Self-organization is a common way of forming functional structures in biology. It involves biochemical signalling pathways, triggered by a host of external conditions, that alter the mechnical properties of the individual constituents. These changes then propagate up in scale and, via changes in the self-organization, alter the biological function. In this work, we investigate self-organization and pattern formation due to self-propulsion in biological systems. The aim is to understand and map the collective dynamics in terms of mechanical properties of single constituents. We study dense ensembles of self-propelled vesicles that act as models for motile cells and ensembles of self-propelled semiflexible filaments that mimic actin filaments and microtubules in motility assays. Both systems are made of polar and active objects that possess extended shapes with an associated flexibility. We explore the collective dynamics in both systems as a function of activity, flexibility, and interactions between objects.
Epithelial tissue serves as barrier for tissues and organs. To achieve this function, epithelial cells are typically tightly-packed, spatially well-ordered, and non-motile. However, a set of conditions can turn epithelial cells motile. During vertebrate embryonic development, wound healing, and cancer metastasis, cells become motile to rearrange the tissue, heal the wound, or travel away from the primary tumor, respectively. In vitro experiments on motile cell monolayers furthermore revealed a jamming transition in which an initially motile, fluid-like tissue undergoes a dynamic arrest. We study such motility transitions of dense cell monolayers in a minimal model approach. We go beyond existing models by including finite extension and flexibility of cells. To this end, we develop a novel computational model of cells as active vesicles that incorporates cell motility, cell-cell adhesions, compressibility, and flexibility. Increasing motility strength and decreasing cell-cell adhesions, area compression modulus, and bending rigidity lead to fluidization of the monolayer. In between the jammed and completely fluid- like states, we identify an active turbulence regime where cell motion is dominated by the formation of vortices. We thus uncover deformability-driven motility transitions and predict an active turbulent state for motile cell monolayers.
In a second part, we study the collective behaviour of self-propelled semiflexible filaments by introducing self-propulsion as a constant magnitude force acting tangentially along the bonds of each filament. The combination of polymer properties, excluded-volume interactions, and self-propulsion leads to distinct phases as a function of rigidity, activity, and aspect ratio of individual filaments. We identify a transition from a free-swimming phase to a frozen steady state wherein strongly propelled filaments form spirals at a regime of low rigidity and high aspect ratio. Filaments form clusters of various sizes depending on rigidity and activity. In particular, we observe that filaments form small and transient clusters at low rigidities while stiffer filaments organize into giant clusters. However, as activity increases further, the clustering of filaments displays a reentrant phase behaviour where giant clusters melt, due to the strong propulsion forces bending the filaments.
Our results highlight the role of mechanical properties and the finite extent of the constituents on the collective motion patterns. Cells and filaments display different symmetry properties at high densities due to structural differences. Filaments show an effective nematic symmetry, which results in an active turbulence regime characterized by half-integer topological defects. Cells, with polar symmetry, exhibit an active turbulence phase dominated by vortices
