Self-aligning polar active matter

Self-alignment describes the property of a polar active unit to align or antialign its orientation toward its velocity. In contrast to mutual alignment, where the headings of multiple active units tend to directly align with each other—as in the Vicsek model—self-alignment impacts the dynamics at the individual level by coupling the rotation and displacements of each active unit. This enriches the dynamics even in the absence of interactions and allows, for example, a single self-propelled particle to orbit in a harmonic potential. At the collective level, self-alignment modifies the nature of the transition to collective motion already in the mean-field description and can lead to other forms of self-organization such as collective actuation in dense or solid elastic assemblies of active units. This has significant implications for the study of dense biological systems, metamaterials, and swarm robotics. Here a number of models are reviewed that were introduced independently to describe the previously overlooked property of self-alignment and some of its experimental realizations are identified. The aim of this review is threefold: (i) to underline the importance of self-alignment in active systems, especially in the context of dense populations of active units and active solids; (ii) to provide a unified mathematical and conceptual framework for the description of self-aligning systems; and (iii) to discuss the common features and specific differences of the existing models of self-alignment. The review concludes by discussing promising research avenues in which the concept of self-alignment could play a significant role.

Reviews Of Modern Physics

By: Paul Baconnier, Olivier Dauchot, Vincent Démery, Gustavo Düring, Silke Henkes, Cristián Huepe, and Amir Shee.

Rev. Mod. Phys. 97, 015007 – Published 20 March, 2025

DOI: https://doi.org/10.1103/RevModPhys....