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Shaping fluid-fluid interfaces : Bubbles formed by blowing on soap films and mazes created by evaporating drops in porous media
Thin liquid films have been studied extensively over the centuries as they impact a wide range of phenomena, in physics, chemistry and engineering. We will present two experiments in which we study the creation, stability and rupture of such fluid structures.
Firstly, we discuss the formation of soap bubbles by blowing air through a nozzle onto a soap film. We work either with circular bubble wands or vertically-falling soap films. We vary key geometric parameters and gas density, and we measure the critical air speed above which bubbles are formed. The response is sensitive to containment, i.e., the ratio between film and nozzle sizes, and dissipation in the turbulent gas jet. We rationalize the observed four different hydrodynamic regimes by comparing the dynamic pressure exerted by the jet on the film and the Laplace pressure needed to create the curved surface of a bubble.
Secondly, we investigate the evaporation of drops of complex fluids (colloidal suspensions or surfactant solutions) confined in transparent and quasi-two dimensional porous media. Experiments show that evaporation creates pore-scale solid deposits that compose the walls of a centimeter-sized maze. This process occurs via a sequence of Haines jumps at the scale of a pore and is driven by simple rules. We rationalize these results with a cellular automaton that acts as a maze generator. This model well describes experimental findings, i.e., formation dynamics and final structure of mazes, as functions of the geometry and wettability of a porous medium.