Electric field-induced instabilities in ferrofluid microflows

Abstract

Ferrofluids have been increasingly used to manipulate particles and cells in microfluidic devices via negative magnetophoresis. They have also been recently exploited to achieve a fast microfluidic mixing through magnetic field-induced flow instabilities at the ferrofluid/water interface. This work presents the first demonstration of electric field-induced instabilities in electroosmotic ferrofluid/water co-flows through a T-shaped microchannel. With the increase in electric field, instability waves and even chaotic flows can be formed when the two fluids merge at the T-junction due to the significant mismatch of their electrical conductivities. The experimentally observed dynamic behaviors of the ferrofluid/water interface are qualitatively captured by the ferrofluid concentration distribution obtained from a 2D numerical model. The measured threshold electric field for observing sustainable flow instabilities is found to decrease with the increase in ferrofluid concentration. While this trend is correctly predicted by the numerical model, the threshold electric field values are substantially under-predicted. The parametric effects that may be responsible for this discrepancy are discussed.