The flow behaviour of powders depends on many factors. The following applies in general: Coarser, dry grain types flow easily, but the fine the grains get, the stronger is the impact of morphology and various adhesive forces on the flow behaviour:
Van der Waals forces
Liquid bridges
Electrostatic interaction
The 4 most common methods for determining flow properties are
Shear cell
Angle of repose
Compressibility index or Hausner ratio
Flow rate through a nozzle
A product is usually considered as having good flow properties if it flows out of a silo without interruption or does not solidify during storage or transport.