Heterogeneity, suspension, and yielding in sparse microfibrous cellulose gels 1. Bubble rheometer studies

Abstract

Microstructural effects on suspension and yielding are studied in aqueous dispersions of bacterial cellulose fibers using a small suspended air bubble as a sensitive probe particle. An external pressure field is used to control the applied stress and characterize very early stages of fluid yielding, as well as more developed flow. The bubble allows sensitive measurement of small yield stress values but also indicates a discrepancy between bulk and microscale yield stress values. Image analysis and flow visualization provide a measurement of the deformation, yielding, and flow of low-concentration microfiber dispersions at length scales comparable to that of the fibers. Tracking of trapped tracer particles indicates local restructuring occurs in fiber networks, driving heterogeneous yielding and flow. The observed heterogeneity effects decreased as fiber concentration increased, reducing network restructuring. The size of the yielded region in the gels varied inversely with fiber concentration, but did not fully account for the bulk-microscale discrepancies, indicating the gels are restructuring, responsive fluids. We suggest a two-fluid description of sparse fiber gels is necessary to fully account for the heterogeneity and suspension performance.