Solid-state characterization, powders humidity

Engineers and scientists are use to dealing with liquids and gases whose properties can be readily measured, tabulated and even calculated. The boiling point of pure benzene at one atmosphere pressure can be safely relied upon to remain at 80.1 °C. The viscosity of water at 20 °C can be confidently predicted to be 0.001 Pas. The thermal conductivity of copper at 100°C is 377W/m K. With particulate solids, the picture is quite different. The flow properties of sodium bicarbonate powder, for example, depends not only on the particle size distribution, the particle shape and surface properties, but also on the humidity of atmosphere and the state of the compaction of the powder. These variables are not easy to characterize and so their influence on the flow properties is difficult to predict with any confidence. 

---