Porosimetry

Short description: Measurement and characterization of the porosity of a material

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:

[math]\displaystyle{ P_L - P_G = -\frac{4 \sigma \cos \theta}{D_P} }[/math]

[math]\displaystyle{ P_{L} }[/math] = pressure of liquid

[math]\displaystyle{ P_{G} }[/math] = pressure of gas

[math]\displaystyle{ \sigma }[/math] = surface tension of liquid

[math]\displaystyle{ \theta }[/math] = contact angle of intrusion liquid

[math]\displaystyle{ D_{P} }[/math] = 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:

[math]\displaystyle{ D_P = \frac{1470 \ \text{kPa} \cdot \mu \text{m}}{P_L} }[/math]

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.

See also

• BET theory, measurement of specific surface
• Evapoporometry
• Porosity
• Wood's metal, also injected for pore structure impregnation and replica