MSC Seminar. Wednesday 23/10/2024. C. Patrick Royall (Gulliver, ESPCI): “Spatial organisation in active matter: amoebae, a labyrinth and a three-state system.”

We present results from three active matter model systems: Quincke Rollers (2d colloids) metaldielectric Janus particles (3d colloids) and zebrafish. In the bulk, Quincke Rollers exhibit an intriguing phase behavior in which two regimes of mesophase separation similar to passive systems such as diblock copolymers and short range attraction-long range repulsion systems are found. At low driving, activity suppresses phase separation while at high driving, activity-induced banding leads to mesophase separation [1]. Clusters of Quincke Rollers exhibit a hierarchy of novel bound states, reminiscent of excited states of molecules, only here the coupling is a combination of steric and hydrodynamic interactions [2,3].
In passive matter, dimensionality has a profound effect upon phase behaviour. To realise 3d active colloids, we use metal-dielectric Janus particles, and explore their dynamics and phase behaviour. The latter is somewhat akin to a passive system of dipolar colloids, which forms an array of polymorphic colloidal crystal structures. Beyond this already rich and complex behaviour, the active system forms novel long-lived structures such as sheets and a labyrinthine phase [4].