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Abstract The transient behavior of rate-dependent adhesion in poro-viscoelastic contact is more complex than crack propagation in Mode I opening due to time-dependent material behavior, crack acceleration from nonlinear kinematics, and variation in contact radius. This study revisits our previous experiment, where a spherical glass probe is unloaded on flat gelatin, and investigates crack velocity ($$V_\text {c}$$ $V_{\text{c}}$) and energy release rate (ERR). For a given unloading rate,$$V_\text {c}$$ $V_{\text{c}}$increases monotonically by one order of magnitude, and the wide range of unloading rates ensures that$$V_\text {c}$$ $V_{\text{c}}$spans 3–4 orders of magnitude. ERR remains almost unchanged at 2–3 times the thermodynamic work of adhesion at slow rates. At fast rates, ERR initially increases to 4–8, then decreases until full separation. We hypothesize that the decreasing ERR trend is due to finite-size effects: the hysteretic energy dissipation zone grows with crack acceleration, while the material volume decreases during peeling. To explain these trends and the finite-size effect, we adapt de Gennes’ viscoelastic crack propagation model, modifying it to account for crack acceleration and the reduction in contact radius. Under the given time scales (peeling time and viscoelastic relaxation time) and length scales (crack tip radius and initial contact radius), we simulate the evolution of ERR as peeling proceeds and compare the results with experimental data. The model’s results show good qualitative agreement with the experiments. Finally, we discuss the model’s limitations, assumptions, and directions for future research. 

Abstract Surface performance is critically influenced by topography in virtually all real-world applications. The current standard practice is to describe topography using one of a few industry-standard parameters. The most commonly reported number is$$R$$ $R$a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a “rough” and a “smooth” surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking. 

Hydrogels, polymeric networks swollen with water, exhibit time/rate-dependent adhesion due to their poroviscoleastic constitution. 

Water loss in clamped gelatin disks leads to built-up of in-plane stress (A) and increase in elastic modulus (B), as characterized by the laser vibrometry of the first two axisymmetric vibration modes of the disks (C). 

Past decades featured significant advancements in additive and micromanufacturing that facilitated the creation of functional patterned surfaces with impressive spatial resolutions. However, these techniques are expensive and require a considerable amount of time and energy, and hence lack scalability to practical surfaces. Recent techniques employing spinodal decomposition and instabilities amplified via centrifugal acceleration offer viable and cheaper alternatives. The patterns created by those techniques, however, vary randomly in geometry. When interfacing those patterned surfaces with other components and under self-contact scenarios, geometric variations lead to stress concentration and abrupt failure around the contact. In this study, we investigate numerically real contact areas, contact tractions, and stress concentration. We generate patterned surfaces in congruence with actual surfaces created by those techniques. Then, we conduct normal-contact analyses of those surfaces boundary element method (BEM) under nominal mean pressures ranging from 0.001E* toE*, whereE* is the contact modulus. We record real contact areas and stress concentration as a function of nominal mean pressures. We compare these values with the analytical solutions from sinusoidally-patterned and randomly rough surfaces. Randomness in pattern geometry is primarily influenced by the processing parameters such as the degree of anisotropy in spinodal decomposition and acceleration in amplified instabilities. To understand the influence of the processing parameters, we perform a parametric study. We find isotropic spinodal decomposition creates patterns that deliver contact area and traction distributions similar to randomly rough surfaces, and lead to high-stress concentrations. Such high-stress concentrations are expected to occur under self-contact loading scenarios, and thus can explain the compromised resilience and strength in recently-proposed spinodal metamaterials. For patterned surfaces created by amplified instabilities, high-stress concentrations are obtained for the surfaces created at high accelerations. At high accelerations, increased elastic instabilities and stochastic growth result in a more skewed and broader distribution in heights. Therefore, high-stress concentrations are inevitable. To account for combined loading scenarios, we conduct additional simulations on the same surface patterns with frictional pre-sliding contacts. We find the frictional tractions play a secondary role in stress concentrations where the primary factor is the processing parameters determining the degree of randomness in pattern geometry.