The nonlinear motion of cells subject to external forces

TL;DR

This paper models how active matter droplets respond non-linearly to boundary forces, revealing distinct motion modes and sharp transitions influenced by different self-propulsion mechanisms.

Contribution

It introduces a minimal model for active matter cell-like droplets under external forces, highlighting non-linear responses and mode transitions based on propulsion mechanisms.

Findings

Two main motion modes: long thin with zero bulk traction and parabolic with finite traction.

Sharp transition between modes depending on force strength and type.

Indications of drop break-up under large stretching forces.

Abstract

To develop a minimal model for a cell moving in a crowded environment such as in tissue, we investigate the response of a liquid drop of active matter moving on a flat rigid substrate to forces applied at its boundaries. We consider two different self-propulsion mechanisms, active stresses and treadmilling polymerisation, and we investigate how the active drop motion is altered by these surface forces. We find a highly non-linear response to forces that we characterise using drop velocity, drop shape, and the traction between the drop and the substrate. Each self-propulsion mechanism gives rise to two main modes of motion: a long thin drop with zero traction in the bulk, mostly occurring under strong stretching forces, and a parabolic drop with finite traction in the bulk, mostly occurring under strong squeezing forces. In each case there is a sharp transition between parabolic, and long thin drops as a function of the applied forces and indications of drop break-up where large forces stretch the drop.