Ferrofluid transport analysis for thermal, biomedical and MEMS application

Ferrofluids are stable colloidal suspensions of single domain superparamagnetic nanoparticles suspended in a nonmagnetic liquid. Flowfields established with these fluids can be altered by applying external magnetic fields to realize enhanced heat transfer, controlled mass transfer or field-assisted ferrofluid aggregation to form three dimensional structures.; Unlike the conventional heat transfer, ferrohydrodynamic convection is not yet well characterized. This is addressed through simulation of two-dimensional forced and free convections in a ferrofluid under the influence of magnetic field created by a line-source dipole. In a nonisothermal ferrofluid system, local gradient in fluid susceptibility alters the flowfield leading to an additional advective mode of energy transport. For a forced flow system, the heat transfer enhancement by thermomagnetic convection depends on the magnetic dipole strength. Thermomagnetic convection in a differentially heated square enclosure increases with increasing magnetic dipole strength and the temperature difference, but decreases with increasing fluid viscosity. Also, the heat transfer increases when the length scale is reduced. This makes thermomagnetic convection a promising option for microscale heat transfer applications.; The ability to control ferrofluid transport in a host liquid medium can be harnessed for magnetic drug targeting, where chemotherapeutic agents bonded to the nanoparticles of a biocompatible ferrofluid are injected into the vasculature and then magnetically guided in vivo to the target location. To get an insight of the yet limited understanding of magnetic drug targeting hydrodynamics, investigations are conducted at a laboratory-scale idealized geometry in steady and pulsatile forced flow configurations. Targeted localization of ferrofluid under an imposed magnetic field and the time evolution of the ferrofluid aggregate size and location are characterized numerically and experimentally.; In the context of MEMS, field-assisted ferrofluid aggregation at target location on a substrate or microchannel is proposed for mask-less etching or deposition. Formation of free-standing conical structures of a sessile droplet on a flat substrate is demonstrated and is proposed for manipulating MEMS scale devices and for liquid bridge switches.