Numerical investigations on viscosity-driven droplet deformation and coalescence in a dual dispersion inlet microchannel

The reported study focuses on understanding the merging of droplets in continuous-phase media in a microchannel. An open-source solver, ESI-OpenFOAM v2312, is used, utilizing the interIsoFoam solver with a piecewise linear interface reconstruction scheme for accurate interface capture. The fixed dispersed-phase flow through branched inlets and the variable continuous-phase flow in the main channel enabled extensive exploration of parameters such as the viscosity ratios (λ), the flow rate ratios (Q′), and the continuous phase Capillary number (Cac). Initially, geometrical attributes of droplets and liquid films in branching microchannels in Newtonian fluid flow are reported. The second inlet allows for synchronized coalescence and doubling of the droplet volume. The merging time decreases with Q′ due to the enhanced hydrodynamic collision rates, while post-merger relaxation occurs rapidly via visco-capillary dynamics. The influence of channel wall wettability variations on liquid film thinning and frontal curvature blunting of droplets is also investigated. Flow regimes transitioned systematically from parallel flow to dripping flow of Cac between 10−4 and 10−2 in different viscosity ratios, with a decrease in normalized droplet length (LD/Wc) affecting the droplet morphology. The results provide design principles for microfluidic devices targeting regime-selective emulsification, synchronized multi-stage reactors, and fouling-resistant droplet platforms. Such applications in chemical synthesis and biomedical diagnostics are directly governed by droplet formation, coalescence, and interfacial transport.