Coalescence of immiscible sessile droplets on a partial wetting surface

TL;DR

This study investigates the coalescence process of immiscible sessile droplets on partial wetting surfaces, revealing three stages with new insights into interface effects, oscillation dynamics, and retraction behavior through experimental and theoretical analysis.

Contribution

It introduces a comprehensive three-stage model of immiscible droplet coalescence, including a new capillary timescale and a modified Ohnesorge number, advancing understanding beyond previous miscible droplet studies.

Findings

Liquid bridge growth follows a 2/3 power law regardless of immiscibility.

Interfacial tension influences oscillation periods by resisting droplet oscillation.

Viscous resistance during retraction is localized near the contact line.

Abstract

Droplet coalescence is a common phenomenon and plays an important role in multi-disciplinary applications. Previous studies mainly consider the coalescence of miscible liquid, even though the coalescence of immiscible droplets on a solid surface is a common process. In this study, we explore the coalescence of two immiscible droplets on a partial wetting surface experimentally and theoretically. We find that the coalescence process can be divided into three stages based on the timescales and force interactions involved, namely (I) the growth of the liquid bridge, (II) the oscillation of the coalescing sessile droplet, and (III) the formation of a partially-engulfed compound sessile droplet and the subsequent retraction. In stage I, the immiscible interface is found not to affect the scaling of the temporal evolution of the liquid bridge, which follows the same 2/3 power law as that of miscible droplets. In Stage II, by developing a new capillary timescale considering both surface and interfacial tensions, we show that the interfacial tension between the two immiscible liquids functions as a nonnegligible resistance to the oscillation which decreases the oscillation periods. In Stage III, a modified Ohnesorge number is developed to characterize the visco-capillary and inertia-capillary timescales involved during the displacement of water by oil; a new model based on energy balance is proposed to analyze the maximum retraction velocity, highlighting that the viscous resistance is concentrated in a region close to the contact line.