Initial foam volume, calculation

The simplest technique for determination of the average by volume expansion ratio (and density) is the direct measurement of the total foam volume and the liquid volume in it (or its mass). Using barbotage methods for foam production the foaming process can be run until the initial solution is completely transformed into foam. Foam expansion ratio and its density are calculated by the formula... 

The physical meaning of the constant V. o is the initial liquid volume in a foam. It has been experimentally proved [67] that its value is close to the real liquid volume in a foam. The deference between V o values obtained experimentally from t/AV t) dependence and the real values does not usually exceed 5% [68], Therefore, for approximated calculation it can be assumed that Eq. (5.46) contains only one unknown constant wo. 

Foam compressibility has been considered by Siehr [3], He studied a foam of expansion ratio 100 prepared from a diluted potato juice. 100 cm3 of the foam were placed into a vertical pipe, closed at the bottom and was subjected to compression. At pressure dump the foam volume was reversed to its initial value, i.e. the process was entirely reversible. At Ap= 12.5, 24.0 and 48.0 kPa, the volume decreased from 100 cm3 to 90, 81 and 69 cm3, respectively. At ambient atmospheric pressure of 101.3 kPa and the above values of Ap, the calculated foam volume (not accounting for pa and the foam liquid volume) should be 89, 80.8 and 67.8 cm3, respectively. The calculated volumes, corresponding to the experimentally obtained, are derived at po = 106.5 kPa (Ap = 106.5 - 101.3 = 5.2 kPa), accounting for the liquid volume. Therefore, foam compressibility is in accord with the equation of state of the foam (the error being ca. 5%). 

The densities of quite stable foams can be measured by pycnometer, or by calculation from known volumes of initial liquid and final foam, from equation 21. In some cases, conductivity measurements can be used. 

As mentioned earlier, coalescence gives rise to a bubble size distribution in an initially monodispersed foam. The number of bubbles decreases and the mean bubble volume increases. Owing to the complexity involved in computing the evolution of the bubble size distribution, we restrict our treatment to the calculation of the mean bubble volume F, which is required in the formulation of the bulk conservation equations. This is considered next. 

Cellular plastics are used extensively in flotation applications because of their ability to maintain a low buoyancy factor. The buoyancy factor is directly affected by the amount of water a particular foam plastic will absorb. The test method developed to determine water absorption of rigid cellular plastics is fully described in the ASTM Standards Manual. Basically, the test consists of determining the volume of initial dry weight of the object and calculating the initial buoyancy force. The object is then immersed in water and, at the end of the immersion period, the final buoyancy force is measured with an underwater weighing assembly. The difference between the initial and final buoyancy force is the weight of water absorbed. This difference is expressed in terms of water absorbed per unit of specimen volume. The test results are seriously affected if proper steps are not followed closely and variables are not controlled carefully. 

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