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Pressure dependence of viscosity.

Jürn W P Schmelzer¹, Edgar D Zanotto, Vladimir M Fokin

¹Institut für Physik der Universität Rostock, Universitätsplatz, 18051 Rostock, Germany.

The Journal of Chemical Physics

|March 4, 2005

Summary

The pressure dependence of liquid viscosity is generally positive but can be negative, especially for water and silicate melts, due to free volume and structural changes. This study provides a general tool to estimate viscosity pressure dependence.

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Area of Science:

Physical Chemistry
Materials Science
Thermodynamics

Background:

The relationship between viscosity and free volume in liquids is a key factor in understanding their mechanical properties.
The pressure dependence of viscosity is crucial for various applications, but existing models often fail to explain observed behaviors, particularly negative pressure dependence.

Purpose of the Study:

To reanalyze the pressure dependence of viscosity in liquids of constant composition under isothermal conditions.
To develop a general theoretical framework for predicting viscosity pressure dependence, accounting for free volume effects and structural transformations.

Main Methods:

Analysis based on general considerations of viscosity and free volume relationships.
Derivation of a quantitative equation for pressure dependence using temperature dependence of viscosity, thermal expansion coefficient, and isothermal compressibility.
Extension of theoretical analysis to include non-equilibrium states (glasses) and pressure-induced structural changes.

Main Results:

Viscosity generally increases with pressure in stable or metastable liquids at moderate pressures.
Negative pressure dependence is observed and explained for water (0-4°C) and silicate liquids, attributed to negative thermal expansion coefficients.
At high pressures (GPa), structural transformations become significant, potentially leading to decreased viscosity even with positive thermal expansion.

Conclusions:

A general tool is provided to estimate the pressure dependence of viscosity, reconciling existing literature controversies.
The model accounts for both free volume effects and pressure-induced structural changes in equilibrium and non-equilibrium states.
The findings offer a unified approach to understanding viscosity behavior under varying pressure conditions.

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