Publications

Confluent epithelial tissues can be viewed as soft active solids, as their individual cells contract in response to local conditions. Little is known about the emergent properties of such materials. Empirical observations have shown contraction waves propagation in various epithelia, yet the governing mechanism, as well as its physiological function, is still unclear. Here we propose an experiment-inspired model for such dynamic epithelia. We show how the widespread cellular response of contraction-under-tension is sufficient to give rise to propagating contraction pulses, by mapping numerically and theoretically the consequences of such a cellular response. The model explains observed phenomena but also predicts enhanced rip-resistance as an emergent property of such cellular sheets. Unlike healing post-rupture, these sheets avoid it by actively re-distributing external stresses across their surface. The mechanism is relevant to a broad class of tissues, especially such under challenging mechanical conditions, and may inspire engineering of synthetic materials.

Programmable shape-shifting materials can take different physical forms to achieve multifunctionality in a dynamic and controllable manner. Although morphing a shape from 2D to 3D via programmed inhomogeneous local deformations has been demonstrated in various ways, the inverse problem-finding how to program a sheet in order for it to take an arbitrary desired 3D shape-is much harder yet critical to realize specific functions. Here, we address this inverse problem in thin liquid crystal elastomer (LCE) sheets, where the shape is preprogrammed by precise and local control of the molecular orientation of the liquid crystal monomers. We show how blueprints for arbitrary surface geometries can be generated using approximate numerical methods and how local extrinsic curvatures can be generated to assist in properly converting these geometries into shapes. Backed by faithfully alignable and rapidly lockable LCE chemistry, we precisely embed our designs in LCE sheets using advanced top-down microfabrication techniques. We thus successfully produce flat sheets that, upon thermal activation, take an arbitrary desired shape, such as a face. The general design principles presented here for creating an arbitrary 3D shape will allow for exploration of unmet needs in flexible electronics, metamaterials, aerospace and medical devices, and more.

We study the geometry of defects in amorphous materials and their elastic interactions. Defects are defined and characterized by deviations of the material's intrinsic metric from a Euclidian metric. This characterization makes possible the identification of localized defects in amorphous materials, the formulation of a corresponding elastic problem, and its solution in various cases of physical interest. We present a multipole expansion that covers a large family of localized 2D defects. The dipole term, which represents a dislocation, is studied analytically and experimentally. Quadrupoles and higher multipoles correspond to fundamental strain-carrying entities. The interactions between those entities, as well as their interaction with external stress fields, are fundamental to the inelastic behavior of solids. We develop analytical tools to study those interactions. The model, methods, and results presented in this work are all relevant to the study of systems that involve a distribution of localized sources of strain. Examples are plasticity in amorphous materials and mechanical interactions between cells on a flexible substrate.

We present a limiting model for thin non-Euclidean elastic rods. Originating from the three-dimensional ( 3D) reference metric of the rod, which is determined by its internal material structure, we derive a 1D reduced rod theory. Specifically, we show how the spontaneous twist and curvature of a rod emerge from the reference metric derivatives. Thus, the model allows calculating the unconstrained equilibrium configuration of a thin rod directly from its internal structure. The model is applied to the study of cells from members of the Geraniaceae plant family and their configurational response to dehydration. We show how the geometrical arrangement of cellulose fibrils on the cell walls determines the helical shapes of isolated cells.

Crumpling occurs when a thin deformable sheet is crushed under an external load or grows within a confining geometry. Crumpled sheets have large resistance to compression and their elastic energy is focused into a complex network of localized structures(1). Different aspects of crumpling have been studied theoretically(2,3), experimentally(4,5) and numerically(6,7). However, very little is known about the dynamic evolution of three-dimensional spatial configurations of crumpling sheets. Here we present direct measurements of the configurations of a fully elastic sheet evolving during the dynamic process of crumpling under isotropic confinement. We observe the formation of a network of ridges and vertices into which the energy is localized. The network is dynamic. Its evolution involves movements of ridges and vertices. Although the characteristics of ridges agree with theoretical predictions, the measured accumulation of elastic energy within the entire sheet is considerably slower than predicted. This could be a result of the observed network rearrangement during crumpling.

We present an experimental study of the three-dimensional (3D) configurations that result from non-uniform lateral growth/shrinking of thin elastic sheets. We build gel sheets that undergo inducible differential shrinking. The non-uniform shrinking prescribes a non-Euclidean metric on a disc, and thus a non-zero Gaussian curvature. To minimize their elastic energy the free sheets form three-dimensional structures that approximate the imposed metric. We show how both large scale buckling and wrinkling-type structures can be generated, depending on the nature of possible embeddings of the imposed metric in Euclidean space.