particles) occurred in the reduced height system. As previously
mentioned, this band acts similar to the crystallized state presented
by Vanel et al.
[10]
. Even though this lower band appears crystallized,
the solids fraction is well removed from that of a crystalline state
(0.6416comparedto0.7405a13%difference)duetothepresenceofthe
larger particles. However, the full height simulation demonstrated
convection, as shown in
Fig. 8
, so our system exists on the boundary
between convection and percolation
[10]
. A more comprehensive
parametric investigation is necessary to determine the location of this
transition asa function of operatingparameters aswell asin eachof the
additional mixtures presented here.
Also shown in
Fig. 9
are the diffusivities of all species in the vertical
direction, calculated by
[44]
D
ij
=
Δ
x
i
−
Δ
x
i
Δ
x
j
−
Δ
x
j
=
2
Δ
t
,where
D
ij
isthediffusivityinthe
i
-directionduetogradientsinthe
j
-direction,
Δ
x
i
is the displacement of a particle in a time period
Δ
t
and
Δ
x
i
is the average
displacement of all particles in that same time period. This quantity
captures movement on a
‘
microscopic
’
particle level, whereas convection
characterizes the global
motion of the system. Correspondingly, a larger
number of particle sizes also results
in larger diffusivities, further
enhancing the mixing. Plotted in
Fig. 11
is the mean-square displacement
versus number of cycles for each of o
ur three systems, which can be used
to characterize the type of diffusive movement. All slopes are within 10%
of unity for all three cases considered, 0.9267 for binary, 1.023 for ternary
and1.025for11particlesizes,i.e.
Δ
x
i
−
Δ
x
j
Δ
x
i
−
Δ
x
j
αΔ
t
n
with
n
≈
1.
If the slope on a log-log plot is larger than unity, the mean motion of the
particles is considered superdiffusive which corresponds to large
fl
uctuations of particle positions between time points. If the slope is less
than unity, the movement is conside
red subdiffusive, which means that
the particles are trapped and motion is restricted to small, infrequent
jumps. If the mean-square displacement is directly proportional to the
time element (as it is for our case), then the diffusion is ordinary and
average particle movement is similar to traditional Brownian motion.