Collision of immiscible droplets on a solid surface

While the impact of a single drop on a surface has been extensively studied, the more complex scenario of a drop impacting a sessile drop, particularly when the two liquids are immiscible, remains relatively unexplored. This study employs a multiple-relaxation-time phase-field lattice Boltzmann model to numerically investigate the impact of an immiscible droplet onto a sessile droplet. The findings show that compared to the better-known case of miscible impact, immiscible impact exhibits reduced spreading on the substrate. A key determinant of impact outcome is the interfacial tension between the droplets, quantified by the spreading factor. Simulations across varying spreading factors reveal different outcomes: droplet sticking, bouncing, engulfment of the sessile by the impacting droplet, and vice versa. Furthermore, the maximum spreading exhibits a dependence on the Weber number and contact angle, analogous to miscible impact. Liquid property mismatches, particularly density disparities, significantly influence the impact dynamics, while viscosity variations have a lesser effect. Finally, the study examines off-center impact and demonstrates the influence of initial misalignment on the impact dynamics.