@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 {56, title = {Nutations in growing plant shoots: The role of elastic deformations due to gravity loading}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {136}, year = {2020}, pages = {103702}, abstract = {

The effect of elastic deformations induced by gravity loading on the active circumnutation movements of growing plant shoots is investigated. We consider first a discrete model (a gravitropic spring-pendulum system) and then a continuous rod model which is analyzed both analytically (under the assumption of small deformations) and numerically (in the large deformation regime). We find that, for a choice of material parameters consistent with values reported in the available literature on plant shoots, rods of sufficient length may exhibit lateral oscillations of increasing amplitude, which eventually converge to limit cycles. This behavior strongly suggests the occurrence of a Hopf bifurcation, just as for the gravitropic spring-pendulum system, for which this result is rigorously established. At least in this restricted set of material parameters, our analysis supports a view of Darwin{\textquoteright}s circumnutations as a biological analogue to structural systems exhibiting flutter instabilities, i.e., spontaneous oscillations away from equilibrium configurations driven by non-conservative loads. Here, in the context of nutation movements of growing plant shoots, the energy needed to sustain oscillations is continuously supplied to the system by the internal biochemical machinery presiding the capability of plants to maintain a vertical pose.\ 

}, keywords = {Circumnutations, Flutter instability, Gravitropism, Hopf bifurcation}, doi = {10.1016/j.jmps.2019.103702}, author = {Agostinelli, Daniele and Lucantonio, Alessandro and Noselli, Giovanni and DeSimone, Antonio} }