Collision between chemically-driven self-propelled drops

TL;DR

This paper analyzes head-on collisions between chemically-driven self-propelled drops, revealing how chemical reactions and Marangoni flows influence their interactions, resulting in elastic collisions or fusion depending on system parameters.

Contribution

The study provides a combined analytical and numerical analysis of chemically-driven drop collisions, highlighting the dominance of concentration-mediated interactions over hydrodynamics.

Findings

Drops exhibit elastic collision or fusion depending on parameters.

Concentration-mediated interactions dominate hydrodynamic effects.

A reduced model for collision dynamics is derived and analyzed.

Abstract

We consider analytically and numerically head-on collision between two self-propelled drops. Each drop is driven by chemical reactions that produce or consume the concentration isotropically. The isotropic distribution of the concentration field is destabilized by motion of the drop which is itself made by Marangoni flow from concentration-dependent surface tension. This symmetry-breaking self-propulsion is distinct from other self-propulsion mechanisms due to the intrinsic polarity such as squirmers and self-phoretic motion; there is a bifurcation point below which the drop is stationary and above which it moves spontaneously. When two drops moving along the same axis with opposite direction, the interactions arise both from hydrodynamics and concentration overlap. We found that two drops exhibit either elastic collision or fusion depending on the distance from the bifurcation point controlled, for instance, by viscosity. The elastic collision results from the balance between dissipation and energy injection by chemical reactions. We derive the reduced equations for the collision between two drops and analyze the contributions from the two interactions. The concentration-mediated interaction is found to dominate the hydrodynamic interaction.