  Fluidized beds are suspensions of grains by ascending fluids in tubes, and
are commonly used in industry given their high rates of mass and heat transfers
between the solids and fluid. Although usually employed in large scales (tube
diameter of the order of the meter), fluidized beds have a great potential in
much smaller scales (order of the millimeter) for processes involving powder
and fluids. Of particular interest is the pharmaceutical industry, which can
take advantage of mm-scale fluidized beds for promoting diffusion of species,
classifying grains, or peeling individual particles. This paper reports
experiments with a mm-scale gas-solid fluidized bed, which consisted of
0.5-mm-diameter glass particles suspended by an air flow in a 3-mm-ID glass
tube. We filmed the bed with a high-speed camera and processed the images with
a numerical code for tracking both the entire bed and individual particles. We
found that instabilities in the form of alternating high- and low-compactness
regions (known respectively as plugs and bubbles) appear in the bed, and that
the fluctuating energy of particles (known as granular temperature) is
relatively low within plugs. Therefore, mm-scale beds have much reduced
agitation and transfer rates when compared to their m-scale counterparts. We
show also that increasing the flow velocity does not avoid the appearance of
plugs, though the granular temperature increases, mitigating the problem. Our
results shed light on detailed mechanisms taking place within the miniaturized
bed, providing insights for chemical and pharmaceutical processes involving
powders.
