Three-dimensional conservative phase-field lattice Boltzmann simulations of droplet generation in a flow-focusing microdevice

Microfluidic droplet devices have been extensively studied because of their wide range of applications, particularly in canonical geometries, such as T-junctions and flow-focusing configurations. In this work, we present a three-dimensional phase-field lattice Boltzmann framework based on the conservative Allen–Cahn equations, capable of accurately simulating both binary and ternary fluid systems in contact with arbitrary shaped solid surfaces. To the best of our knowledge, this is the first study to perform three-dimensional simulations of droplet generation in flow-focusing microfluidic devices with three immiscible phases and wettability using a phase-field approach. The method applies non-equilibrium bounce-back boundary conditions at inlets and outlets to properly enforce mass conservation and employs a method based on the bounce-back scheme on rigid walls that are not conformal to the lattice grid to handle wettability on complex geometries. This treatment ensures mass conservation even in the presence of staircase approximations on curved boundaries. The wettability model is validated through droplet spreading tests on both flat and curved surfaces, and spurious currents near triple junctions are analyzed to assess numerical accuracy. Then, a comprehensive parametric study on droplet generation in a flow-focusing microdevice is presented, exploring the effects of capillary number, flow rate ratio, viscosity ratio, and contact angle on droplet size, morphology, and production frequency. Finally, a preliminary study on the generation of multicomponent droplets is conducted to demonstrate that the developed solver is a robust and versatile tool, well suited to advancing the understanding of multiphase microfluidic flows and facilitating the interpretation of the experimental data.