Mercury Intrusion Porosimetry (MIP)

Mercury intrusion porosimetry (MIP) is used to evaluate porosity, pore size distribution, and pore volume of various solid and powder materials. The technique involves the intrusion of a non-wetting liquid (often mercury) at high pressure into a material through the use of an instrument, known as a porosimeter. The instrument employs a pressurized chamber to force mercury to intrude into the voids in a porous substrate.  As pressure is applied, mercury fills the larger pores first.  As pressure increases, the filling proceeds to smaller and smaller pores.  Both the inter-particle pores (between the individual particles) and the intra-particle pores (within the particle itself) can be characterized using this technique.

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. 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. 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.