@article {71, title = {Breaking the left-right symmetry in fluttering artificial cilia that perform nonreciprocal oscillations}, journal = {Meccanica}, volume = {In press}, year = {2024}, abstract = {

Recent investigations on active materi- als have introduced a new paradigm for soft robotics by showing that a complex response can be obtained from simple stimuli by harnessing dynamic instabili- ties. In particular, polyelectrolyte hydrogel filaments actuated by a constant electric field have been shown to exhibit self-sustained oscillations as a consequence of flutter instability. Owing to the nonreciprocal nature of the emerging oscillations, these\ artificial cilia\ are able to generate flows along the stimulus. Building upon these findings, in this paper we pro- pose a design strategy to break the left-right symme- try in the generated flows, by endowing the filament with a natural curvature at the fabrication stage. We develop a mathematical model based on morphoe- lastic rod theory to characterize the stability of the equilibrium configurations of the filament, proving the persistence of flutter instability. We show that the emerging oscillations are nonreciprocal and gener- ate thrust at an angle with the stimulus. The results we find at the level of the single cilium open new\ perspectives on the possible applications of artificial ciliary arrays in soft robotics and microfluidics.\ 

}, keywords = {Active matter, Artificial cilia, Flutter instability, left-right symmetry breaking, soft-robotics}, doi = {10.1007/s11012-024-01765-7}, author = {Ariel S. Boiardi and Roberto Marchello} } @article {69, title = {Nonreciprocal oscillations of polyelectrolyte gel filaments subject to a steady and uniform electric field}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {173}, year = {2023}, pages = {105225}, abstract = {

Soft actuators typically require time-varying or spatially modulated control to be operationally effective. The scope of the present paper is to show, theoretically and experimentally, that a natural way to overcome this limitation is to exploit mechanical instabilities. We report experiments on active filaments of polyelectrolyte (PE) gels subject to a steady and uniform electric field. A large enough intensity of the field initiates the motion of the active filaments, leading to periodic oscillations. We develop a mathematical model based on morphoelasticity theory for PE gel filaments beating in a viscous fluid, and carry out the stability analysis of the governing equations to show the emergence of flutter and divergence instabilities for suitable values of the system{\textquoteright}s parameters. We confirm the results of the stability analysis with numerical simulations for the nonlinear equations of motion to show that such instabilities may lead to periodic self-sustained oscillations, in agreement with experiments. The key mechanism that underlies such behaviour is the capability of the filament to undergo active shape changes depending on its local orientation relative to the external electric field, in striking similarity with gravitropism, the mechanism that drives shape changes in plants via differential growth induced by gravity. Interestingly, the resulting oscillations are nonreciprocal in nature, and hence able to generate thrust and directed flow at low Reynolds number. The exploitation of mechanical instabilities in soft actuators represents a new avenue for the advancement in engineering design in fields such as micro-robotics and micro-fluidics.\ 

}, keywords = {Artificial cilia, Flutter and divergence instability, Polyelectrolyte gels, Self-sustained oscillations}, doi = {10.1016/j.jmps.2023.105225}, author = {Giancarlo Cicconofri and Valentina Damioli and Giovanni Noselli} }