Oscillating drop method

A recently developed method allows harmonic changes of the drop surface area in a wide range of frequencies. Figure 19 shows the principle set-up for the oscillating-drop method (29). 

Keywords Oscillating drop method, surfactant solutions, dilatioiuil elasticity, microgravity... 

M. Przyborowski, T. Hibiya, M. Eguchi, and I. Egry, 1995, Surface tension measurement of molten silicon by the oscillating drop method using electromagnetic levitation , J. Cryst. Growth 151,... 

H. Fujii, T. Matsumoto, S. Izutani, S. Kiguchi, K. Nogi, Surface tension of molten silicon measured by microgravity oscillating drop method and improved sessile drop method. Acta Mater. 54(5), 1221-1225 (2006)... 

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. 

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. 

The most recently developed methods to investigate the surface relaxation of soluble adsorption layers due to harmonic disturbances is the oscillating bubble or drop method. The technique involves the generation of radial oscillations of a gas bubble or a liquid drop at the top of a capillary immersed into the solution under study. The first set-up was described by Lunkenheimer Kretzschmar [150] and Wantke et al. [151] followed by a number of new designs of apparatus using novel pressure transducers to monitor the pressure changes inside a bubble or a drop [67, 152, 153, 154]. 

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 instrument shown schematically in Fig. 26 is suitable for slow oscillation experiments, as it was performed for the first time by Miller et al. in 1993. The frequency limit of the oscillations is given by the condition for the liquid meniscus shape, which has to be Laplacian. Under too fast deformations this condition is not fulfilled and hence the method does not provide reliable results. To reach higher frequencies of oscillation, the above mentioned oscillating drop or bubble experiments are suitable, because the shape of the menisci is spherical due to the small diameter. The instrument of Fig. 26 can be designed such that a pressure sensor and piezo translator are built in and the video system serves as optical control and determines the drop/bubble diameter accurately. 

As mentioned above the oscillating drop or bubble method, based on profile analysis tensiometry, is the most recently developed method to investigate the surface relaxation of soluble adsorption layers. By increasing/decreasing the volume of a pendent drop or bubble, a variety of area changes can be performed, such as step, square pulse, ramp type, trapezoidal, and of course harmonic area changes at low frequencies. 

Measuring principle of oscillating bubble and drop methods.489... 

These two equations allow the understanding of the measuring principle of the oscillating bubble or drop method. Oscillation can be generated either by external pressure variation 8 , or by volume variation in the cell, while the measured signal is the meniscus volume 5V (which is equivalent to the volume passing through the capillary 6V = ) or the... 

Drop and bubble methods have been developed significantly during the last years. These techniques provide access to dynamic properties of liquid interfaces. The drop and bubble shape technique as well as the fast oscillating drops and bubbles are described essentially as tools for dilational surface rheology. 

A third group of methods is the so-called dynamic methods, such as the oscillating jet, capillary wave, or oscillating drop/bubble methods. These methods are typically based on the evalnation of periodically applied interfacial stresses followed by interface relaxation and provide the means for determining the dilational rheology of the liquid-gas and liquid-liquid interfaces. 

Interfacial dilatational stress is measured in processes of isotropic expansion (compression) of an interface. Such processes are realized in the maximum bubble pressure method [176-180], the oscillating-bubble method [181-183], the pulsed-drop method [184], and the drop-expanding method [39,83,84,185-187]. Because of the simple spherical sjrmmetry equation (81), together with the projection of Eq. (80) along n, yields... 

Another oscillatory method makes use of a drop acoustically levitated in a liquid. The drop is made to oscillate in shape, and the interfacial tension can be calculated from the resonance frequency [113]. 

There are other, less commonly used, methods for measuring hardness. One is an impact method in which an indenter is dropped from a known height onto a specimen, and either the size of the indentation, or the coefficient of restitution, is measured. Another is the pendulum method in which a rocking pendulum is applied to a specimen surface. The damping of the pendulum s oscillations is a measure of the hardness. Still another is Moh s scratch method in which the ability of one specimen to scratch another is observed. These methods are described in various books (McColm, 1990), but only the... 

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