PHD Defense - Paul Baconnier - 23/01/2023 - Active elastic solids: collective motion, collective actuation, and polarization

Paul Baconnier’s PhD Defense, conducted under the supervision of Olivier Dauchot, will take place on:
Monday January 23rd at 3:30pm
IPGG amphitheatre
6 Rue Jean Calvin
75005 Paris

Active elastic solids: collective motion, collective actuation, and polarization

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. Moreover, their shape-preserving property and their intrinsic non-equilibrium nature make active solids a promising framework to create multifunctional metamaterials with bona fide autonomy. Yet, the feedback mechanism between elastic and active forces, and the possible emergence of collective behaviors remains 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.

In this presentation, I will start by introducing collective actuation, a new collective behavior specific to active solids, that takes place when activity is large enough. I will discuss three different collective actuation regimes observed in experiments and numerical simulations, and propose a framework to understand better the nature of these self-oscillations and how they emerge. We will see that most of the physics can be understood by analyzing the behavior of a single particle trapped in elliptic harmonic potentials. We also propose a hydrodynamic theory of active solids to describe their large-scale properties, and analyze some of its consequences.

Altogether, this work attempt to unveil the nature of the oscillations observed in dense living aggregates, like dense cell monolayers, dense bacterial suspensions, bacterial bio-films, and much more.