On breakup in droplet collision and coalescence: Surfactant-induced suppression

For collision between two droplets with sufficiently large Weber numbers (We), regimes of coalescence (III), reflexive separation (IV), and stretching separation (V) after temporary coalescence have been identified in accordance with specific ranges of the impact parameter (B) and Ohnesorge number (Oh). Recognizing that prevention of droplet breakup is critical for various applications, here we discuss the mechanism governing the transition between the breakup regimes and experimentally study the influence of surfactants on the response in the various regimes. It is shown that, compared to the surfactant-free case, adding surfactant substantially delays the occurrence of regime IV, requiring notably larger critical We (Wec) for the transition between regimes of coalescence III and IV. By increasing the surfactant concentration (Φ) over the critical micelle concentration (CMC), Wec decreases although it is still significantly larger than that of surfactant-free cases, even when Φ is 40 times the CMC. Similar suppression with surfactant addition is also observed for the transition between regimes III and V, showing at least 45% larger B required to reach regime V as compared to the surfactant-free cases. Furthermore, by comparing the collision sequences with and without surfactant, with similar values of B, Oh, and We, the nondimensional maximum length of relaxation (Lm) in the surfactant-free cases is substantially larger than that with surfactant addition. The results demonstrate significant effects of surfactant addition in droplets on the suppression of further breakup after merging.