Formation mechanism and size prediction of millimeter-scale double emulsion in an inverted co-flowing device

Abstract

An inverted co-flowing device was utilized to overcome challenges such as incomplete tube flow and bubble accumulation in large-diameter channels, enabling the successful generation of double emulsions exceeding 4 mm in diameter. The formation process of the double emulsion was visually analyzed and classified into five distinct stages: growth, transition, squeezing, stretching, and pinch-off. During the transition, squeezing, and pinch-off stages, the double emulsion length and minimum neck radius exhibited a linear relationship with time, whereas in the growth and stretching stages, both parameters followed a power-law relationship. Additionally, the study investigated the effects of key parameters, including the capillary number of the continuous phase, the flow rate ratio of the dispersed phase, and the outer tube diameter, on the resulting double emulsion size. A predictive equation was formulated to estimate the double emulsion size within the inverted co-flowing device, requiring no fitting parameters and demonstrating applicability across various outer tube diameters. This equation achieved a prediction error of less than 9 %, significantly enhancing size control precision in the fabrication of large double emulsions.