Active chiral molecules in activity gradients

TL;DR

This paper investigates how active chiral dimers respond to activity gradients, revealing a transition from antichemotactic to chemotactic behavior driven by active torques and cooperative particle exploration.

Contribution

It provides an analytical and comparative study of active chiral dimers in activity gradients, highlighting the role of active torques in chemotactic switching and contrasting with charged Brownian particles under magnetic fields.

Findings

Increasing torque induces a switch from antichemotactic to chemotactic behavior.

Active dimers explore activity gradients cooperatively, leading to net bias.

Charged active particles under magnetic fields do not exhibit the same chemotactic behavior.

Abstract

While the behavior of active colloidal molecules is well studied by now for a constant activity, the effect of activity gradients is much less understood. Here we explore one of the simplest molecules in activity gradients, namely active chiral dimers composed of two particles with opposite active torques of the same magnitude. We show analytically that with increasing torque, the dimer switches its behavior from antichemotactic to chemotactic. The origin of the emergent chemotaxis is the cooperative exploration of activity gradient by the two particles. While one of the particles moves into higher activity regions, the other moves towards lower activity region resulting in a net bias in the direction of higher activity. We do a comparative study of chiral active particles with charged Brownian particles under magnetic field and show that despite the fundamental similarity in terms of their odd-diffusive behavior, their dynamics and chemotactic behavior are generally not equivalent. We demonstrate this explicitly in a dimer composed of oppositely charged active particles, which remains antichemotactic for any magnetic field.