Light-actuated interfacial flows via photosurfactants: transient effects*

Photo-surfactants, also known as light-actuated surfactants, are notable because they provide a means of external control for interfacial flows via light illumination. These surfactants are made up of a hydrophobic tail group and a hydrophilic head group separated by a light-responsive molecule that can reversibly toggle between two orientations (trans or cis) upon light absorption. Because the dominant orientation in a system is primarily a function of the wavelengths of incident light and the two states are characterized by markedly different interfacial properties, photo-surfactant systems can display significantly different values of equilibrium surface tension under different illumination wavelengths. This phenomenon has led to research on so-called chromo-capillarity, i.e. the use of light to drive flows via the formation of surfactant gradients on fluid interface. Experiments have shown systems with photo-surfactants can achieve non-negligible interfacial velocities immediately after illumination of an interface, but nearly all experiments observe a decay of these velocities to near zero at long term steady states. In this talk we discuss the system of PDEs that governs the fluid and surfactant dynamics and investigate this important transient regime via hybrid analytical and numerical methods, discussing among other things the potential of using photo-surfactants in combination with a superhydrophobic surface architecture to pump liquids.