Micro particle image velocimetry forms the basis of modern microscale flow measurement techniques. However, the motion of tracking particles that have been introduced into the fluid to measure flow velocity in micro fluidic flows can be affected by a number of different body forces that may not affect the carrier fluid. Under these conditions, molecular tagging velocity (MTV) has a distinct advantage in resolving fluid motion. A MTV approach, based on the photobleaching mechanism, is presented that allows any desired pattern of flow markers to be written into the region-of-interest to track the flow. This allows standard particle master shadowgraphy algorithms to be used to track groups of tagged molecules in the flow resolving two components of the bulk fluid motion. The MTV techniques developed so far for microfluidic application provide only a one dimensional flow measurement, typically along the axial direction perpendicular to the tagged region. Other tagging methods include the use of a grid and a structured mask for macro-scale and micro-scale flows respectively. One of the limitations of these approaches is that velocity information for only a predefined pattern is available. The MTV technique presented here is capable of resolving two dimensional flow velocity information with the limitations associated with the application of a mask. The salient feature of the presented approach is the use of a laser scanner which allows unrestricted, repeatable and accurate movement of the write laser within the field-of-view. The temporal displacement of the tagged region is captured onto an imaging device. The proof of concept of the technique is presented in the current work. A known flow velocity through a curved section of a 750 μm wide serpentine channel is measured using the designed system. The tagged regions are in the form of dark dots which advect with the flow. The temporal and spatial evolution of these tagged regions is tracked to reveal the flow information. The aim of this technique is to perform velocity measurements in a dielectrophoretic flow of a mixture of nanoparticles and the caged fluorescent dye, which may show different flow behavior, possibly in two opposite directions due to the charge on them.

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