Effects of Junction Angle and Viscosity Ratio on Droplet Formation in Microfluidic Cross-Junction

Abstract

This study describes the dynamic behaviors of droplet formation in microfluidic cross-junction devices (MFCDs) based on a two-dimensional numerical model using the volume of fluid (VOF) method. The effects of the junction angle (phi = 30 to 90 deg) between the main and side channels and the viscosity ratios (beta = 10(-5) to 2.0) are considered. The numerical results indicate that the active area for droplet formation in the alternating digitized pattern formation (ADPF) generally increases with the decrease of phi at the same water fraction (w(f)). A junction angle of around 60 deg was predicted as the most efficient angle at which alternating droplets are still formed at lower capillary numbers (Ca). In addition, the droplet size in ADPF decreases as phi increases with the same flow conditions. When phi is less than 90 deg and prior to approaching the equilibrium state, there always exists a periodic deviation in the relative distance between droplets. The frequency of droplet generation in ADPF decreases as phi decreases, and it decreases more quickly when phi is less than 60 deg. In addition, the throughput of MFCDs can be controlled effectively as a function of phi, w(f), and Ca. The droplet formation in MFCDs depends significantly on the viscosity ratio beta, and the ADPF becomes a jetting formation (JF) when beta is greater than unity. Furthermore, the droplet size in ADPF decreases with the increase of beta. The understanding of droplet formation in MFCDs is very useful for many applications, such as nanoparticle synthesis with different concentrations, hydrogel bead generation, or cell transplantation in biomedical therapy.