Active Dipolar Colloids in Three Dimensions: Strings, Sheets, Labyrinthine Textures and Crystals

TL;DR

This study explores the complex phase behavior of active dipolar colloids in three dimensions, revealing diverse structures such as strings, sheets, labyrinths, and crystals, influenced by electric fields and particle interactions.

Contribution

It provides the first detailed experimental investigation of 3D active colloids at high concentrations, uncovering novel phases and dynamic behaviors not observed in 2D systems.

Findings

Identification of phase transitions from active gas to labyrinthine structures.

Observation of large-fluctuation active sheets resembling membranes.

Complex dependence of particle dynamics on electric field and state diagram position.

Abstract

Active matter exhibits striking behaviour reminiscent of living matter and molecular fluids, and has promising applications in drug delivery or mixing at the micron scale. Active colloidal systems provide important models with simple and controllable interactions amenable to theory and computer simulation. Experimental work is dominated by (quasi) two--dimensional (2d) systems at relatively low concentration, and rather less is known of the 3d case at concentrations pertinent to motility induced phase separation or to mimic morphogenesis. Here we investigate a 3d experimental system of active colloids in a dense suspension up to volume fractions of 0.5. The particles in our system are self-propelled in the lateral plane under an AC electric field. The field in addition induces an electric dipole, and the competition between activity and both steric and dipolar interactions gives rise to phase behaviour ranging from an active gas to a dynamic labyrinthine phase as well as dense tetragonal and hexagonal crystals. Intermediate volume fractions are characterised by two--dimensional sheets with large fluctuations reminiscent of active membranes. These active sheets break symmetry in a direction perpendicular to the applied field. Moreover, the relationship between electric field and the particle dynamics depends in a complex and unexpected manner upon the position in the state diagram.