2007 Volume 35 Issue 5 Pages 265-271
Surface-active agents (surfactants) have been used in washing, printing, and medical science to improve efficiency in processing but often cause complicated problems that remain unresolved. As these problems have marked effects on the quality of the final product, basic research to determine solutions is necessary. 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. Several aqueous solutions of surfactants were used: polyoxyethylene (10) lauryl ether, polyoxyethylene (100) stearyl ether, etc. The momentum equation applied to the falling drop was simplified and an equation was obtained for measuring the dynamic surface tension. It was shown that this equation is useful and dynamic surface tensions were measured for several kinds of aqueous surfactant solutions. The dynamic surface tension was found to depend on the molecular weight of the hydrophilic group and the normalized dynamic surface tension is correlated with the parameter (surface age)×(molecular weight of the hydrophilic group).