Soft Deployable Structures via Core-Shell Inflatables.

Deployable structures capable of significant geometric reconfigurations are ubiquitous in nature. While engineering contraptions typically comprise articulated rigid elements, soft structures that experience material growth for deployment mostly remain the handiwork of biology, e.g., when winged insects deploy their wings during metamorphosis. Here we perform experiments and develop formal models to rationalize the previously unexplored physics of soft deployable structures using core-shell inflatables. We first derive a Maxwell construction to model the expansion of a hyperelastic cylindrical core constrained by a rigid shell. Based on these results, we identify a strategy to obtain synchronized deployment in soft networks. We then show that a single actuated element behaves as an elastic beam with a pressure-dependent bending stiffness which allows us to model complex deployed networks and demonstrate the ability to reconfigure their final shape. Finally, we generalize our results to obtain three-dimensional elastic gridshells, demonstrating our approach's applicability to assemble complex structures using core-shell inflatables as building blocks. Our results leverage material and geometric nonlinearities to create a low-energy pathway to growth and reconfiguration for soft deployable structures.

Instability mediated self-templating of drop crystals.

The breakup of liquid threads into droplets is prevalent in engineering and natural settings. While drop formation in these systems has a long-standing history, existing studies typically consider axisymmetric systems. Conversely, the physics at play when multiple threads are involved and the interaction of a thread with a symmetry breaking boundary remain unexplored. Here, we show that the breakup of closely spaced liquid threads sequentially printed in an immiscible bath locks into crystal-like lattices of droplets. We rationalize the hydrodynamics at the origin of this previously unknown phenomenon. We leverage this knowledge to tune the lattice pattern via the control of injection flow rate and nozzle translation speed, thereby overcoming the limitations in structural versatility typically seen in existing fluid manipulations paradigms. We further demonstrate that these drop crystals have the ability to self-correct and propose a simple mechanism to describe the convergence toward a uniform pattern of drops.

Formation of Pixelated Elastic Films via Capillary Suction of Curable Elastomers in Templated Hele-Shaw Cells.

Natural materials are highly organized, frequently possessing intricate and sophisticated hierarchical structures from which superior properties emerge. In the wake of biomimicry, there is a growing interest in designing architected materials in the laboratory as such structures could enable myriad functionalities in engineering. Yet, their fabrication remains challenging despite recent progress in additive manufacturing. In particular, soft materials are typically poorly suited to form the requisite structures consisting of regular geometries. Here, a new frugal methodology is reported to fabricate pixelated soft materials. This approach is conceptually analogous to the watershed transform used in image analysis and allows the passive assembly of complex geometries through the capillary-mediated flow of curable elastomers in confined geometries. Emerging from sources distributed across a Hele-Shaw cell consisting of two parallel flat plates separated by an infinitesimally small gap, these flows eventually meet at the "dividing lines" thereby forming Voronoi tesselations. After curing is complete, these structures turn into composite elastic sheets. Rationalizing the fluid mechanics at play allows the structural geometry of the newly formed sheets to be tailored and thereby their local material properties to be tuned.

Bubble casting soft robotics.

Inspired by living organisms, soft robots are developed from intrinsically compliant materials, enabling continuous motions that mimic animal and vegetal movement1. In soft robots, the canonical hinges and bolts are replaced by elastomers assembled into actuators programmed to change shape following the application of stimuli, for example pneumatic inflation2-5. The morphing information is typically directly embedded within the shape of these actuators, whose assembly is facilitated by recent advances in rapid prototyping techniques6-11. Yet, these manufacturing processes have limitations in scalability, design flexibility and robustness. Here we demonstrate a new all-in-one methodology for the fabrication and the programming of soft machines. Instead of relying on the assembly of individual parts, our approach harnesses interfacial flows in elastomers that progressively cure to robustly produce monolithic pneumatic actuators whose shape can easily be tailored to suit applications ranging from artificial muscles to grippers. We rationalize the fluid mechanics at play in the assembly of our actuators and model their subsequent morphing. We leverage this quantitative knowledge to program these soft machines and produce complex functionalities, for example sequential motion obtained from a monotonic stimulus. We expect that the flexibility, robustness and predictive nature of our methodology will accelerate the proliferation of soft robotics by enabling the assembly of complex actuators, for example long, tortuous or vascular structures, thereby paving the way towards new functionalities stemming from geometric and material nonlinearities.

Wetting and wrapping of a floating droplet by a thin elastic filament.

We study the wetting of a thin elastic filament floating on a fluid surface by a droplet of another, immiscible fluid. This quasi-2D experimental system is the lower-dimensional counterpart of the wetting and wrapping of a droplet by an elastic sheet. The simplicity of this system allows us to study the phenomenology of partial wetting and wrapping of the droplet by measuring angles of contact as a function of the elasticity of the filament, the applied tension and the curvature of the droplet. We find that a purely geometric theory gives a good description of the mechanical equilibria in the system. The estimates of applied tension and tension in the filament obey an elastic version of the Young-Laplace-Dupré relation. However, curvatures close to the contact line are not captured by the geometric theory, possibly because of 3D effects at the contact line. We also find that when a highly-bendable filament completely wraps the droplet, there is continuity of curvature at the droplet-filament interface, leading to seamless wrapping as observed in a 3D droplet.

Laser Generation of Sub-Micrometer Wrinkles in a Chalcogenide Glass Film as Physical Unclonable Functions.

