In this paper we present and demonstrate a technique for mapping fluid flow rates in microfluidic systems with sub-micrometer resolution using confocal microscopy in conjunction with fluorescence correlation spectroscopy (FCS). Flow velocities ranging from ϳ50 m/s to ϳ10 cm/s can be recorded using fluorescent polymer nanospheres as fluid motion tracers. Velocity profiles and images of the flow in poly(dimethylsiloxane)-glass microchannels are presented and analyzed. Using the method, velocity

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... ages along the horizontal (top view) and vertical planes within a microdevice can be obtained. This is, to our knowledge, the first report of FCS for producing velocity maps. The high-resolution velocity maps can be used to characterize and optimize microdevice performance and to validate simulation efforts.