@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 {70, title = {Flutter instability in solids and structures, with a view on biomechanics and metamaterials}, journal = {Proceedings of the Royal Society A}, volume = {479}, year = {2023}, pages = {20230523}, abstract = {

The phenomenon of oscillatory instability called "flutter" was observed in aeroelasticity and rotor dynamics about a century ago. Driven by a series of applications involving non-conservative elasticity theory at different physical scales, ranging from nanomechanics to the mechanics of large space structures and including biomechanical problems of motility and growth, research on flutter is experiencing a new renaissance. A review is presented of the most notable applications and recent advances in fundamentals, both theoretical and experimental aspects, of flutter instability and Hopf bifurcation. Open problems, research gaps and new perspectives for investigations are indicated.

}, keywords = {elasticity, Hopf bifurcation, non-conservative systems, non-Hermitian mechanics, non-holonomic constraints}, doi = {10.1098/rspa.2023.0523}, author = {Davide Bigoni and Francesco Dal Corso and Oleg N. Kirillov and Diego Misseroni and Giovanni Noselli and Andrea Piccolroaz} } @article {58, title = {On polymer network rupture in gels in the limit of very slow straining or a very slow crack propagation rate}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {136}, year = {2020}, pages = {103754}, abstract = {

The J-integral is formulated in a direct manner for a gel consisting of a cross-linked polymer network and a mobile solvent. The form of the J-integral is given for a formulation that exploits the Helmholtz energy density of the gel and expressions are provided for it in both the unswollen reference configuration of the polymer network and in the current swollen configuration of the gel when small strains are superimposed on the swollen state. Similarly, the form of the J-integral is developed for an approach that exploits the Landau energy density of the gel and its reference and current configuration expressions are also developed. The Flory-Rehner model of the gel is used to obtain expressions for both the densities of Helmholtz energy and the Landau energy, with the chemical potential of the solvent derived from the Helmholtz energy used in the Legendre transformation that generates the Landau energy. Both the Helmholtz and Landau energies are expanded asymptotically for small strains superimposed on the swollen state of the gel. The results for the various forms of the energies are then used to obtain the elasticity law and the incompressibility constraint for the gel, each derived from both the Helmholtz and the Landau energies. The results are then inserted into the J-integral and fracture mechanics insights obtained for the rapid and slow loading of a gel body with a stationary crack and for a gel body with a crack that is experiencing slow, steady propagation. It is found that the Landau energy form of the J-integral is particularly useful for the slow loading of stationary cracks and for the slow steady propagation of the crack. It is noted that solvent flux during crack growth can cause an increase in the effective fracture toughness of the gel. However, it is found that there is an absence of such diffusional toughening in the rapidly loaded stationary crack case, the very slowly loaded stationary crack case and for the crack experiencing extremely slow but steady propagation. It is further found that, for cracks propagating very slowly, diffusional toughening rises linearly with crack propagation rate up to a critical crack growth rate, above which the diffusional toughening becomes insensitive to the crack propagation rate. The critical crack propagation rate for this transition is found to be dependent on the linear dimension of the gel body and on constitutive parameters for the gel elasticity and solvent diffusion.\ 

}, keywords = {J-integral, polymer gel, Rupture, Slow crack growth, Toughness}, doi = {10.1016/j.jmps.2019.103754}, author = {Robert M McMeeking and Alessandro Lucantonio and Giovanni Noselli and Vikram S Deshpande} } @article {53, title = {A fluorescent dye method suitable for visualization of one or more rat whiskers}, journal = {Bio-protocol}, volume = {8}, year = {2018}, chapter = {e2749}, abstract = {

Visualization and tracking of the facial whiskers is critical to many studies of rodent behavior. High-speed videography is the most robust methodology for characterizing whisker kinematics, but whisker visualization is challenging due to the low contrast of the whisker against its background. Recently, we showed that fluorescent dye(s) can be applied to enhance visualization and tracking of whisker(s) (Rigosa et al., 2017), and this protocol provides additional details on the technique.\ 

