In the classic theory of solid adhesion, surface energy drives deformation to increase contact area whereas bulk elasticity opposes it. Recently, solid surface stress has been shown also to play an important role in opposing deformation of soft materials. This suggests that the contact line in soft adhesion should mimic that of a liquid droplet, with a contact angle determined by surface tensions. Consistent with this hypothesis, we observe a contact angle of a soft silicone substrate on rigid silica spheres that depends on the surface functionalization but not the sphere size. However, to satisfy this wetting condition without a divergent elastic stress, the gel phase separates from its solvent near the contact line. This creates a four-phase contact zone with two additional contact lines hidden below the surface of the substrate. Whereas the geometries of these contact lines are independent of the size of the sphere, the volume of the phase-separated region is not, but rather depends on the indentation volume. These results indicate that theories of adhesion of soft gels need to account for both the compressibility of the gel network and a nonzero surface stress between the gel and its solvent.wetting | adhesion | soft matter | surface tension | phase separation S olid surfaces stick together to minimize their total surface energy. However, if the surfaces are not flat, they must conform to one another to make adhesive contact. Whether or not this contact can be made, and how effectively it can be made, are crucial questions in the study and development of solid adhesive materials (1, 2). These questions have wide-ranging technological consequence. With applications ranging from construction to medicine, and large-scale manufacturing to everyday sticky stuff, adhesive materials are ubiquitous in daily life. However, much remains unknown about the mechanics of solid adhesion, especially when the solids are very compliant (3-5). This limits our understanding and development of anything that relies on the mechanics of soft contact, including pressure-sensitive adhesives (6, 7), rubber friction (8), materials for soft robotics (9-12), and the mechanical characterization of soft materials, including living cells (13)(14)(15)(16)(17).Adhesion is favorable whenever the adhesion energy, W = γ 1 + γ 2 − γ 12 , is positive, where γ 1 and γ 2 are the surface energies of the free surfaces and γ 12 is the interfacial energy in contact. When W > 0, the solids are driven to deform spontaneously to increase their area of contact, but at the cost of incurring elastic strain. The foundational and widely applied Johnson-Kendall-Roberts (JKR) theory of contact mechanics (18, 19) was the first to describe this competition between adhesion and elasticity. However, it was recently shown that the JKR theory does not accurately describe adhesive contact with soft materials because it does not account for an additional penalty against deformation due to solid surface stress, ! (4). Unlike a fluid, the surface stress of a solid is not a...

An adhesively stressed thin film of a soft hydrogel confined between two rigid flat substrates auto-roughens with its dominant wavelength () exhibiting pronounced dependence on the film thickness (H). A linear stability analysis confirmed that this long wavelength instability (~7H) is due to an elasto-capillary effect, the implementation of which required direct measurements of the surface tension and the elasticity of the gel. The surface tension of the gel was estimated from the fundamental spherical harmonic of a hemispherical cap of the gel that was excited by an external noise. The shear modulus () of the gel was determined from its resonant shear mode in a confined geometry. During the course of this study, it was found that a high density steel ball submerges itself inside the gel by balancing its excess weight with the accumulated strain induced elastic force that allows another estimation of its elastic modulus. The large ratio (1.8 mm) of the surface tension to its elasticity ascertains the role of elasto-capillarity in the adhesion induced pattern formation with such gels. Experimental results are in accord with a linear stability analysis that predicts that the rescaled wavelength 27 . 0

ABSTRACT.A liquid drop moves on a solid surface if it is subjected to a gradient of wettability or temperature. However the pinning defects on the surface manifesting in terms of a wetting hysteresis, or a first order non-linear friction, limits the motion in the sense that a critical size has to be exceeded for a drop to move. The effect of hysteresis can, however, be mitigated by an external vibration that can be either structured or stochastic, thereby creating a directed motion of the drop. Many of the well-known features of rectification, amplification and switching that are generic to electronics can be engineered with such types of movements. A specific case of interest is the random coalescence of drops on a surface that gives rise to a self-generated noise.This noise overcomes the pinning potential diffusively, thereby generating a random motion of the coalesced drops. Randomly-moving coalesced drops themselves exhibit a purely diffusive flux when a boundary is present to eliminate them by absorption. With the presence of a bias, the coalesced drops execute a diffusive drift motion that can have useful application in various water and thermal management technologies.2

ABSTRACT. We study the interaction of small hydrophobic particles on the surface of an ultra-soft elastic gel, in which a small amount of elasticity of the medium balances the weights of the particles. The excess energy of the surface of the deformed gel causes them to attract as is the case with the generic capillary interactions of particles on a liquid surface.The variation of the gravitational potential energies of the particles resulting from their descents in the gel coupled with the superposition principle of Nicolson allow a fair estimation of the distance dependent attractive energy of the particles. This energy follows a modified Bessel function of the second kind with a characteristic elastocapillary decay length that decreases with the elasticity of the medium. An interesting finding of this study is that the particles on the gel move towards each other as if the system possesses a negative diffusivity that is inversely proportional to friction. This study illustrates how the capillary interaction of particles is modified by the elasticity of the medium, which is expected to have important implications in the surface force driven self-assembly of particles. In particular, this study points out that the range and the strength of the capillary interaction can be tuned in by appropriate choices of the elasticity of the support and the interfacial tension of the surrounding medium. Manipulation of the particle interactions is exemplified in such fascinating mimicry of the biological processes as the tubulation, phagocytic engulfment and

ABSTRACT:We report, for the first time, some experimental observations regarding a new type of long range interaction between rigid particles that prevails when they are suspended in an ultrasoft elastic gel. A denser particle submerges itself to a considerable depth inside the gel and becomes elasto-buoyant by balancing its weight against the elastic force exerted by the surrounding medium. By virtue of a large elasto-capillary length, the surface of the gel wraps around the particle and closes to create a line singularity connecting the particle to the free surface of the gel. Substantial amount of tensile strain is thus developed in the gel network parallel to the free surface that penetrates to a significant depth inside the gel. The field of this tensile strain is rather long range owing to a large gravito-elastic correlation length and strong enough to pull two submerged particles into contact. The particles move towards each other with an effective force following an inverse linear distance law. When more monomers or dimers of the particles are released inside the gel, they orient rather freely inside the capsules they are in, and attract each other to form close packed clusters. Eventually, these clusters themselves interact and coalesce. This is an emergent phenomenon in which the gravity, the capillarity and the elasticity work in tandem to create a long range interaction. We also present the results of a related experiment, in which a particle suspended inside a thickness graded gel moves accompanied by the continuous folding and the relaxation of the gel's surface.