Capillary mechanisms in membrane emulsification: oil-in-water emulsions stabilized by Tween 20 and milk proteins

We investigate the process of membrane emulsification in the presence of the nonionic surfactant Tween 20, and the milk proteins Na-caseinate and beta-lactoglobulin (BLG). Our goal is to examine the factors which control the drop-size distribution in the formed emulsions. The drops are produced at the outer surface of a cylindrical microporous glass membrane, so that the process of their formation and detachment can be directly observed by an optical microscope. In the case of 2 wt.% aqueous solution of Tween 20 we obtain a relatively fine and monodisperse oil-in-water emulsion with a mean drop diameter about three times that of the pore. The microscopic observations show that in this case the oil drops intensively pop out of separate pores. In contrast, for the lower concentrations of Tween 20, as well as for the investigated solutions of Na-caseinate and BLG, we observe that the membrane is covered by a layer of growing attached emulsion drops, which are polydisperse, with a relatively large mean drop size. This fact can be explained with a greater dynamic contact angle solid-water-oil. In such a case, after a drop protrudes from an opening, it does not immediately detach, but instead, the contact area drop/membrane expands over several pore openings. The smaller drop size in the emulsions stabilized by BLG, in comparison with those stabilized by Na-caseinate, is related to the circumstance that BLG adsorbs faster at the oil-water interface than Na-caseinate. In the investigated emulsions we did not observe any pronounced coalescence of oil drops. Hence, the generation of larger and polydisperse oil drops in some of the studied solutions is attributed mostly to the effect of expansion of the drop contact line and formation of hydrophobized domains on the membrane surface. Therefore, any factor, which leads to decrease of the dynamic three-phase contact angle, and thus prevents the contact-line expansion, facilitates the production of fine and monodisperse emulsions.