Coalescence of water drops at air–water interfaces with surface tension gradients induced by absorbed surfactants

Surface tension gradients induced by absorbed surfactants significantly affect drop coalescence at a gas–liquid interface. By independently adjusting contents of fluorinated anionic surfactant at the water drop surfaces and the bulk air–water interfaces, drop coalescence processes were investigated when the initial surface–tension ratio R σ = σ b / σ d ranged from 0.29 to 3.69. It finds that partial coalescence occurred at R σ ≥ 0.90, complete coalescence at 0.40 ≤ R σ < 0.90, and drop ejection at R σ < 0.40. For R σ > 1, Marangoni flow directed from the drop surface toward the air–water interface, promoting axial liquid drainage toward the interface and favoring neck contraction until pinch‐off. For R σ < 1, the Marangoni flow was reversed, weakening neck contraction and resulting in complete coalescence. For low R σ , strong reverse flow promoted liquids to flow toward the drop region, leading to breakup of a rather thin liquid column due to Rayleigh–Plateau instability.