Two-phase crystallisation in a carpet of inertial spinners

TL;DR

This paper investigates how hydrodynamic interactions among inertial spherical spinners lead to concentration-dependent crystallization patterns, including uniform hexagonal structures and phase separation at higher densities.

Contribution

It reveals the role of non-monotonic hydrodynamic repulsion and spinning frequency variations in the crystallization and phase separation of inertial spinners.

Findings

Rapid formation of uniform hexagonal structures at semi-dilute concentrations

Coexistence of high and low density spinner phases at higher surface coverage

Crystallization driven by concentration-dependent hydrodynamic interactions

Abstract

We study the dynamics of torque driven spherical spinners settled on a surface, and demonstrate that hydrodynamic interactions at finite Reynolds numbers can lead to a concentration dependent and non-uniform crystallisation. At semi-dilute concentrations, we observe a rapid formation of a uniform hexagonal structure in the spinner monolayer. We attribute this to repulsive hydrodynamic interactions created by the secondary flow of the spinning particles. Increasing the surface coverage leads to a state with two co-existing spinner densities. The uniform hexagonal structure deviates into a high density crystalline structure surrounded by a continuous lower density hexatically ordered state. We show that this phase separation occurs due to a non-monotonic hydrodynamic repulsion, arising from a concentration dependent spinning frequency.