Measurement of Dynamic Surface Tension by a Growing Drop Technique

Abstract

The recently developed growing drop technique for measuring dynamic surface tension in which a drop is grown at the end of a fine capillary into another immiscible fluid has been improved by the use of an ultra-high-speed video camera and motion analysis system. When the relative importance of the surface tension force is large compared to gravitational and viscous forces, i.e., the gravitational Bond number, G, and the capillary number, Ca, are much smaller than unity, shapes of growing drops approach sections of spheres, and the difference in pressure between the interior and the exterior of the drop, Δp, is related to the surface tension, σ and the radius of curvature, R, by the static Young-Laplace formula Δp = 2σ/R. Because the new apparatus described in this paper is capable of measuring transient drop shapes and pressures in 16 to 1 ms, it permits detailed observation of profiles of growing drops and pressures inside them virtually from zero surface age and thereby allows exploration of the limitations of the growing drop method. In contrast to related work, the results demonstrate that the present technique can determine the surface tension of an interface with a surface age as low as 20 ms. Although the present technique is capable of measuring tension prior to the hemispherical stage, measurements in the very early stages of drop growth are rendered impossible due to oscillations of the newly forming drop. The high speed of data acquisition that is possible with the new system has resulted in the discovery of a disparity between the extremum in drop pressure and that in drop radius in surfactant-laden drops, which can be attributed to the changing of tension with time. The effect of drop viscosity on the measurement of dynamic surface tension with the growing drop technique is also studied for the first time by forming drops of mixtures of water and glycerol into air.