The fluidization phenomena of gas-solids systems strongly depend on the types of particles. In 1973, Derek Geldart created a chart classifying different particulate materials into four so-called "Geldart Groups" based on their fluidized bed behavior. He classified the particulate materials according to their mean particle size dp and the difference between the solid and gas phase densities ρp - ρg, as the figure below shows.
Group A - Aeratable
Group B - Sand like
Group C - Cohesive
Group D - Spoutable