Deterministic fractionation of binary suspensions moving past a line of microposts

We investigate the motion of suspended particles past a single line of equally spaced cylindrical posts that is slanted with respect to the driving force. We show that such a line of posts can fractionate particles according to their size, with small particles permeating through while the larger particles are deflected by the steric barrier created by the posts, even though the gaps between posts are larger than the particles. We perform characterization experiments driving monodisperse suspensions of particles of different size past the line of posts over the entire range of forcing orientations and present both the permeation probability through the individual gaps between the posts as well as the fraction of permeating particles through the one-dimensional array. In both cases, we observe a sharp transition from deflection to permeation mode that is a function of particle size, thus enabling separation. We then drive binary mixtures at selected orientations of the line of posts and demonstrate high separation purity and efficiency.