Anomalous mechanics of polar active solids.

Active solids pave the way for novel mechanical responses, such as the so-called “odd elasticity”. For such response to take place, the individual agent must obey a chiral dynamics. Accordingly, experimental investigations have so far focused on materials composed of active spinners [1, 2,3], where the spinning vector is orthogonal to the space describing the material.

Here, I will discuss the mechanical response of another kind of active solids based on polar active particles. In that case, the propulsion vector is embedded in the same physical space as the elastic strain, allowing for a non-trivial elasto-active coupling, e.g. polar particles can reorient in response to displacements, which active spinner cannot. In the past years, it was demonstrated that for a strong enough activity, such solids experience spontaneous collective actuation dynamics [4-5]. For low enough activity, the solid is at rest and ressemble a bona-fide passive elastic solid. However we show that the mechanical response to a simple traction experiment is truly exotic: not only forces transverse to the direction of traction are non zero, but they also exhibit a rich oscillatory dynamics.

[1] Bililign, E. S., et al. Nature Phys. 18.2 (2022), 212–218.

[2] Tan, T. H., Mietke, et al. (2022). Nature, 607(7918), 287-293.

[3] Brandenbourger, M., et al. Nature Comm. 10.1 (2019), 1–8.

[4] Baconnier, P., et al. Nature Phys. 503.18 (2022), 1234–1239.

[5] Hernández-López, C.et al. Phys. Rev. Lett. 132, 238303 (2024)