Revealing foam-fibre interactions in the production of lightweight materials: captive bubble study with model silica and cellulose surfaces

Foam forming technology enables the production of versatile cellulose fiber materials extending from thick, porous and lightweight structures to stiff 3D forms, thin nonwovens and layered hybrid products. These renewable materials can find their use in many current industry sectors as an alternative for plastics, including packaging, high-efficiency air filters and substrates for biocatalytic conversion. Due to these wide application possibilities, controlling the final material properties is highly important. The stability and bubble size of the foam provide tools to tailor the density and pore size distribution of the formed fiber network [1]. Additional effects come from surfactant chemistry and fiber surface characteristics. Thus, it is necessary to understand the attachment of bubbles with distinct fiber types with varied surfaces and link that knowledge with foam architecture, stability and final microporous structure. To reveal the basic fundamental aspects related to the bubble-fiber interactions, a simple model surface approach was used. The air bubbles were contacted with either highly hydrophobic or highly hydrophilic silica surfaces as well as with amphiphilic cellulose model surfaces in the presence of sodium dodecyl sulfate (SDS) using the captive bubble method. Characterization of SDS adsorption on model surfaces was done using the quartz crystal microbalance. Generally, air bubbles had a repulsive interaction with hydrophilic silica surfaces and attractive one with hydrophobic surfaces. The role of various interaction components was analyzed in terms of calculated interface energy. For hydrophobic silica surfaces, we observed a transition from attraction to repulsion with increasing SDS concentration. The attachment tendency showed significant scatter near the transition due to metastable states of the system. Moreover, the critical SDS concentration was affected by electrolyte concentration. For the hygroscopic and amphiphilic cellulose model surface, the behavior was similar to the hydrophilic silica surface when using SDS as the surfactant. In applications like nonwovens with both natural and man-made fibers, the foam-formed structure is expected to be sensitive not only to the used fibers but also to the type and concentration of surfactant.