On the role of capillary instabilities in the sandcastle effect

Using acoustic resonators, we have investigated the mechanical stiffness of contacts between rough surfaces. In the first part, the underlying acoustic model is validated with experiments showing a transition from elastic to inertial loading. The second part is concerned with the increase in contact stiffness induced by transient exposure to a humid environment. A novel mode of surface deformation is proposed, which builds on a capillary instability. Under certain conditions, a slight decrease in the mean distance between the two surfaces may induce a rather strong increase in capillary attraction, while leaving the elastic forces of repulsion almost constant. The thus-created negative differential spring constant induces a collapse of the gap in-between neighboring load-bearing asperities. The initial decrease in distance may either be induced by local asperity creep or by distortions of the contacting surfaces on a larger scale, which improve the interlock of the asperities at a small cost of strain energy.