What is "elutriation"

Elutriation \E*lu`tri*a"tion\, n. The process of elutriating; a decanting or racking off by means of water, as finer particles from heavier.

n. The process of separating the lighter particles from the heavier ones by means of an upward directed stream of gas or liquid.

'''Elutriation ''' is a process for separating particles based on their size, shape and density, using a stream of gas or liquid flowing in a direction usually opposite to the direction of sedimentation. This method is mainly used for particles smaller than 1 μm. The smaller or lighter particles rise to the top (overflow) because their terminal sedimentation velocities are lower than the velocity of the rising fluid. The terminal velocity of any particle in any medium can be calculated using Stokes' Law if the particle's Reynolds number is below 0.2.