Pressure technique, maximum bubble

A new method of surface tension determination has been developed which is continuous, automated, compatible with computer data acquisition systems, and capable of monitoring flowing process streams. The method is a variant of the well-known maximum bubble pressure technique. To illustrate the principles, we will describe the simplest initial configuration of the instrument here. Further details and a description of a refined version of the instrument will be reported later. 

Only the two first methods allow measurement of the temperature coefficient of the surface energy. The maximum bubble pressure technique is well-adapted for metals with low and intermediate melting points and specially for oxidizable metals, while the sessile drop technique has been applied with success to measure ctlv values up to 1500°C. The drop weight method is particularly useful for very high melting-point metals because it avoids liquid contact with container materials. This is also true for the recently developed levitation drop technique that analyses the oscillation spectrum of a magnetically levitated droplet. 

In the method of the Jailing meniscus a liquid-wetted tapering tube is placed vertically in a reservoir, as in fig. 1.26. Inside the tube liquid is held by the capillary pressure. The tube is now moved upwards - or the liquid in the vessel downwards - to increase the hydrostatic pressure head, and this is continued until the liquid in the capillary collapses. From the hydrostatic head the Laplace pressure is obtained and from that the surface tension. The method is very simple and may be considered as the counterpart of the maximum bubble pressure technique there are also similarities to the situation sketched in fig. 1.8a. The idea is rather old... 

A variant is the micro-pipette method, which is also similar to the maximum bubble pressure technique. A drop of the liquid to be studied is drawn by suction into the tip of a micropipette. The inner diameter of the pipette must be smaller than the radius of the drop the minimum suction pressure needed to force the droplet into the capillary can be related to the surface tension of the liquid, using the Young-Laplace equation [1.1.212). This technique can also be used to obtain interfacial tensions, say of individual emulsion droplets. Experimental problems include accounting for the extent of wetting of the inner lumen of the capillary, rate problems because of the time-dependence of surfactant (if any) adsorption on the capillary and, for narrow capillaries accounting for the work needed to bend the interface. Indeed, this method has also been used to measure bending moduli (sec. 1.15). 

The maximum bubble pressure technique is a classical method in interfacial science. Due to the fast development of new technique and the great interest in experiments at very small adsorption times in recent years, commercial set-ups were built to make the method available for a large number of researchers. Rehbinder (1924, 1927) was apparently the first who applied the maximum bubble pressure method for measurement of dynamic surface tension of surfactant solutions. Further developments of this method were described by several authors (Sugden 1924, Adam Shute 1935, 1938, Kuffiier 1961, Austin et al. 1967, Bendure 1971,... 

The effective surface age has been determined on the basis of a diffusion controlled adsorption model. Similar to the maximum bubble pressure technique, where the bubble time is longer than teff, the drop formation time t is also significantly longer than the effective age teff and can be obtained as teff=3t/7 [185], This estimation assumes a radial flow inside and outside the growing drop and a homogeneous expansion of the drop surface. 

Actually, the maximum bubble pressure technique is not suitable for long time experiments. Due to the small size of a single bubble, the experiment is very sensitive to smallest temperature changes. However, for relatively large system volumes, a self-generation of bubbles with longer lifetimes is possible. This method as known as the so-called stopped flow method (Fainerman and Miller 1998). 

The data shown in Fig. 21 are partially those of Fig. 20, however, plotted as a function of the effective surface age and restricted to the short time interval t < 5 ms. This demonstrate that the maximum bubble pressure technique is able to provide results in the sub-millisecond time range. 

Fainerman V.B. and R. Miller, The maximum bubble pressure technique, monograph in "Drops and Bubbles in Interfacial Science", in Studies of Interface Science , D. Mobius and R. Miller (Eds.), Vol. 6, Elsevier, Amsterdam, 1998, p. 279-326. 

Pandit, A., Miller, C.A., and Quintero, L., Interfacial tensions between bitumen and aqueous surfactant solutions by maximum bubble pressure technique, Colloids Surfaces A, 98, 35, 1995. 

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. 

FIGURE 19.1 Dynamic surface-tension data for n-alkyldimethylphosphine oxides, as measured by maximum bubble-pressure technique (O), drop-volume tensiometry ( ), and de Nolly ring tensiometry (A), and model fit ignoring reorientation (dotted line) and incorporating reorientation (solid line). (From Fainerman, V. B., et al. 2000. Adv. Colloid Interface Sci. 86 (1-2) 83-101. With permission.)... 

Liquids Since y and a are numerically equal for liquids, only the determination of y is necessary. There are various techniques for measuring y for liquids. These include the capillary-rise, sessile-drop, pendant-drop, drop-weight and maximum-bubble-pressure techniques. Several of these techniques will be described in Chapters 3 and 6. Table 2.1 presents typical values for the surface energies of various types of liquids. 

The maximum bubble pressure technique has been developed in various directions over recent years. One of the most promising of these is the use of this measurement technique in medicine. In a recent book, the capacity of dynamic surface tension measurements is shown as a new diagnostic tool, as well as for monitoring therapies in medicine (2). Quite a number of statistically significant relationships between selected surface tension values and biochemical data have been proven to exist. 

