Solid-Liquid Transition of Deformable and Overlapping Active Particles

TL;DR

This study investigates how overlaps in deformable cell monolayers affect solid-liquid transitions, revealing a shift from continuous to discontinuous melting and linking cell extrusion to topological defects.

Contribution

It introduces a multi-phase field model accounting for overlaps, showing novel transition behaviors and the role of disclinations in cell extrusion.

Findings

Transition changes from continuous to discontinuous with overlaps.

Intermittent regime with alternating solid and liquid states.

Cell extrusion correlates with 5-fold disclinations.

Abstract

Experiments and theory have shown that cell monolayers and epithelial tissues exhibit solid-liquid and glass-liquid transitions. These transitions are biologically relevant to our understanding of embryonic development, wound healing, and cancer. Current models typically consider purely two-dimensional monolayers with no overlaps between neighboring cells. In reality, overlaps are important, and they may be precursors of cell extrusion -- a key biophysical process to maintain homeostasis in epithelial tissues. Here, we use a multi-phase field model to study the solid-liquid transition in a confluent monolayer of deformable cells which can overlap. When cells overlap rather than deform, we find that the melting transition changes from continuous to discontinuous, and that there is an intermittent regime close to the transition, where solid and liquid states alternate over time. By studying the dynamics of $5$- and $7$-fold disclinations in the hexagonal lattice formed by the cell centers, we observe that these correlate with spatial fluctuations in the cellular overlap, and that cell extrusion tends to initiate near $5$-fold disclinations.