Hydrodynamic signatures of stationary Marangoni-driven surfactant spreading*

Surfactant spreading at air-water interfaces is driven by flow setup by surface tension gradients (Marangoni stress) established by surfactants themselves. We experimentally probe the nature of steady surfactant transport on the interface when the resulting flow is strongest in a thin boundary layer near the interface. In particular, we present three experimental hydrodynamic signatures to distinguish between two limiting cases, viz. adsorption versus dissolution dominated transport, without invoking the surfactant's physico-chemical properties. In a region much larger than the surfactant source, but much smaller than the interfacial area, the steady-state fluid velocity assumes a self-similar form whose magnitude decays as a power-law with the distance from the source. We experimentally demonstrate that this power-law possesses an exponent -3/5 in adsorption and -1 in dissolution dominated flow. Explicit measurement of boundary layer and shear stress provide additional hydrodynamic signatures of surfactant transport mechanisms in the two limiting cases. We test this criterion against two known surfactants, Sodium Dodecyl Sulfate and Tergitol 15-S-9, and apply the results to camphoric acid, with unknown surface properties.