Surface Roughness and Hydrodynamic Boundary Conditions

TL;DR

This study investigates how nanorough surfaces affect the hydrodynamic forces during high-speed thin film drainage, revealing no-slip boundary conditions despite reduced resistance, contrasting prior slip reports.

Contribution

It demonstrates that nanoroughness causes a shift in hydrodynamic boundary conditions without slip, challenging previous claims of shear-dependent boundary slip in similar systems.

Findings

Hydrodynamic resistance decreases compared to Taylor's prediction.

Measured forces align with no-slip boundary conditions at shifted positions.

Shift in hydrodynamic thickness is independent of separation and shear rate.

Abstract

We report results of investigations of a high-speed drainage of thin aqueous films squeezed between randomly nanorough surfaces. A significant decrease in hydrodynamic resistance force as compared with predicted by Taylor's equation is observed. However, this reduction in force does not represents the slippage. The measured force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at the intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to be independent on the separation and/or shear rate. Our results disagree with previous literature data reporting very large and shear-dependent boundary slip for similar systems.