Using Light to Control Dynamic Surface Tensions of Aqueous Solutions of Water Soluble Surfactants

We report the use of UV light to control the dynamic surface tensions of mixed surfactant systems containing sodium dodecyl sulfate, SDS, and 4,4′-bis(trimethylammoniumhexyloxy)azobenzene bromide, BTHA. The light influences the dynamic surface tension of these solutions by driving the isomerization of the azobenzene moiety from cis to trans. By combining use of the du Nouy ring, maximum bubble pressure, and Wilhelmy plate methods to measure dynamic and equilibrium surface tensions, the dynamic surface tension of an illuminated aqueous solution of these surfactants is demonstrated to be up to 25 mN/m lower than the dynamic surface tension of a solution not previously exposed to UV light. In contrast, the equilibrium surface tensions of these solutions change by less than 2 mN/m upon illumination. Measurements of quasi-elastic and static light scattering from these mixed surfactant solutions support our hypothesis that illumination influences the dynamic surface tension through its effect on the state of aggregation of the surfactant in bulk solution and thus the rate of mass transport of surfactant to the surface of the solution. A decrease in the number density of aggregates in solution upon illumination is observed. The decrease in dynamic surface tension that follows illumination is used to trigger the release of droplets of aqueous solution from a capillary at specified times. Patterned illumination is used to select the release of droplets from an array of droplets.