Session G35: Deformation of Soft Solid Surfaces Driven by Surface Stress

Elastocapillary Adhesion of Soft Gel Microspheres

While elasticity has long been understood to be important in determining the relative “stickiness” between non-conformal surfaces, in recent years capillary forces have emerged as playing key roles in adhesion with highly compliant materials. For example, recent studies have demonstrated that solid surface tension can compete with or dominate over bulk elasticity in governing contact mechanics on small length scales, and mounting evidence suggests that the internal free fluid phase of compliant polymer gels also contributes significantly to mechanical response via both poroelasticity and classic capillary wetting. In this work, we experimentally investigate the adhesion between polydimethylsiloxane gel microspheres and rigid glass substrates. By varying the asperity size, adhesion energy, and gel material properties, we observe a continuous transition from classic elastic to capillary-like adhesion, with contact line deformation mediated by a fluid contact zone that phase separates from the gel. Interestingly, the intermediate-compliance gels demonstrate a broad range of equilibrium contact deformations, reflecting a shallow energetic minimum that likely contributes to the robustness of everyday adhesives. We develop a new model incorporating elastocapillary and poroelastic mechanics that captures the complete range of adhesive behavior.   

Shape of the capillary ridge on ideal elastomeric substrates

Soft solids play an important role in stretchable electronics, cellular membranes and water collection. Upon introduction of a liquid contact line, soft solids can deform substantially causing changes to geometry and dynamics. On the nanoscale, the deformation at the liquid/solid contact line is a capillary ridge. We study these capillary ridges for a system which consists of a thin polymer film in the melt state atop an elastomeric poly(dimethylsiloxane) (PDMS) film. We use a thorough washing procedure to create our PDMS films which creates a true elastomer composed of only a crosslinked network. Our bilayer polymer films sit atop a solid silicon substrate. The liquid polymer layer dewets on the soft elastomer PDMS base. We vary the thickness of the underlying elastomer film, which changes the effective stiffness, therefore changing the size of the capillary ridge. We use atomic force microscopy to directly measure the shape of the capillary ridge in our system.   

Capillary Forces Drive Deformation and Break-up of Soft Elastic Beams

Concentrated packings of highly swollen granular hydrogels, commonly referred to as microgels, undergo a jamming-like transition and exhibit solid-like behavior under small strains. The rheological properties of these packed microgels, including the yield stress and elastic shear modulus, can be tuned over several orders of magnitude through small changes in the overall polymer concentration. Recently, these packed microgels have been utilized in the 3D-printing community as a sacrificial support bath to provide mechanical stability to fluid phases as they are structured into complex geometries and solidify into soft elastic solids. However, capillary forces acting at the interface between immiscible pairs of printed material and support bath can lead to destabilizing instabilities, resulting in the break-up of the fluid phase prior to solidification or deformations of the soft solids after solidification. Understanding and controlling these instabilities is crucial for guiding the manufacturing of soft materials with low moduli and yield stresses. Here, we use packed granular microgels swollen in aqueous and organic solvents to explore the interfacial instabilities of 3D-printed fluid and soft elastic microbeams within immiscible support baths. By leveraging the highly tunable rheological properties of these packed microgel systems and the control offered by 3D-printing, we systematically explore the stability of fluid and soft elastic microbeams across a range of yield stresses and beam diameters. For fluid beams within immiscible support materials, we find the stability can be predicted through the interfacial tension and the feature size of the beam. For soft elastic beams printed within viscous fluid support baths we observe the emergence of new instabilities when the surface stresses are comparable to the yield stress of the microbeam. We observe the microbeams axially contracting under capillary forces resulting in the coiling of the ends and spontaneous buckling of the center of the beam.   

Clustering of micropillars in soft solids

Surface stresses can deform soft solids. Recent experiments have explored this effect using a compliant material textured with an array of cuboid micropillars. Elastocapillarity can cause isolated pillars to flatten and become rounded. Interestingly, when pillar arrays are sufficiently dense, clustering and long-range pattern formation is observed. We investigate this behavior with both theory and finite element simulations, probing the clustering transition as a function of pillar geometry and the thickness of the base layer. We compare this clustering effect to related instabilities observed in soft solids and discuss how our results can be used to guide experimental design.   

