Collective motions of externally driven colloids with different sizes

The collective motions of externally driven N_p-colloidal systems (2 ≤ N_p ≤ 3) in a confined viscous fluid between two parallel plates with a separation distance Lz have been investigated using three-dimensional direct numerical simulations with fully resolved hydrodynamics. We consider a system which consists of colloid particles with two different sizes (L: Large, S: Small, the size ratio is 1.25). In the two-particle system consisting of L- and S- particle, a stable doublet cluster whose leading particle is the small particle is always formed. In a three-particle system consisting of the two large particles and one small particle, although only the LSL-cluster have been observed in an experiment, we found that two types of stable clusters (SLL, LSL) appears and LLS-cluster does not appear, where (XYZ) means that X is the leading particle, Y the middle and Z the rear particle. We also found that SLL more probably appears in a higher external driven force and a larger L_z.