Slip and friction at fluid-solid interfaces: Concept of adsorption layer

TL;DR

This paper introduces the concept of an Adsorption Layer at fluid-solid interfaces, providing a thermodynamically consistent model that explains slip phenomena and matches experimental and molecular dynamics data.

Contribution

It develops a self-consistent thermodynamic framework for interfacial slip, coupling adsorption, friction, and pressure gradients, advancing beyond classical slip models.

Findings

Explains water slippage in carbon nanotubes quantitatively.

Captures slip variations near moving contact lines.

Shows slip length depends on geometry and composition.

Abstract

When a fluid flows past a solid surface, its macroscopic motion arises from a subtle interplay between microscopic hydrodynamic and thermodynamic effects at the fluid-solid interface. Classical hydrodynamic models often rely on an unphysical no-slip boundary condition or an arbitrarily prescribed slip length, yet both approaches lack a rigorous physical foundation. This work introduces the concept of an Adsorption Layer (AL), an interfacial region of thickness delta l, where fluid-solid molecular interactions regulate both surface adsorption/depletion and interfacial slip. By applying the energy minimization principle, we derive balance equations within the AL that couple fluid-solid friction, viscous stresses, and surface adsorption dynamics. This framework establishes a self-consistent thermodynamic coupling between the AL and the bulk fluid, unlike conventional sharp-interface models. A key finding is the often-overlooked role and coupling of pressure and chemical potential gradients in the direction normal to the interface. This theoretical advance successfully explains the confinement-induced enhancement of water slippage in carbon nanotubes, quantitatively agreeing with molecular dynamics and experimental data -- an effect classical slip models fail to reproduce. Furthermore, when extended to binary liquids, the theory captures spatial variations in slip velocity near moving contact lines, highlighting the role of interfacial friction in shaping local flow. Our results demonstrate that the slip length is not a fixed material constant but rather an emergent, geometry- and composition-dependent property arising from coupled interfacial thermodynamics and hydrodynamics. This framework provides a physically grounded description of interfacial momentum transfer, with significant implications for microfluidics and surface engineering.