Microscale flow visualization velocimetry

As mentioned earlier, most microscale flow visualization techniques have focused on determination of flow velocity in liquids (or velocimetry). Fluorescence-based techniques, e.g., LIF and PF, have been extensively used for this application. However, in this section, we will look at another microfluidic application for which the development of microscale flow visualization techniques have been critical-micromixing. 

Microscale Flow Visualization Particle Image Velocimetry (PIV) Shear Stress Sensors... 

Particle based flow visualization techniques introduce marker particles into the flow field to study the motion of the bulk by determining the velocity vectors associated with these particles. Laser Doppler velocimetry (LDV), optical Doppler tomography (ODT) and particle image velocimetry (PIV) are the different methods that have been developed in this regard. However, of these methods microscale PIV ( x-PIV) is the most well-developed and popular microscale flow visualization technique. 

A promising device for measuring nanoscale and microscale channel flows is micro PIV, as mentioned in the previous Sect. How Characteristics in Microchannels. Santiego et al. [62] developed a micro PIV for measuring the instantaneous and mean liquid velocities around a circular cylinder of a diameter of 30 p,m placed in a microchannel. The diameter of the tracer particles ranged from 100 to 300 nm. Visualization of flow field in microchannels [63,64] and Brownian motion of a tracer particle [65] can also provide useful insight. Micro PIV is also used for observing the motion of blood cell in a capillary [66,67]. Micro hot-wire anemometer [68] and Doppler velocimetry [69] have also been recently developed for such studies in which bulk carbon nanotube were used as a wire. 

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