The Activation Energy for Wall Slip

TL;DR

This paper introduces a new method to measure the temperature dependence of slip length at liquid-solid interfaces, revealing how molecular properties influence wall slip behavior through activation energy analysis.

Contribution

It presents a novel experimental approach to determine the quadruple activation energy for shear and wall slip, validated across various hydrocarbons with different molecular structures.

Findings

Slip length increases with molar mass.

Molecular structure affects slip length and activation energy.

Adding polar molecules decreases slip length and increases activation energy.

Abstract

The Navier slip boundary condition is interpreted as an equilibrium of shear rate and slip rate. From the argument that the slip rate shall be proportional to the molecules' collision rate, the temperature dependence of the Navier slip boundary condition is derived. The model for the temperature dependence of the slip length is validated by slip measurements of liquid hydrocarbons in a novel Couette typ tribometer being introduced. The essence of the gained experimental data for one fluid-solid-interface is the quadruple activation energy for shear and wall slip together with the viscosity and slip length at a reference temperature. This quadruple is determined for four different hydrocarbon liquids of different molecular mass, structure and polarity proving the applicability of the new measurement method. From the executed systematic measurements three conclusions regarding the slip length dependence are pointed out: (i) the slip length increases with increasing molar mass; (ii) changing the molecular structure from saturated hydrocarbon to unsaturated affects the slip length as well as the activation energy for slip; (iii) adding a small fraction of polar molecules to the hydrocarbon decreases the slip length and increases the activation energy for wall slip due to the polar end-groups of the liquid.