}, keywords = {barrel cortex, Dye, Fluorescence, Tactile perception, Tracking, Whisker}, doi = {10.21769/BioProtoc.2749}, author = {Jacopo Rigosa and Alessandro Lucantonio and Giovanni Noselli and Arash Fassihi and Fabrizio Manzino and Francesca Pulecchi and Mathew E Diamond} } @article {44, title = {Dye-enhanced visualization of rat whiskers for behavioral studies}, journal = {eLife}, volume = {6:e25290}, year = {2017}, abstract = {

Visualization and tracking of the facial whiskers is required in an increasing number of rodent studies. Though many approaches have been employed, only high-speed videography has proven adequate for measuring whisker motion and deformation during interaction with an object. However, whisker visualization and tracking is challenging for multiple reasons, primary among them the low contrast of the whisker against its background. Here we demonstrate a fluorescent dye method suitable for visualization of one or more rat whiskers. The process makes the dyed whisker(s) easily visible against a dark background. The coloring does not influence the behavioral performance of rats trained on a vibrissal vibrotactile discrimination task, nor does it affect the whiskers{\textquoteright} mechanical properties.

}, keywords = {behavioral studies, dye-enhanced visualization, rat whiskers}, doi = {10.7554/eLife.25290}, author = {Jacopo Rigosa and Alessandro Lucantonio and Giovanni Noselli and Arash Fassihi and Erik Zorzin and Fabrizio Manzino and Francesca Pulecchi and Mathew E Diamond} } @article {41, title = {Poroelastic toughening in polymer gels: A theoretical and numerical study}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {94}, year = {2016}, pages = {33-46}, abstract = {

We explore the Mode I fracture toughness of a polymer gel containing a semi-infinite, growing crack. First, an expression is derived for the energy release rate within the linearized, small-strain setting. This expression reveals a crack tip velocity-independent toughening that stems from the poroelastic nature of polymer gels. Then, we establish a poroelastic cohesive zone model that allows us to describe the micromechanics of fracture in gels by identifying the role of solvent pressure in promoting poroelastic toughening. We evaluate the enhancement in the effective fracture toughness through asymptotic analysis. We confirm our theoretical findings by means of numerical simulations concerning the case of a steadily propagating crack. In broad terms, our results explain the role of poroelasticity and of the processes occurring in the fracturing region in promoting toughening of polymer gels.

}, keywords = {crack propagation, fracture, polymer gel, swelling, toughening}, doi = {10.1016/j.jmps.2016.04.017}, author = {Giovanni Noselli and Alessandro Lucantonio and Robert M McMeeking and Antonio DeSimone} } @article {31, title = {Reverse engineering the euglenoid movement}, journal = {Proceedings of the National Academy of Sciences of USA}, volume = {109}, year = {2012}, pages = {17874{\textendash}17879}, abstract = {

Euglenids exhibit an unconventional motility strategy amongst unicellular eukaryotes, consisting of large-amplitude highly concerted deformations of the entire body (euglenoid movement or metaboly). A plastic cell envelope called pellicle mediates these deformations. Unlike ciliary or flagellar motility, the biophysics of this mode is not well understood, including its efficiency and molecular machinery. We quantitatively examine video recordings of four euglenids executing such motions with statistical learning methods. This analysis reveals strokes of high uniformity in shape and pace. We then interpret the observations in the light of a theory for the pellicle kinematics, providing a precise understanding of the link between local actuation by pellicle shear and shape control. We systematically understand common observations, such as the helical conformations of the pellicle, and identify previously unnoticed features of metaboly. While two of our euglenids execute their stroke at constant body volume, the other two exhibit deviations of about 20\% from their average volume, challenging current models of low Reynolds number locomotion. We find that the active pellicle shear deformations causing shape changes can reach 340\%, and estimate the velocity of the molecular motors. Moreover, we find that metaboly accomplishes locomotion at hydrodynamic efficiencies comparable to those of ciliates and flagellates. Our results suggest new quantitative experiments, provide insight into the evolutionary history of euglenids, and suggest that the pellicle may serve as a model for engineered active surfaces with applications in microfluidics.

}, keywords = {active soft matter, microswimmers, self-propulsion, stroke kinematics}, doi = {10.1073/pnas.1213977109}, author = {Marino Arroyo and Luca Heltai and Daniel Mill{\'a}n and Antonio DeSimone} }