The Effects of Surfactant on Certain Air—Sea Interaction Phenomena

A comprehensive laboratory study has been made on the following typical phenomena at the air-sea interface both for ordinary tap water and for water containing a soluble surfactant (NaC12H25SO4): (1) generation of wind waves; (2) wind shear stress and wind setup; and (3) growth of regular waves by the wind. The addition of the surfactant to the water shows a large suppression of the wind-generated waves, and its effect increases with increasing concentration of the surfactant within the range of our experiment. When the wind waves attenuate partially, the spectral density near the dominant frequency region shows similarity. For the maximum concentration used (2.6 x 10-2%) wind waves are almost completely suppressed up to a wind speed U10 ≅ 15 m/s, where U10 is the wind speed at height z = 10m. For U10>19m/s, wind waves are generated which are similar to those on tap water. A large decrease of the wind shear stress is observed when the wind waves are suppressed almost completely by the surfactant. An empirical relation for the drag coefficient has been obtained for this case, which is slightly different from that by Van Dorn (1953). An empirical relation for the drag coefficient covering a very wide range of wind speed (U10:8–35 m/s) has also been obtained for ordinary tap water. The surface slope has been measured and related to the friction velocity of the wind. It is shown that the same relation holds for tap water and for water containing surfactant, if the friction velocities measured for the respective waters are used in the relation. The measured growth rate of the fundamental frequency component of regular waves on tap water is greater, by a factor of 2, than Miles’ (1959) growth rate, and a little greater than Snyder and colleagues’ (1981) growth rate. The growth rate of the regular waves is greatly reduced by the addition of surfactant. However, the relation between the growth rate and the friction velocity of the wind is little affected by the surfactant, because the friction velocity is diminished by the presence of surfactant.