  We employ micro-particle image velocimetry ($\mu$-PIV) to investigate laminar
micro-flows in hydrophobic microstructured channels, in particular the slip
length. These microchannels consist of longitudinal micro-grooves, which can
trap air and prompt a shear-free boundary condition and thus slippage
enhancement. Our measurements reveal an increase of the slip length when the
width of the micro-grooves is enlarged. The result of the slip length is
smaller than the analytical prediction by Philip et al. [1] for an infinitely
large and textured channel comprised of alternating shear-free and no-slip
boundary conditions. The smaller slip length (as compared to the prediction)
can be attributed to the confinement of the microchannel and the bending of the
meniscus (liquid-gas interface). Our experimental studies suggest that the
curvature of the meniscus plays an important role in microflows over
hydrophobic micro-ridges.
