Remote control of the stability of bubbles, thin-films and foams using photosensitive surfactants

Our goal is to control the stability of liquid interfaces using a photoswitchable azobenzen surfactant which can change shape and hydrophobicity under UV or blue light. We have studied experimentally and theoretically the influence of light on the adsorption and desorption fluxes of these surfactants at the scale of a single air-water interface. We found that light stimulation induces a flux of desorption of the surfactants from the interface. The desorption flux is limited by the rate of photoconversion of the photosurfactant at the interface, hence it can be tuned by the light intensity [1]. At the scale of a vertical soap film we observed that UV light induces an upward Marangoni flow, which tends to oppose to the downward drainage flow caused by gravity (see Figure). This is due to the fact that there is a reservoir of non converted surfactants in the large liquid meniscus formed below the film whereas there is no such reservoir in the confined thin-liquid film [2]. Hence there is a gradient in surface tension between these two zones, which causes a Marangoni flow upward. We show experimentally and theoretically that the light intensity controls the velocity of the upward flow. We also find that the electrostatic repulsion between the two surfactant monolayers of the film can be weakened by the UV light. At the macroscopic scale, we observe that the foam can be destabilized using UV light [3]. We will discuss the mechanisms leading to the foam collapse based on the observations made at smaller scales