Using light-actuated photosurfactants for liquid mixing and sculpting

Photosurfactants are soluble surface active agents that can change conformation under light of different frequencies. Under visible light they conform to the trans state, while illumination by shorter wavelength UV light transforms trans surfactants to the cis state by bending of their hydrocarbon tails. The process is reversible. This talk has two objectives. First, to introduce mathematical models that can describe the two surfactant species, their exchange kinetics in the bulk due to light switching, their adsorption/desorption from interfaces (e.g. air-water), and the exchange between them at the interface. Second, to use these models to study theoretically the fundamental problem the utilization of non-uniform light gradients to induce mixing and interfacial non-uniformities in horizontal liquid layers covering a substrate. It is shown that the induced Marangoni flows can produce vortical structures that enhance mixing. In addition, we solve the inverse problem to determine the required light gradients required to produce a pre-determined interfacial amplitudes.