Active chiral molecules in activity gradients

Abstract

While the behavior of active colloidal molecules is well studied now for 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 an activity gradient by the two particles. While one of the particles moves into higher activity regions, the other moves towards lower activity regions, 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 a 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 to any magnetic field.