An experimental study of the mixing of liquids in a continuous-flow microchannel under conditions of Rayleigh-Taylor instability

Abstract

This study experimentally investigates the influence of Rayleigh-Taylor instability on the liquid mixing process in a flow-type microchannel. We investigated a two-layer liquid-liquid system consisting of a denser aqueous solution of potassium permanganate (upper layer) and less dense pure water (lower layer) with a fixed density gradient. A colorimetric method based on the dependence of light absorption by solutions on the concentration of the dissolved substance was used to reconstruct the two-dimensional concentration field. Based on the obtained data, we analyzed the spatial structure of convective flows and the degree of liquid mixing along the channel. The studies were carried out at low Reynolds numbers, which precludes the development of turbulent flow, and across a wide range of Richardson numbers that indicate the ratio of buoyancy to inertial forces in the formation of convective structures. We found that at high Richardson numbers, the development of Rayleigh-Taylor instability ensures almost complete mixing of liquids within five calibers from the channel inlet. Reducing the Richardson number by increasing the flow rate through the channel leads to a slowdown in the instability development and, consequently, in a significant decrease in mixing efficiency along the channel. We demonstrated that with optimal values of governing parameters, the characteristic liquid mixing length can be reduced by an order of magnitude compared to pure diffusive mixing scenarios. The obtained results demonstrate the effectiveness of convective instability mechanisms in controlling mass transfer processes in flow-type microchannel systems.