Effects of viscosity, interfacial tension, and flow geometry on droplet formation in a microfluidic T-junction

Precise control of monodisperse micron-sized liquid droplet emulsions produced in a microfluidic T-junction has far reaching implications in several mechanical, biomedical, and optical applications. This paper is an experimental study in a T-junction microfluidic device, allowing large ranges of interfacial tension between the two immiscible fluids, viscosity ratios, channel geometries, and their impact on droplet formation. Classification of the droplet formation regimes, droplet in T-junction (DTJ), droplet in channel (DC), and parallel flow (PF), is further clarified based on experiments in this study and in the literature. Our experiments show that the droplet volume decreases and the production frequency increases as the channel aspect ratio (h/wc) is increased, consistent with conservation laws. In addition, the transition flow rate ratio (Qd/Qc) for a given capillary number decreases with decreasing aspect ratio for both DTJ–DC and DC–PF transitions, with subscripts d and c referring to the dispersed and continuous phases. Larger viscosity ratios (μd/μc) and interfacial tension values also tend to decrease this transition flow rate ratio for a given capillary number. The viscosity ratio, interfacial tension, and channel geometry have an impact on the DTJ–DC and DC–PF droplet transition regions, and a new parameter, the dimensionless interfacial tension (Ω), is used to further characterize these transition locations. Using this parameter, the channel aspect ratio, flow rate ratio, and viscosity ratio, we develop empirical correlations for predicting the DC–PF transition regions for both the squeezing and dripping regimes. These correlations predict the capillary number at which the DC–PF transition will occur with less than 8.5 % error in the dripping regime and less than 2.8 % in the squeezing regime.