The paper presents a wettability difference-driven passive manipulation technique for numerically exploring the dynamics of head-on collisions between compound pendant droplets. Depending on the size of the shell drop on the surface junction having a weak hydrophobic jump, four different regimes are observed after merging, namely, (a) complete coalescence (CC) without pinch-off, (b) outer coalescence without pinch-off, (c) CC before pinch-off, and (d) CC after pinch-off. A regime map is developed by varying the oil drop diameter (do) and water drop diameter (dw), along with their corresponding Bond numbers. The effect of extreme hydrophobicity combined with increasing shell drop size leads to two interesting phenomena: lens configuration with CC without pinch-off, and compound droplet pinch-off. Further, depending on the gap between, merging of equal-sized pendant–sessile compound droplets result pendant–sessile compound drops of unequal sizes and a double core sessile compound drop.
Pathak et al. (2026) studied this question.