Porosity: Types And Measurement

Importance of Porosity

Porosity is defined as the ratio between the volume of voids and the total volume of a material. It indicates the extent of empty space within a substance, which can influence its physical properties, such as strength, permeability and liquid storage capacity.

Types of Porosity

There are several types of porosity, each associated with different characteristics and origins. The following sections describe the main types of porosity:

Open Porosity:

Definition: In open porosity the pores are interconnected. Consequently, liquids or gases can flow through the material. This property is critical in materials such as rock and ceramic, where permeability is required.

Example: Sponges, certain rocks (e.g. sandstone) and some synthetic foams.

Closed Porosity:

Definition: Closed porosity refers to isolated pores that are not interconnected. These pores do not allow the flow of liquids. Such a pore structure is observed in materials like glass and certain types of foam.

Example: Certain types of rubber and Ceramic, where the pores are enveloped within the material without interconnection.

Total Porosity:

Definition: Total porosity is the ratio of the volume of all pores (both open and closed) to the total volume of the material. It quantifies the overall void space, but does not provide information about the movement of liquids or gases.

Example: Total porosity as measured in soil samples, taking into account both interconnected and isolated pores.

Effective Porosity:

Definition: Effective porosity refers to the proportion of total porosity in which the pores are interconnected and permit the movement of liquids or gases. This parameter is used to determine how fluids move through materials such as rock or soil.

Example: Effective porosity in aquifers, which is used to determine the rate at which groundwater can move through rock formations.

Measurement of Porosity

The accurate determination of porosity is essential for various scientific and industrial applications. Several methods are used to measure porosity, each appropriate for different materials and pore sizes:

• Mercury Intrusion Porosimetry: In this method, mercury is forced into the pores under controlled pressure, thereby measuring the pore size distribution and total porosity.
• Gas Adsorption (BET method): This technique utilises gas molecules adsorbed onto the surface of the material. It produces data on both the surface area and pore dimensions.
• Optical Microscopy: This method allows for the direct visualisation and measurement of pores using high-resolution imaging techniques.

Methods for Measuring Porosity

Further information is available at Stanford Advanced Materials (SAM).

Frequently Asked Questions

What is porosity?
Porosity is the measure of void spaces in a material, expressed as a percentage of the total volume.

Why is porosity important?
Porosity influences the properties of a material, including strength, permeability and the ability to store liquids. It is therefore significant in fields such as geology and engineering.

How does porosity differ from permeability?
Porosity describes the volume of voids in a material, whereas permeability indicates the ease with which liquids can flow through these spaces.

Which factors influence the porosity of a material?
The factors include the composition of the material, the size and distribution of its pores and the processes to which the material was subjected during formation.

Can the porosity of materials be altered?
Yes, porosity can be modified through procedures such as compaction, heat treatment or chemical alteration to achieve the required properties.