Capillarity of Soft Interfaces

Soft materials are among the world's most useful and versatile engineering materials. Indeed, Nature has used soft materials for millions of years to create sophistication far beyond our current engineering capacity. We find soft materials everywhere, from living tissue, to rubbers (e.g. car tyres), foods, cushioning (e.g. shoes, electronic devices), flexible adhesives and gels. Recently, their use has boomed due to their flexibility, ease of manufacture, and as they help in the cheap, accurate and fast production of micro- and nanotechnology.

Our ability to manipulate and engineer soft materials fundamentally depends on having a good understanding of their mechanical behaviour.

However, recent work has shown that soft materials often display unexpected and unusual phenomena, frequently disobeying well-established laws of mechanics. For instance, soft solids spontaneously wrinkle or show creasing instabilities, they exhibit unusual new fracturing phenomena, and Young's law of wetting and JKR's law of adhesion break down on soft surfaces.

A key reason for such anomalous behaviour in soft materials is the presence of surface capillary forces. Just as surface tension acts on liquids, so too does it act on solids: by minimising surface area. In stiff solids, surface forces are too weak to deform the solid. However, in soft materials such as rubbers and gels, they can significantly change the solid's shape. Intriguingly, this means that soft materials can exhibit a whole spectrum of new behaviour -- sometimes acting like solids, sometimes more like liquids (when capillarity is important), and often exhibiting a whole new class of intermediate phenomena.

The perspective that capillary interactions can both reshape and stabilize soft materials also opens new routes to design. These could include new modes of actuation in soft robotics, the use of functionalized colloids to tune interfacial forces, or the deployment of chemical groups at the surface of elastomers.

Understanding and engineering soft solids relies crucially on better insight into the basic physical laws that govern the capillarity of soft interfaces -- yet these have only very recently started to be explored. An improved understanding will also allow for a better understanding of how biological systems are constrained by, and could even exploit, elastocapillary effects. For example, durotaxis of cells on substrates and interfacial controls during morphogenesis or tumor growth are poorly-understood.

With this workshop we wish to develop a common view on interfacial forces in soft matter: Can one capture the capillarity of soft interfaces by a unifying framework? What details of the underlying physical chemistry are important in classifying systems? Can we establish common benchmarks to characterize mechanical properties for a broad range of soft interfaces? To achieve this synthesis, we aim to bring together specialists from chemistry, physics, biology, engineering, and industry, with each group presenting the state-of-the-art in their respective fields.