Precursors to Molecular Slip on Smooth Hydrophobic Surfaces

Experiments and simulations suggest that simple liquids can experience slip while flowing near a smooth, solid surface. Hydrophobic surfaces are expected to enhance slip at high shear rates due to a depleted density in the liquid near the solid. 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, yet excess transverse motion near hydrophobic surfaces due to elastic deformation of liquid molecules in local potential wells. We also show that this dynamic effect can be easily missed in simulations with finite-ranged interaction potentials.