Electrokinetic instabilities in ferrofluid microflows

Lab-on-a-chip devices have been increasingly used in the past two decades for chemical and biomedical analysis. These devices employ the concepts of microfluidics and offer the promise of incorporating multiple laboratory processes onto a single portable chip. Electric field has been often employed in microfluidic devices for the ease of fluid and sample control as well as the convenience of chip integration and interfacing. Flow instabilities can take place when two fluids of unequal electrical properties are pumped through a microchannel under the application of an adequately strong electric field. The study of these electrokinetic instabilities in microfluidic devices is not only significant to fundamental research but also relevant to practical applications such as sample mixing.;In this work an experimental investigation of the electrokinetic instability between co-flowing ferrofluid and DI water in a T-shaped microchannel is carried out. The effects of the applied DC electric field and the ferrofluid concentration on the flow pattern are examined. For each concentration of ferrofluid, pure diffusion happens until a certain value of electric field, above which instability waves are generated at the interface of the ferrofluid and DI-water flows and convected downstream. Moreover, these waves become more irregular and even chaotic with the increase in electric field. This threshold electric field is found to decrease with the increase in ferrofluid concentration.;Meanwhile, a two-dimensional transient numerical model using commercial solver COMSOL 4.3b is also developed to simulate the electrokinetic instability phenomenon by employing the electrical conductivity difference between DI water and ferrofluid. Theoretical analysis of the equations governing electrokinetic flows show that free charges are generated in a fluid with conductivity gradients in the presence of electric field. The action of electric field on these free charges result in Coulomb force that generates instability when strong enough. The effect of electric field on the flow is simulated and the threshold electric field is found through a series of simulations for each ferrofluid concentration. The simulation result trend is in good agreement with the experiments, but the numerical model under predicts the threshold electric field found through experiments.;Furthermore, the effect of permittivity variation between ferrofluid and DI-water is included in the numerical model to understand its influence on the electrokinetic instability pattern and threshold electric field. Theoretical analysis shows that the presence of permittivity gradient can also induce an electrical force in the bulk fluid in the presence of electric field. This force is opposite to that generated by the conductivity gradient, and hence serves to stabilize the flow, which should lift the threshold electric field for electrokinetic instability. Such an influence is, however, found to be insignificant through the numerical model accounting for both conductivity and permittivity gradients.;The numerical model assumes ferrofluid as a continuous fluid and hence the electrophoretic and magnetophoretic forces experienced by the nanoparticles are not incorporated. A brief study about the effects of these factors on the threshold electric field indicates their insignificant influence. The possible deviation in the diffusion coefficient of ferrofluid is also investigated in the numerical model, whose influence is also found to be inconsequential. It is supposed that the top and bottom wall effects on the electrokinetic instability should be taken into consideration by the use of a three-dimensional numerical model.