Direct measurement of microscale flow structures induced by inertial focusing of single particle and particle trains in a confined microchannel

Abstract

Understanding the flow structures induced by inertial focusing of particles is essential in microfluidics-based applications. In spite of numerous studies described in the literature, such microscale flows have, until today, not been subject to quantitative experimental study. This paper describes the construction and validation of a micro-particle image velocimetry-based experimental setup to investigate particle-induced flows in a confined microchannel. The flow structures around a single inertially focused particle are first visualized and quantitatively measured at Reynolds numbers Re from 21 to 525. A ring-like vortex flow is observed to form in front of the particle at Re = 63 owing to an increased particle lag effect, and finally the reverse flow regime is replaced by a vortex flow regime (at Re ≥ 105). This vortex flow produces a strong wall repulsive force and pushes the equilibrium position of the particle toward the channel center. Then, flows induced by both in-line and staggered particle trains are investigated (for 21 ≤ Re ≤ 105). For in-line particle trains, single-vortex flows are present between two neighboring particles on both sides of the channel. For staggered particle trains, two vortices rather than one are present between two neighboring particles at small Re (Re = 21), but this double-vortex flow develops into a single-vortex flow at relatively high Re (Re = 105). The present investigation helps in understanding particle dynamics and the mechanisms of interaction among particles, fluid, and channel walls. The experimental results presented here also provide validation data for further numerical and analytical studies.