature was simulated using COMSOL Multiphysics and found to be in good agreement with the experimental findings. Simulated surface velocity vector plots revealed ferrofluid vortices near the heat load, these vortices resulted in enhanced mixing of hot and cold ferrofluid, leading to increased cooling. The thermal resistance of our multi-torus magnetic cooling device was determined analytically. The present device offers lower thermal resistance per unit length and lower ferrofluid thermal resistance per unit volume of ferrofluid compared to conventional heat pipes. A self pumping, self regulating passive heat pump, capable of kilowatt level cooling, has been fabricated and its cooling performance assessed.

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

The advantages of ferrofluids in microfluidic lab-on-a-chip applications include remote control of the fluid flow within the chips, e.g., mixing of the species using an external uniform magnetic field. Hence, three-stream flow systems, consisting of a ferrofluid core clad by two streams of diamagnetic silicone oil were studied. The instability of the ferrofluid, subjected to an external uniform magnetic field, was studied. When the strength of this magnetic field was increased to a critical value, the ferrofluid was spread toward the silicone oil and a transient instability developed at the ferrofluid-silicone oil interface. Further increasing magnetic field resulted in periodic instability structures and permanent instability. The effect of magnetic field strength, flow rate and flow rate ratio were determined. With a higher flow rate ratio, the permanent instability was observed only at the larger magnetic field strength. Our modeling results were consistent with these experimental results. Our work shows that an external uniform magnetic field of only a few millitesla can lead to instability and mixing, thus it is relevant to mixing in practical microfluidic devices. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Magnetic droplets, consisting of magnetic nanoparticles in a carrier fluid, are of high interest due to applications such as remote and wireless control in a microfluidic environment. We investigated the influence of magnetic field on the control of ferrofluid droplet size in a nonmagnetic carrier fluid. Generation of larger droplets by a re-pumping mechanism was studied. The magnetic field leads to coalescence and mixing of the magnetic droplets. A significant response of the ferrofluid droplets to changes in flow rate ratio, the viscosity of the carrier medium and magnetic field intensity was observed. The droplet size can be increased by three times of its initial diameter by tuning the magnetic field intensity. Our modeling results show a similar trend to the experimental findings. Such control, mixing, and re-pumping of droplets is relevant to novel Lab-on-a-Chip applications. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Magnetic droplets are versatile tools for a range of lab-on-a-chip (LoC) applications. The combination of a uniform magnetic field and magnetic droplet offers wireless and programmable manipulation. We performed LoC experiments and numerical studies on ferrofluid droplets under the influence of a uniform magnetic field. The dynamic behavior of flowing ferrofluid droplets was examined. The droplet size, shape, inter-droplet spacing and velocity could be controlled by tuning the magnetic susceptibility of the ferrofluid, the viscosity of the carrier medium, and the flow rates. Our droplet-based micromagnetofluidic numerical model is in good agreement with our experiments. These studies are useful for magnetic droplet control and mixing in a LoC using a uniform magnetic field. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Magnetofluidic spreading (MFS) is a phenomenon in which a uniform magnetic field is used to induce spreading of a ferrofluid core cladded by diamagnetic fluidic streams in a three-stream channel. Applications of MFS include micromixing, cell sorting and novel microfluidic lab-on-a-chip design. However, the relative importance of the parameters which govern MFS is still unclear, leading to non-optimal control of MFS. Hence, in this work, the effect of various key parameters on MFS was experimentally and numerically studied. Our multi-physics model, which combines magnetic and fluidic analysis, showed excellent agreement between theory and experiment. It was found that spreading was mainly due to cross-sectional convection induced by magnetic forces, and can be enhanced by tuning various parameters. Smaller flow rate ratio, higher magnetic field, higher core stream or lower cladding stream dynamic viscosity, and larger magnetic particle size can increase MFS. These results can be used to tune magnetofluidic spreading in microchannels. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Spreading of a water based ferrofluid core, cladded by a diamagnetic fluid, in three-stream micromixer channels was studied. This spreading, induced by an external magnetic field, is known as magnetofluidic spreading (MFS). MFS is useful for various novel applications where control of fluid-fluid interface is desired, such as micromixers or micro-chemical reactors. However, fundamental aspects of MFS are still unclear, and a model without correction factors is lacking. Hence, in this work, both experimental and numerical analyses were undertaken to study MFS. We show that MFS increased for higher applied magnetic fields, slower flow speed of both fluids, smaller flow rate of ferrofluid relative to cladding, and higher initial magnetic particle concentration. Spreading, mainly due to connective diffusion, was observed mostly near the channel walls. Our multi-physics model, which combines magnetic and fluidic analyses, showed, for the first time, excellent agreement between theory and experiment. These results can be useful for lab-on-a-chip devices. Published version