Foam forming leads to sheet structures with exceptional volume of large pores. The link between fibre network structure and foam properties is investigated by comparing pore structure with measured bubble-size distribution. In foams produced by mechanical mixing, higher rotor speed leads to smaller average bubble size, whereas the effects coming from air content and surfactant are smaller and non-systematic. A significant drop in the average bubble size is seen when mixing fibres to foam. In sheets made with foam forming, there are more large pores compared to the water formed sheets. The size of these pores is affected by the sizes of the bubbles in the foam. Overall, pore size distribution is more strongly affected by the fibre type than by small changes in bubble size distribution.
Recent developments in making fibre materials using the foam-forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure-characterization methods, both two-dimensional and three-dimensional techniques were used to explore the porous structure of foam-formed samples made with two different types of cellulose fibre. These techniques included X-ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X-ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two-dimensional imaging techniques. The two-dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam-fibre interaction.
Foam physics is a field of study that scientists and researchers are interested in due to the vast range of uses, e.g. foam-foamed materials, oil extraction, and food processing. This study proposes a new equation for the drainage of wet foam that could add to the science of foam. To improve our comprehension of the intricate behaviour of wet foam, this model expands on a theoretical derivation. The usage of a bubble size formula that was proposed using the experimental data is one of the model's distinguishing characteristics. The size of foam bubbles can be predicted using this formula more precisely. A thorough derivation of the theoretical model is provided in the paper. Finally, this work presents a novel wet foam drainage model that has the potential to enhance the field of foam physics. The results of this work have important implications for industries. Therefore, more study is needed for developing a two dimensional drainage equation.
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