  Experiments and simulations suggest that simple liquids may experience slip
while flowing near a smooth, hydrophobic surface. Here we show how precursors
to molecular slip can be observed in the complex response of a liquid to
oscillatory shear. We measure both the change in frequency and bandwidth of a
quartz crystal microbalance (QCM) during the growth of a single drop of water
immersed in an ambient liquid. By varying the hydrophobicity of the surface
using self-assembled monolayers, our results show little or no slip for water
on all surfaces. However, we observe excess transverse motion near hydrophobic
surfaces due to weak binding in the corrugated surface potential, an essential
precursor to slip. We also show how this effect can be easily missed in
simulations utilizing finite-ranged interaction potentials.
