Complexity of crack front geometry enhances toughness of brittle solids.

Brittle solids typically fail by growth and propagation of a crack from a surface flaw. This process is modelled using linear elastic fracture mechanics, which parameterizes the toughness of a material by the critical stress intensity factor, or the prefactor of the singular stress field. This widely used theory applies for cracks that are planar, but cracks typically are not planar, and instead are geometrically complex, violating core tenets of linear elastic fracture mechanics.

Elastocapillary menisci mediate interaction of neighboring structures at the surface of a compliant solid.

Surface stress drives long-range elastocapillary interactions at the surface of compliant solids, where it has been observed to mediate interparticle interactions and to alter transport of liquid drops. We show that such an elastocapillary interaction arises between neighboring structures that are simply protrusions of the compliant solid. For compliant micropillars arranged in a square lattice with spacing p less than an interaction distance p^{*}, the distance of a pillar to its neighbors determines how much it deforms due to surface stress: Pillars that are close together tend to be rounder and flatter than those that are far apart.

Influence of the Degree of Swelling on the Stiffness and Toughness of Microgel-Reinforced Hydrogels.

The stiffness and toughness of conventional hydrogels decrease with increasing degree of swelling. This behavior makes the stiffness-toughness compromise inherent to hydrogels even more limiting for fully swollen ones, especially for load-bearing applications. The stiffness-toughness compromise of hydrogels can be addressed by reinforcing them with hydrogel microparticles, microgels, which introduce the double network (DN) toughening effect into hydrogels.

Revealing the complexity of ultra-soft hydrogel re-swelling inside the brain.

The brain is an ultra-soft viscoelastic matrix. Sub-kPa hydrogels match the brain's mechanical properties but are challenging to manipulate in an implantable format. We propose a simple fabrication and processing sequence, consisting of de-hydration, patterning, implantation, and re-hydration steps, to deliver brain-like hydrogel implants into the nervous tissue.

Superspreading events suggest aerosol transmission of SARS-CoV-2 by accumulation in enclosed spaces.

Viral transmission pathways have profound implications for public safety; it is thus imperative to establish a complete understanding of viable infectious avenues. Mounting evidence suggests SARS-CoV-2 can be transmitted via the air; however, this has not yet been demonstrated. Here we quantitatively analyze virion accumulation by accounting for aerosolized virion emission and destabilization.

Air mediates the impact of a compliant hemisphere on a rigid smooth surface.

Fleeting contact between solids immersed in a fluid medium governs the response of critically important materials, from coffee to soil. Rapid impact of soft solids occurs in systems as diverse as car tires, soft robotic locomotion and suspensions, including soil and coffee. In each of these systems, the dynamics are fundamentally altered by the presence of a fluid layer mediating solid contact.

Surface textures suppress viscoelastic braking on soft substrates.

A gravity-driven droplet will rapidly flow down an inclined substrate, resisted only by stresses inside the liquid. If the substrate is compliant, with an elastic modulus < 100 kPa, the droplet will markedly slow as a consequence of viscoelastic braking. This phenomenon arises due to deformations of the solid at the moving contact line, enhancing dissipation in the solid phase.

How Fast Cracks in Brittle Solids Choose Their Path.

While we fundamentally understand the dynamics of simple cracks propagating in brittle solids within perfect (homogeneous) materials, we do not understand how paths of moving cracks are determined. We experimentally study strongly perturbed cracks that propagate between 10% and 95% of their limiting velocity within a brittle material. These cracks are deflected by either interaction with sparsely implanted defects or via an intrinsic oscillatory instability in defect-free media.

Universal Soft Robotic Microgripper.

Here, a soft robotic microgripper is presented that consists of a smart actuated microgel connected to a spatially photopatterned multifunctional base. When pressed onto a target object, the microgel component conforms to its shape, thus providing a simple and adaptive solution for versatile micromanipulation. Without the need for active visual or force feedback, objects of widely varying mechanical and surface properties are reliably gripped through a combination of geometrical interlocking mechanisms instantiated by reversible shape-memory and thermal responsive swelling of the microgel.

How Supertough Gels Break.

Fracture of highly stretched materials challenges our view of how things break. We directly visualize rupture of tough double-network gels at >50% strain. During fracture, crack tip shapes obey a x∼y^{1.

Internal Stresses Lead to Net Forces and Torques on Extended Elastic Bodies.

A geometrically frustrated elastic body will develop residual stresses arising from the mismatch between the intrinsic geometry of the body and the geometry of the ambient space. We analyze these stresses for an ambient space with gradients in its intrinsic curvature, and show that residual stresses generate effective forces and torques on the center of mass of the body. We analytically calculate these forces in two dimensions, and experimentally demonstrate their action by the migration of a non-Euclidean gel disc in a curved Hele-Shaw cell.

Lift-off instability during the impact of a drop on a solid surface.

We directly measure the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. Upon impact, the air separating the liquid from the solid surface fails to drain and wetting is delayed as the liquid rapidly spreads outwards over a nanometer thin film of air. We show that the approach of the spreading liquid front toward the surface is unstable and the spreading front lifts off away from the surface.

Skating on a film of air: drops impacting on a surface.

The commonly accepted description of drops impacting on a surface typically ignores the essential role of the air that is trapped between the impacting drop and the surface. Here we describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads.

Shape and motion of a ruck in a rug.

The motion of a ruck in a rug is used as an analogy to explain the role of dislocations in crystalline solids. We take literally one side of this analogy and study the shape and motion of a bump, wrinkle or ruck in a thin sheet in partial contact with a rough substrate in a gravitational field. Using a combination of experiments, scaling analysis and numerical solutions of the governing equations, we quantify the static shape of a ruck on a horizontal plane.