Tractionless Self-Propulsion of Active Drops

TL;DR

This paper introduces a novel tractionless self-propulsion mechanism for active liquid drops on substrates, driven by nematic director field configurations, with potential implications for biological cell migration.

Contribution

It demonstrates, through analytical and numerical methods, a new tractionless self-propulsion mode for active nematic drops, linking topological nematic configurations to motion.

Findings

Identifies a simple self-propelling solution with no surface traction.

Shows the direction of motion is controlled by nematic director winding.

Suggests relevance to efficient cell migration in tissues.

Abstract

We report on a new mode of self-propulsion exhibited by compact drops of active liquids on a substrate which, remarkably, is tractionless, i.e., which imparts no mechanical stress locally on the surface. We show, both analytically and by numerical simulation, that the equations of motion for an active nematic drop possess a simple self-propelling solution, with no traction on the solid surface and in which the direction of motion is controlled by the winding of the nematic director field across the drop height. The physics underlying this mode of motion has the same origins as that giving rise to the zero viscosity observed in bacterial suspensions. This topologically protected tractionless self-propusion provides a robust physical mechanism for efficient cell migration in crowded environments like tissues.