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We report the results of direct measurements of velocity profiles in a microchannel with hydrophobic and hydrophilic walls, using a new high-precision method of double-focus spatial fluorescence cross correlation under a confocal microscope. In the vicinity of both walls the measured velocity profiles do not go to zero by supplying a plateau of constant velocity. This apparent slip is proven to be due to a Taylor dispersion, an augmentation by shear diffusion of nanotracers in the direction of flow. Comparing the velocity profiles near the hydrophobic and hydrophilic walls for various conditions shows that there is a true slip length due to hydrophobicity. This length, of the order of several tens of nanometers, is independent of the electrolyte concentration and shear rate.

We present an experimental approach to flow profiling within femtoliter sample volumes, which allows the high-precision measurements at the solid interface. The method is based on the spatial cross-correlation of the fluorescence response from labeled tracer particles (latex nanospheres or single dye molecules). Two excitation volumes, separated by a few micrometers, are created by two laser foci under a confocal microscope. The velocity of tracer particles is measured in a channel about 100 microm wide within a typical accuracy of 0.1%, and the positions of the walls are estimated independently of any hydrodynamic data. The underlying theory for the optical method is given for an arbitrary velocity profile, explicitly presenting the numerical convolutions necessary for a quantitative analysis. It is illustrated by using the Poiseuille flow of a Newtonian liquid with slip as an example. Our analysis yields a large apparent fluid velocity at the wall, which is mostly due to the impact of the colloidal (electrostatic) forces. This colloidal lift is crucially important in accelerating the transport processes of molecules and nanoparticles in microfluidic devices.

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