Soft wetting: surface effects in highly deformable amorphous materials
As largely demonstrated in the last two decades, elasticity plays an important role in surface physics. Phenomena such as surface reconstruction, surface segregation, surface adsorption, elastic instabilities, self assembly, and nano-structuration of crystalline solids are directly induced by surface stresses. However, it is only recently that the soft condensed matter community has discovered this field of physics and started to extend concepts to soft amorphous solids. I will review the important thermodynamic results of solid surface physics, which make the distinction between two quantities that turn out to be equal in an incompressible liquid: surface energy and surface stress. I will then show that important qualitative phenomena are not correctly described by macroscopic theories and require a treatment were nanoscopic aspects are explicitely introduced. These concepts will be illustrated by recent experimental results obtained on gels and elastomers, answering qualitative questions like: Why is the spreading of a drop much slower on an soft solid than on a rigid substrate? Does elasticity modifies the contact angle made by a drop on a soft solid? How does a drop deform a soft solid? To conclude, I will open perspectives on the functionalisation of soft solids.