Hydrodynamics of liquid–liquid Taylor flow in microchannels

Abstract

An understanding of the hydrodynamics of the flow of long liquid droplets in another liquid phase is necessary for droplet-based microfluidics and other Lab-on-a-Chip devices. In this work, we use high speed photography/brightfield microscopy and CFD simulations to study the hydrodynamics of vertically upward flow of Taylor droplets of water dispersed in a continuous hexadecane phase in a channel of diameter 1.06mm. The volume-of-fluid (VOF) method is used to model two-phase flow and the shapes and velocities of the droplets measured experimentally are in excellent agreement with those obtained from CFD simulations. It is observed that the film thicknesses and the dynamic pressure drop caused by the presence of the droplets for these low viscosity droplets (viscosity ratio=0.3) can be predicted by the Bretherton's expressions for an inviscid bubble. For a sufficiently long droplet, the flow field in the uniform thickness film region can be predicted by an ideal annular flow solution. A model to predict the overall pressure drop in a unit cell, consisting of a Taylor droplet surrounded by the two halves of continuous phase slugs is also presented by summing up the pressure drop contributions from the continuous phase liquid slugs, uniform thickness film region and interfacial pressure drops at the droplet front and rear. The model was accurate for long droplets and slugs but was not reliable for shorter droplets and slugs, for which the slug and film flows may not be fully-developed and the shapes of the front and rear of the droplets interact.