Surfactants have been used in many fields for improving efficiency in processing but often cause some problems complicated and unsolved. We observed liquids dripping out of an aperture at relatively low Reynolds numbers using a CCD camera, measured mass and velocity of falling drops, and estimated the dynamic surface tension of several aqueous solutions of surfactants: polyoxyethylene (23) lauryl ether, polyoxyethylene (20) stearyl ether, etc. It was concluded that the molecular weight of hydrophilic group in surfactants greatly affects the dynamic surface tension of the solutions at the same mol concentration. We provided the normalized dynamic surface tension and it was shown to be correlated with the surface age multiplied by the molecular weight of the hydrophilic group of the surfactant. A model was presented by considering the dynamic behavior of surfactant molecules approaching a static state to predict the normalized dynamic surface tension of surfactants. The predictions of this model agreed well with the experimental results. V_z: flow velocity; r: arbitrary radius; D: outer diameter of the capillary tube; d: inner diameter of the capillary tube; M: mass of the liquid droplet; g: acceleration due to gravity; ρ: liquid density; γ: dynamic surface tension; V_m: mean flow velocity; C: concentration; σ: static surface tension; σ_w: surface tension of water; σ_s: static surface tension of samples; γ_θ: normalized dynamic surface tension; T: surface age
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