Experimental evidence that water has no dynamic surface tension

Abstract

The study of surface phenomena is receiving more and more attention as scientists in many fields realize the importance of this area. In studying liquid surfaces one of the experimental techniques that has great potential is the oscillating (or vibrating) let method of measuring dynamic surface tension. The method has never reached its full potential because various investigators have raised legitimate questions about the interpretation of data obtained from this method. In recent years we have been using the oscillating jet in a study of surfactant adsorption and have performed some experiments that we feel will put the interpretation of data on a sounder basis. At the symposium on dynamic surface tension at the 145th National Meeting of the American Chemical Society in 1963, a controversy developed over whether or not distilled water exhibits a dynamic surface tension. This controversy stemmed from the fact that the surface tension of water measured with certain dynamic methods, notably the oscillating jet, was apparently a function of time. The surface tension appeared to decrease with increasing time from a value of 80 or 90 dynes/cm, down to the static value. Therefore, either the technique gives erroneous values at lower times or water does have a dynamic surface tension. The oscillating jet method consists of forcing a stream of liquid under constant pressure through an elliptical orifice. The surface tension forces cause the stream to oscillate between two ellipses the major axes of which are perpendicular to each other. A standing wave pattern is formed on the stream, and the wavelength of this wave pattern was related mathematically to the surface tension of the liquid by Bohr (1). Bohr's mathematical treatment of the oscillating jet was for static surface tension measurements of a liquid the surface of which would be free of impurities because it was freshly formed. The assumptions he made, in order to solve the fluid flow equations, precluded the use of the first few waves in calculating the surface tension, tIowever, these first few waves are just the ones that give the high values of the surface tension of water. These high values indicate that either water has a dynamic surface tension or Bohr's model is inadequate for the first few waves of the stream. IIansen and co-workers (2) have developed a semitheoretical method of correcting Bohr's model near the orifice, and Thomas and Potter (3) have developed a purely empirical correction method. Both of these methods work quite well, and Hansen's method tends to prove that the high values for the surface tension of water near the orifice are due to the inadequacy of Bohr's model. However, a definitive experimental proof would be more satisfying. Those who contend that water has a dynamic surface tension in the time range greater than 1 millisecond, attribute it to the associated structure of water due to hydrogen bonding. It is assumed that the structure in the surface has to attain an equilibrium configuration which requires more than 1 millisecond. To test this theory experimentally, one should investigate a liquid with the same physical properties as water but with no possibility for association of any kind. Since it is not possible to obtain such a