Measurement of Bubble Size Distributions

FIGURE 2.5 Schematic ray diagram illustrating foam imaging arrangement using prism. Ray AB achieves total internal reflection at air-water snrface of wall-water-air film. Ray CD is hy contrast refracted away at Plateau border. 

FIGURE 2.7 Schematic illustration of sampling bias in favor of larger bubbles at plane of observation when imaging foams. (From Garrett, P.R., Foams and antifoams, in Food Colloids, Fundamentals of Formulation, Dickinson, E., Miller, R., eds.. The Royal Society of Chemistry, Special Publication No. 258, p 55, 2001.) 

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Toramaru A (1990) Measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes. J Vole Geotherm Res 43 71-90... 

Greaves, M., Kobbacy, K.A.H., Measurement of bubble size distribution in turbulent gas-liquid dispersions, Chem. Eng. Res. Des. 62 (1984), 3-12. 

Turbulence stirring for a standardized time - measurement of bubble-size distribution on a glass slide by using a microscope... 

Weissenborn, P. and Pugh, R. J., Measurement of bubble size distribution in froths and its application to studying froth structures and stability, in Proceedings of the 12th Scandinavian Symposium on Surface Chemistry, Ste-nius, P. and Sarvaranta, L. (Eds), Department of Forestry Production, Helsinki University of Technology, Publication C6, Helsinki, 1994, pp. 152-154. 

Various methods of foam generation that have been used in this context are first described, together with an indication of their respective advantages, disadvantages, and limitations. Although rarely considered in studies of antifoam action, we briefly include the issue of measurement of bubble size distributions because few methods of foam generation conveniently produce monodisperse foam. 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

One optical imaging technique that circumvents the problem of multiple fight scattering is to estimate the bubble size distribution from the area individual foam bubbles occupy ai a glass surface. Such experiments, and the systematic differences between bulk and surface bubble distributions, have been reviewed. Another technique that also directly measures the bubble size distribution is ihe use Of a Coulter counter, where individual bubbles are drawn through a small lube and counted. This yields a direct measure of the bubble size distribution, hut it is invasive and cannot probe the structure of the foam. 

Foam stability is most commonly monitored by following its collapse and liquid drainage. Both are macroscopic properties and can easily be measured. These properties are not directly related to microscopic events such as drainage occurring from lamellae, but they allow a description of foam behaviour over time, and, in conjunction with the transient development of bubble size distributions (BSD), yield a complete description of foam destabilization mechanisms (Hailing 1981 Bisperink et al. 1992 Patino 1995 Lau and Dickinson 2005). The common parameter characterising foam stability is half-life time for liquid drainage or foam collapse (Deeth and Smith... 

This can be followed by measuring the bubble size distribution as a function of time, by using microphotography or by counting the number of bubbles. Alternatively, the specific surface area or average bubble size can be measured as a function of time. Other techniques such as light scattering or ultrasound can also be applied. 

It is possible to model acoustic emission production from a population of bubbles of different sizes giving good correlation with physical measurements. The inverse problem has also been tackled that is, the prediction of bubble size distributions from the acoustic emission spectrum. Good correlation (within 20% across the distribution) for bubble sizes from 0.2 to 10 mm has been obtained. 

The method of dynamic gas disengagement (Sriram and Mann, 1977 Patel et al., 1989) to obtain an estimate of bubble size distribution is worthy of mention since it is convenient to use, sometimes even in real systems, especially for lower gas fractions. The impeller is stopped and the trend in the level measured against time. This trend indicates the bubble size distribution if terminal rise velocities are known and if coalescence is negligible. 

The vesicles (bubbles) in basaltic lava flows can be used to determine paleoelevation at the time of eruption. In the repertoire of paleoelevation proxies presently available to the research community, it represents one of very few direct proxies of elevation. The technique is based on the sizes of vesicles at the tops and bottoms of lava flows. We assume that bubbles do not know a priori that they will reside in one part of the flow or another when they are erupted from a volcanic vent. As such, the mass of gas is evenly distributed throughout the flow. The volume of the bubbles will therefore depend on pressure, which at the top of the flow is just atmospheric pressure, and at the bottom is atmospheric plus hydrostatic pressure from lava overburden. Since lava thickness can be measured in the field, and bubble size distributions (most notably the modal size) can be measured in the lab, a simple relation can be solved for atmospheric pressure, and using the standard atmosphere, elevation can be determined. 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

The local gas holdup and bubble behavior were measured by a reflective optic fiber probe developed by Wang and co-workers [21,22]. It can be known whether the probe is im-merging in the gas. The rate of the time that probe immerg-ing in the gas and the total sample time is gas holdup. Gas velocity can be got by the time difference that one bubble touch two probes and the distance between two probes. Chord length can be obtained from one bubble velocity and the time that the probe stays in the bubble. Bubble size distribution is got from the probability density of the chord length based on some numerical method. The local liquid velocity in the riser was measured by a backward scattering LDA system (system 9100-8, model TSI). Details have been given by Lin et al. [23]. 

An important condition which has to be fulfilled when using this method for foam dispersity determination is the absence of an excess hydrostatic pressure in the foam liquid phase. This pressure is equalized to a considerable extent when an equilibrium distribution the foam expansion ratio and the border pressure along the height of the foam column is established. This can be controlled by measuring the pressure in the Plateau borders at a certain level of the foam column by means of a micromanometer. However, if this condition is overlooked, the hydrostatic pressure can introduce a considerable error in the results of bubble size measurements, especially in low expansion ratio foams. Probably, it is the influence of the unrecorded hydrostatic pressure that can explain the lack of correspondence between the bubble size in the foam and the excess pressure in them, observed by Aleynikov[49]. The... 

If measurements are carried out in the same sample at consecutive time intervals the rate of drainage, the rate of foam collapse, the changes in foam volume and the gas fraction in the foam can be determined in addition to the evolution of the bubble-size distribution. 

Bubble size distribution is one the most diffieult parameters to measure and analyze. However, its evolution with time is of great importance, because it is closely linked to the product quahty as perceived by a consumer (Hailing 1981, Campbell and Mougeot 1999). In some systems, a uniform bubble size may be desirable (e.g., bread dough and eake batter) which improve baking characteristics, while in others, a wide spread in the distribution may be advantageous to achieve specific mouth-feel responses. 

One other parameter critical to product properties is the size distribution of the bubbles in the expanded product. Comparable bulk densities will be measured either with a few large bubbles or a large number of small ones. However, the rehydration and textural properties of the two structures will be markedly different. The distribution of bubble sizes relates to nucleation rather than growth. Frequently, the presence of insoluble particles in the melt is sufficient to cause multisite nucleation as shown in the above figure, but when this is not the case, small amounts of finely divided powder can be added to the formulation. Calcium carbonate is frequently used, acting as a weak point in the continuous melt, and also releasing gaseous carbon dioxide (personal communication, Charles Chessari, Food Science Australia, N. Ryde, Australia). 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

The bubble size distribution was also measured at a cell potential of 3.3V and is given in figure 3. To that purpose backlighting or shadowgraphy, being an in situ and non-intrusive optical measurement technique, was used. 

However, in other cases the model predictions deviate much more from each other and were in poor agreement the experimental data considering the measurable quantities like phase velocities, gas volume fractions and bubble size distributions. An obvious reason for this discrepancy is that the breakage and coalescence kernels rely on ad-hoc empiricism determining the particle-particle and particle-turbulence interaction phenomena. The existing param-eterizations developed for turbulent flows are high order functions of the local... 

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