Nanoscale flow on a bubble mattress: effect of surface elasticity

We present an experimental study of the elastic properties of a superhydrophobic surface in the Cassie regime, due to the gas bubbles trapped at the liquid-solid interface. We use a surface force apparatus to measure the force response to an oscillating drainage flow between a sphere and the surface. We show that the force response allows to determine the surface elasticity without contact, using the liquid film as a probe. The elasticity of the bubble mattress is dominated by the meniscii stiffness, and its determination enables us to probe the shape of these meniscii. Another effect of surface elasticity is to decrease the viscous friction. We show that this effect can be wrongly attributed to rate dependant boundary slippage if elastohydrodynamics is not taken into account.