Collective Dynamics and Motility of Soft Elliptical Particles

Swarming behaviour is abundant in nature. Over many different length scales, in for example herds, flocking birds and swimming bacteria, roughly identical individuals interact locally to achieve group behaviour. The similarities between these examples suggests the existence of a general underlying principle. We propose here a local interaction model for self-propelling, elliptical particles that results in collective motion. Any particle interacts with its neighbours only, experiences noise on its orientation and pushes inwards if it is in the outer layer of the group. Initially, alignment between particles is the result of steric repulsion. We observe two types of group behaviour. The first type is a migrating group, where particles in the bulk are aligned over large length scales, but do not rearrange. The second type has very little net motion. The elliptical particles form smaller regions of aligned and antialigned particles, effectively cancelling the net motion of the group. Finally, we compare the group behaviour of elliptical particles to circular ones and investigate the importance of polar alignment. We conclude that polar alignment is a requirement for large-scale collective dynamics, like collective migration and rotation.