Abstract: 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.