Conversion of drops into bubbles upon impact on a porous membrane with gas discharge*

The impact of drops on a porous membrane with high contact-angle hysteresis and gas discharge is studied experimentally and based on CFD simulations. Four different impact modes are identified, ranging from complete repulsion to fast immobilization of a drop. The rebound time of a drop is proportional to the inertial-capillary timescale, similar to the case of drop impact in the Leidenfrost regime. The most remarkable aspect of the dynamics is the transformation of a drop into a bubble, which occurs when the drop just overcomes the repulsion by the gas flow and wets the surface. In that case, the contact line of the drop on the membrane surface gets immobilized, while the bottom surface of the drop gets displaced away from the membrane, resulting in a bubble with a diameter significantly larger than the diameter of the impacting drop. The inflation of the gas bubble is due to the expanding gas cushion below the drop, fed by the gas discharge through the membrane. The transition to the regime in which a drop is transformed into a bubble is well described by a simple scaling relationship based on a balance between inertia and the repulsive force due to the gas flow.