Research Projects

Soft materials are increasingly finding wider and newer applications in research laboratories, in industries, as well as in our daily lives. Their increasing importance and relevance in critical technologies have motivated researchers to examine their behavior closely and to develop suitable theoretical frameworks that may help us understand the mechanisms underlying their behavior as well as allow us to predict their response robustly with high precision. These studies have revealed a rich range of interesting phenomenology when two or more soft phases come together at an interface. One particular example of such a soft interface that occurs ubiquitously in nature is that of a thick soft solid covered with a thin bendable sheet covering, e.g. in human, animal, and fruit skins. The broken symmetry at the interface between two different materials, results in interfacial forces, which, for a large class of soft materials, may be strong enough to cause significant deformations leading to a glossary of intriguing, often non-intuitive effects usually classified as elastocapillary phenomena. While it is well appreciated that the surfaces of soft solids entail large surface energies that play a significant role in determining the deformations of the surface, the nature and magnitude of the corresponding force, often called the surface tension, continue to remain debatable. The conceptual difficulty involved can be summarized in the following manner. For the case of liquids at thermodynamic equilibrium, surface tension (force per unit length) is a well-understood concept—it is isotropic and has a value numerically equal to the surface energy (energy per unit area). However, solids are fundamentally different from liquids, as they have a finite shear rigidity along with a well-defined ‘target metric’ that corresponds to the stress-free configuration. As a result, the molecules of a solid at an interface cannot flow like a liquid but are displaced from their preferred relative distances at the cost of a restoring elastic energy, giving rise to the surface stress. The situation is further complicated by the presence of dissipative frictional stresses at the interface in any case where dynamics is involved. The nature of the frictional response of a soft interface differs in important ways from the well-established laws of friction for two rigid solid surfaces moving relative to each other. In particular, recent studies have shown the existence of novel effects of geometrical origin that depend on the relative shape of the two surfaces involved. One of the difficulties in settling the basic conceptual question in the field is due to the limitations in the techniques available to unambiguously measure surface stresses experimentally. The most popular method for measuring the surface tension of soft solids has been based on measuring the contact angle made by a sessile liquid drop deposited on the surface of a soft solid. While the contact angle method is known to work very well for rigid substrates, the measurement of contact angle on a soft substrate and its interpretation become extremely complicated due to the non-trivial and singular deformation of the substrate near the three-phase contact line. In this project, we propose to develop novel techniques that can be used to measure both surface tension and frictional stresses at a soft interface. These techniques will then be used to study the nature of the surface stresses at these interfaces and systematically measure their dependence on the elastic and viscoeleastic properties of the two surfaces, their relative geometries, and the relative velocity at the interface. The technique proposed will particularly be suitable for measurement of subtle effects of geometry on the nature of the surface stresses.