The capillary interaction between pairs of granular rafts.

When an object is placed at the surface of a liquid, its weight deforms the interface. For two identical spherical objects, such a deformation creates an attractive force, leading to the aggregation of the two-body system. Here, we experimentally investigate the interaction between two granular rafts, formed by the aggregation of dense millimeter-sized beads placed at an oil-water interface. The interfacial deformation created by such a two-dimensional object exceeds by at least an order of magnitude the deformation of a single bead. This leads to unusually high capillary forces which strongly depend on the number of particles. Likewise, because the raft grows in size as more particles are added, the viscous drag experienced increases along with the capillary attraction, leading to a non-trivial dependence of the balance of forces on the number of beads. By studying the relative motion of two granular rafts in relation with the interfacial deformation they generate, we derive a model for the observed speed profiles. With this work, we generalize how the capillary interaction between two non-identical complex structures evolves with their respective geometry.