Response of floating grains on a capillary Faraday wave in the dense limit

When macroscopic grains float on a water-air interface, they aggregate into a static cluster due to attractive gravity-induced capillary interaction between the grains. Here we study what happens when the grains in the cluster are excited using capillary Faraday waves with a wavelength comparable to the grain size. The grains are found to exhibit cooperative motion in domains which tend to become larger as the particle concentration $\phi$ increases towards the jamming point. We measure the average grain velocity and the velocity-velocity correlations of the grains in space and time as a function of $\phi$ and the driving amplitude $a$.