Precursors to Molecular Slip on Smooth Hydrophobic Surfaces

TL;DR

This study investigates the early signs of molecular slip in liquids near hydrophobic surfaces by measuring oscillatory shear responses, revealing weak binding effects that precede slip, which are often missed in simulations.

Contribution

It demonstrates how precursors to slip can be observed through oscillatory shear measurements and highlights the importance of surface potential corrugation.

Findings

No slip observed for water on tested surfaces.

Excess transverse motion indicates weak binding near hydrophobic surfaces.

Finite-ranged potentials in simulations may overlook these precursors.

Abstract

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.