Nonreciprocal dynamics of active structures - Ariel Surya Boiardi (SISSA, Trieste, Italy)

Reciprocity is a property of linear physical systems that manifests as symmetry relations; breaking these constraints can open exciting new possibilities for both science and technology. Active materials, which convert external stimuli into mechanical work, provide a platform to explore nonreciprocal phenomena. Using polyelectrolyte (PE) hydrogels as a model active material, we investigate their dynamics under electric stimuli through theoretical, numerical, and experimental approaches.

The movements of aquatic micro-organisms need to break time-reversal symmetry in order to produce fluid flows and achieve locomotion, as dictated by the constraints of low Reynolds number hydrodynamics. Inspired by this, we demonstrated that flutter instability in polyelectrolyte (PE) hydrogel filaments can produce self-sustained nonreciprocal oscillations, enabling undulatory swimming under minimal actuation from a steady electric field [1].

Beyond instabilities, the responsiveness of PE hydrogels to environmental cues breaks spatial symmetry, enabling nonreciprocal transmission in both static and dynamic regimes. We focus on a periodic structure of PE hydrogel rods, exhibiting nonreciprocal behaviors, predicted by a simple mathematical model. Furthermore, the ability of PE hydrogel to harvest energy from the external stimulus allows to overcome viscous damping from the surrounding fluid and suppress attenuation, leading to unidirectional wave amplification [2].

[1] Boiardi, A. S. & Noselli, G. Minimal actuation and control of a soft hydrogel swimmer from flutter instability. Journal of the Mechanics and Physics of Solids 191, 105753 (2024).
[2] Boiardi, A. S. & Noselli, G. Nonreciprocal dynamics of active structures. In preparation

Contact: Emmanuel Siéfert