Puncturing polymer membranes: Non-wetting organic liquids (see scheme; red) can form a mesoscopic wetting layer on a water surface (light blue), ifthey are mixed with suitable particles (dark blue). Photochemical cross-linking of the liquid in the mixed layer and transfer to metal grids or porous supports produces composite membranes. Subsequent removal of the particles yields free-standing porous membranes with uniform pore size and high porosity (see scheme).