Dewetting on a free-standing ideal elastomer film

The dewetting of liquids from soft surfaces offers a powerful method to probe pattern formation in systems where elasticity and capillarity compete. Here we present the dewetting of a thin polymer film on top of a free-standing elastomeric poly(dimethylsiloxane) (PDMS) film. Upon preparation, the PDMS film is a gel. To prepare a true elastomer film, the sample must be subjected to a series of solvent washes to remove uncrosslinked chains. The film is then made free-standing by transferring it from a substrate onto the surface of a water bath and subsequently onto a support washer. This system allows us to study the dewetting of a liquid polymer on a soft elastomer film without the influence of a rigid underlying substrate. We investigate the elastocapillary interactions by which droplets can interact at a distance, mediated by the elastic film.   

Soft Materials at surfaces and interfaces: Elastocapillarity

In recent years, there has been a significant interest in understanding the interaction between a liquid’s surface tension and a solid’s elasticity: elastocapillarity. In particular, liquids can generate significant deformations of highly compliant materials. The capillary interaction between liquids and soft deformable solids can provide novel avenues for self-assembly and self-organization. These elastocapillary interactions are highly relevant in a wide variety of systems including capillary origami, kirigami, and folding, soft tissues, wetting of fibers and hair, and micro-patterning of soft surfaces. In this talk I will summarize recent work on the capillary interactions of liquid droplets with elastic surfaces.   

Master Curves for Poroelastic Spherical Indentation

Theoretical and numerical analyses are conducted to rigorously construct master curves that can be used for interpretation of spherical indentation test for poroelasticity characterization. Poroelastic contact between a rigid sphere and a fully saturated porous medium consisting of slightly compressible solid and fluid phases is considered in this work. Both displacement-controlled load relaxation test and force-controlled creep test with a surface drainage condition assumed to be fully permeable, fully impermeably or impermeable over the contact area but permeable everywhere else are examined.

 

We may view such contact problems as the poroelastic extension of the Hertzian contact. In principle, mechanical properties could be determined from the undrained and drained limits while the hydraulic diffusivity is reflected in the transient response. Theoretical solutions are first derived within the framework of Biot's theory using the McNamee-Gibson displacement function method. We show that for this class of poroelastic contact problems, relaxation of the normalized indentation force or displacement is affected by material properties through a single derived material constant only. Finite element analysis is then performed in order to examine the differences between the theoretical solutions, obtained by imposing the normal displacement or force over the contact area, and the numerical results where frictionless contact between a rigid sphere and the poroelastic medium is explicitly modeled. Master curves based on the theoretical solutions with the validity supported by the numerical analysis are proposed. Our analyses indicate that between the two testing approaches, the normalized indentation response from the displacement-controlled method appears to be less sensitive to uncertainties in material properties, suggesting that the displacement-controlled method is more reliable for determining hydraulic diffusivity from a theoretical point of view. Application of these master curves for the ramp-hold loading scenario is also discussed.

    

The blob is back: diffusion of an active elastic membrane

This work deals with the free motion of an active elastic membrane defined as a two-dimensional network of active stochastic particles interacting by nonlinear hard springs. Notice that in this model, the rotational dynamics of each particle is stochastic and independent from the others, and yet it moves. By proposing a discrete model, it is numerically found that irrespective of the active membrane's size, its long-time mean-square displacement --with respect to its center of mass-- behaves linearly in time. However, at short times, it is seen that 'the blob' has a superdiffusive dynamics (with a power in time alpha around four and five) which depends on the blob's size. For example, for a small blob (N=25 particles) it is observed that alpha is around five, whereas for N=961, alpha is around four. The effects of rotational noise, propulsion speed and nonlinearity on the motion of this 'blob' are also studied.   

Cracks or pulses? Investigating slip nucleation at soft frictional interfaces

The transition from stationary to dynamic sliding between two contacting elastic bodies can occur via the propagation of unsteady interface rupture fronts. These fronts can broadly be manifest either as crack-like or pulse-like modes, depending on the local elastic fields, and can propagate with a wide range of velocities. Understanding the nucleation and propagation of these rupture modes at elastic contacts has attracted significant attention over the past two decades, especially given their consequences for systems as diverse as soft interfaces and crustal earthquakes. In this work, we focus on comprehending fundamental nucleation mechanisms for rupture fronts and link them to the remote boundary conditions applied while sliding an elastic block against a rigid substrate. We develop a two-dimensional elastic model comprised of bulk lattice-spring networks that dynamically break and readhere interface bonds via distance-based criteria. We closely investigate the characteristics of rupture modes for a homogeneous interface, tracing their evolution from nucleation up until complete traversal through the interface. We show that the selection of the rupture modes also depends intimately on the local stress fields at the interface.