Breakup-based preparation of ultra-thin solid-in-water-in-oil conformal droplets in a microchannel

Abstract

Droplets encapsulating solid microparticles with a thin coating layer have extensive applications in the fields of biochemical, advanced materials, and inertial confinement fusion. In this work, the droplet break-up technique is employed to prepare solid–water–oil (S/W/O) conformal droplets with an ultra-thin coating layer. A microfluidic chip, consisting of a T-junction and a Y-junction, was designed and constructed for the controlled preparation of ultra-thin S/W/O conformal droplets by generating–splitting integration. The flow pattern, regime, and dynamic mechanisms of the S/W/O droplet break-up were also experimentally investigated. The results show that there are three break-up regimes: breakup, non-breakup, and transition. Two different modes are observed in the break-up regime: without solid core stagnation and with solid core stagnation. In the case of the solid core without stagnation, the neck goes through three stages: squeezing, transition, and pinch-off. When the solid core stagnates, the neck goes through one more solid core stagnation stage after squeezing. The stagnation percentage decreases as the dispersed phase capillary number increases and increases as the continuous phase capillary number increases. The coating thickness of the S/W/O droplet increases and then decreases as the continuous phase flow rate increases. The coating thickness of the daughter S/W/O droplet was significantly reduced and was less affected by the continuous phase flow rate.