Tortuosity

Tortuosity is widely used as a critical parameter to predict transport properties of porous media, such as rocks and soils.

But unlike other standard microstructural properties, the concept of tortuosity is vague with multiple definitions and various evaluation methods introduced in different contexts.

In acoustics and following initial works by Maurice Anthony Biot in 1956, the tortuosity is used to describe sound propagation in fluid-saturated porous media.

In such media, when frequency of the sound wave is high enough, the effect of viscous drag force between the solid and the fluid can be ignored.

In the case of GPC the separation is based on molecular size and it works by the use of stationary media with an appropriate porous microstructure and adequate pore dimensions and distribution.

Insoluble matrix formers, such as ethyl cellulose, certain vinyl polymers, starch acetate and others control the permeation of the drug from the preparation and into the surrounding liquid.

The rate of mass transfer per area unit is, among other factors, related to the shape of polymeric chains within the dosage form.

Higher tortuosity or curviness retards mass transfer as it acts obstructively on the drug particles within the formulation.

A tortuous river ( meander of Nowitna River , Alaska )
Tortuosity calculation from an x-ray tomography reconstruction of a porous sandstone (the pores are shown): [ 9 ] the color represents the shortest distance within the pore space from the left limit of the image to any point in the pores. Comparing this distance to the straight-line distance shows that the tortuosity is about 1.5 for this sample. It has been demonstrated that the tortuosity increases when the porosity decreases. [ 10 ]