Abstract

Self-propelled particles are a fundamental constituent of active matter, be it biological or synthetic. Using electric fields, researchers have been able to create a model system of self-propelling particles, albeit confined to two dimensions. These particles are powered by an electrohydrodynamic instability called as Quincke rotation which converts spontaneous rotation to translation on a no-slip plane. Many such particles have been shown to interact with each other electrohydrodynamically and give rise to collective motion (Bricard et al, Nature (2013)).

Naturally, this leads us to ask the question: what happens in a three-dimensional fluid environment? Do the particles display collective motion, i.e. spontaneously flow in a particular direction? I will show how a fundamental hydrodynamic phenomenon prevents particles from showing any collective motion in three-dimensions. This phenomenon remains hidden as a secondary effect in driven systems like suspensions in shear flow and hence has not been discussed in literature. It only emerges as a leading order effect in suspensions where particles can spontaneously spin in any direction.