Active Matter at high density

Active matter is a rapidly growing field that has potential applications to biological systems from the organism scale to the sub-cellular level. A key outstanding question relevant e.g. to biological tissues remains the behaviour of active particle systems at high density. We perform a numerical study of a dense collection of self-propelled particles with soft repulsive interactions in two dimensions. In a first system with alignment, we observe a jammed phase at high density and low self-propulsion speed. The dynamics of this phase is controlled by the low-frequency modes of the underlying jammed packing. The non-aligning system was recently shown to exhibit active phase separation in two dimensions in the absence of any attractive interaction or breaking of the orientational symmetry. We construct a phase diagram in terms of activity and packing fraction and identify three distinct regimes: a homogeneous liquid with anomalous cluster size distribution, a phase-separated state both at high and at low density, and a frozen phase. Finally, I will briefly discuss two applications of this area: Active matter on a Sphere (with Rastko Sknepnek, Dundee), and Cellular Migration in the corneal epithelium (with Martin Collinson, Aberdeen).