A new system is described here, a photo-foam. A UV stimulus triggers a fast and localized destabilization of such a foam, still quite stable in ambient white light. This photo foam is made of a solution of a photo-surfactant synthesized in the lab. The hydrophobic tail containing an azobenzene group is able to change its conformation reversibly depending on the light stimulus (UV or blue). First the two isomers of the surfactant (cis and trans) are shown to have a very different affinity to the air-water interface. Second, this work establishes that the light stimulus generates a flux of desorption of the surfactant from the interface. This molecular flux triggers a series of effects at different scales of the foam : 1. At an air-water interface, light controls the adsorption of the surfactant so that surface tension can be finely tuned by the light intensity. 2. For a soap film, electrostatic interactions are modified in few seconds. Thus, the initial thickness of the film becomes instable and various objects are thus created. 3. When soap thick films are interconnected, Marangoni flows are developed and can prevent the drainage to occur. Their dynamics is directly determined by the molecular desorption flux, and controlled by light intensity. 4. The study of these different fluxes at different lengthscales brings a new perspective to understand the fast destabilization of the photo-foam, but also its local drainage retardation.
