  We present a microfluidic platform for the continuous separation of suspended
particles based on their size and settling velocity. The separation method
takes advantage of the flow field in the vicinity and inside a parallel array
of slanted open cavities. These cavities induce flow along them, which deflects
the suspended particles to a different degree depending on the extent to which
they penetrate into the cavities. The cumulative deflection in the periodic
array ultimately leads to vector chromatography, with the different species in
the sample moving in different directions. We demonstrate density and size
based separation over a range of flow rates by separating polystyrene and
silica particles and show that purities nearing 100% can be achieved for
multicomponent mixtures. We also demonstrate the potential of the platform to
separate biological cells by fractionating different blood components. We
discuss the presence of two regimes, which can be distinguished depending on
the ratio between the settling velocity and the velocity of the particles
across the open cavities. The proposed platform could also integrate additional
separative force fields in the direction normal to the plane of the cavities to
fractionate specific mixtures based on the distinguishing properties of the
component species.
