# Self-Propelled Rods: Insights and Perspectives for Active Matter

## TL;DR

This paper reviews experimental and theoretical insights into self-propelled rods, highlighting their collective behaviors, pattern formations, and the bridging of different active matter regimes.

## Contribution

It provides a comprehensive survey of self-propelled rods, emphasizing emergent patterns and the connection between dry and wet active matter systems.

## Key findings

- Observation of diverse collective phenomena such as polar clusters and turbulence
- Identification of pattern formation mechanisms in dry and wet systems
- Bridging of polar particles and active nematics regimes

## Abstract

A wide range of experimental systems including gliding, swarming and swimming bacteria, in-vitro motility assays as well as shaken granular media are commonly described as self-propelled rods. Large ensembles of those entities display a large variety of self-organized, collective phenomena, including formation of moving polar clusters, polar and nematic dynamic bands, mobility-induced phase separation, topological defects and mesoscale turbulence, among others. Here, we give a brief survey of experimental observations and review the theoretical description of self-propelled rods. Our focus is on the emergent pattern formation of ensembles of dry self-propelled rods governed by short-ranged, contact mediated interactions and their wet counterparts that are also subject to long-ranged hydrodynamic flows. Altogether, self-propelled rods provide an overarching theme covering many aspects of active matter containing well-explored limiting cases. Their collective behavior not only bridges the well-studied regimes of polar self-propelled particles and active nematics, and includes active phase separation, but also reveals a rich variety of new patterns.