Cluster and conquer: The morphodynamics of invasion of a compliant substrate by active rods

TL;DR

This study uses simulations to explore how active rod-shaped particles colonize soft substrates, revealing that clustering and substrate stiffness critically influence colonization patterns, growth dynamics, and furrow network morphology.

Contribution

It introduces a minimal 2D simulation model showing how motility-induced clustering affects colonization morphodynamics on compliant substrates.

Findings

Clustering enhances colony spread rate.

Colonization rate peaks at an optimal substrate stiffness.

Furrow networks exhibit fractal-like structures with stiffness-dependent dimensions.

Abstract

The colonisation of a soft passive material by motile cells such as bacteria is common in biology. The resulting colonies of the invading cells are often observed to exhibit intricate patterns whose morphology and dynamics can depend on a number of factors, particularly the mechanical properties of the substrate and the motility of the individual cells. We use simulations of a minimal 2D model of self-propelled rods moving through with a passive compliant medium consisting of particles that offer elastic resistance before being plastically displaced from their equilibrium positions. It is observed that the motility-induced clustering of active (self-propelled) particles is crucial for understanding the morphodynamics of colonisation. Clustering enables motile colonies to spread faster than they would have as isolated particles. The colonisation rate depends non-monotonically on substrate stiffness with a distinct maximum at a non-zero value of substrate stiffness. This is observed to be due to a change in the morphology of clusters. Furrow networks created by the active particles have a fractal-like structure whose dimension varies systematically with substrate stiffness but is less sensitive to particle activity. The power-law growth exponent of the furrowed area is smaller than unity, suggesting that, to sustain such extensive furrow networks, colonies must regulate their overall growth rate.