an apparatus and method for controlling the pore structure of thin polymeric sheets , such as porous membranes and thin films , during solvent casting or interfacial polymerization . the invention may be used to reduce or eliminate macrovoid pore defects from solvent - cast or interfacially polymerized polymeric membranes used in a variety of separations or controlled release tasks or to create a desired pore structure in porous thin films , such as may be used in breathable garments , surgical dressings , and screen printing . a nonuniform electric field is generated at one or more locations in proximity to a liquid film in which a desired pore structure is to be formed . due to the difference in dielectric constant between the liquid within the pores and that in the surrounding liquid , the electric field causes an attractive or repulsive force on the evolving pores .