Soft matter at interfaces: Adhesion, wetting, and capillary interactions

We attempt to improve existing adhesives by patterning materials in order to serve as a gecko mimic. Two practical attempts are described here: clustering, as a hierarchy mimic, and ordering, as a means of directing crack front propagation. The results of our experiments mostly suggest that the peak force per unit area Fmax/A is approximately constant, hence structured adhesives at the scale investigated here do not improve adhesive properties. We then perform computer simulations to address a larger parameter space. Depending on simulation parameters we are able to distinguish two different pillar detachment regimes; a stochastic regime, where individual pillars detach in a random sequence, and a deterministic regime similar to the ones found in our laboratory experiments, where detachment occurs by crack propagation. Further simulations are performed in the deterministic regime. We find that the peak force, normalised by pillar count, at the onset of adhesive failure Fmax/N scales is approximately constant at low separation velocities, and develops as Fmax/N ~ log v at higher separation velocities. We find a positive effect of clustering, with a reported increase in Fmax/N of approximately 4–12% for square clustered samples, compared to a homogeneous square array of pillars. Patterns with random pillar placement perform on par with samples with square pillar arrangement.

We study the wetting properties of a range of different hydrogels. Hydrogel chemistry and morphology are tuned through the choice of monomer (acrylamide, Nisopropylacrylamide, and poly(ethylene glycol) methyl ether methacrylate). The effect of charge and salt is investigated by copolymerization of charged monomers with the hydrogel. pH-sensitive, switchable hydrogels are synthesised by copolymerising amino groups. Post-synthesis swelling depends on crosslink density and the presence of charge, either permanent or pH-induced. Counterintuitively, we find that oils spread completely, and that water only partially wets the hydrogel/air interface, regardless of the hydrophilic nature of these materials. We investigate this effect through static and dynamic contact angle measurements of air and oil at hydrogel/water interfaces. We observe unusually high contact angle hysteresis of oil on N-isopropylacrylamide gels, and demonstrate that this is a result of the adsorption of polymer chains at the air/water interface. Hysteresis is significantly reduced in permanently charged gels, but partly restored upon addition of salt to the water phase. To our knowledge, this is the first report of salt-induced contact angle hysteresis. Although the swelling behaviour of switchable, pH-sensitive hydrogels at pH << pKa is comparable to that of permanently charged hydrogels, their contact angle hysteresis is not significantly different from neutral N-isopropylacrylamide gels, or switchable gels at pH >= pKa.

Spherical colloidal particles at liquid interfaces with non-zero deviatoric curvature cause a quadrupolar deformation of the interface at the three-phase contact line. We measure the resulting capillary pair interactions as a function of interfacial curvature for microgel core-shell particles at mineral oil/water interfaces. The long-range capillary attraction is found from analysis of approach trajectories of pairs of particles. These data are complemented in the short-range from the Boltzmann statistics of bond length fluctuations to obtain the full pair potential. We show that the capillary interactions resulting from the interfacial anisotropy are strongly dependent on interfacial curvature. As a consequence of the strong interactions in quadrupolar symmetry, the self-assembly process proceeds as diffusion-limited aggregation on a square lattice.

Previous experiments suggest a correlation between the deviatoric curvature of an interface, and the strength of capillary interactions between particles adsorbed to that interface, but fall short to reproduce theoretical predictions. In this chapter we attempt to replicate the experiment, using a potentially more straightforward approach with solid spherical colloids, which lack the deformability of the microgels used previously. However, we find an unforeseen short range and low strength of capillary interactions. A bond angle analysis shows that interactions depend only weakly on the deviatoric curvature of the interface, and are significantly influenced by the chemical composition of the oil phase.