ABSTRACT
The first of our series of experiments (Valverde et al. 2001) demonstrated that both regimes, the solidlike, for gas velocities Vg below a critical value Vc, and the fluidlike, for Vg larger than Vc, could be found in a non-bubbling fluidized bed of toner. In the solidlike regime, the mixing of two fluidized color samples, with the initial separation plane parallel to the gas flow, was impossible. In this regime, the gas pressure drop fell below the weight of the bed, i.e. a fraction of the particles support derived from their mutual contact, indicating that the particles remained in a stationary static state. Accordingly we could measure a nonvanishing tensile strength. Stresses in this state were clearly carried by enduring interparticle contacts. A confirmation of the existence of a mechanical strength in the solidlike regime could be obtained from slowly tilting the fluidized bed. In agreement with Rietema’s experiments the bed remained stable while the top surface was tilted as well. However, when the gas velocity was increased above the critical value Vc, a radically different behavior was found. In this fluidlike regime the whole bed weight was supported by the gas flow and there was no tensile yield stress. Stresses in this state were carried by particle collisions due to fluctuations in their motion. The powder then took on many of the properties of a low-viscosity liquid, its upper surface remaining horizontal when the container was tilted. Both color samples mixed successfully and the mixing process could be described by an effective self-diffusion coefficient that increased with gas velocity. When the gas velocity was further increased above a critical value Vb, instability in the form of rising large gas bubbles appeared, but certainly the source of bubbling could not be attributed to the loss of mechanical strength in contradiction with Rietema’s arguments. The fluid-to-solid transition became also clear when we investigated the response of the fluidized bed to vertical vibrations (Valverde et al. 2001). In the fluidlike regime the bed expanded reversibly as vibration amplitude was increased. At a critical amplitude the fluidized state destabilized and large gas bubbles appeared rising across the bed. On the contrary in the solidlike regime the bed behaved like a weak granular solid relaxing irreversibly toward a more compact state by eliminating low-density metastable configurations.
