Mechanics at soft interfaces

Most of us have noticed that paperclips, breakfast cereals, and small bubbles tend to float in clusters on a liquid surface. This phenomenon is a classical example of capillary interaction governed by surface tension of the liquid. If we replace the liquid with a bowl of soft solid e.g. jello and place tiny liquid drops on top of it, the drops attract and coalesce. Even though solid by nature, behavior of the soft interface in this case is analogous to a liquid surface.

Soft, polymeric solids possess some very exciting properties and that’s why their use in science and technology has seen unprecedented growth over the last decade. These solids can respond to a wide range of external stimulus such as pH, humidity, temperature, electric field or swelling, and consequently act as scaffolds in tissue engineering, valves in microfluidic devices, self-cleaning surfaces, and active components in soft robots and metamaterials. One of the distinguishing features of soft solids is the existence of an interfacial tension strong enough to govern the response of these materials at macroscopic scales, which has traditionally been attributed to elasticity alone. In this thesis, we explored the interaction of a soft interface with liquid drops, solid particles, and liquid flow to understand the wetting, adhesion, lubrication of soft solids. We also brought a new perspective to the classical creasing instability of a compressed soft solid where the interface folds onto itself and creates a self-contact.