Suppressing Breakups in Binary Droplet Collisions by Surfactant

In collisions of two droplets, reflexive separation (IV), rotational separation (VI) and stretching separation (V) usually occur after temporary coalescence at sufficiently large Weber number (We) within various ranges of impact parameter (B) and Ohnesorge number (Oh). Here we present the influence of surfactant on various separation regimes by experiments. Compared to the surfactant-free cases in head-on collisions, adding surfactant delays the occurrence of regime (IV), showing larger values of the critical We for transition (denoted by Wec) between the regimes of coalescence (III) and reflexive separation (IV). By increasing surfactant concentration over the critical micelle concentration (CMC), Wec decreases and approaches the theoretical value predicted by energy conservation. Regarding the off-center droplet collisions without adding surfactant, we have rederived the theoretical model proposed by Jiang et al. to predict the transition boundary between regimes (III) and (V). The results show that our model, in a dimensionless form, describes the existing experimental data with a higher precision compared to the previous model having dimensional fitting coefficients and it is valid for surfactant-free drops in a range of Oh = 0.0089-0.0592. When surfactant is added, however, about 30 % larger B is required to reach regime (V). Our experiments exclusively demonstrate the influence of surfactant on suppressing the separations in binary droplet collisions. The cause can be interpreted by the non-uniform distributions of surfactant on the droplet surfaces, which induce Marongoni stresses that oppose droplet deformation and further breakup.