# Competing Active and Passive Interactions Drive Amoeba-like Living Crystallites and Ordered Bands

## TL;DR

This study investigates how active and passive interactions in electrically-driven colloids lead to diverse collective phases, including crystallites, active matter, and band formations, revealing the microscopic mechanisms behind these behaviors.

## Contribution

The paper introduces a comprehensive experimental and simulation study of colloids under electric fields, uncovering novel phase behaviors driven by competing active and passive interactions.

## Key findings

- Electrohydrodynamic flows induce hexagonal crystallites at low fields.
- Self-propulsion causes dynamic active phases with shape-changing crystallites.
- Higher fields lead to an active gas and activity-driven band formation.

## Abstract

Swimmers and self-propelled particles are physical models for the collective behaviour and motility of a wide variety of living systems, such as bacteria colonies, bird flocks and fish schools. Such artificial active materials are amenable to physical models which reveal the microscopic mechanisms underlying the collective behaviour. Here we study colloids in a DC electric field. Our quasi-two-dimensional system of electrically-driven particles exhibits a rich and exotic phase behaviour. At low field strengths, electrohydrodynamic flows lead to self-organisation into crystallites with hexagonal order. Upon self-propulsion of the particles due to Quincke rotation, we find an ordered phase of active matter in which the motile crystallites constantly change shape and collide with one another. At higher field strengths, this "dissolves" to an active gas. We parameterise a particulate simulation model which reproduces the experimentally observed phases and, at higher field strengths predicts an activity-driven demixing to band-like structures.