Emergent clusters in strongly confined systems

TL;DR

This study investigates how strong confinement influences the emergent structures and density fluctuations in driven colloidal suspensions, revealing that boundaries significantly impact mesoscale organization through flow-induced effects.

Contribution

The paper demonstrates that confinement induces large-scale density fluctuations in driven colloidal systems, with experimental and simulation results showing boundary effects on mesoscale structures.

Findings

Large-scale density fluctuations depend on confinement degree.

Simulations match experimental pattern quantitatively.

Flow generated by rotating particles causes mesoscale structure changes.

Abstract

Driven suspensions, where energy is input at a particle scale, are a framework for understanding general principles of out-of-equilibrium organization. A large number of simple interacting units can give rise to non-trivial structure and hierarchy. Rotationally driven colloidal particles are a particularly nice model system for exploring this pattern formation, as the dominant interaction between the particles is hydrodynamic. Here, we use experiments and large-scale simulations to explore how strong confinement alters dynamics and emergent structure at the particle scale in these driven suspensions. Surprisingly, we find that large-scale (many times the particle size) density fluctuations emerge as a result of confinement, and that these density fluctuations sensitively depend on the degree of confinement. We extract a characteristic length scale for these fluctuations, demonstrating that the simulations quantitatively reproduce the experimental pattern. Moreover, we show that these density fluctuations are a result of the large-scale recirculating flow generated by the rotating particles inside a sealed chamber. This surprising result shows that even when system boundaries are far away, they can cause qualitative changes to mesoscale structure and ordering.