The influence of surface-active contamination on the initiation of wind waves

Abstract

A small-scale apparatus was used to examine the effects of surface materials on the initiation of waves on water by the action of wind. Rigorous procedures were followed to ensure freedom from contamination of the initial water surface, and the effect of various forms of subsequent contamination were investigated, using a sensitive moire fringe wave slope observation technique. Provision was made in the design of the water container for insoluble films to circulate, avoiding the downwind buildup of contamination.In measurements on pure water surfaces, no evidence of a ‘critical wind velocity’ was found, although slightly contaminated water was found to be appreciably less responsive to the wind excitation at wind speeds up to 4 m s−l. Measurements on monolayer-covered surfaces showed that, at these wind speeds, a compressed monolayer of relatively high surface tension and low dilational elasticity can have a greater stilling effect than one of low surface tension and high dilational elasticity. Measurements on a series of solutions of surface-active material, covering a range of surface tensions from 63 to 26 mN m−l at a concentration expected to reduce dilational elasticity by bulk-to-surface diffusion, indicated that the damping effect becomes steadily greater as the surface tension decreases.