Theory of wetting and capillarity for mercury porosimetry

The experimental method of mercury porosimetry for the determination of the porous properties of solids is dependent on several variables. One of these is the wetting or contact angle between mercury and the surface of the solid. 

When a liquid is placed in contact with a surface of a porous solid the question arises as to whether it will penetrate into the pores. The answer must be pursued in the realm of capillarity which deals with the equilibrium geometries of liquid-solid interfaces and the angle of contact between the liquid and the pore wall. 

The surface tension, y, of a substance is identical to the free surface energy G per unit area and is the work required to alter the surface area by 1 cm. Therefore, y has the dimensions of energy per unit area and is usually expressed in ergs cm or dynes cm 

According to the preceding, a bubble should collapse in order to minimize its free surface energy. In the process of shrinking, however, the gas pressure within the bubble will increase preventing any further radius reduction. When the bubble radius decreases from r to r — dr, the free energy decreases by 

When the bubble shrinks, the volume change is 47cr dr. The gas within the bubble undergoes compression while the external atmosphere undergoes expansion. The net work associated with the compression and expansion processes is given by 

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