Figure 10. Comparison of the solid fraction from experiments and DEM
simulations.
4.3 Large particle rotation
Videos taken during the shear tests (available in the supplementary
information) reveal drastically different macroscopic flow behavior for
the monodispersed and binary mixtures. For the monodispersed smaller
sized beads, a bulk motion of the system, with coordinated movement of
particles in a wave-like fashion is observed. For the monodispersed
larger size beads, a start-stop and rotational motion of individual
particles is observed within the bulk. However, these individual
movements of large particles are not observed in the video with 0.50
large glass bead mass fraction, suggesting that at lower concentrations,
small particles hinder the ability for the larger particles to rotate.
The 0.75 system exhibited the highest degree of segregation of all the
mixtures tested. For the 0.75 system, in regions of low levels of
particle segregation, the macroscopic flow behavior of the binary
mixture resembles that of the 0.5 system. However, in regions of high
levels of particle segregation, with a higher concentration of large
glass beads near the surface, rotation of individual large glass beads
is observed. Time-lapse images of the video for the 0.75 system are
extracted and shown in Figure 11. In the images, a few particles are
outlined to indicate their movement with time. From these images, it is
evident that the highlighted large glass bead is rotating. As the larger
glass bead rotates, multiple small glass beads on the surface of the
larger particle can be identified. This behavior indicates that the
addition of small glass beads, even at low concentrations, can adhere to
the larger glass beads.
While spinning movement is observed for the large particles, the small
particles move as a bulk in a wave-like motion. Furthermore, for the
50:50 binary mixture, the larger particles show no evidence of spinning
movement, and the bulk flow is similar to that of the small particles.
Therefore, an explanation for the unique shear stress trend is proposed
based on this observed macroscopic flow behavior. The hypothesis is that
large particles can spin, rather than move transversally, minimizing the
bed dilatation during shearing. And, the addition of smaller, more
cohesive particles and the adhesion between large and small particles
minimizes or prevents the spinning of the large particles. This, in
turn, results in a larger shear stress of the binary blend required to
induce flow.