A numerical model-assisted experimental design study of inertia-based particle focusing in stepped microchannels

Focusing methods based on fluid inertia have been demonstrated to be able to concentrate particles with a high precision and predictability offering great potential for practical application as manipulation of bodily fluids as blood for clinical diagnostics. However, to perform focusing to one single position at the center location of a channel with high focusing quality on the one hand and low pressure losses and shear stresses on the other hand still is a challenge in microchannel design. Three different stepped microchannels are analyzed via bright-field microscopy to investigate the impact of various geometric parameters on the focusing behavior of spherical particles. Reynolds numbers within a range of \( 8 \le Re \le 75\) are measured to evaluate the impact of flow conditions on the focusing characteristics. The microchannels show the ability to focus particles to a single stream with a maximum focusing efficiency of 97.1\(\%\) and purity of 99.1\(\%\). The focusing strongly depends on the channel geometry, that is, step length, step height, and settling length. A semiempirical model is developed to predict force-induced focusing ability at maximum shear stresses that are reduced significantly compared to previous systems. This model is demonstrated to be in very good agreement with experimental results and therefore can be utilized for future device design. Finally, guidelines for the design of stepped single-stream focusing devices at low pressure losses and low maximum shear stresses are derived based on experimental, numerical, and semiempirical data.