A technique for quantifying velocity profiles of fluids flowing in circular microchannels is presented. The primary puipose of this technique is to provide a robust method for quantifying the effect of surface character on the bulk fluid behaviour. A laser-scanning confocal microscope has been used to obtain fluorescent particle images from a 1 micron thick plane along the centreline of hydrophobic and hydrophilic glass capillaries. The velocities of fluorescent particles being carried in pressure-driven laminar flow of a Newtonian fluid have been evaluated at the centreplane of 57.5 micron capillaries using a variation of particle tracking velocimetry (PTV). This work aims to clarify inconsistencies in previously reported slip velocities observed in water over hydrophobically modified surfaces at micron and sub-micron lengthscales. A change in the velocity profile is observed for water flowing in hydrophobic capillaries, although the behaviour appears to be a result of an optical distortion at the fluid-wall interface. This may point to previous suggestions of a thin layer of air adsorbing to the surface. Notwithstanding, the results do not confidently suggest evidence of slip of water on hydrophobic surfaces in microchannels.
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