@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 = {<p align="justify">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.\&nbsp;</p>},
	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}
}
@article {67,
	title = {Transient shape morphing of active gel plates: Geometry and physics},
	journal = {Soft Matter},
	volume = {18},
	year = {2022},
	pages = {5867-5876},
	chapter = {5867},
	abstract = {<p align="justify">The control of shape in active structures is a key problem for the realization of smart sensors and actuators, which often draw inspiration from natural systems. In this context, slender structures, such as thin plates, have been studied as a relevant example of shape morphing systems where curvature is generated by in-plane incompatibilities. In particular, in hydrogel plates these incompatibilities can be programmed at fabrication time, such that a target configuration is attained at equilibrium upon swelling or shrinking. While these aspects have been examined in detail, understanding the transient morphing of such active structures deserves further investigation. In this study, we develop a geometrical model for the transient shaping of thin hydrogel plates by extending the theory of non-Euclidean plates. We validate the proposed model using experiments on gel samples that are programmed to reach axisymmetric equilibrium shapes. Interestingly, our experiments show the emergence of non-axisymmetric shapes for early times, as a consequence of boundary layer effects induced by solvent transport. We rationalize these observations using numerical simulations based on a detailed poroelastic model. Overall, this work highlights the limitations of purely geometrical models and the importance of transient, reduced theories for morphing plates that account for the coupled physics driving the evolution of shape. Computational approaches employing these theories will allow to achieve accurate control on the morphing dynamics and ultimately advance 4D printing technologies.\&nbsp;</p>},
	keywords = {active materials, hydrogel plates, Transient morphing},
	doi = {10.1039/D2SM00669C},
	author = {Valentina  Damioli and Erik Zorzin and Antonio DeSimone and Giovanni Noselli and Alessandro Lucantonio}
}
