Julia Yeomans OBE FRS

Abstract:

We develop a continuum theory to describe the collective dynamics of deformable epithelial cells, distinguishing the force-generating active filaments in the cells from their shape. The theory demonstrates how active flows driven by active filaments can create nematic domains and topological defects in the cell shape field. We highlight the role of the filament flow-aligning parameter, λQ, a rheological quantity that determines the response of the filaments to velocity gradients in the active flows, and plays a significant, to date unappreciated, role in determining the pattern of extensional and compressional active flows. In a contractile cell layer, local flows are expected to align elongated cells perpendicular to the active filaments. However, with increasing λQ, long-range correlations in the active turbulent flow field lead to extended regions where this alignment is parallel, consistent with recent experiments on confluent Madin-Darby canine kidney (MDCK) cell layers. Further, we distinguish defects in the filament director field, which contribute to the active driving, and those in the shape director field, measured in experiments, which are advected by the active flows. By considering the shape-filament orientation, we explain the unexpected motion of +1/2 defects towards their head in contractile cell layers, consistent with recent experiments on epithelial layers examining stress around shape defects.

Abstract:

We coarse-grain a model of closely packed ellipses that can vary their aspect ratio to derive continuum equations for materials comprising confluent deformable particles such as epithelial cell layers. We show that contractile nearest-neighbor interactions between ellipses can lead to their elongation and nematic ordering. Adding flows resulting from active hydrodynamic stresses produced by the particles also affects the aspect ratio and can result in active turbulence. Our results, which agree well with multiphase field simulations of deformable isotropic cells, provide a bridge between models that explicitly resolve cells and continuum theories of active matter.

Abstract:

We use a two-fluid model to study a confined mixture of an active nematic fluid and a passive isotropic fluid. We find that an extensile active fluid preferentially accumulates at a boundary if the anchoring is planar, whereas its boundary concentration decreases for homeotropic anchoring. These tendencies are reversed if the active fluid is contractile. We argue that the sorting results from gradients in the nematic order, and show that the behaviour can be driven by either imposed boundary anchoring or spontaneous anchoring induced by active flows. Our results can be tested by experiments on microtubule-kinesin motor networks, and may be relevant to sorting to the boundary in cell colonies or cancer spheroids.

Abstract:

We describe channel flows in a continuum model of deformable nematic particles. In a simple shear flow, deformability leads to a nonlinear coupling of strain rate and vorticity, and results in shape oscillations or flow alignment. The final steady state can depend on initial conditions, and we explain this behavior by considering a phase space representation of the dynamics. In Poiseuille flow, particle deformability and nematic elasticity induce banding, where particles near the walls are aligned, and those near the center of the channel oscillate in direction and shape. Our results show that particle deformability can lead to complex behavior even in simple flows, suggesting new microfluidic experiments.