Drop and bubble shape technique

The methods used in this study are mainly the drop and bubble shape technique and the torsion pendulum rheometry. The drop and bubble shape technique, which is based on fitting the shape of an axisymmetric liquid meniscus to the respective Gauss Laplace equation, allows to continuously monitor the... 

The drop and bubble shape technique is superior over other methods due to quite a number of advantages. Most of all it is an absolute method and is applicable to any liquid interface. In many practical cases it is the only method of choice when only a small amount of the sample is available. Moreover, temperature control is simple to organise and even cells for measurements at several hundreds degrees [3] or at very high pressures [4] are known. The drop and bubble... 

Experimental set-lips for the drop and bubble shape technique... 

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. 

The experimental results shown in Figure 12.10 demonstrate the capacity of the drop and bubble shape technique. After the adsorption process has reached an equilibrium state, over a period of time of about 6 h, some square pulses of the drop area are subsequently, produced. Such area perturbations are suitable for determining the surface dilational elasticity of the interfacial layer. Efficient dosing systems even allow a sinusoidal area change, again providing information... 

Another experimental example of the use of the drop and bubble shape technique is given in Figure 12.11. The latter shows the dynamic surface tensions of blood serum as measured by the maximum bubble pressure (filled symbols) and drop shape techniques (open symbols). It becomes evident that both methods complement each other perfectly when the data from the MPT2 studies are plotted as a function of /efi- The results impressively demonstrate that during the radiotherapy process the dynamic surface tension parameters return to the normal values (2), i.e. approach those characteristic of healthy females from the respective control group. 

Another field of application of the drop and bubble shape techniques is in studying the penetration of soluble surface-active molecules into spread insoluble monolayers. In general, the obtaining of quantitative information on penetrated layers under dynamic and equilibrium conditions requires much attention... 

Loglio G, Pandolfini P, Tesei U, and Noskov B (1998b) Measurements of interfacial properties with the axisymmetric bubble-shape analysis technique effects of vibrations. Colloids Surfaces A 143 301-310 Loglio G, Pandolfini P, Miller R, Makievski AV, Ravera F, Ferrari M and Liggieri L (2001) "Drop and Bubble Shape Analysis as Tool for Dilational Rheology Studies of Interfacial Layers", in "Novel Methods to Study Interfacial Layers", Studies in Interface Science, Vol. 11, D. Mobius and R. Miller (Eds.), Elsevier, Amsterdam, pp 439-485... 

The surface tension of surfactant solutions is the easiest accessible experimental quantity and hence the most frequently used method to study the adsorption process at liquid interfaces. As earlier shown the rate of adsorption is a function of surface activity and bulk concentration. This explains why a broad time interval has to be experimentally covered to study the large variety of surfactants. A single method cannot provide a sufficiently broad interval so that different complementary methods are needed. Some methods are particularly developed for the short adsorption times, such as the bubble pressure method providing data from less than 1 ms up to some minutes. On the contrary, so-called static methods like the Wilhelmy plate or drop and bubble shape methods give access to very large times, starting from few seconds and reaching up to hours and even days. Both techniques complement each other perfectly. 

This paragraph summarised the state of the art of the bubble pressure tensiometry and presents experimental results showing the capacity of this technique for a deeper understanding of adsorption kinetic mechanisms. The subsequent paragraph describes the drop and bubble shape methodology and demonstrates the complementarity of the two experimental techniques. 

Among the methods for measuring the interfacial tension of liquid interfaces the drop and bubble shape tensiometry is known for a long time [1]. However, it became applicable with an acceptable accuracy only about 15 years ago with the availability of a video technique that can be directly linked to a high performance computer [2]. The first set-ups were run with expensive work stations, while today typically PCs are used as their performance is much higher now than the work stations of the first instruments. The development of this experimental technique was extremely fast and quite a number of commercial instruments are at present available on the market. 

If the approximation is good, within a single leap a jump occurs from the current trial of the parameters to the minimising ones. Actually, from the first estimation of the parameter values, just 3 - 4 successive loops are usually required. Note, the absolute accuracy of drop and bubble shape methods depends mainly on the main properties of the video technique and the... 

