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 mi xed surfactant systems containing sodium dodecyl sulfate, SDS, and 4,4'-bis (trimethylammoniumhexyloxy)azobenzene bromide, BTHA. The light influences t he 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 dynam ic and equilibrium surface tensions, the dynamic surface tension of an illu minated aqueous solution of these surfactants is demonstrated to be up to 2 5 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 qu asi-elastic and static light scattering from these mixed surfactant solutio ns 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 sur face of the solution. A decrease in the number density of aggregates in sol ution upon illumination is observed. The decrease in dynamic surface tensio n that follows illumination is used to trigger the release of droplets of a queous solution from a capillary at specified times. Patterned illumination is used to select the release of droplets from an array of droplets.