Rectification in a mixture of active and passive particles subject to a ratchet potential

TL;DR

This study uses simulations to explore how mixtures of active and passive particles in a ratchet potential exhibit current rectification and reversal, influenced by activity levels, temperature, and particle interactions.

Contribution

It reveals the conditions under which current reversal occurs in active-passive particle mixtures, highlighting the role of temperature and particle interactions in rectification phenomena.

Findings

Current can reverse direction depending on particle temperature and concentration.

High-temperature passive particles can push active particles, causing current reversal.

Active-passive mixtures exhibit complex rectification behavior influenced by activity and thermal effects.

Abstract

We study by simulation a mixture of active (run-and-tumble) and passive (Brownian) particles with repulsive exclusion interactions in one dimension, subject to a ratchet (smoothed sawtooth) potential. Such a potential is known to rectify active particles at one-body level, creating a net current in the `easy direction'. This is the direction in which one encounters the lower maximum force en route to the top of a potential barrier. The exclusion constraint results in single-file motion, so the mean velocities of active and passive particles are identical; we study the effects of activity level, Brownian diffusivity, particle size, initial sequence of active and passive particles, and active/passive concentration ratio on this mean velocity (i.e., the current per particle). We show that in some parameter regimes the sign of the current is reversed. This happens when the passive particles are at high temperature and so would cross barriers relatively easily, and without rectification, except that they collide with `cold' active ones, which would otherwise be localized near the potential minima. In this case, the reversed current arises because hot passive particles push cold active ones preferentially in the direction with the lower spatial separation between the bottom and top of the barrier. A qualitatively similar mechanism operates in a mixture containing passive particles of two very different temperatures, although there is no quantitative mapping between that case and the systems studied here.