Session L06: Wetting and Adhesion of Soft Materials: Dynamics and Instability I

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Thirty five years ago, two theories for the pinning of elastic lines by heterogeneities have appeared almost simultaneously (Joanny & de Gennes, J. Chem. Phys. 81 (1984) 552; Rice, J. Appl. Mech 52 (1985) 571). Starting from these results, a large number of models and simulations have greatly advanced our understanding of complex line phenomena, and especially wetting hysteresis and fracture toughness of heterogeneous materials. However, these theories provide quasistatic pictures and they do not tell us much about what happens beyond the depinning treshold in the rather ubiquitous case where the response of the material itself is dissipative (e.g. viscous liquids or viscoelastic solids). In fact, even for homogeneous systems, evaluating the dissipation is still often a problematic question and the most simple cases - dewetting newtonian liquid or adhesion rupture for a linear viscoelastic solid - are far from being completely understood, especially when confrontion with experimental results is intended... Here we consider the dynamics of a front in a dissipative material moving on a heterogeneous surface at finite velocity. Based on recent numerical results for periodic substrates, we will first show how heterogeneities renormalize the dynamics of newtonian fluids near the dynamic wetting transition and actually obliterate some of the details of the wetting problem. We will then discuss the generalization to the case of dissipative soft solids.   

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A common feature in adhesion and wetting systems is a meniscus. Yet, most adhesive models do not account for the role it plays. Historically, contact models have been treated separately from models of capillarity due to the complexity of elasticity at large deformations. Here, we consider a system at the intersection of these two extremes. We explore the adhesion of a gel sphere to a rigid substrate. As the gel is deformed, a liquid fraction is expelled to form a meniscus with the surface. This meniscus adds additional contact area between the gel and substrate, changing the energetic contributions due to adhesion and surface tension. In comparison with experiment, we find that established contact models underestimate the contact area, especially at low Young’s moduli. To better understand this system, we have built on previous contact and capillary models to develop a formulation which includes elasticity, adhesion, and surface tension. The model accounts for the formation of a meniscus and is applicable at even large contact radii.   

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Capillary forces can play an important role in adhesion between rigid surfaces and highly compliant polymer gels in multiple ways. Recent studies have demonstrated that solid surface tension can compete with or even dominate over bulk elasticity on small length scales, leading to phenomena unanticipated by classic solid mechanics. Furthermore, mounting evidence suggests that the internal free fluid phase of gels can also contribute significantly to the mechanical response of highly compliant materials, from phase separation in static equilibria to the most extreme dynamics. In this work, we investigate the adhesion between polydimethylsiloxane (PDMS) gel microspheres and rigid glass substrates. We vary the stiffness and size of the microspheres as well as the surface energy of the substrate and directly image the adhered microspheres using brightfield microscopy. Our measurements show a range of adhesive contact geometries, from classic elastic to quantitatively wetting-like behavior. We understand our data with a model that incorporates adhesion, elasticity, surface tension, and phase separation.   

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Establishing a new adhesive contact is a highly dynamic process. The strong drive to conform into contact can rapidly deform a soft, sticky substrate, even when the contact is made quasi-statically. In compliant gels, establishing an adhesive contact may be governed by a complex interplay between adhesive, elastic, and capillary forces interacting with both viscoelastic and poroelastic dissipation mechanisms. We report a series of experiments designed to capture the three-dimensional evolution of a nascent adhesive contact between a compliant silicone gel and a rigid spherical indenter. We use interference, brightfield, and confocal microscopy, respectively, to directly measure the 3D, time-dependent deformation of the gel surface from high-speed sub-micrometer deformations at the initiation of contact to longer-time structural equilibration on the scale of tens of micrometers. These results provide new insights into the governing physics of dynamic adhesion across a broad range of length and time scales.   

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Adhesion of thrombus to wrinkled arterial lumen has attracted increasing attention as researchers are striving to resolve fouling issues in biological and artificial surfaces. While thrombus is viscoelastic, the majority of current research assumes only the limiting elastic behavior. This work therefore studies the delamination of a viscoelastic patch, as a model for realistic thrombus from a wrinkling surface. Specifically, a viscoelastic patch is attached to the surface of an elastic bilayer which undergoes wrinkling with increasing amplitude under increasing applied compression on the two ends. At a critical wrinkling amplitude, the patch starts to de-adhere from the wrinkled surface. Finite element and analytical models based on energy minimization are developed to undertake a parametric study with varying relaxation properties, adhesion energies, and loading rates to determine their correlations with critical delamination amplitudes. This allows the investigation of how viscoelastic characteristics such as relaxation time (related to thrombus age) and the loading rate (related to pulse rate) influence the de-adhesion process, providing insight into native arterial topographic renewal mechanisms and design strategies for topographic vascular grafts.   

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To get insights into the process of liquid-phase exfoliation of graphite into graphene, we study numerically and theoretically the dynamics of a peeling front in a system of two adhered carbon nanosheets immersed in water. The crack propagation is induced by lifting one of the edges with an assigned velocity v. A continuum model based on the equation of the elastica coupled with a model for the hydrodynamic pressure is compared to non-equilibrium molecular dynamics (MD) simulations of a graphene-water system. We quantify the viscous-dependent contribution to the external peeling force as a function of v, as separated from the adhesive contribution.
Contrary to primary expectations, we find that the lubrication forces have a negligible effect on the peeling force owing to the large slip length characterising our system. The inclusion of an entrance pressure drop due to converging flow streamlines upstream of the peeling edge is found to be necessary. Despite this inclusion, while the shape of the sheet in MD agrees with that in the continuum model, the viscous contribution to the peeling force predicted by MD is larger than that predicted by the continuum model. The mechanism underlying this discrepancy is explained by studying the drag forces on the peeling edge.   

