Tunable Assembly of Confined Janus Microswimmers in Sub-kHz Electic Fields under Gravity

TL;DR

This study demonstrates how electric field frequency can control the structural states of confined Janus microswimmers, enabling reversible transitions between gas-like and crystalline arrangements for potential reconfigurable material design.

Contribution

It introduces a method to manipulate microswimmer assemblies using AC electric fields, revealing new structural transformation mechanisms distinct from motility-induced phase separation.

Findings

Structural transitions controlled by electric field frequency

Reversible switching between gas-like and crystalline states

Distinct clustering mechanism from motility-induced phase separation

Abstract

Active systems comprising micron-sized self-propelling units, also termed microswimmers, are promising candidates for the bottom-up assembly of small structures and reconfigurable materials. Here we leverage field-driven colloidal assembly to induce structural transformations in dense layers of microswimmers driven by an alternating current (AC) electric field and confined in a microfabricated trap under the influence of gravity. By varying the electric field frequency, we realize significant structural transformations, from a gas-like state at high frequencies to dynamically rearranging dense crystalline clusters at lower frequencies, characterized by vorticity in their dynamics. We demonstrate the ability to switch between these states on-demand, showing that the clustering mechanism differs from motility-induced phase separation. Our results offer a valuable framework for designing high-density active matter systems with controllable structural properties, envisioned to advance the development of artificial materials with self-healing and reconfiguration capabilities.