Laser interaction with solids is routinely used for functionalizing materials' surfaces. In most cases, the generation of patterns/structures is the key feature to endow materials with specific properties like hardening, superhydrophobicity, plasmonic color-enhancement, or dedicated functions like anti-counterfeiting tags. A way to generate random patterns, by means of generation of wrinkles on surfaces resulting from laser melting of amorphous Ge-based chalcogenide thin films, is presented. These patterns, similar to fingerprints, are modulations of the surface height by a few tens of nanometers with a sub-micrometer periodicity. It is shown that the patterns' spatial frequency depends on the melted layer thickness, which can be tuned by varying the impinging laser fluence. The randomness of these patterns makes them an excellent candidate for the generation of physical unclonable function tags (PUF-tags) for anti-counterfeiting applications. Two specific ways are tested to identify the obtained PUF-tag: cross-correlation procedure or using a neural network. In both cases, it is demonstrated that the PUF-tag can be compared to a reference image (PUF-key) and identified with a high recognition ratio on most real application conditions. This paves the way to straightforward non-deterministic PUF-tag generation dedicated to small sensitive parts such as, for example, electronic devices/components, jewelry, or watchmak.

Collaborative Oscillatory Fracture.

We report a new oscillatory propagation of cracks in thin films where three cracks interact mediated by two delamination fronts. Experimental observations indicate that delamination fronts joining the middle crack to the lateral crack tips swap contact periodically with the crack tip of the middle crack. A model based on a variational approach analytically predicts the condition of propagation and geometrical features of three parallel cracks. The stability conditions and oscillating propagation are found numerically and the predictions are in favorable agreement with experiments. We found that the physical mechanism selecting the wavelength structure is a relaxation process in which the middle crack produces a regular oscillatory path.

Printing on liquid elastomers.

In recent years the research community has paid significant attention to geometrically engineered materials. These materials derive their unique properties from their structure rather than their chemistry alone. Despite their success in the laboratory, the assembly of such soft functional materials remains an outstanding challenge. Here, we propose a robust fluid-mediated route for the rapid fabrication of soft elastomers architected with liquid inclusions. Our approach consists of depositing water drops at the surface of an immiscible liquid elastomer bath. As the elastomer cures, the drops are encapsulated in the polymer and impart shape and function to the newly formed elastic matrix. Using the framework of fluid mechanics, we show how this type of composite material can be tailored.

Curvature Regularization near Contacts with Stretched Elastic Tubes.

Inserting a rigid object into a soft elastic tube produces conformal contact between the two, resulting in contact lines. The curvature of the tube walls near these contact lines is often large and is typically regularized by the finite bending rigidity of the tube. Here, it is demonstrated using experiments and a Föppl-von Kármán-like theory that a second, independent, mechanism of curvature regularization occurs when the tube is axially stretched. In contrast with the effects of finite bending rigidity, the radius of curvature obtained increases with the applied stretching force and decreases with sheet thickness. The dependence of the curvature on a suitably rescaled stretching force is found to be universal, independent of the shape of the intruder, and results from an interplay between the longitudinal stresses due to the applied stretch and hoop stresses characteristic of curved geometry. These results suggest that curvature measurements can be used to infer the mechanical properties of stretched tubular structures.

An unbounded approach to microfluidics using the Rayleigh-Plateau instability of viscous threads directly drawn in a bath.

We study the droplet-forming instability of a thin jet extruded from a nozzle moving horizontally below the surface of an isoviscous immiscible fluid bath. While this interfacial instability is a classic problem in fluid mechanics, it has never been studied in the context of the deposition of a thread into a reservoir, an open-sky version of microfluidics. As the nozzle translates through the reservoir, drops may form at the nozzle (dripping) or further downstream (jetting). We first focus on rectilinear printing paths and derive a scaling law to rationalize the transition between dripping and jetting. We then leverage the flexibility of our system and study the dynamics of breakup when printing sinusoidal paths. We unravel a methodology to control both the size of the drops formed by the instability and the distance that separates them.

Buckling patterns in biaxially pre-stretched bilayer shells: wrinkles, creases, folds and fracture-like ridges.

We introduce a new experimental system to study the effects of pre-stretch on the buckling patterns that emerge from the biaxial compression of elastomeric bilayer shells. Upon fabrication of the samples, releasing the pre-stretch in the substrate through deflation places the outer film in a state of biaxial compression and yields a variety of buckling patterns. We systematically explore the parameter space by varying the pre-stretch of the substrate and the ratio between the stiffness of the substrate and film. The phase diagram of the system exhibits a variety of buckling patterns: from the classic periodic wrinkle to creases, folds, and high aspect ratio ridges. Our system is capable of readily transitioning between these buckling patterns, a first for biaxial systems. We focus on the wrinkle to ridge transition. In the latter, we find that pre-stretch plays an essential role and that the ridge geometry (width, height) remains nearly constant throughout their formation process. For the localized ridged patterns, we find that the propagation of the ridge tip depends strongly on both strain and stiffness ratio, in a way that is akin to hierarchical fracture.

Self-replicating cracks: a collaborative fracture mode in thin films.

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings, or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness. Depending on triggering mechanisms, crescent alleys, spirals, or long bands are generated over a wide range of experimental parameters. We describe with a simple physical model, the selection of the fracture path and provide a configuration diagram displaying the different failure modes.

Local stresses in the Janssen granular column.

We study experimentally the distribution of local stresses in a granular material confined inside a vertical cylinder. We use an image correlation technique to measure the displacement field of the container induced by the forces exerted by the grains on the inner wall. We describe an optimization procedure based on the linear theory of elastic shells to deduce the distribution of these forces from the measured displacement field. They correspond to the stress field of the granular material close to the container's inner wall. We first confirm the validity of Janssen's description for various experiments, including the influence of the bead diameter and the effect of an additional mass on top of the granular column. We then apply this method to determine the stress field during the gravity-driven discharge of a silo through an aperture.