Electrohydrodynamic instabilities in a microchannel under an AC field

In this study, the electrohydrodynamic instability between two immiscible liquids for two different liquid couples, Newtonian/Newtonian and non-Newtonian/Newtonian is experimentally studied. The liquid couples are pumped into a rectangular cross-section microchannel with electrodes on the side walls. An alternating current (AC) electric field is applied with sinusoidal or square waves. As Newtonian liquids, silicone oils with different viscosities, ethylene glycol and 85% glycerol-water solution are used. For the non-Newtonian liquid, 250 ppm xanthan gum is dissolved in 85% glycerol solution. At a certain voltage, called the hitting voltage, the interface that formed between the two immiscible liquids flowing in the microchannel loses its flat form and hits the channel walls. The hitting voltage value for various thickness ratios, total flow rates and AC field frequencies are investigated. The instability provides microdroplets if the applied voltage is approximately 200 Vpp more than the hitting voltage. Increasing the thickness ratio, defined as the thickness of the dispersed phase to that of the continuous phase, has an increasing effect on the hitting voltage for both liquid couples. The total flow rate has also an increasing effect on the hitting voltage but the effect is more significant for the non-Newtonian/Newtonian liquid couple than the non-Newtonian/Newtonian liquid couple. The AC field frequency shows a jump for the hitting voltage at some critical frequency. The applied function type has a significant effect on the hitting voltage. It is observed that the hitting voltages for the square function are not only lower than the sinusoidal values, but also has a linear whilst sinusoidal has an exponential trend. Particle encapsulation experiments are also performed and it is seen that encapsulating particles is possible but controlling it requires very high precision tools.