Ratchet Effects in Active Matter Systems

TL;DR

This paper reviews the development of active matter ratchets, highlighting how self-propelled particles like bacteria and colloids can generate directed motion on asymmetric substrates without external forces, with potential applications in sorting and micromachines.

Contribution

It provides a comprehensive overview of active matter ratchets, emphasizing recent advances, collective effects, and future research directions involving deformable systems and non-dissipative phenomena.

Findings

Active matter can produce net motion without external forces.

Collective effects can reverse ratchet directions.

Active ratchets can transport passive particles.

Abstract

Ratchet effects can arise for single or collectively interacting Brownian particles on an asymmetric substrate when a net dc transport is produced by an externally applied ac driving force or by periodically flashing the substrate. Recently, a new class of active ratchet systems has been realized through the use of active matter, which are self-propelled units that can be biological or non-biological in nature. When active materials such as swimming bacteria interact with an asymmetric substrate, a net dc directed motion can arise even without external driving, opening a wealth of possibilities such as sorting, cargo transport, or micromachine construction. We review the current status of active matter ratchets for swimming bacteria, cells, active colloids, and swarming models, focusing on the role of particle-substrate interactions. We describe ratchet reversals produced by collective effects and the use of active ratchets to transport passive particles. We discuss future directions including deformable substrates or particles, the role of different swimming modes, varied particle-particle interactions, and non-dissipative effects.