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Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface (cf. closed-cell foam). There are many ways to test porosity in a substance or part, such as industrial CT scanning. The term porosity is used in multiple fields including pharmaceutics, ceramics, metallurgy, materials, manufacturing, hydrology, earth sciences, soil mechanics and engineering.


Void fraction in two-phase flowEdit


Porosity in earth sciences and constructionEdit



where VV is the volume of void-space (such as fluids) and VT is the total or bulk volume of material, including the solid and void components. Both the mathematical symbols   and   are used to denote porosity.




where   is the surface porosity,   is the compaction coefficient (m−1) and   is depth (m).

A value for porosity can alternatively be calculated from the bulk density  , saturating fluid density   and particle density  :


If the void space is filled with air, the following simpler form may be used:


Normal particle density is assumed to be approximately 2.65 g/cm3 (silica), although a better estimation can be obtained by examining the lithology of the particles.

Porosity and hydraulic conductivityEdit


Sorting and porosityEdit

Effects of sorting on alluvial porosity. Black represents solids, blue represents pore space.


Porosity of rocksEdit


Porosity is the ratio of pore volume to its total volume. Porosity is controlled by: rock type, pore distribution, cementation, diagenetic history and composition. Porosity is not controlled by grain size, as the volume of between-grain space is related only to the method of grain packing.


Porosity of soilEdit



Soil porosity is complex. Traditional models regard porosity as continuous. This fails to account for anomalous features and produces only approximate results. Furthermore, it cannot help model the influence of environmental factors which affect pore geometry. A number of more complex models have been proposed, including fractals, bubble theory, cracking theory, Boolean grain process, packed sphere, and numerous other models. The characterisation of pore space in soil is an associated concept.

Types of geologic porositiesEdit

Primary porosity
Secondary porosity
Fracture porosity
Vuggy porosity
Effective porosity (also called open porosity)
Ineffective porosity (also called closed porosity)
Dual porosity
In solids (i.e. excluding aggregated materials such as soils), the term 'macroporosity' refers to pores greater than 50 nm in diameter. Flow through macropores is described by bulk diffusion.
In solids (i.e. excluding aggregated materials such as soils), the term 'mesoporosity' refers to pores greater than 2 nm and less than 50 nm in diameter. Flow through mesopores is described by Knudsen diffusion.
In solids (i.e. excluding aggregated materials such as soils), the term 'microporosity' refers to pores smaller than 2 nm in diameter. Movement in micropores is activated by diffusion.

Porosity of fabric or aerodynamic porosityEdit


Die casting porosityEdit

Casting porosity is a consequence of one or more of the following: gasification of contaminants at molten-metal temperatures; shrinkage that takes place as molten metal solidifies; and unexpected or uncontrolled changes in temperature or humidity.

While porosity is inherent in die casting manufacturing, its presence may lead to component failure where pressure integrity is a critical characteristic. Porosity may take on several forms from interconnected micro-porosity, folds, and inclusions to macro porosity visible on the part surface. The end result of porosity is the creation of a leak path through the walls of a casting that prevents the part from holding pressure. Porosity may also lead to out-gassing during the painting process, leaching of plating acids and tool chatter in machining pressed metal components.[4]

Measuring porosityEdit


Several methods can be employed to measure porosity:

  • Imbibition methods,[5] i.e., immersion of the porous sample, under vacuum, in a fluid that preferentially wets the pores.


VV is the effective volume of the pores,
VT is the bulk volume of the sample,
Va is the volume of the container containing the sample,
Vb is the volume of the evacuated container,
P2 is final pressure present in the entire system.
The porosity follows straightforwardly by its proper definition
  • Thermoporosimetry and cryoporometry. A small crystal of a liquid melts at a lower temperature than the bulk liquid, as given by the Gibbs-Thomson equation. Thus if a liquid is imbibed into a porous material, and frozen, the melting temperature will provide information on the pore-size distribution. The detection of the melting can be done by sensing the transient heat flows during phase-changes using differential scanning calorimetry – (DSC thermoporometry),[6] measuring the quantity of mobile liquid using nuclear magnetic resonance – (NMR cryoporometry)[7] or measuring the amplitude of neutron scattering from the imbibed crystalline or liquid phases – (ND cryoporometry).[8]

See alsoEdit


  • Glasbey, C. A.; G. W. Horgan; J. F. Darbyshire (September 1991). "Image analysis and three-dimensional modelling of pores in soil aggregates". Journal of Soil Science. 42 (3): 479–86. doi:10.1111/j.1365-2389.1991.tb00424.x.
  • Horgan, G. W.; B. C. Ball (1994). "Simulating diffusion in a Boolean model of soil pores". European Journal of Soil Science. 45 (4): 483–91. doi:10.1111/j.1365-2389.1994.tb00534.x.
  • Horgan, Graham W. (1996-10-01). "A review of soil pore models" (PDF). Retrieved 2006-04-16.
  • Horgan, G. W. (June 1998). "Mathematical morphology for soil image analysis". European Journal of Soil Science. 49 (2): 161–73. doi:10.1046/j.1365-2389.1998.00160.x.
  • Horgan, G. W. (February 1999). "An investigation of the geometric influences on pore space diffusion". Geoderma. 88 (1–2): 55–71. Bibcode:1999Geode..88...55H. doi:10.1016/S0016-7061(98)00075-5.
  • Nelson, J. Roy (January 2000). "Physics of impregnation" (PDF). Microscopy Today. 8 (1). Archived from the original (PDF) on 2009-02-27.
  • Rouquerol, Jean (December 2011). "Liquid intrusion and alternative methods for the characterization of macroporous materials (IUPAC Technical Report)*" (PDF). Pure Appl. Chem. 84 (1): 107–36. doi:10.1351/pac-rep-10-11-19.


  3. ^ Effective and Ineffective Porosity or Total and Effective Porosity Explained at E&P
  4. ^ "How to Fix Die Casting Porosity?". Godfrey & Wing.
  6. ^ Brun, M.; Lallemand, A.; Quinson, J-F.; Eyraud, C. (1977). "A new method for the simultaneous determination of the size and the shape of pores: The Thermoporometry". Thermochimica Acta. Elsevier Scientific Publishing Company, Amsterdam. 21: 59–88. doi:10.1016/0040-6031(77)85122-8
  7. ^ Mitchell, J.; Webber, J. Beau W.; Strange, J.H. (2008). "Nuclear Magnetic Resonance Cryoporometry" (PDF). Phys. Rep. 461 (1): 1–36. Bibcode:2008PhR...461....1M. doi:10.1016/j.physrep.2008.02.001
  8. ^ Webber, J. Beau W.; Dore, John C. (2008). "Neutron Diffraction Cryoporometry – a measurement technique for studying mesoporous materials and the phases of contained liquids and their crystalline forms". Nucl. Instrum. Methods A. 586 (2): 356–66. Bibcode:2008NIMPA.586..356W. doi:10.1016/j.nima.2007.12.004

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