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Why study granular mixing? The simple answer lies in its
undeniable industrial importance. A more subtle reason is that mixing
flows generate unique qualitative patterns that are easily visualized
and verified; because of this, mixing studies allow relatively modest
computational experiments to effectively probe complex flows and to
test the robustness of model theories. In particular, particle dynamics
simulations allow access to information which would be very difficult
or impossible to measure in a physical experiment such as,
instantaneous force distributions, three dimensional concentration and
velocity profiles, etc. This project ostensibly focuses on cohesive
mixing, yet insight into granulation and aggregate break-up processes --
with implications far beyond granular mixing -- will be gained.
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- Varying Wetting Angles: Our theories predict that particles with differing surface properties (red-hydrophilic and green-hydrophobic) but
otherwise identical in all other respects will mix perfectly when (a) tumbled dry; however, (b) when interstitial water is added, they will
instead segregate.
Many of the industries which deal with particulate
materials are in some way affected by cohesion; most notably the
pharmaceutical, metallurgical, and pigment industries. Cohesion between
particles arises from a variety of sources: van der Waals forces,
electrostatic forces, and liquid bridging (capillary forces), to name a
few. These interparticle forces become increasingly important as
particle size decreases and, as cohesion becomes important, substantial
departure from the behavior of free-flowing particulate systems becomes
evident. For the case of free-flowing powders, it is necessary to
attempt to restrict particle motion in order to minimize segregation or
de-mixing. In contrast, in cohesive systems, particles tend to
segregate into aggregates that must be broken apart to attain a quality
mixture. Despite this fundamental difference, some of the tools that
have proven effective for the study of cohesionless materials can still
be employed for cohesive systems -- specifically, particle dynamics
simulations. To that end, a cohesive-solid particle dynamics code is
being developed to study the change in stable heap angle as a function
of global Bond number (the ratio of particle weight to attractive
force) as well as the evolution of mixing in a rotated drum. Here, we
are concerned with moderately sized particles, ~1mm, and so we devote
our attention to the forces attributable to interstitial liquid -- the
dominant interaction effect at these length scales.
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- Varying Size Ratio and Wetting Angle. Our theories predict a wealth of behavior when multiple particle proerties vary. From top to bottom the
rows correspond to size ratios of 0.25, 0.53 and 0.75, respectively. From left to right the images represent results when dry -- (a), (b) and
(c); wet with the smaller particle (green) being more hydrophilic -- (d), (e), and (f); wet with both particles hydrophilic -- (g), (h) and (i);
and wet with the smaller particle (red/orange) being more hydrophobic -- (j), (k) and (l). Results are in agreement with the predictions from
theory.
By taking a discrete view of cohesion, we develop a particle-level model which can accurately predict the extent of particle
mixing and segregation in cohesive (wet) granular systems. Our model is based on a discrete characterization tool and is used to generate phase
diagrams of the predicted particle behavior. These phase diagrams exhibit both mixed and segregated phases where the boundary is determined by
the mechanical and surface properties of the particles, such that manipulation of surface properties and/or size/density ratios provides a novel
method to control cohesive particle mixing and segregation A detailed description of the phase diagram development process as well as
quantitative validation of the theoretical results are reported here for the first time. These results have implications for pharmaceutical
formulations, industrial mixing/separation processes, and novel particle production methods (e.g., engineered agglomerates with precisely
prescribed compositions) (Visit out Papers List for the latest information!)
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