23 research outputs found
Mechanics of bio–hybrid systems
Bio–hybrid system are morphing structures whose shaping can be electrically driven and strongly depends on the geometrical and mechanical characteristics of the system. The estimation of those characteristics which allow for getting target shapes is a great challenge. We present and discuss an approximate model for narrow bio–hybrid strips which works well in plane bending. A generalization towards three–layers bio–hybrid system is presented
Morphing of Geometric Composites via Residual Swelling
Understanding and controlling the shape of thin, soft objects has been the
focus of significant research efforts among physicists, biologists, and
engineers in the last decade. These studies aim to utilize advanced materials
in novel, adaptive ways such as fabricating smart actuators or mimicking living
tissues. Here, we present the controlled growth--like morphing of 2D sheets
into 3D shapes by preparing geometric composite structures that deform by
residual swelling. The morphing of these geometric composites is dictated by
both swelling and geometry, with diffusion controlling the swelling-induced
actuation, and geometric confinement dictating the structure's deformed shape.
Building on a simple mechanical analog, we present an analytical model that
quantitatively describes how the Gaussian and mean curvatures of a thin disk
are affected by the interplay among geometry, mechanics, and swelling. This
model is in excellent agreement with our experiments and numerics. We show that
the dynamics of residual swelling is dictated by a competition between two
characteristic diffusive length scales governed by geometry. Our results
provide the first 2D analog of Timoshenko's classical formula for the thermal
bending of bimetallic beams - our generalization explains how the Gaussian
curvature of a 2D geometric composite is affected by geometry and elasticity.
The understanding conferred by these results suggests that the controlled
shaping of geometric composites may provide a simple complement to traditional
manufacturing techniques
Geometry and Mechanics of Thin Growing Bilayers
We investigate how thin sheets of arbitrary shapes morph under the isotropic
in-plane expansion of their top surface, which may represent several stimuli
such as nonuniform heating, local swelling and differential growth. Inspired by
geometry, an analytical model is presented that rationalizes how the shape of
the disk influences morphing, from the initial spherical bending to the final
isometric limit. We introduce a new measure of slenderness that
describes a sheet in terms of both thickness and plate shape. We find that the
mean curvature of the isometric state is three fourth's the natural curvature,
which we verify by numerics and experiments. We finally investigate the
emergence of a preferred direction of bending in the isometric state, guided by
numerical analyses. The scalability of our model suggests that it is suitable
to describe the morphing of sheets spanning several orders of magnitude.Comment: 5 pages, 4 figure
A geometrically exact model for thin magneto-elastic shells
We develop a reduced model for hard-magnetic, thin, linear-elastic shells
that can be actuated through an external magnetic field, with geometrically
exact strain measures. Assuming a reduced kinematics based on the
Kirchhoff-Love assumption, we derive a reduced two-dimensional magneto-elastic
energy that can be minimized through numerical analysis. In parallel, we
simplify the reduced energy by expanding it up to the second-order in the
displacement field and provide a physical interpretation. Our theoretical
analysis allows us to identify and interpret the two primary mechanisms
dictating the magneto-elastic response: a combination of equivalent magnetic
pressure and forces at the first order, and distributed magnetic torques at the
second order. We contrast our reduced framework against a three-dimensional
nonlinear model by investigating three test cases involving the indentation and
the pressure buckling of shells under magnetic loading. We find excellent
agreement between the two approaches, thereby verifying our reduced model for
shells undergoing nonlinear and non-axisymmetric deformations. We believe that
our model for magneto-elastic shells will serve as a valuable tool for the
rational design of magnetic structures, enriching the set of reduced magnetic
models.Comment: 24 pages, 5 figure
Fluid--structure interactions of bristled wings: The trade-off between weight and drag
The smallest flying insects often have bristled wings resembling feathers or
combs. We combined experiments and three-dimensional numerical simulations to
investigate the trade-off between wing weight and drag generation. In
experiments of bristled strips, a reduced physical model of the bristled wing,
we found that the elasto-viscous number indicates when reconfiguration occurs
in the bristles. Analysis of existing biological data suggested that bristled
wings of miniature insects lie below the reconfiguration threshold, thus
avoiding drag reduction. Numerical simulations of bristled strips showed that
there exist optimal numbers of bristles that maximize the weighted drag when
the additional volume due to the bristles is taken into account. We found a
scaling relationship between the rescaled optimal numbers and the dimensionless
bristle length. This result agrees qualitatively with and provides an upper
bound for the bristled wing morphological data analyzed in this study.Comment: 14 pages, 7 figure
Curvature-Induced Instabilities of Shells
Induced by proteins within the cell membrane or by differential growth,
heating, or swelling, spontaneous curvatures can drastically affect the
morphology of thin bodies and induce mechanical instabilities. Yet, the
interaction of spontaneous curvature and geometric frustration in curved shells
remains still poorly understood. Via a combination of precision experiments on
elastomeric spherical bilayer shells, simulations, and theory, we show a
spontaneous curvature-induced rotational symmetry-breaking as well as a
snapping instability reminiscent of the Venus fly trap closure mechanism. The
instabilities and their dependence on geometry are rationalized by reducing the
spontaneous curvature to an effective mechanical load. This formulation reveals
a combined pressurelike bulk term and a torquelike boundary term, allowing
scaling predictions for the instabilities in excellent agreement with
experiments and simulations. Moreover, the effective pressure analogy suggests
a curvature-induced buckling in closed shells. We determine the critical
buckling curvature via a linear stability analysis that accounts for the
combination of residual membrane and bending stresses. The prominent role of
geometry in our findings suggests the applicability of the results over a wide
range of scales.Comment: 12 pages, 9 figures (including Supporting Information
Snapping of Bistable, Prestressed Cylindrical Shells
Bistable shells can reversibly change between two stable configurations with
very little energetic input. Understanding what governs the shape and
snap-through criteria of these structures is crucial for designing devices that
utilize instability for functionality. Bistable cylindrical shells fabricated
by stretching and bonding multiple layers of elastic plates will contain
residual stress that will impact the shell's shape and the magnitude of
stimulus necessary to induce snapping. Using the framework of non-Euclidean
shell theory, we first predict the mean curvature of a nearly cylindrical shell
formed by arbitrarily prestretching one layer of a bilayer plate with respect
to another. Then, beginning with a residually stressed cylinder, we determine
the amount of the stimuli needed to trigger the snapping between two
configurations through a combination of numerical simulations and theory. We
demonstrate the role of prestress on the snap-through criteria, and highlight
the important role that the Gaussian curvature in the boundary layer of the
shell plays in dictating shell stability.Comment: 8 pages, 8 figure
Thermodynamically based multiphysic modeling of ionic polymer metal composites
The modeling of the complex response of the IPMC-like body to electrical and mechanical stimuli is set within the context of the 3-D theory of linear elasticity. A field of chemically induced distortions is included in the model; these mechanical distortions and the derivation of the final PDE equations of the multiphysics problem are thermodynamically consistent. Some results of the numerical experiments are revisited through an original analysis of the stress distribution along the IPMC-like body