Geometrical control of interface patterning underlies active matter invasion

TL;DR

This study uncovers how active bacterial matter confined by deformable boundaries forms complex patterns driven by local curvature, revealing a new mode of active matter invasion and pattern engineering.

Contribution

It demonstrates that interface patterning in active matter is governed by collective curvature sensing, linking interface geometry to active stress distribution and pattern formation.

Findings

Ordered interfacial protrusions emerge with bacterial clusters behind them.

Hierarchical transition from protrusions to self-similar branching patterns.

Active stresses are enhanced near high-curvature regions, driving pattern formation.

Abstract

Interaction between active materials and the boundaries of geometrical confinement is key to many emergent phenomena in active systems. For living active matter consisting of animal cells or motile bacteria, the confinement boundary is often a deformable interface, and it has been unclear how activity-induced interface dynamics might lead to morphogenesis and pattern formation. Here we studied the evolution of bacterial active matter confined by a deformable boundary. We discovered that an ordered morphological pattern emerged at the interface characterized by periodically-spaced interfacial protrusions; behind the interfacial protrusions, bacterial swimmers self-organized into multicellular clusters displaying +1/2 nematic defects. Subsequently, a hierarchical sequence of transitions from interfacial protrusions to creeping branches allowed the bacterial active drop to rapidly invade surrounding space with a striking self-similar branch pattern. We found that this interface patterning is controlled by the local curvature of the interface, a phenomenon we denote as collective curvature sensing. Using a continuum active model, we revealed that the collective curvature sensing arises from enhanced active stresses near high-curvature regions, with the active length-scale setting the characteristic distance between the interfacial protrusions. Our findings reveal a protrusion-to-branch transition as a novel mode of active matter invasion and suggest a new strategy to engineer pattern formation of active materials.