John Martin Kolinski

Elastomers can be engineered as stretchable, compliant materials capable of bearing load, making them attractive for soft robotics, wearable electronics, and biomedical devices. Unfortunately, single-network elastomers are constrained by a stiffness-toughness compromise. While multinetwork elastomers can mitigate this limitation, tough multinetwork elastomers typically exhibit a limited fatigue resistance. Here, we demonstrate that double-network granular elastomers (DNGEs) composed of stiff elastomer microparticles connected through a softer second network combine toughness and fatigue resistance. Their locally varying composition and structure enable DNGEs to efficiently deconcentrate stress and repetitively dissipate energy if stretched to moderate strains, imparting them a good fatigue resistance. Leveraging the three-dimensional printability of DNGEs, we spatially vary the composition to combine stiffness, toughness, and fatigue resistance within one elastomer. We anticipate that DNGEs will unlock opportunities in soft robotics, wearables, and biomedicine, where current soft materials remain limited by the stiffness-toughness-fatigue resistance trade-off.

During brittle crack propagation, a smooth crack front curve frequently becomes disjoint, generating a stepped crack and a material ligament that unites the newly formed crack fronts. These universal features fundamentally alter the singular field structure and stability of propagating cracks; however, a quantitative analysis of their mechanics is lacking. Here, we perform in situ 3D measurements to resolve the deformation field around stepped cracks, and crucially, within the ligament feature. The 3D kinematic data are obtained by scanning a thin laser sheet through the brittle hydrogel samples, while recording the scattered intensity from the embedded tracer particles. We find that the ligament concentrates the strain energy density. We introduce the "Ligament Equivalent Energy" and demonstrate that the macroscopic fracture energy increases linearly with this volumetric quantity when normalized by the ligament rupture area. Furthermore, by extracting an effective traction-separation law from the local stress fields, we reveal that the ligament functions as an effective cohesive zone.

Direct crack tip strain imaging reveals irreversible plastic zones governing soft material fatigue, both below and above the fatigue threshold.

A liquid drop impacting a non-wetting rigid substrate spreads, retracts and rebounds, while an elastic sphere deforms slightly and bounces off. This work bridges these two limits and offers a framework for liquid-to-elastic transitions.

On the surface of a vibrating liquid bath, instead of coalescing, a drop will continually bounce on a thin film of air between the drop and the free surface, giving rise to rich chaotic dynamics and quantum analog behavior. However, perpetual bouncing is yet to be demonstrated on a vibrating rigid solid, where the control of the drop’s motion is not limited by the bath dynamics. Here we show that vibration of an atomically smooth mica surface prolongs a drop’s hovering state by several orders of magnitude, increasing the time to wet from less than a second to several minutes. The excitation of the second spherical harmonic mode determines a transition between a bouncing state with high-amplitude rebounds, and a bound state, where the drop’s motion is locked onto the vibrating solid above a thin air layer. We further develop a coupled linear spring model, accounting for the drop’s deformation, that predicts bouncing trajectories without fitting parameters. Our results extend the scope of bouncing drop studies beyond the liquid bath and highlight the potential for precision manipulation of small liquid quantities in air.

During brittle crack propagation, a smooth crack front curve frequently becomes disjoint, generating a stepped crack and a material ligament that unites the newly formed crack fronts. These universal features fundamentally alter the singular field structure and stability of propagating cracks; however, a quantitative analysis of their mechanics is lacking. Here, we perform in-situ 3D measurements to resolve the deformation field around stepped cracks, and crucially, within the ligament feature. The 3D kinematic data are obtained by scanning a thin laser sheet through the brittle hydrogel samples, while recording the scattered intensity from the embedded tracer particles. We find that the ligament concentrates the strain energy density, and moreover, the apparent fracture energy increases proportionally to the strain energy within the ligament.

A droplet falling toward a solid surface displaces the surrounding air until it encounters a defect, and contact nucleates. On atomically smooth surfaces devoid of defects, contact can be delayed until the droplet rebounds; however, above a critical impact velocity the droplet always contacts the surface. Here we show that for alcohol droplets in a humid atmosphere, the surface of the droplet above the solid develops patterns as a consequence of an instability; consequently, the liquid approaches the surface more closely in some places than others, accelerating contact formation. We demonstrate the attenuation and even suppression of this instability by varying the liquid composition, and characterize the growth rate and length scale of the patterns on liquid-air interface.

Surface roughness significantly modifies the liquid film thickness entrained when dip coating a solid surface, particularly at low coating velocity. Using a homogenization approach, we present a predictive model for determining the liquid film thickness coated on a rough plate. A homogenized boundary condition at an equivalent flat surface is used to model the rough boundary, accounting for both flow through the rough texture layer, through an interface permeability term, and slip at the equivalent surface. While the slip term accounts for tangential velocity induced by viscous shear stress, accurately predicting the film thickness requires the interface permeability term to account for additional tangential flow driven by pressure gradients along the interface. We find that a greater degree of slip and interface permeability signifies less viscous stress that would promote deposition, thus reducing the amount of free film coated above the textures. The model is found to be in good agreement with experimental measurements, and requires no fitting parameters. Furthermore, our model may be applied to arbitrary periodic roughness patterns, opening the door to flexible characterization of surfaces found in natural and industrial coating processes.

A viscous, lubrication-like response can be triggered in a thin film of fluid squeezed between a rigid flat surface and the tip of an incoming projectile. We develop a scaling for this viscous approach stage of fluid-mediated normal impact, applicable to soft impactors. Under the assumption of mediating fluid being incompressible, the impacting solid displays two limit regimes: one dominated by elasticity, and the other by inertia. The transition between the two is predicted by a dimensionless parameter, which can be interpreted as the ratio between two time scales that are the time that it takes for the surface waves to warn the leading edge of the impactor of the forthcoming impact, and the characteristic duration of the final viscous phase of the approach. Additionally, we elucidate why nearly incompressible solids feature (a) substantial ‘gliding’ prior to contact at the transition between regimes, (b) the largest size of entrapped bubble between the deformed tip of the impactor and the flat surface, and (c) a sudden drop in entrapped bubble radius past the transition between regimes. Finally, we argue that the above time scale ratio (a dimensionless number) can govern the different dynamics reported experimentally for a fluid droplet as a function of its viscosity and surface tension.