Active elastic solids – a classification for collective actuations - Paul Baconnier (AMOLF, Amsterdam)

Active solids consist of elastically coupled out-of-equilibrium units performing work. They are central to autonomous processes in biological systems, e.g. locomotion, self-oscillations and morphogenesis. Yet, the feedback mechanism between elastic and active forces, and the possible emergence of collective behaviors in such systems remain poorly understood. We take advantage of centimetric models of self-propelled active units and introduce a minimal realization of an active elastic solid. Polar active agents exert forces on the nodes of a two-dimensional elastic lattice, and the resulting displacement field nonlinearly reorients the active agents. From this so-called elasto-active feedback emerges numerous new collective behaviors, which shall be called collective actuation.

In this talk, I will show how the structure's vibrational properties control the emergence and the properties of collective actuation. Crucially, collective actuation is selective: only a few normal modes are activated by the active dynamics, and they are not necessarily the lowest energy ones. Combining experiments with the numerical and theoretical analysis of an agents model, we unveil the bifurcation scenario and the selection mechanism by which the collective actuation takes place. Eventually, we propose a classification of collective actuations, and draw analogies with the self-oscillating behaviors observed in living active solids, such as cell monolayers and bacterial bio-films.