Separation of large droplets from an oil-in-water emulsion using a deterministic lateral displacement (DLD) microfluidic chip.

Long-term stability of oil-in-water (O/W) emulsions is often compromised by the presence of larger droplets, which act as seeds for coalescence and phase separation. Conventional separation methods like filtration and centrifugation face challenges such as high energy consumption and filter clogging. In this study, we present a deterministic lateral displacement (DLD) microfluidic chip for the continuous and passive separation of larger oil droplets to enhance emulsion homogeneity and stability. A PDMS-based DLD chip, featuring a micropillar array with a pillar diameter of 20 μm and a gap of 5 μm, was designed to achieve a theoretical critical separation diameter (Dc) of approximately 1.7 μm. The separation mechanism was validated using a numerical estimation and experiments with fluorescent polystyrene beads. We successfully demonstrated that the DLD chip effectively removes larger droplets from ultrasonically prepared nanoemulsions, reducing the median particle diameter (D50) from 1.103 μm to 0.768 μm without using any surfactants. The results confirm that the DLD-based post-processing is a promising method for improving the quality of O/W emulsions, although challenges related to the fabrication of high-aspect-ratio structures and the pressure tolerance of PDMS need to be overcome for high-throughput applications.