Controlling Droplet Breakup and Routing in Microfluidic T-Junctions via Wettability Contrast

In this study, we investigate how the asymmetric wetting conditions affect the droplet dynamics in microfluidic T-junctions using a three-dimensional multicomponent lattice Boltzmann method based on the two-component Shan--Chen model. The model has been validated against theoretical and benchmark cases, including the Young--Laplace law and classical symmetric droplet breakup. Through a series of simulations, we have explored the effects of contact angle contrast, droplet length, capillary number, channel geometry, and viscosity ratio on the margins that split breakup and non-breakup regimes. The results demonstrate that spatial wettability asymmetry significantly alters droplet evolution, which promotes controlled redirection or suppression of breakup. Increasing the upper-lower contact angle difference promotes droplet steering toward the more wettable side, whereas higher capillary numbers and longer droplets tend to experience more breakup. Additionally, geometric parameters such as aspect ratio and side-to-main channel width ratio regulate the competition between surface and hydrodynamic forces, critically influencing droplet final state. The viscosity ratio affects the droplet dynamics in two ways: impacting internal recirculation and interfacial stress resistance. This work aims to provide a physics-based framework for passive droplet control by utilizing wettability engineering and geometrical design to offer practical insights for lab-on-a-chip devices and multiphase flow systems. Future work will incorporate non-Newtonian effects and dynamic wetting to extend applicability to more complex flow regimes.