Wettability-mediated dynamics of two-phase flow in microfluidic T-junction

Abstract

Fundamental understanding of two-phase flow and the concomitant implications of wall wettability and viscosity are critical in areas like microfluidics and lab on chip applications. In this work, numerical investigation of the displacement of two immiscible fluids in a T-junction is presented. The study reveals the interplay of critical physicochemical determinants like capillarity, viscosity, and wettability on the dynamics of two-phase flow. Temporal evolution of the displacement of dispersed phase resulting into different regimes like squeezing, dripping, necking, droplet formation, and jetting for a combination of capillary numbers, viscosity ratios, and wettability scenarios is furnished in detail in order to elucidate the mechanism of droplet formation through the displacement behavior of two-phase flow. The findings establish the surface wettability to be the dominating factor in determining the time evolution of the dispersed liquid interface at a lower capillary number. With the increase in the hydrophobicity of the surface, the liquid interface transits from squeezing to dripping and then to droplet formation at a low capillary number. However, irrespective of wettability, the regime changes from jetting to parallel flows due to an increase in capillary number beyond a critical limit. Furthermore, the phenomenon of squeezing, necking, and breakage occurs relatively earlier with the increase in viscosity ratio and hydrophobicity of the surface. These results may bear significant implications toward designing of droplet dispensing systems with the substrate wettability as a critical controlling parameter.