In addition to the maximum bubble pressure technique discussed above, a group of methods based on static or growing drops and bubbles exists which give access to interfacial tensions at short adsorption times, i.e. parts of a second, up to several minutes and even hours (3). These methods are based on the measurement of the capillary pressure however, in this case, the entire process of the drop or bubble formation is used to study the adsorption processes at the respective interfaces. 

The maximum bubble pressure technique is the most useful technique for measuring adsorption kinetics at short times, particularly if a correction for the so-called "dead time , x, is made. The dead time is simply the time required to detach the bubble after it has reached its hemispherical shape. A schematic representation of the principle of maximum bubble pressure is shown in Fig. 3.7, which shows the evolution of a bubble at the tip of a capillary. The figure also shows the variation of pressure p in the bubble with time. At t = 0 (initial state), the pressure is low (note that the pressure is equal to 2y/r since r of the bubble is large, p is small). At t = x (smallest bubble radius that is equal to the tube radius) p reaches a maximum. At t = x, (detachment... 

There are numerous ways in which surface tension can be measured. The technique that is perhaps the most suitable for electrolyte solutions is the Maximum Bubble Pressure technique, as the continuous generation of fresh interface assists in acquiring data and is not influenced by low levels of contamination. Typically a bubble is produced from a fine capillary every five to ten seconds and as it grows the pressure is monitored. When the pressure reaches a maximum, the radius of the bubble is equivalent to the inside radius of the hydrophilic capillary tip. At this time the radius of the bubble and the internal pressure are known, therefore the surface tension can easily be calculated using the Laplace equation. This bubble then detaches and a new bubble is created, allowing for easy repeat measurements of surface tension at a freshly generated interface. 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

The oil-water dynamic interfacial tensions are measured by the pulsed drop (4) technique. The experimental equipment consists of a syringe pump to pump oil, with the demulsifier dissolved in it, through a capillary tip in a thermostated glass cell containing brine or water. The interfacial tension is calculated by measuring the pressure inside a small oil drop formed at the tip of the capillary. In this technique, the syringe pump is stopped at the maximum bubble pressure and the oil-water interface is allowed to expand rapidly till the oil comes out to form a small drop at the capillary tip. Because of the sudden expansion, the interface is initially at a nonequilibrium state. As it approaches equilibrium, the pressure, AP(t), inside the drop decays. The excess pressure is continuously measured by a sensitive pressure transducer. The dynamic tension at time t, is calculated from the Young-Laplace equation... 

Surface tension and density of liquid alloys have been studied by Moser et al. (2006). Measurements by maximum bubble pressure and dilatometric techniques were carried out in an extensive range of temperatures on liquid alloys close to the ternary eutectic Sn3 3Ag0 76Cu with different Sb additions, which decrease surface tension and density. The experimental data were discussed in comparison also with values calculated on the basis of different models. 

Figure D3.5.6 Adsorption kinetics of a small molecule surfactant. Surface tension of polyoxyethylene (10) lauryl ether (Brij) at the air-water interface decreases as time of adsorption increases. Brij concentration is 0.1 g/liter, as measured by the drop volume technique and the maximum bubble pressure method (UNITD3.6).

An almost overwhelmingly large number of different techniques for measuring dynamic and static interfacial tension at liquid interfaces is available. Since many of the commercially available instruments are fairly expensive to purchase (see Internet Resources), the appropriate selection of a suitable technique for the desired application is essential. Dukhin et al. (1995) provides a comprehensive overview of currently available measurement methods (also see Table D3.6.1). An important aspect to consider is the time range over which the adsorption kinetics of surface-active substances can be measured (Fig. D3.6.5). For applications in which small surfactant molecules are primarily used, the maximum bubble pressure (MBP) method is ideally suited, since it is the only... 

The Wilhelmy plate technique has also been modified to measure surface or interfacial tensions under special conditions. By way of illustration we mention the application to the measurement of electrocapillanj curves for the mercuiy-aqueous electrolyte solution interface by Montgomery and Anson ), extending earlier work by Smith ). Electrocapillary curves are plots of y as a function of the potential applied across the interface, which should be polarizable. In fig. II.3.48 we already gave a few such curves. Montgomery and Anson found that their curve in 10 2 M NaF agreed within 0.005 mN m- with data obtained using the maximum bubble pressure. 

Table 1.2. Surface tensions of water in mN m , obtained by various investigations using different techniques. Temperatures in degrees Celsius. Abbreviations for methods CR = capillary rise, WP = Wilhelmy plate, DNR = Du Nouy ring, DM = other detachment method or object in the surface. HD = hanging (pendent) drop, SD = sessile drop, MBP = maximum bubble pressure DW = drop weight.

Under certain circumstances, the contact angle between Hg and the solution can change with applied potential. For this reason, the maximum bubble pressure method which is independent of contact angle is preferred (see section 8.2). This technique is easily adapted to computer-controlled experiments. 

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