In this review we discuss the state of the art of the optical whole-field velocity measurement technique micro-scale Particle Image Velocimetry (µPIV). µPIV is a useful tool for fundamental research of microfluidics as well as for the detailed characterization and optimization of microfluidic applications in life science, lab-on-a-chip, biomedical research, micro chemical engineering, analytical chemistry and other related fields of research. An in depth description of the µPIV method is presented and compared to other flow visualization and measurement methods. An overview of the most relevant applications is given on the topics of near-wall flow, electrokinetic flow, biological flow, mixing, two-phase flow, turbulence transition and complex fluid dynamic problems. Current trends and applications are critically reviewed. Guidelines for the implementation and application are also discussed.

We derive analytical expressions for the three-dimensional (3D) acoustophoretic motion of spherical microparticles in rectangular microchannels. The motion is generated by the acoustic radiation force and the acoustic streaming-induced drag force. In contrast to the classical theory of Rayleigh streaming in shallow, infinite, parallel-plate channels, our theory does include the effect of the microchannel side walls. The resulting predictions agree well with numerics and experimental measurements of the acoustophoretic motion of polystyrene spheres with nominal diameters of 0.537 and 5.33 $\mu \text{m}$. The 3D particle motion was recorded using astigmatism particle tracking velocimetry under controlled thermal and acoustic conditions in a long, straight, rectangular microchannel actuated in one of its transverse standing ultrasound-wave resonance modes with one or two half-wavelengths. The acoustic energy density is calibrated in situ based on measurements of the radiation dominated motion of large 5-$\mu \text{m}$-diameter particles, allowing for quantitative comparison between theoretical predictions and measurements of the streaming-induced motion of small 0.5-$\mu \text{m}$-diameter particles.

A General Defocusing Particle Tracking (GDPT) method is proposed for tracking the three-dimensional motion of particles in Lab-on-a-chip systems based on a set of calibration images and the normalized cross-correlation function. In comparison with other single-camera defocusing particle-tracking techniques, GDPT possesses a series of key advantages: it is applicable to particle images of arbitrary shapes, it is intuitive and easy to use, it can be used without advanced knowledge of optics and velocimetry theory, it is robust against outliers and overlapping particle images, and it requires only equipment which is standard in microfluidic laboratories. We demonstrate the method by tracking the three-dimensional motion of 2 μm spherical particles in a microfluidic channel using three different optical arrangements. The position of the particles was measured with an estimated uncertainty of 0.1 μm in the in-plane direction and 2 μm in the depth direction for a measurement volume of 1510 × 1270 × 160 μm³. A ready-to-use GUI implementation of the method can be acquired on http://www.unibw.de/lrt7/gdpt.

The paper shows experiments revealing the surface and bulk flow on evaporating droplets using a 3D-PTV. The measurements permit to calculate the surface tension gradients and show how different surfactants can turn the droplet's surface either rigid or elastic.

The flow of a charged-stabilized suspension through a single constricted channel is studied experimentally by tracking the particles individually. Surprisingly, the behavior is found to be qualitatively similar to that of inertial dry granular systems: For small values of the neck-to-particle size ratio ($D/d<3$), clogs form randomly as arches of the particle span the constriction. The statistics of the clogging events are Poissonian as reported for granular systems and agree for moderate particle volume fraction ($\varphi \approx 20\%$) with a simple stochastic model for the number of particles at the neck. For larger neck sizes ($D/d>3$), even at the largest $\varphi (\approx 60\%)$ achievable in the experiments, an uninterrupted particle flow is observed, which resembles that of an hourglass. This particularly small value of $D/d(\simeq 3)$ at the transition to a practically uninterrupted flow is attributed to the low effective friction between the particles, achieved by the particle's functionalization and lubrication.