Direct velocity measurements of hydrodynamically confined microflows

Hydrodynamically confined microflows (HCMs) can be created underneath microfluidic probes (MFPs) in open liquid environments for local processing of surfaces, cells, and tissues. The behavior of the flow inside the gap between the probe and a substrate has been widely characterized by monitoring the size and shape of the confined flow using fluorescent dyes. However, velocity vectors within the flow, which are critical to a full understanding of fluid mechanics, have not been measured directly. Here, we report direct measurement of in-plane velocity vectors in an HCM using a micrometer-resolution particle image velocimetry. The effects of probe geometry, gap height, flow rate, and flow rate ratio on HCMs are investigated using fluorescent particles and dyes. Multi-port polydimethylsiloxane microfluidic probes (MFPs) are used to generate microflows under various conditions. Acquisition of images from different x–y planes parallel to the substrate provides information on the three-dimensional (3D) nature of the confined flows with two components (2C). The in-plane shape and velocity measurements of the HCMs, along with computational fluid dynamics (CFD) simulations, are used to estimate a complete 3D three-component (3C) shape of the confined flow. The results presented here provide a more comprehensive understanding of fluid mechanics in HCMs and will facilitate their application in biology, chemistry, medicine, and engineering.