Physical gels are swollen polymer networks consisting of transient crosslink junctions associated with hydrogen or ionic bonds. Unlike covalently crosslinked gels, these physical crosslinks are reversible thus enabling these materials to display highly tunable and dynamic mechanical properties. In this work, we study the polymer composition effects on the fracture behavior of a gelatin gel, which is a thermoreversible biopolymer gel consisting of denatured collagen chains bridging physical network junctions formed from triple helices. Below the critical volume fraction for chain entanglement, which we confirm via neutron scattering measurements, we find that the fracture behavior is consistent with a viscoplastic type process characterized by hydrodynamic friction of individual polymer chains through the polymer mesh to show that the enhancement in fracture scales inversely with the squared of the mesh size of the gelatin gel network. Above this critical volume fraction, the fracture process can be described by the Lake-Thomas theory that considers fracture as a chain scission process due to chain entanglements.

We report experimental observations of the dynamic behavior of single, magnetically tagged 3–4 mm particles varying in density from 0.55 g/cm3 to 1.2 g/cm3 as they migrate freely in a bubbling air-fluidized bed of 177–250 μm glass beads of 2.5 g/cm3 density over a range of air flows. The densities of the tracer particles (made by imbedding small magnets in wooden particles) were chosen to span a range typical for many biomass materials and exhibited both segregated and well-mixed behavior. Using high-speed measurements from externally mounted magnetic probes, we were able to reconstruct three-dimensional spatial and temporal information about the tracers’ trajectories over periods of five minutes. Based on this information, we describe general trends in how the tracers moved and redistributed themselves as functions of their density, fluidization air flow, and the overall concentration of low density particles present. One key finding was that the time average vertical probability distribution of the tracer particles locations is consistent with a Weibull distribution. The effective Weibull parameters appear to vary systematically with the degree of fluidization and particle density. Also, we observed that temporal autocorrelations in the vertical position of the tracer particles vary systematically with fluidization intensity and reveal important information about the dominant bed circulation time scales. Our results suggest that it may be possible to develop relatively simple statistical models or correlations for describing the spatial distribution and circulation of mm sized particles in bubbling beds of this type. Such tools should be useful for simulating some types of fluidized biomass processing and for validating kinetic-theory models of fluidized bed systems.