1,720,997 research outputs found
Nutations in growing plant shoots as a morphoelastic flutter instability
No full textGrowing plant shoots exhibit spontaneous oscillations that Darwin observed, and termed ‘circumnutations’. Recently, they have received renewed attention for the design and optimal actuation of bioinspired robotic devices. We discuss a possible interpretation of these spontaneous oscillations as a Hopf-type bifurcation in a growing morphoelastic rod. Using a three-dimensional model and numerical simulations, we analyse the salient features of this flutter-like phenomenon (e.g. the characteristic period of the oscillations) and their dependence on the model details (in particular, the impact of choosing different growth models) finding that, overall, these features are robust with respect to changes in the details of the growth model adopted
Dispositivo strumentato per una caratterizzazione di una struttura interna di un blocco di formaggio
No full textLa presente divulgazione si riferisce ad dispositivo strumentato (1) per una caratterizzazione di una struttura interna di un blocco di formaggio (9). Il dispositivo (1) comprende un percussore (2), un ricevitore (3) ed una sede (18) di 5 alloggiamento del blocco di formaggio (9); il percussore (2) ed il ricevitore (3) si affacciano su differenti zone della sede di alloggiamento (18). Il ricevitore (3) è configurato per essere disposto a contatto con una prima regione superficiale (91) del blocco di formaggio (9) ed il percussore (3) è configurato per colpire una seconda regione superficiale (92) del blocco di formaggio (9). Inoltre il ricevitore (3) 10 è configurato per ricevere un’onda di percussione (99) e generare un corrispondente primo segnale (61). Durante l’uso un’onda di percussione (99) è prodotta da un colpo del percussore (2) sulla seconda regione superficiale (92) e l’onda di percussione (99) si propaga dalla seconda regione superficiale (92) alla prima regione superficiale (91) attraverso il blocco di formaggio (9). 15 Vengono inoltre divulgate una apparecchiatura di caratterizzazione (6, 78, 87) di una forma di formaggio (9) includente un dispositivo strumentato (1), e
apparecchiature di movimentazione (7, 8) per una forma di formaggio (9), ciascuna includente una apparecchiatura di caratterizzazione (78, 87). È inoltre divulgato un metodo per determinare una caratterizzazione di una struttura 20 interna di un blocco di formaggio (9
Discrete one-dimensional crawlers on viscous substrates: achievable net displacements and their energy cost
No full textWe study model one-dimensional crawlers, namely, model mechanical systems that can achieve self-propulsion by controlled shape changes of their body (extension or contraction of portions of the body), thanks to frictional interactions with a rigid substrate. We evaluate the achievable net displacement and the related energetic cost for self-propulsion by discrete crawlers (i.e., whose body is made of a discrete number of contractile or extensile segments) moving on substrates with either a Newtonian (linear) or a Bingham-type (stick-slip) rheology. Our analysis is aimed at constructing the basic building blocks towards an integrative, multi-scale description of crawling cell motility
The biomechanical role of extra-axonemal structures in shaping the flagellar beat of euglena gracilis
No full textWe propose and discuss a model for flagellar mechanics in Euglena gracilis. We show that the peculiar non-planar shapes of its beating flagellum, dubbed ’spinning lasso’, arise from the mechanical interactions between two of its inner components, namely, the axoneme and the paraflagellar rod. The spontaneous shape of the axoneme and the resting shape of the paraflagellar rod are incompatible. Thus, the complex non-planar configurations of the coupled system emerge as the energetically optimal compromise between the two antagonistic components. The model is able to reproduce the experimentally observed flagellar beats and the characteristic geometric signature of spinning lasso, namely, traveling waves of torsion with alternating sign along the length of the flagellum
A Theoretical Study on the Transient Morphing of Linear Poroelastic Plates
No full textBased on their shape-shifting capabilities, soft active materials have enabled new possibilities for the engineering of sensing and actuation devices. While the relation between active strains and emergent equilibrium shapes has been fully characterized, the transient morphing of thin structures is a rather unexplored topic. Here, we focus on polymer gel plates and derive a reduced linear model to study their time-dependent response to changes in the fluid environment. We show that independent control of stretching and bending deformations in stress-free conditions allows to realize spherical shapes with prescribed geometry of the mid-plane. Furthermore, we demonstrate that tensile (compressive) membrane stresses delay (accelerate) swelling-induced shape transitions compared to the stress-free evolution. We believe that these effects should be considered for the accurate design of smart systems and may contribute to explain the complexity of natural shapes
On polymer network rupture in gels in the limit of very slow straining or a very slow crack propagation rate
Full text linkThe 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
Morphable structures from unicellular organisms with active, shape-shifting envelopes: Variations on a theme by Gauss
No full textWe discuss some recent results on biological and bio-inspired morphing, and use them to identify promising research directions for the future. In particular, we consider issues related to morphing at microscopic scales inspired by unicellular organisms. We focus on broad conceptual principles and, in particular, on morphing approaches based on the use of Gauss’ theorema egregium (Gaussian morphing). We highlight some connections with biological cell envelopes containing filaments and motors, and discuss ideas for the implementation of Gaussian morphing in surfaces actuated by active shearing or stretching
Poroelastic toughening in polymer gels: A theoretical and numerical study
No full textWe 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
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