Experimental study on the electrohydrodynamic instability between two immiscible liquids flowing in a microchannel

Abstract

The aim of this study is to investigate experimentally the electrohydrodynamic instability between two Newtonian immiscible liquids in a microchannel. When two immiscible liquids flow in laminar regime, a flat interface is formed between them. If an electric field is applied on the system, the interface may deform, i.e. may become unstable. This instability, called electrohydrodynamic instability (EHD) is used for forming mono dispersed droplets. In this study, the fabrication methods of microchannels with electrodes are investigated. A microchannel of depth 350 μm, width 1.1 mm, electrode length 10 mm, and total length 20 mm, with double sided conductive and non-conductive tape on PMMA is fabricated. Silicone oil with different viscosities and ethylene glycol are injected into the channel via a syringe pump and a DC electric field in the range of 200-1600 V is applied normal to the flow direction. The effect of the parameters such as the flow rate ratio, depth ratio, viscosity ratio and the total flow rate of the liquid couple, on the critical voltage at which the interface starts to destabilize is investigated. Then the effect of total flow rate on the time elapsed for the interface to be ruptured to form droplets by hitting the wall of the channel is analyzed. Finally, the effect of each parameter on the size of droplets at various voltages is investigated. It is observed that an increase in the viscosity or the flow rate ratio of the silicone oil to the ethylene glycol has a stabilizing effect, i.e. a higher voltage is needed while the total flow rate has no effect on it. However, it is observed that an increase in the total flow rate results in shortening of the elapsed time for the interface to hit the wall. Moreover, the droplet size decreases down to 0.1 μL with an increase in the applied voltage, the viscosity ratio or the total flow rate or a decrease in the flow rate ratio. In addition to these observations, two empirical models for determining the critical electric number, i.e., the dimensionless voltage and the droplet size and another model which is a combination of both models, for determining the droplet size at the critical voltage are established.