Mercury intrusion porosimetry is a destructive technique that renders the sample non-reusable. Its popularity stems from its ability to probe voids over a very wide size range, down to 4 nm for high pressure instruments.
The technique is dependent on the behaviour of non-wetting mercury as it inundates (intrudes) a porous object under the action of external pressure. The Laplace equation 1, itself derived from the Young-Laplace equation for the pressure across a curved liquid boundary held in tension, governs the behaviour of non wetting liquids in a capillary:
[1]
where d is the pore equivalent capillary diameter, γ is the interfacial tension between mercury and air, P is the applied pressure and θ is the contact angle. (This equation is also referred to as the Washburn equation, but that equation is time-dependent.)
Porosimetry measurements are nearly always carried out using mercury, since it is the only liquid that is sufficiently inert and non-wetting for general applications. Equation 1 is, therefore, widely applicable, and the non-wetting nature of mercury means that it is necessary to apply pressure in inverse proportion to pore diameter, such that a record of the pressure provides the analysis of pore diameter.
After intrusion, the reverse procedure, with pressure reduction, allows the drainage or extrusion curve to be obtained, as the mercury emerges from the regions of the sample where it is possible to do so. The commonly used values for γ and θ, for mercury entering an evacuated sample, are 0.485 N m-1 and 140°, respectively. There are, however, uncertainties and variations in these values, the consequences of which have been discussed by Van Brakel.
The void size distribution, obtained by the first derivative of the cumulative volume-pressure intrusion curve, subsequently applying the Laplace equation, is representative of a one-dimensional model of a porous solid, consisting of parallel equivalent capillaries, as described in the capillary bundle model section. However, this model cannot fully explain some of the typically encountered geometrical pore network features, which result in important subtleties arising in the data obtained from mercury intrusion porosimetry. For example, the intrusion and extrusion curves typically differ. The 'shielding' of larger void spaces, or pores, by narrower void spaces, or throats leading to them, causes this hysteresis effect. The shielding requires a higher than expected pressure for many of the features to fill. Similarly, rapid extrusion of mercury can lead to detachment, known as 'snap off', within the mercury column as the strictures imposed by throats prevents the flow of mercury needed to replenish that being extruded.
In order to understand how PoreXpert overcomes the shielding problem, you should download and read our article published in Transport in Porous Media in 2018.