STATIC AND DYNAMIC-RESPONSE OF NATURAL MULTICOMPONENT OCEANIC SURFACE-FILMS TO COMPRESSION AND DILATION - LABORATORY AND FIELD OBSERVATIONS

The quasi-static elasticity (Gibbs' elasticity) of sea surface films c ollected in North American coastal waters has been examined using a la rge number of surface pressure-area isotherms. The films examined show considerable variability in elastic properties. Differences among fil ms appear to be the result of chemical variability due to differences in source; they are also affected by dynamic physical processes (e.g., film compression). Elasticities of adsorbed films from subsurface and microlayer water are observed to exhibit a bimodal distribution at 0. 5 mN m-1 reference spreading pressure; films from organized banded sli cks are shown to have generally higher elastic moduli. Mechanisms of s urface selectivity and competitive adsorption are suggested to explain this effect; results of film aging and work hardening studies are pre sented in support. Significant hysteresis effects are observed for cyc lical compression and dilation of the films suggesting the prevalence of relaxation processes that lead to compositional changes and higher elastic moduli. Field measurements of capillary and ripple spectra are presented to infer information on wave damping and dynamic viscoelast icity. Results obtained in regions where surface convergence is absent imply damping enhancement over a broad frequency range consistent wit h theory for films of low elastic moduli. Films found in surface conve rgence zones, the result of surface upwelling, exhibit damping enhance ments consistent with higher elastic moduli. Damping maxima are observ ed in multiple frequency bands; this banding is not readily explained with current theories. Results similar to these have been observed in laboratory studies with other chemical systems. In both the laboratory and field experiments, the tendency of the films to group into a few subsets with similar surface characteristics implies that natural film s might be modeled by a small number of representative end members. Th is would facilitate remote sensing applications by providing a basis f or determining the effects of films on surface waves under various con ditions (e.g., diurnal variation, history of variability of wind stres s).