The relation between bubble-fibre interaction and material properties in foam forming

Foam forming of cellulose fibre materials is based on an interaction between fibres and bubbles, which can take several material properties to new levels. To control the formed structure, the mechanisms of this interaction have been systematically investigated. This started with captive bubble studies where we analysed the interaction of a single bubble with various smooth cellulose and silica model surfaces. The bubbles adhered only to hydrophobic surfaces, and this attraction was sensitive to the surface tension. From this simplest case, the studied system gradually became more complex. We found that a bubble adheres weakly also to a submerged cellulose nanofibre (CNF) film, which could be explained by nanoscale surface roughness capturing nanobubbles. The interaction with real fibres was studied by pressing a single bubble against a fibre bed in water and sodium dodecyl sulphate (SDS) solution. Fibre type and surface tension had all apparent effects on the attachment. In the case of natural fibres, the presence of hydrophobic lignin clearly increased the fibre attachment on a bubble, while added SDS decreased the attachment with all fibre types. These findings agreed with the mechanisms found earlier using the model surfaces. Finally, when forming thick nonwoven materials using hydrophilic and hydrophobic viscose fibres, differences in fibre network structure and strength properties depended on the fibre hydrophobicity and surfactant type, as suggested by the results obtained in simpler systems.