Capillary buckling of a floating annulus†
We describe the out-of-plane buckling of a flexible annulus floating on a bath of water as surfactant molecules are added outside the annulus. The difference in surface tension induces compressive stresses, which result in regular orthoradial wrinkles beyond a critical difference in surface tension. The wrinkles first appear in the vicinity of the inner edge of the annulus and progressively grow as the concentration of surfactant is increased. Conversely, the wavenumber remains constant and relies on a simple balance between gravity and the bending stiffness of the membrane within the range of our experimental parameters. Our experiments fall outside the regimes explored in the literature for similar situations, and we propose an approximate analytical description that completes the existing theoretical grounds.