Many biological systems form colonies at high density. Passive granular systems will be jammed at such densities, yet for the survival of biological systems it is crucial that they are dynamic. We construct a phase diagram for a system of active particles interacting via Vicsek alignment, and vary the density, self-propulsion force, and orientational noise. We find that the system exhibits four different phases, characterized by transitions in the effective diffusion constant and in the orientational order parameter. Our simulations show that there exists an optimal noise such that particles require a minimal force to unjam, allowing for rearrangements.

Due to an unfortunate error occurred during production, refs. [21] and [22] have been wrongly renumbered in the bibliography on the last page of the paper. The correct numbering is given here below: [21] Stukowski A., Visualization and analysis of atomistic simulation data with OVITO-the Open Visualization Tool, Modelling Simul. Mater. Sci. Eng., 18 (2010) 015012. [22] Kosterlitz J. M., J. Phys. C: Solid State Phys., 7 (1974) 1046. The reference numbering within the main text need no correction. We deeply apologize to the authors for the unwanted mistake.

A key process in the life of any multicellular organism is its development from a single egg into a full grown adult. The first step in this process often consists of forming a tissue layer out of randomly placed cells on the surface of the egg. We present a model for generating such a tissue, based on mechanical interactions between the cells, and find that the resulting cellular pattern corresponds to the Voronoi tessellation of the nuclei of the cells. Experimentally, we obtain the same result in both fruit flies and flour beetles, with a distribution of cell shapes that matches that of the model, without any adjustable parameters. Finally, we show that this pattern is broken when the cells grow at different rates.