Droplet suction on porous media

We study the forced aspiration of small (approximate to mm) and large (approximate to cm) liquid drops, deposited on prewetted porous membranes, and pumped mechanically with a constant current J. Two kinds of membranes are used where the pores are i) disconnected, cylindrical and calibrated or ii) interconnected "sponge-like". Whatever the size of the drops and the intensity J of the current, two suction regimes are observed versus time: 1) a "locked" regime, when the drop is pinned, with a dynamic contact angle decreasing from advancing (theta (a)) to finite receding (theta (r)) contact angle; 2) an "unlocked" regime, where the contour line recedes with a constant contact angle closed to theta (r). In both regimes, the shape of the drop remains quasistatic, during the suction process, i.e. a spherical cap for small drops and a flat "gravity pancake" for large ones.