Collision of surfactant-laden droplets: insights from molecular dynamics simulation

We study the collision dynamics of surfactant-laden droplets and compare it with that of pure water droplets, with a focus on the bridge growth rate, energy balance, and disk dynamics, distinguishing the cases of head-on and off-centre collisions. By using molecular dynamics simulation of a coarse-grained model, it is found that initial linear scaling describes the first stage of the collision process, which is followed by power-law dynamics, in contrast to an initial thermal regime and a subsequent power-law behaviour observed for droplet coalescence. The transition between the two regimes occurs faster for surfactant-laden droplets. At higher collision velocities, the linear regime dominates the process with a gradual reduction of the power-law behaviour, reaching a situation in which the bridge growth is fully characterised by linear dynamics. The different behaviour of the droplets is presented in the form of a diagram of different scenarios, namely coalescence, separation, and splattering. In particular, it is found that higher velocities and larger offsets increase the likelihood of separation and splattering, with water droplets producing a greater number of satellite droplets due to reduced viscous damping. Also, a disk-like structure is observed as a result of collision, but it is less pronounced in the case of surfactant-laden droplets, due to higher dissipation of energy.