Micro-resolution particle image velocimetry

Particle image velocimetry (PIV), a technique commonly used at the macroscopic level to measure velocity vectors of particle-seeded flows, is adapted to measure both instantaneous and ensemble-averaged flow fields in microfluidic MEMS devices, where micron-scale spatial resolution is critical. Adapting PIV to the microscopic level presents a number of challenges, including: (1) visualizing tracer particles that are smaller than the wavelength of light, (2) minimizing errors due to the Brownian motion of the tracer particles, and (3) recording particle images with short exposure times, so that their motion does not cause particle streaking in the image field. The PIV technique is used to measure a low Reynolds number Hele-Shaw flow around a roughly 30 μm elliptical obstruction and a low Reynolds number flow through a 20 × 200 μm capillary tube. Velocity vector fields are presented with a spatial resolution of 6.9 × 6.9 × 1.5 μm. In principle, super-resolution particle tracking velocimetry (PTV) can be used to extend the spatial resolution of the velocity measurements down to approximately 1.5 × 1.5 × 1.5 μm.