surface first in contact with the non solvent mixture methanol has the lowest surface tension 18 dynes cm besides ether 17 dynes cm but ether has very high vapor pressure at room temperature therefore the final amount of ether in the mixture can not be known precisely with methanol as the low surface tension liquid and water as the high surface tension liquid varying the concentration ratio provides a way of controlling the non solvent surface tension this allows pre stressing of the membrane during solidification from compression all the way to tension fig7 shows the sem pictures of a membrane in which the dmac solution has been subjected to pure methanol alcohol a the matte surface b the porous structure and c the surface first in contact with the non solvent there is no thin skin layer and the pores are relatively large the membrane porous structure is subject to tension so bending it adds tensile stress to the surface and it breaks this membrane is as stiff as cardboard because of tension on the surface this membrane is useful in filtration applications but is not suitable for fabric applications fig8 illustrates the differences between hydrophobic and hydrophilic non solvent interactions with solute normally the droplets that form on a solid surface manifest the hydrophobic interaction of a liquid with a solid surface if the contact angle between the liquid and the solid is smaller than 90 degrees the surface interaction is hydrophobic if it is greater than 90 degrees it is hydrophilic it is commonly described in textbooks in terms of a capillary tube inserted into the.