Emulsions are widely used across various industries but often require surfactants for stabilization

leading to complex product formulations and environmental concerns. Continuous preparation of stable emulsions without surfactants is still a significant challenge. Herein

a secondary membrane emulsification system was developed to produce stable oil-in-water emulsions under surfactant-free conditions

where the first membrane module employed the cross-flow membrane emulsification to generate the primary emulsion

while the second membrane module employed the premix membrane emulsification to further reduce the droplet size

thereby improving the emulsion stability. The morphology and droplet size of the secondary emulsion significantly depend on the membrane pore size

membrane surface wettability

primary emulsion flow

and primary emulsion quality. The secondary membrane emulsification system demonstrates excellent operational stability over 12 h

with no obvious changes in the emulsion characteristics. The prepared secondary emulsion remains relatively stable up to 10 min without surfactants

significantly superior to the primary emulsion. Furthermore

the developed system successfully produces the Janus emulsion with homogeneous internal phase morphology. This research provides an effective approach for the continuous preparation of stable emulsions without surfactants.