SOLUTOCAPILLARY-CONVECTION-DRIVEN MACRO VOID DEFECT FORMATION IN POLYMERIC MEMBRANES

Large (10-50 jum) macrovoid defects (MVs), which form during the fabrication of polymeric membranes, limit the wider use of membranes in separation applications ranging from desalination to fuel cells. MVs are generally considered undesirable because they lead to excessive compression and reduced flux through the membrane. Two competing hypotheses have been proposed to account for MV formation in evaporative-c ast membranes. The diffusive model contends that MVs grow by molecular diffusion of primarily nonsolvent to MV nuclei whereas the modified solutocapillary convection (SC) hypothesis suggests that Marangoni or solutocapillary convection along the MV interface facilitates large-scale defect growth. Several microgravity and ground-based experiments have been conducted in order to determine which of the two hypotheses is most appropriate. In this paper, we report new results from video microscopy flow visualization (FV) studies, and compare these to previous test results from KC-135 buoyancy and ground-based penetration-depth experiments. The results of the FV studies indicate that flow velocities along the MV interface are greater than bulk-solution flow velocities. FV data have also provided a detailed qualitative description of MV birth, growth, and collapse. Taken together, the results of these experiments