Migration and Spreading of Droplets across a Fluid-Fluid Interface in Microfluidic Coflow.

Interfacial migration of droplets in microfluidic confinements has significant relevance in cell biology and biochemical assays. So far, studies on passive interfacial migration of droplets are limited to co-flow interfaces having small interfacial tension (IFT ∼ 1 mN/m). Here, we elucidate the migration and spreading of droplets (SiO-1000, SiO-100, FC40, and castor oil as phase 3, P3) across the interface between a pair of coflowing streams (PEG as P1, SiO-100, SiO-20, FC40, and olive oil as P2) having large IFT (∼10 mN/m), with the three different phases immiscible. Interfacial migration involving interfaces of large IFT is facilitated by confining droplets between the channel wall and coflow interface. We find that contact between droplets and the coflow interface is governed by the confinement ratio (i.e., the ratio of drop size to stream width) and the ratio of the capillary numbers of the coflowing streams. Depending on the sign of the spreading parameter (