The drop and bubble shape tensiometer allows one to perform oscillation experiments. However, in contrast to similar studies with small spherical drops it is limited to slow oscillations. Depending on the hydrodynamic conditions, mainly the rheological behaviour of the bulk phases, oscillation from a certain frequency onwards do not provide drops or bubbles with a Laplacian shape, so that the data analysis will fail and yield unrealistic data. Even for liquids of high grade of purity interfacial tension changes are simulated and hence misinterpretations can be the consequence. Thus, the use of high speed video technique is not really relevant for shape analysis tensiometry (although provided by several companies), as the hydrodynamic relaxation can take much time to yield Laplacian menisci. 

The apparatus used in the present study is based on the axi-symmetric bubble shape analysis, i.e., a firmly established technique for the measurement of static and dynamic surface tension as well as of the geometrical properties of the bubble (Loglio et al. 1996, 2001, Kovalchuk et al. 2001, Miller et al. 2000, Rusanov and Prokhorov 1996, Neumann and Spelt 1996, Cheu et al. 1998). In essence, the shape of a bubble (or of a drop) is determined by a combination of surface tension and gravity effects. Surface forces tend to make drops and bubbles spherical whereas gravity tends to elongate them. 

A new method for the measurement of surface and interfacial tension has been developed based on a video digitising technique to measure the drop profQe of pendant and sessile drops [19]. The method gives a standard deviation of 0.5%, so the resolution, at present, is less than the maximum bubble pressure technique. It is an extremely useful technique for studying the liquid-liquid interface, and electrocapillary curves of similar shape to those obtained on mercury have been obtained for the water-nitrobenzene interface. 

The surface tension measurement techniques can be divided into the following three categories (i) Force Methods, which include the truly static methods of the capillary rise and Wilhelmy plate methods, as well as the dynamic detachment methods of the Du Nouy ring and drop weight, (ii) Shape Methods, which include the pendant or sessile drop or bubble, as well as the spinning drop methods, and (iii) Pressure Methods, which are represented by the maximum bubble pressure method. These techniques are summarized in the following sections of this chapter. 

In theory, liquid/vapour measurements can be made by using this approach, where a vapour bubble replaces the less dense liquid drop. However, this would be rarely done for such measurements due to the difficulty of implementation, as discussed below. As with the other shape techniques described above, this is a static measurement and only small amounts of the liquids of interest are required. The primary disadvantage to this technique is that it is very difficult to set up and perform. Thus, this approach is usually only used to make measurements of extremely low interfacial tensions, where it would be the only reliable method available. 

Figure 12.11. Dynamic surface tensions of blood serum samples as measured by the drop shape technique (PATl, SIN-TECH-Berlin, Germany, (0, )) and maximum bubble pressure method (MPT2, LAUDA, Germany, ( , )), samples taken from a 43 years old woman suffering from cervical carcinoma , , before radiotherapy o, at the end of treatment...

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. 

Pendant or Sessile Drop Method The surface tension can be easily measured by analyzing the shape of a drop. This is often done by optical means. Assuming that the drop is axially symmetric and in equilibrium (no viscous and inertial effects), the only effective forces are gravity and surface or interfacial forces. In this case, the Young-Laplace equation relates the shape of the droplet to the pressure jump across the interface. Surface tension is, then, measured by fitting the drop shape to the Young-Laplace equation. Either a pendant or a sessile drop can be used for surface tension measurement. The pendant drop approach is often more accurate than the sessile drop approach since it is easier to satisfy the axisymmetric assumption. Similar techniques can be used for measuring surface tension in a bubble. 

There are a variety of simple and inexpensive techniques for measuring contact angles, most of which are described in detail in various texts and publications and will be mentioned only briefly here. The most common direct methods (Fig. 17.4) include the sessile drop (a), the captive bubble (b), the sessile bubble (c), and the tilting plate (d). Indirect methods include tensiometry and geometric analysis of the shape of a meniscus. For solids for which the above methods are not applicable, such as powders and porous materials, methods based on capillary pressures, sedimentation rates, wetting times, imbibition rates, and other properties, have been developed. 

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