Oscillatory flow improves hydrodynamic ordering of soft suspensions in rectangular channels

A computational study is presented that examines the hydrodynamic ordering of soft-particle suspensions within rectangular channels undergoing both steady and oscillatory flow. In these conditions, particles assemble into one-dimensional train-like configurations aligned in the flow direction. The results indicate that oscillatory flow facilitates a significant improvement in the ordering process, particularly for the assembly of multiple side-by-side trains within the channel. Several key parameters are systematically varied, including the Wolmersley number (Wo) representing the oscillatory frequency, the capillary number (Ca) representing the particle deformability, and the particle volume fraction (ϕ). It is found that optimal ordering occurs for a particular range of Wo number, and that this range is dependent on Ca. Finally, polydisperse suspensions are also considered, whereby dispersity in the particle size is varied. The simulations reveal that oscillatory flow is more robust (relative to steady flow) for ordering polydisperse suspensions into side-by-side train structures. This study provides an alternative strategy for reliably ordering biological cells, vesicles, droplets, or other deformable particles into train-like configurations without the use of flow-focusing fluidic channel features.