Droplet actuation induced by coalescence: experimental evidences and phenomenological modeling

TL;DR

This study investigates droplet interactions on a substrate, demonstrating how coalescence can induce droplet actuation, and presents a phenomenological model that explains and predicts this behavior for potential microfluidic applications.

Contribution

The paper provides experimental evidence of droplet actuation due to coalescence and introduces a simple model that captures the dynamics of this phenomenon.

Findings

Droplet coalescence can induce controlled displacement.

A phenomenological model accurately predicts droplet motion.

Hydrophilic stripe confinement enables actuation control.

Abstract

This paper considers the interaction between two droplets placed on a substrate in immediate vicinity. We show here that when the two droplets are of different fluids and especially when one of the droplet is highly volatile, a wealth of fascinating phenomena can be observed. In particular, the interaction may result in the actuation of the droplet system, i.e. its displacement over a finite length. In order to control this displacement, we consider droplets confined on a hydrophilic stripe created by plasma-treating a PDMS substrate. This controlled actuation opens up unexplored opportunities in the field of microfluidics. In order to explain the observed actuation phenomenon, we propose a simple phenomenological model based on Newton's second law and a simple balance between the driving force arising from surface energy gradients and the viscous resistive force. This simple model is able to reproduce qualitatively and quantitatively the observed droplet dynamics.