Surface wave on a particle raft

The mechanical properties of particle laden interfaces are investigated by studying capillary wave propagation along the interface. Interfaces are coated with monodisperse silica hydrophobic particles with diameters ranging from 35 m to 159 m. The surfaces are prepared with a particle density just above that required for random close packing, so that no macroscopic wrinkles can be observed. Measurements of wave celerity versus wavelength and a comparison of the results with theory allow us to define the stretching modulus and the bending stiffness of such an interface. These quantities are for the first time measured independently and under a large range of frequencies (100 Hz900 Hz). Particle diameters and contact angles have been varied systematically. We observe that the stretching modulus does not depend on the particle size, while the bending stiffness exhibits variations with the square of particle diameter. These results are confronted with previous results on quasistatic assays and with theoretical predictions on the capillary origin of bending stiffness, showing a good agreement. Similarities and differences are discussed. Capillary wave propagation along a liquid/vapour interface is modified by the presence of particles.