Elutriation

Elutriation, also known as air classification, is a process for separating lighter particles from heavier ones using a vertically-directed stream of gas or liquid (usually upwards). This method is predominately used for particles with size (>1μm). The smaller or lighter particles rise to the top (overflow) because their terminal velocities are lower than the velocity of the rising fluid. The terminal velocites of any particle in any media can be calculated using Stokes' Law if the particle Reynolds number is below .2.

Read more about Elutriation:  Air Elutriation