Pinched-Flow Fractionation of Spherical Particles

The fractionation of spherical particles according to size and mass density (under gravity and/or centrifugal forces).

Particles are entering the main channel from an inlet with lower inflow rate. The fluid from sample inlet is reduced to a thin layer at the wall in the subsequent part of the main channel. In this design, an additional gravitational/centrifugal force pushes the particles down onto the wall leading to an ordering even before the pinch. In the main channel, particles that are larger than this layer are significantly pushed more towards the center of the main channel due to their contact interaction with the walls. As a result, in very simple terms, they are pushed onto different flow lines. In an expansion of the flow behind this pinch the particles exit through different outlet channels according to their size. In the animation above there are also lighter (red) and heavier (grey) small particles. In this initial design they still exit through the same outlet but a tendency of the lighter particles being more right than the grey ones can be observed. They simply follow the flow lines better than the heavier particles on which larger gravitational/centrifugal forces act downwards. The separation effect is small (there's only a factor 2 in the mass density of the particles) and it can be observed that any hydrodynamic interaction between the particles can easily diminish this weak effect. This means, going to larger particle volume fractions will be difficult here. However, the particle inlet could be modified in a way that particles entering the flow expansion after the pinch are sufficently seperated.