porosimetry

{{short description|Measurement and characterization of the porosity of a material}}

{{More citations needed|date=September 2020}}

Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous structure, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.

The technique involves the intrusion of a non-wetting liquid (often mercury) at high pressure into a material through the use of a porosimeter. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension.

A force balance equation known as Washburn's equation for the above material having cylindrical pores is given as:{{cite journal|last1=Abell|first1=A.B.|last2=Willis|first2=K.L.|last3=Lange|first3=D.A.|title=Mercury intrusion porosimetry and image analysis of cement-based materials|journal=Journal of Colloid and Interface Science|volume=211|issue=1|year=1999|pages=39–44|issn=0021-9797|doi=10.1006/jcis.1998.5986}}

:$P\_L\; -\; P\_G\; =\; -\backslash frac\{4\; \backslash sigma\; \backslash cos\; \backslash theta\}\{D\_P\}$

:$P\_\{L\}$ = pressure of liquid

:$P\_\{G\}$ = pressure of gas

:$\backslash sigma$ = surface tension of liquid

:$\backslash theta$ = contact angle of intrusion liquid

:$D\_\{P\}$ = pore diameter

Since the technique is usually performed within a vacuum, the initial gas pressure is zero. The contact angle of mercury with most solids is between 135° and 142°, so an average of 140° can be taken without much error. The surface tension of mercury at 20 °C under vacuum is 480 mN/m. With the various substitutions, the equation becomes:

:$D\_P\; =\; \backslash frac\{1470\; \backslash \; \backslash text\{kPa\}\; \backslash cdot\; \backslash mu\; \backslash text\{m\}\}\{P\_L\}$

As pressure increases, so does the cumulative pore volume. From the cumulative pore volume, one can find the pressure and pore diameter where 50% of the total volume has been added to give the median pore diameter.