A simple model for self-propulsion of microdroplets in surfactant solution

TL;DR

This paper introduces a simple hydrodynamic model explaining how microdroplets self-propel in surfactant solutions, highlighting the role of surfactant distribution and flow instability in spontaneous motion.

Contribution

It presents a novel analytical and numerical model linking surfactant dynamics and flow instability to droplet self-propulsion, emphasizing surface micelle generation as a key factor.

Findings

Self-propulsion arises from symmetry breaking due to surfactant distribution.

Flow instability is driven by Marangoni stress from non-uniform surfactant coverage.

The first flow mode becomes unstable via a supercritical bifurcation, enabling droplet swimming.

Abstract

We propose a simple active hydrodynamic model for the self-propulsion of a liquid droplet suspended in micellar solutions. The self-propulsion of the droplet occurs by spontaneous breaking of isotropic symmetry and is studied using both analytical and numerical methods. The emergence of self-propulsion arises from the slow dissolution of the inner fluid into the outer micellar solution as filled micelles. We propose that the surface generation of filled micelles is the dominant reason for the self-propulsion of the droplet. The flow instability is due to the Marangoni stress generated by the non-uniform distribution of the surfactant molecules on the droplet interface. In our model, the driving parameter of the instability is the excess surfactant concentration above the critical micellar concentration which directly correlates with the experimental observations. We consider various low-order modes of flow instability and show that the first mode becomes unstable through a supercritical bifurcation and is the only mode contributing to the swimming of the droplet. The flow fields around the droplet for these modes and their combined effects are also discussed.