Measurement of dynamic surface tension by the oscillating droplet method

An optical measuring method has been applied to determine the dynamic surface tension of aqueous solutions of heptanol. The method uses the frequency of an oscillating liquid droplet as an indicator of the surface tension of the liquid. Droplets with diameters in the range between 100 and 200 m are produced by the controlled break-up of a liquid jet. The temporal development of the dynamic surface tension of heptanol-water solutions is interpreted by a diffusion controlled adsorption mechanism, based on the three-layer model of Ward and Tordai. Measured values of the surface tension of bi-distilled water, and the pure dynamic and static (asymptotic) surface tensions of the surfactant solutions are in very good agreement with values obtained by classical methods.List of symbols a coefficient of intermolecular forces, Nm^-1 - B adsorption constant - c _o bulk concentration, mol m^-3 - D apparent diffusion coefficient, m²s^-1 - t time, s - T absolute temperature, K - R universal gas constant=8.314, J mol^-1 K^-1 - (, t) droplet contour function - _o droplet equilibrium radius, m Greek symbols maximum surface excess concentration, mol m^-2 - (t) droplet volume normalization function - azimuth of the polar coordinate system - density, kgm^-3 - surface tension, N m^-1 - (t) concentration in the subsurface, molm^-3 - droplet oscillation frequency Daimler-Benz AG, Produktion & Umwelt, D-89081 UlmOn leave of absence from the Institute of Fundamental Technological Research, Polish Academy of Sciences, PL-00-049 Warszawa