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Flat-sheet membranes for membrane distillation (MD) were prepared from solutions of polyvinylidene fluoride (PVDF) in dimethylacetamide by the phase inversion technique. Various amounts of water were added to the solutions as a non-solvent additive. The surface of the membranes were investigated by tapping mode atomic force microscopy (TM-AFM), to obtain mean pore size, pore size distribution, nodule size, pore density, surface porosity and roughness parameters. Pore sizes fitted well to the log-normal distribution function for all membranes studied. The pore sizes, surface porosity, nodule sizes and roughness parameters were found to be higher for membranes prepared with higher water content in the casting solution. The mean pore sizes obtained with TM-AFM were 1.2 to 2.1 times larger than those determined from gas permeation test. To study the effect of pore size distribution on the water vapor …