Membrane emulsification: Droplet size and uniformity in the absence of surface shear

A series of tests with membrane pore sizes between 7 and 60 μm and uniform spacing between the pores of 80 and 200 μm, conducted under conditions of zero surface shear at the membrane, show that an additional force to the buoyancy and capillary forces exists in membrane emulsification. A push-off force, derived by consideration of the geometry of the drops as they deform at the surface of the pores under high injection rates when most of the pores are passing liquid, is shown to model the size of the drops formed in the emulsification. In the tests, sunflower oil was injected into water and as the emulsification injection rate increased it was noticeable that there was a point at which the resulting drop distribution is at its narrowest. For the two pore spacings studied: 80 and 200 μm, the point at which the distribution was at its narrowest was at a Weber number of 1.5 × 10−2, where the Weber number is defined using the drop diameter rather than the pore diameter.