Dynamic mechanisms for apparent slip on hydrophobic surfaces.

Recent experiments [Y. Zhu and S. Granick, Phys. Rev. Lett. 87, 096105 (2001)] have measured a large, shear-dependent fluid slip at partially wetting fluid-solid surfaces. We present a simple model for such a slip, motivated by the recent observations of nanobubbles on hydrophobic surfaces. The model considers the dynamic response of bubbles to change in hydrodynamic pressure, due to the oscillation of a solid surface. Both the compression and diffusion of gas in the bubbles decrease the force on the oscillating surface by a "leaking mattress" effect, thereby creating an apparent shear-dependent slip. With bubbles similar to those observed by recent atomic force microscopy, the model predicts a force decrease consistent with the experimental measurements of Zhu and Granick.