Apparent fluid slip at hydrophobic microchannel walls

TLDR

Micron‑resolution particle image velocimetry measures water velocity profiles in 30 × 300 μm channels to within 450 nm of the wall. On hydrophilic glass the profiles agree with Stokes’ equation and no‑slip, whereas on a 2.3 nm hydrophobic OTS monolayer the flow shows an apparent slip of ~10 % of the free‑stream velocity, corresponding to a slip length of ~1 μm that is negligible above 1 mm but must be considered at micro‑ and nano‑scales.

Abstract

Micron-resolution particle image velocimetry is used to measure the velocity profiles of water flowing through 30×300 μm channels. The velocity profiles are measured to within 450 nm of the microchannel surface. When the surface is hydrophilic (uncoated glass), the measured velocity profiles are consistent with solutions of Stokes’ equation and the well-accepted no-slip boundary condition. However, when the microchannel surface is coated with a 2.3 nm thick monolayer of hydrophobic octadecyltrichlorosilane, an apparent velocity slip is measured just above the solid surface. This velocity is approximately 10% of the free-stream velocity and yields a slip length of approximately 1 μm. For this slip length, slip flow is negligible for length scales greater than 1 mm, but must be considered at the micro- and nano scales.