Interplay of Micellar Architecture and Viscosity Governs Active Droplet Motility

TL;DR

This study investigates how micellar structure and viscosity influence the autonomous motion of liquid crystal oil droplets, revealing a non-monotonic velocity dependence on salt concentration and developing a model to explain these effects.

Contribution

The paper uncovers the role of micellar architecture and viscosity in droplet motility, introducing a theoretical model that links salt-induced micellar transitions to propulsion speed.

Findings

Droplet velocity shows a non-monotonic dependence on salt concentration.

Salt induces a transition from spherical to rod-like micelles.

A theoretical model accurately predicts propulsion speeds based on micellar properties.

Abstract

The autonomous motion of liquid crystal oil droplets in micellar media arises from spontaneous breaking of time reversal symmetry via nonlinear coupling between Marangoni stresses and surfactant transport. While this phenomenon has been widely studied, the influence of micellar solute structure remains unexplored. By modifying micellar architecture using a structure forming salt, we uncover a pronounced non monotonic dependence of droplet velocity on salt concentration. Increasing salt simultaneously raises the medium viscosity and drives a transition of micelles from spherical to rod-like or worm like morphologies. Using complementary experiments, we quantify the viscosity and micellar interaction lengthscale as functions of the salt to surfactant ratio and develop a theoretical model that consistently reproduces the measured propulsion speeds. Flow fields around the droplets are characterized by particle image velocimetry. Our results demonstrate that salt surfactant composition governs active droplet propulsion by jointly controlling micellar solute interaction lengthscales and medium viscosity.