Oil-in-water segmented flow in the optimized microfluidic system for surfactant-free ultrasonic emulsification.

Droplet-based microfluidics is a promising technique for generating stable emulsions in various applications, including pharmaceuticals, food, cosmetics, and biosensors. However, conventional methods rely on surfactants, which pose potential toxicity and environmental concerns. To address this issue, we developed a microfluidic device for surfactant-free oil droplet generation, serving as a pre-processing stage for ultrasonic emulsification. Three microfluidic channels were designed: a conventional T-junction, a needle-inserted channel, and a needle-inserted glass capillary channel. Oil-water flow segmentation characteristics of the fabricated devices were analyzed using high-speed camera and image processing techniques. Results demonstrated that the needle-inserted glass capillary exhibited superior stability, effectively generating oil droplets rather than slugs by utilizing a higher water affinity and minimizing oil contact with the channel walls. Furthermore, when integrated with ultrasonic emulsification, the pre-fragmented oil droplets exhibited improved processing efficiency. These findings highlight the potential of combining microfluidic pre-processing with ultrasound emulsification as a viable alternative to surfactant-based methods, offering enhanced precision and sustainability in droplet generation and emulsion formation.