Foam-fiber interaction in tailoring lightweight materials

Foam forming technology enables the production of versatile cellulose fiber materials extending from thick, porous and lightweight structures to stiff 3Dforms, thin nonwovens and layered hybrid products. These renewable cellulosefiber-foam materials find their use in many current industry sectors and futurebionanomaterial architectures for e.g. packaging, high-efficiency air filters andsubstrates for biocatalytic conversion. The stability and bubble size of the foamprovide tools to tailor the density and pore size distribution of the formed fibernetwork. Additional effects come from surfactant chemistry and fiber surfacecharacteristics. Thus, it is important to understand the attachment of bubbles with these distinct fiber types and link that knowledge with foam architecture, stability and final microporous structure. We elaborate the fundamental aspects related to the bubble-fiber interactions using model surface approach. Air bubbles were contacted with either highly hydrophobic or highly hydrophilic silica surfaces as well as with amphiphilic cellulose model surfaces using captive bubble method. Characterization of surfactant adsorption on model surface was done using quartz crystal microbalance.Generally, air bubbles had a repulsive interaction with hydrophilic silica surfacesand attractive one with hydrophobic surfaces. The role of various interactioncomponents was analyzed in terms of calculated interface energy. For hydrophobic silica surfaces, we observed a transition from attraction to repulsion with increasing sodium dodecyl sulfate (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.