Several methods exist to measure and map fluid velocities in microfluidic devices, which are vital to understanding properties on the micro- and nano-scale. Fluorescence correlation spectroscopy (FCS) is a method traditionally exploited for its ability to measure molecular diffusion coefficients. However, several reports during the past decade have shown that FCS can also be successfully used to measure precise flow rates in microfluidics with very high spatial resolution, making it a competitive alternative to other common flow-measurement methods. In 2007 we introduced a modified version of conventional FCS that overcomes many of the artifacts troubling the standard technique. Here we show how the advantages of this method, called dual-focus FCS, extend to flow measurements. To do so, we have measured the velocity flow profile along the cross-section of a square-bore microfluidic channel and compared the result to the theoretical prediction.
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