Exploring the influence of the nanoporous structure of nickel-based superalloy membranes on emulsification performance.

Nanoporous structures made from nickel-based superalloy are fairly new and not thoroughly studied membrane materials for premix membrane emulsification. They show a different kind of pore structure than other membranes typically used in this process with a network of elongated, interconnected pores (150-400 nm). Two different membrane structures, resulting from different creep strains, times and temperatures during production, were investigated for their performance in premix membrane emulsification. The membranes were used in a system with a fixed process pressure, varying specific energy input via pressure or number of emulsification cycles. Furthermore, membranes with different manufacturing parameters and thicknesses were used. Both membrane structures achieved monomodal droplet size distributions with median droplet sizes under 500 nm in one emulsification cycle. The results indicate that while all droplet sizes fall within a comparable range, the pore sizes still play a significant role, with finer pores resulting in smaller droplets but broader droplet size distribution that showed minimal further breakup after repeated passes. The larger, more irregular pores showed the ability to further breakup droplets with increasing emulsification cycles, broadening their distribution. The findings also suggest that a pressure increase activates smaller pores that seem to remain inactive for emulsification at lower pressures, facilitating more transport and droplets breakup. Results underscore the critical role of elongational flow at the membrane inlet in promoting droplet breakup. This study strengthens the theory that droplet breakup in premix membrane emulsification requires droplets to be stretched as they enter the membrane, then breakup either spontaneously by surface instabilities when remaining in this elongated state for a sufficient time or deterministically by mechanical stresses such as shear caused by a tortuous channel.