Dilational Viscosity of Langmuir monolayers

TL;DR

This paper presents a theoretical model for the dilational viscosity of Langmuir monolayers, linking molecular orientation effects to measurable surface viscosity and surface tension behavior.

Contribution

It introduces a model incorporating orientational order via the surface order parameter and Landau expansion, aligning theoretical predictions with experimental data.

Findings

Surface viscosity magnitude matches experimental observations.

Positive sign of dilational viscosity indicates increased in-plane order raises surface tension.

The model successfully explains the relationship between molecular orientation and surface rheological properties.

Abstract

The dilational viscosity $\epsilon'$ of Langmuir monolayer is considered in a theoretical model taking into account an orientational effect of the dilational wave on surface molecules. This orientational order is described by the surface order parameter Q; the orientational part of the free surface energy is given by Landau's expansion in powers of Q. The magnitude of surface viscosity, driven by the surface tension derivative $\partial\gamma/\partial Q$, is in good accord with the experimentally observed $\epsilon'$. The sign of $\epsilon'$ is positive that indicates that increased in-plane ordering increases the surface tension.