Floaters on Faraday waves : clustering and heterogeneous flow

Abstract

In this Thesis, a monolayer of macroscopic particles floating on a surface wave is studied. The liquid is pure water so that the particles are at a water-air interface. The particles are hydrophilic polystyrene spheres with average radius around 0.31 mm and they are slightly heavier than water. The surface wave is a Faraday wave, which is a wave generated at a liquid-air interface when a layer of liquid is exposed to a vertical periodic driving. We demonstrate the role of the particle concentration on the spatial distribution of our hydrophilic heavy spheres floating on a water standing Faraday wave. At low concentration, we observe that our floaters form clusters at the antinodes of the standing wave. On the other hand, at high concentration with the same floaters, inverted patterns, i.e., clustering patterns at the nodal lines of the wave, form. The wave drift and the attractive capillary interaction are the mechanisms driving the clusters. By suggesting an energy calculation, we confirm both the experimentally observed clusters and the transition point from antinode clusters to node clusters. Heterogeneous dynamics of the floaters on nonlinear chaotic waves, namely capillary Faraday wave with a wavelength order of floater size, is the other subject studied experimentally in this Thesis. We observe that the resultant flow is composed of the floater domains, i.e., small floater groups initially moving together, breaking up in a certain way some time later. During the break-up process, the initial floater group morphologically deforms. We develop a morphological approach to quantify the deformation of the domains in time. We show that our suggested morphological characterization of the floater groups is a valuable alternative representation of the dynamical heterogeneities generated by such a particulate flow, which up to now have mainly been quantified by the four-point dynamic susceptibility in the literature.