Order and shape dependence of mechanical relaxation in proliferating active matter

TL;DR

This paper investigates how particle shape and growth dynamics influence collective mechanical relaxation and order in proliferating active matter, revealing shape-dependent regimes that alter traditional alignment behaviors.

Contribution

It introduces a novel regime with oblate particle growth, demonstrating how shape influences alignment and relaxation mechanisms in proliferating active systems.

Findings

Oblate growth can reverse flow-alignment behavior.

Shape-dependent relaxation pathways affect microdomain stability.

New regimes with modified orientation dynamics are identified.

Abstract

Collective dynamics in proliferating anisotropic particle systems arise from an interplay between growth, division, and mechanical interactions, often mediated by particle shape. In classical models of prolate, rod-like growth, flow-induced alignment and division geometry reinforce one another, leading to robust nematic order under confinement. Here we introduce a complementary regime by considering smooth convex particles whose geometry can be oblate for part or all of their growth cycle, creating a tunable competition between these two alignment mechanisms. Using agent-based simulations of elliptical and rounded-rectangular particles in both channel and open-domain geometries, we systematically vary the division aspect ratio to span regimes of cooperation and competition between ordering cues. We find that oblate growth can reverse classical flow-alignment, destabilize microdomain formation in intermediate regimes, and open up new regimes with modified microdomain dynamics in free expansion and sustained orientation dynamics in channel geometry. These findings are explained by an order- and shape-dependent mechanical relaxation interpretation that is supported by explicit measurements. This sheds new light on the available relaxation pathways and therefore provides key ingredients for effective descriptions of collective anisotropic proliferation dynamics.