microPIV

MicroPIV, or micro-particle image velocimetry, is a non-intrusive method for measuring flow fields in microfluidic environments by tracking the motion of tracer particles imaged with optical microscopy. Fluorescent or light-scattering microbeads (typically 0.1–1 μm) are dispersed in the fluid. The sample is illuminated by a pulsed light source, and high-speed cameras capture pairs of images separated by a known time interval. The particle images are analyzed by cross-correlation in overlapping interrogation windows to produce a two-dimensional velocity field in the illuminated plane. Often, a multi-pass interrogation with window deformation is used to improve spatial resolution.

Two-dimensional microPIV provides in-plane velocity components within the focal plane, while three-dimensional information can be obtained with stereo microPIV, which uses two or more cameras, or with volumetric approaches such as tomoPIV or light-sheet–based microPIV. These variants enable estimation of out-of-plane velocities or full three-dimensional, three-component (3D-3C) flow fields, respectively.

Typical measurement volumes in microPIV range from tens to hundreds of micrometers in each dimension, with spatial resolution governed by the magnification and particle image size. Temporal resolution depends on camera frame rate and the chosen Δt between image pairs. Calibration and refractive-index matching are important to minimize optical distortions and systematic errors.

Applications span microfluidics, lab-on-a-chip devices, biological flows in capillaries or microchannels, polymer solutions, and other microscale fluid phenomena. The technique is valued for providing full-field, non-contact velocity information, but it requires careful particle seeding, optical access, and robust calibration to address challenges such as out-of-plane motion and imaging artifacts.