Couple stresses and discrete potentials in the vertex model of cellular monolayers

TL;DR

This paper investigates the discrete stress and couple stresses in vertex models of cellular monolayers, developing potential theory to analyze local stress patterns and the effects of defects and deformations.

Contribution

It introduces a discrete potential theory for vertex models, deriving scalar stress functions and analyzing antisymmetric stress components and couple stresses in disordered tissues.

Findings

Discrete potential theory reveals broad-banded spectra of stress functions.

Couple stresses relate to pressure differences and non-affine deformations.

Simulations show non-affine deformations significantly influence couple stresses.

Abstract

The vertex model is widely used to simulate the mechanical properties of confluent epithelia and other multicellular tissues. This inherently discrete framework allows a Cauchy stress to be attributed to each cell, and its symmetric component has been widely reported, at least for planar monolayers. Here we consider the stress attributed to the neighbourhood of each tricellular junction, evaluating in particular its leading-order antisymmetric component and the associated couple stresses, which characterise the degree to which individual cells experience (and resist) in-plane bending deformations. We develop discrete potential theory for localised monolayers having disordered internal structure and use this to derive the analogues of Airy and Mindlin stress functions. These scalar potentials typically have broad-banded spectra, highlighting the contributions of small-scale defects and boundary-layers to global stress patterns. An affine approximation attributes couple stresses to pressure differences between cells sharing a trijunction, but simulations indicate an additional role for non-affine deformations.