Possible mechanism for multistage coalescence of a floating droplet on the air/liquid interface

We suggest that the existence of the monolayer surfactant film is crucial for the multistage coalescence on a floating droplet of the air/liquid interface and construct the dynamic model of the phenomenon. In this model the surfactant film is considered to be the cause for narrowing and even collapsing of the hole arising after the rupture of the air film which separates the drop and the underlying liquid. The characteristic time of the narrowing is τ=κs/σ, where κs, is the longitudinal viscosity of the film and σ is the surface tension of the system. Our model predicts that for the ratio x*=R1/R0 of two radii R1 and R0 in two successive stages of coalescence the following equation holds: x*=exp(−σ/E), where E is the surface elasticity of the surfactant film. For water drops where x*≈0.5 that gives an estimation E≈50 dyne cm−1 for σ=36 dyne cm−1. We performed the computer calculation of the time dependence of the drop radius during the stage of water drop coalescence. It would be interesting to check our model calculations experimentally.