  Clustering is often presumed to lead to enhanced agglomeration between
cohesive grains due to the reduced relative velocities of particles within a
cluster. Our discrete-particle simulations on gravity-driven, gas-solid flows
of cohesive grains exhibit the opposite trend, revealing a new mechanism we
coin "cluster-induced deagglomeration." Specifically, we examine relatively
dilute gas-solid flows and isolate agglomerates of cohesive origin from overall
heterogeneities in the system, i.e., agglomerates of cohesive origin and
clusters of hydrodynamic origin. We observe enhanced clustering with an
increasing system size (as is the norm for noncohesive systems) as well as
reduced agglomeration. The reduced agglomeration is traced to the increased
collisional impact velocities of particles at the surface of a cluster; i.e.,
higher levels of clustering lead to larger relative velocities between the
clustered and nonclustered regions, thereby serving as an additional source of
granular temperature. This physical picture is further evidenced by a
theoretical model based on a balance between the generation and breakage rates
of agglomerates. Finally, cluster-induced deagglomeration also provides an
explanation for a surprising saturation of agglomeration levels in
gravity-driven, gas-solid systems with increasing levels of cohesion, as
opposed to the monotonically increasing behavior seen in free-evolving or
driven granular systems in the absence of gravity. Namely, higher cohesion
leads to more energy dissipation, which is associated with competing effects:
enhanced agglomeration and enhanced clustering, the latter of which results in
more cluster-induced deagglomeration.
