Probing the erosion and cohesion of a granular raft in motion

Particles at a liquid interface can interact through capillarity, and depending on their size, shape, or surface chemistry, it may lead to their aggregation. Yet the two-dimensional structures thus created can be eroded by strong enough flows or as described in this paper via their own motion along a deformed interface. Here we experimentally highlight an unexpectedly high resistance of such a granular raft to erosion, which cannot be explained by the usual linear calculation of the capillary interaction. To clarify this result, we develop a model experiment in which two particles are horizontally moved along the liquid interface at a chosen velocity until they separate. We deduce again a cohesive force that exceeds the capillary attraction expected by one or two orders of magnitude. A precise description of the geometry of the contact line around a sphere and of its vertical position seems to account rather well for the experimental results.