Magnetic actuation, heat transfer and microsystem applications of iron-oxide nanoparticle based ferrofluids

Official URL: http://risc01.sabanciuniv.edu/record=b1534436 (Table of Contents)

Ferrofluids are colloidal suspensions, in which the solid phase material is composed of magnetic nanoparticles, while the base fluid can potentially be any fluid. The solid particles are held in suspension by weak intermolecular forces and may be made of materials with different magnetic properties. Magnetite is one of the materials used for its natural ferromagnetic properties. They have vital applications in the field of microfluidics such as microscale flow control in microfluidic circuits, actuation of fluids in microscale, and drug delivery mechanisms. Heat transfer performance of such ferrofluids is also one of the crucial properties among many potential coolants that should be analyzed and considered for their wide range of applications. In the first study, different families of devices actuating ferrofluids were designed and developed to reveal this potential. A family of these devices actuates discrete plugs, whereas a second family of devices generates continuous flows in tubes of inner diameter ranging from 254μm to 1.56mm. The devices were first tested with minitubes to prove the effectiveness of the proposed actuation method. The setups were then adjusted to conduct experiments on microtubes. Promising results were obtained from the experiments. Flow rates up to 120μl/s and 0.135μl/s were achieved in minitubes and microtubes with modest maximum magnetic field magnitudes of 300mT for discontinuous and continuous actuation, respectively. The proposed magnetic actuation method was proven to work as intended and is expected to be a strong alternative to the existing micropumping methods such as electromechanical, electrokinetic, and piezoelectric actuation. The results suggest that ferrofluids with magnetic nanoparticles merit more research efforts in micro pumping. In the second study, convective heat transfer experiments were conducted in order to characterize convective heat transfer enhancements with Lauric acid coated ironoxide (Fe3O4) nanoparticle based ferrofluids, which have volumetric fractions between 0%- ~5% and average particle diameter of 25 nm, in a 2.5 cm long hypodermic stainless steel microtube with an inner diameter of 514 μm and an outer diameter of 819 μm. Heat fluxes up to 184 W/cm2 were applied to the system at three different flow rates (1ml/s, 0.62ml/s and 0.36 ml/s). A decrease of around 100% in the maximum surface temperature (measured at the exit of the microtube) with the ferrofluid compared to the pure base fluid at significant heat fluxes (>100 W/cm2) was observed. Moreover, the enhancement in heat transfer increased with nanoparticle concentration, and there was no clue for saturation in heat transfer coefficient profiles with increasing volume fraction over the volume fraction range in this study (0%-5%). The promising results obtained from the experiments suggest that the use of ferrofluids for heat transfer, drug delivery, and biological applications can be advantageous and a viable alternative as new generation coolants and futuristic drug carriers.