Fig. 3.

Mechanism of dynamic self-assembly involves inertial lift forces and viscous disturbance flows induced by rotating particles. Simulated streamlines for Stokes flow (Re = 0) (A) and finite-Reynolds number flow (Re = 48) (B) are not significantly different from each other. Reversing flow can occur solely from confinement effects. The channel dimension in the simulation is 45 μm × 60 μm and particle diameter is 9.9 μm. (C) Existence of this reversing flow was experimentally observed over a range of Reynolds numbers. Tracer particles (1 μm diameter) are observed to follow reversing streamlines with little disturbance. In agreement with simulation, reversing flow appears at near-zero-Reynolds number. Larger particle size is 9.9 μm in diameter. (D) Schematic of two particle interactions. Major components are (i) viscous disturbance flows reflected off the side wall (F_V), (ii) inertial lift force (F_L), and (iii) flow speed distribution (gray arrows). Particles are repelled by F_V and stabilized by F_L. Flow speed distribution fine-tunes the balance of the interactions. The oscillation is initiated by strong F_V when two particles approach close enough for F_V > F_L. (E) Histograms of interparticle distances show that there are preferred spacings. The interparticle spacings shift to larger distances with increasing distance from the inlet, which indicates that small repulsive interactions remain whereas attractive interactions vanish. (F) Time-sequential capture shows defocusing dynamics of two particles entering the expanding region. Average defocusing trajectory ± standard deviation (N = 8) is shown (Inset). Decreasing R_P indicates decreasing inertial interaction.