Oscillating jet method

This is a dynamic method and can be used to measure y from first instants of formation up to greater surface ages depending on the flow rate. 

---

It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

The oscillating jet method is not suitable for the study of liquid-air interfaces whose ages are in the range of tenths of a second, and an alternative method is based on the dependence of the shape of a falling column of liquid on its surface tension. Since the hydrostatic head, and hence the linear velocity, increases with h, the distance away from the nozzle, the cross-sectional area of the column must correspondingly decrease as a material balance requirement. The effect of surface tension is to oppose this shrinkage in cross section. The method is discussed in Refs. 110 and 111. A related method makes use of a falling sheet of liquid [112]. 

Figure 3.11 Illustration of a magnified image of an oscillating jet of liquid that is being injected in front of a grid for measuring the jet s period. This is the oscillating jet method for surface tension measurement.

Figure 4.20. Surface potential relaxation of water and aqueous NaCl solutions. Oscillating Jet method. Temperature 24°C. The electrolyte concentration is indicated. (Redrawn from Kochurova et al.. )...

Various experimental methods for dynamic surface tension measurements are available. Their operational timescales cover different time intervals. - Methods with a shorter characteristic operational time are the oscillating jet method, the oscillating bubble method, the fast-formed drop technique,the surface wave techniques, and the maximum bubble pressure method. Methods of longer characteristic operational time are the inclined plate method, the drop-weight/volume techniques, the funnel and overflowing cylinder methods, and the axisym-metric drop shape analysis (ADSA) " see References 54, 55, and 85 for a more detailed review. 

The oscillating jet method provides dynamic surface tensions in a time interval from 3 ms-50 ms and has been used by many authors (for example Bohr 1909, Addison 1943, 1944, 1945, Rideal Sutherland 1952, Defay Hommelen 1958, Thomas Potter 1975a, b, Fainerman et al. 1993a, Miller et al. 1994d). 

In a recent paper Miller et al. (1994d) discussed parallel experiments with a maximum bubble pressure apparatus and a drop volume method (MPTl and TVTl from LAUDA, respectively), and oscillating jet and inclined plate instruments, performed with the same surfactant solutions. As shown in Fig. 5.27, these methods have different time windows. While the drop volume and bubble pressure methods show only a small overlap, the time windows of the inclined plate and oscillating jet methods are localised completely within that of the bubble pressure instrument. 

A comparison of the bubble pressure method with the oscillating jet method was also performed with aqueous Triton X-100 solutions. Some results are given in Fig. 5.29 as a y/log X3 - plot. In contrast to the inclined plate, the oscillating jet only yields data in the time interval of few milliseconds. Also in this time interval the agreement with the maximiun bubble pressure method is excellent and shows deviations only within the limits of the accuracy of the two methods. 

It has been already indicated (Fig. 7) that micelles can lead to an essential acceleration of the adsorption process. Therefore, special experimental techniques are necessary for its investigation, allowing measurements of the dynamic surface tension in a time interval of milliseconds. The maximum bubble pressure method [78, 81, 83, 89,90,93] and the oscillating jet method [77, 82, 86, 87, 88, 90, 92, 93, 156] are most frequently used for these purposes. The inclined plate method [83, 89, 90, 93], the method of constant surface dilation [85] and the drop volume method [84] have been used also for slow adsorbing surfactants. 

The characteristic times of the slow step of micellisation determined by the maximum bubble pressure and oscillating jet methods have the same orders of magnitude at low concentrations and agree with results of investigation of the bulk phase [139, 157] (Fig. 5.14). At higher... 

There is significant overlap in describing techniques for the measurement of equilibrium surface tension with those for dynamic surface tension, as discussed in the following chapter. Many of those dynamic surface tension methods can be used to measure equilibrium tension simply by performing the experiment over sufficiently long times. The time required for equilibration can range widely, from the practically instantaneous equilibration for pure liquids, to many hours or even days for dilute surfactant or polymer solutions. Thus, some of the dynamic techniques, particularly those that can only be used to study short times, such as the oscillating jet method, are not well suited for equilibrium measurements. 

Figure 12.5. Schematic of the set-up used for the oscillating jet method further details are given in the text (according to Joos (1))...

---