Capillary Wave Propagation on Water Nonuniformly Covered with a Solid Film

Capillary wave propagation has been studied on water nonuniformly covered with a solid film. The wave propagation characteristics have been correlated with the morphology of the film which was observed with a fluorescence microscope. The propagation characteristics were found to be determined by the connectivity of the solid islands rather than by their coverage. When the film consists of solid islands much smaller than the wavelength, the islands are practically invisible to the wave even when they cover half of the water surface; the film made of larger islands shows the properties of a solid sheet at the same coverage. This is because the connectivity is easily established in a film composed of larger islands. When the capillary wave traverses an isolated island much larger than the wavelength, the wave is characterized as the one on a solid film of an infinite extent. When the size of the island is not much larger than the wavelength, however, the apparent amplitude of the wave can increase on coming out of the island. This is the first direct evidence that the capillary wave characteristics depend on the detailed yet macroscopic structure of the film.