Effect of jump in wettability on the hydrodynamics of liquid–liquid two-phase flow in a straight mini capillary tube

Abstract

This experimental study investigates the effect of capillary wall wettability on the hydrodynamics of liquid–liquid two-phase flow. All experiments are performed in a capillary tube with an inner diameter of 2.4 mm, and the length is 40 cm. The half part of three different capillary tubes is dip-coated with 2%, 10%, and 20% wt/wt Polytetrafluoroethylene (PTFE) solution to make it hydrophobic. The major flow patterns observed in hydrophobic and hydrophilic sections are an aqueous slug, aqueous drop, organic slug, organic drop, and encapsulated slug. The encapsulated slug is formed when the contact angle jump is from 169° to 13°. It is observed at high superficial velocities of aqueous and organic phases. The change in wettability of the channel from hydrophobic to hydrophilic enhances the phase inversion. The aqueous slug/droplet formed in a hydrophobic section converts to an organic slug/droplet in the hydrophilic section. Pressure drop was found to increase with an increase in the wettability of the test section. It is maximum during the formation of encapsulated slug flow due to formation of an additional interface. An analytical model is proposed, which shows the critical value of volume fraction during film formation in terms of phase velocities, physical properties, and wettability.