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For survival in extreme environments, organisms have evolved adhesive mechanisms and materials that out-compete those of human technology. While the chemistry of underwater bio-adhesion is a source of valuable insight, the mechanics by which surfaces expel water and come in contact underlies the critical understanding of natural solutions and evaluating biomimetic analogs. Hydrophobicity of an adhesive surface has been shown to be crucial in removing water from a hydrophilic substrate, but the resulting contact is typically heterogeneous, with patches of unevacuated water. Thermodynamically, the hydrophobic part of any adhesive or surface drives the water out in presence of any other akin moiety.
Here, we present an FTIR-based imaging technique to spatially resolve and quantify the thickness of puddles formed between two solids in contact underwater. The technique is validated by comparing measured air gap thickness between a glass lens and glass prism with Hertzian contact theory and then applied to characterize the formation and drainage of patches of water as a soft PDMS lens approaches a functionalized and pristine glass surface. The work paves the way for a better understanding of the contact interfaces involved in the field of tribology, adhesion, and soft coatings.   

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The increasing demand for adhesives that stick to wet tissues is addressed by mimicking the adhesive mechanisms displayed by mussel foot proteins (Mfp). The versatile cohesive and adhesive molecular interactions offered by the pendant variable groups in Mfp is proposed to be the reason for robust underwater adhesion of Mfp. Though appending the Mfp-inspired functional groups to various polymer backbones is a popular strategy to obtain synthetic underwater adhesives, the quantitative aspects of the relationship between polymer structure and interfacial adhesion have gone largely uncorroborated. The main reasons for this obscure corroboration are the scarcities in bioinspired adhesive designs that do not change cohesive parameters upon altering pendant structure and experimental techniques that visualize interactions at the buried contact interfaces. The presentation will introduce the need for underwater adhesives and our synthetic strategies to overcome the challenges in underwater adhesion. Later, the relationship between polymer structure and interfacial adhesion identified using a combination of pull-off force measurements and sum-frequency generation spectroscopy will be revealed.   

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Many reversible adhesive systems have been proposed in the past that closely mimic the adhesion mechanisms in the gecko feet. These adhesive systems comprise an array of compliant micropillars that are fabricated on a substrate, which can be used to fasten two surfaces. Adhesive systems with directional adhesion have also been designed. One such system was proposed by Jeong et al. (PNAS, 2009, v106:5639-5644), where the compliant pillars were inclined with respect to the adherend surface. When the pillars were sheared along with the inclination, the adhesive strength was larger when compared to the case when the pillars were sheared opposite to the pillar orientation. The physical mechanisms that lead to this behavior have not been quantitively detailed yet, and this is the objective of our work. Using finite element simulations, we show that the difference in the strength is due to the nature of singularities in the elastic fields that exist near the interfacial pillar-substrate corners. We also show that this difference exists only when the pillar is compliant with respect to the substrate to which it is attached to. When the substrate is compliant with respect to the pillar, however, we do not observe a direction-dependent strength.   

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Many practical adhesives fail by the growth of interfacial cavities. Conventional contact adhesion measurements relate cavity growth to adhesive performance by recording force, displacement and contact images. Still, the influence of internal pressure within the cavities is an open question. Thus, in-situ cavity pressure measurements will enrich quantitative understanding of adhesive failure mechanisms. Here, we report a contact adhesion testing instrument that applies positive pressure at the contact interface to induce cavitation. We combine experiments with finite element analysis to develop models relating the pressure response in acrylic adhesives and silicone elastomers to their quanitative bulk and interfacial properties, including the elastic modulus and the critical strain energy release rate. Moreover, our experiments demonstrate that modifying interfacial pressure imparts control of failure modes during separation.   

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Hydrogels are elastic polymer-based materials that are highly water absorbent. While they are widely used in industry, e.g., to make contact lenses, microfluidic devices, or flexible electronics, their poroelastic behavior, and in particular their drying dynamics, is not well understood. Here, we investigate how different boundary conditions can lead to different compacting patterns of the skeletal polymer network and subsequent different drying dynamics over time. For example, by modifying only the boundary conditions, we are able to induce a growing, constant, or descending drying rate. We then use confocal microscopy measurements to develop a quantitative analytical model to predict drying under various environmental conditions.   

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The miniaturization of nanostructures and NEMS/MEMS is limited by the dramatic capillary pressures (>100 bar) that build up during drying or wetting. Understanding and controlling the resulting deformations and adhesion is critical for scaling down nanofabrication processes. In this work, we took full advantage of the atomic smoothness, mechanical strength and flexibility of 2D multi-layered van der Waals materials to design switchable nanocontainers/nanochannels by reversibly controlling the interfacial adhesion through capillarity. Our channels have sub-10-nanometre height and a flexible wall, whose mechanical configuration can be switched at will from open to closed by drying-induced capillary forces. Fast imaging of the channel closure reveals a stepwise motion, indicating adhesion front/meniscus depinning. We identified the geometry criterion for channel closure under the capillary pressure and analysed the elasticity-adhesion balance of dry channels to capture the condition for channel opening upon wetting. Our analysis yields the key geometrical and material parameters for the design of such elastocapillarity-switched channels. Finally, we used these channels as switchable nano-doors of nanocontainers entrapping sub-attolitre volumes.