Collective motion in repulsive self-propelled particles in confined geometries

Adhesion-stabilizing long-distance transport of cells on tissue surface

The stable transport of migrating eukaryotic cells is essential in organ development and repair processes. However, the mechanism that preserves transport stability over long distances in organs is not fully understood. As the driving mechanism of cell migration, the expressions of heterophilic cell-cell adhesion between moving cells and scaffolding tissue have been observed in such transport. In this paper, we theoretically investigate this heterophilic adhesion, which is persistently polarized in the migrating cell, as a possible transport stabilization mechanism. The adhesion was examined on the basis of the cellular Potts model, and our results confirm the stabilization of the transport to be an effect of the persistence.

Spontaneously Flowing Crystal of Self-Propelled Particles

We experimentally and numerically study the structure and dynamics of a monodisperse packing of spontaneously aligning self-propelled hard disks. The packings are such that their equilibrium counterparts form perfectly ordered hexagonal structures. Experimentally, we first form a perfect crystal in a hexagonal arena which respects the same crystalline symmetry. Frustration of the hexagonal order, obtained by removing a few particles, leads to the formation of a rapidly diffusing "droplet." Removing more particles, the whole system spontaneously forms a macroscopic sheared flow, while conserving an overall crystalline structure. This flowing crystalline structure, which we call a "rheocrystal," is made possible by the condensation of shear along localized stacking faults. Numerical simulations very well reproduce the experimental observations and allow us to explore the parameter space. They demonstrate that the rheocrystal is induced neither by frustration nor by noise. They further show that larger systems flow faster while still remaining ordered.