Trapping and coalescence of droplets in micro cross-shaped channels

Microfluidic manipulation of droplet dynamics in cross-shaped channels presents significant potential for industrial applications. This study systematically investigates the droplet dynamics of oil-in-water emulsions in microfluidic cross-shaped channels, revealing a coupled mechanism of droplet capture and coalescence driven by vortex breakdown, focusing on the regulatory effects of interfacial tension reduction within the Reynolds number (Re) range of 50-350. The results show that three capture patterns, i.e., symmetric trapping, droplet chain capture, and axial rotation are controlled by the evolution of flow regimes. Surfactant addition significantly raises the critical Re for trapping onset from 90 to 120, with interfacial tension reduction enhancing trapped droplet stability. Increasing surfactant concentration significantly suppresses droplet oscillation frequency and coalescence frequency, while substantially prolonging film drainage time. In addition, based on classical film drainage models, we develop a revised coalescence model incorporating surfactant concentration dependent interfacial transport properties that limits coalescence time prediction errors to within 30 %.