Modelling of coalescence and stability mechanism of particle-coated droplets

We numerically investigate the coalescence dynamics of two equal-sized droplets coated with solid particles using a lattice Boltzmann colour-gradient model coupled with Lagrangian particle tracking. It is shown that the particle coverage critically determines the final morphology of coalescing droplets, giving rise to three distinct regimes: total coalescence, arrested coalescence and total stability. Arrested coalescence arises from dynamic particle jamming at the shrinking interface, where increasing particle coverage generates mechanical resistance against further capillary-driven shape relaxation. A theoretical criterion for the onset of arrested coalescence is derived and validated against a phase diagram constructed from 110 simulation cases. Furthermore, particle wettability governs the formation of inter-droplet bridges: only particles preferentially wetted by the ambient fluid can establish stable bridges. While single-particle bridges are susceptible to flow perturbations, multi-particle bridges enhance stability by entrapping ambient fluid and resisting film drainage. The study is extended to a droplet group under an initial instantaneous shear, where neutrally wetting particles at low coverages are found to promote coalescence, whereas higher coverages lead to arrested coalescence and complex collective morphologies. At moderate coverages, two stabilisation mechanisms are identified: (i) steric hindrance from dense particle layers, and (ii) capillary-force-mediated assembly of droplet chains via multi-particle bridging. These insights provide guidelines for tailoring the morphology and stability of Pickering emulsions in material synthesis, enhanced oil recovery and pharmaceutical applications.