Transport characteristics of liquid-gas interface in a capillary membrane pore

Interface displacement of two phases mainly caused by replacement of one fluid with another one can occur inside the membrane pore during membrane preparation and characterization. This phenomenon is exemplified by measurement of liquid entry pressure (LEP) in membrane distillation, measurement of pore size and pore size distribution by the bubble point method, measurement of pore size and pore size distribution by porometry, wetting of dry membrane and also drying of wet membrane. In most of the theoretical studies, focus was on the transmembrane pressure required for one phase to enter into the membrane pores filled with another phase and little attention was paid to the movement of interface in the capillary. This paper attempts to discuss quantitatively the movement of interface, water/air interface in particular. Therefore, movement of interface was tracked using a well-founded model and consequently the pressure and velocity at the interface and the required time for replacement were studied. In addition, the influence of different parameters including pore radius, contact angle, and membrane thickness was assessed. It was found that the low resistance for the air flow, assumed in Washburn's equation, can be justified as an approximation. Moreover, the model proposed in this work can predict that the pressure at the water/air interface (when air replaces liquid water) may become significantly lower than the atmospheric pressure at the pore exit, causing pore shrinkage. As well, the pressure at the air/water interface (when liquid water replaces air) may become significantly higher than the feed pressure applied at the pore entrance, causing pore expansion.