  Micro fluidized beds are basically suspensions of solid particles by an
ascending fluid in a mm-scale tube, with applications in chemical and
pharmaceutical processes involving powders. Although in many applications beds
are polydisperse, previous works considered only monodisperse beds aligned in
the vertical direction. However, introducing an inclination with respect to
gravity leads to different bed patterns and mixing levels, which can be
beneficial for some applications. In this paper, we investigate experimentally
the behavior of micro gas-solid beds consisting of bidisperse mixtures under
different inclinations. In our experiments, mono and bidisperse beds are filmed
with a high-speed camera and the images are processed for obtaining
measurements at both the bed and grain scales. We show that the degree of
segregation is larger for vertical beds, but mixing varies non-monotonically
with inclination, with an optimal angle of 30$^{\circ}$--50$^{\circ}$ with
respect to gravity. By computing the mean and fluctuation velocities of grains,
we reveal that the mixing layer results from the competition between
segregation by kinetic sieving and circulation promoted by the fluid flow. We
also observe worse fluidization as the angle relative to gravity increases,
accounting then for the non-monotonic behavior. Our results bring new insights
into mixing and segregation in polydisperse beds, which can be explored for
processing powders in industry.
