  We investigate the effect of particle volume fraction on the efficiency of
deterministic lateral displacement (DLD) devices. DLD is a popular passive
sorting technique for microfluidic applications. Yet, it has been designed for
treating dilute suspensions, and its efficiency for denser samples is not well
known. We perform 3D simulations based on the immersed-boundary,
lattice-Boltzmann and finite-element methods to model the flow of red blood
cells (RBCs) in different DLD devices. We quantify the DLD efficiency in terms
of appropriate "failure" probabilities and RBC counts in designated device
outlets. Our main result is that the displacement mode breaks down upon an
increase of RBC volume fraction, while the zigzag mode remains relatively
robust. This suggests that the separation of larger particles (such as white
blood cells) from a dense RBC background is simpler than separating smaller
particles (such as platelets) from the same background. The observed breakdown
stems from non-deterministic particle collisions interfering with the designed
deterministic nature of DLD devices. Therefore, we postulate that dense
suspension effects generally hamper efficient particle separation in devices
based on deterministic principles.
