Comparison of measured wetting behaviour of materials with identical surface energies, presented as particles and plates

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Contact angles have been obtained for four different liquids on surfaces of particles and plates coated with three different polymers, to give three different surface energies. The advancing contact angles on flat plates measured by the sessile drop and Wilhelmy plate methods were in very close agreement (ca 1°) over the entire (wide) range of angles measured (40-80°). Angles measured by liquid penetration using the Studcbaker and Snow method were all overestimates of the true contact angles. A linear relationship was found to exist between the angles measured on plates and those on the particles; this intercepted at the origin, and had a gradient of 0.869. It is argued that the existence of this linear relationship vindicates the use of the Washburn model under certain circumstances, but vividly demonstrates that an effect due to pore geometry causes the results to be artificially high. Contrary to previous publications, it is clear that the overestimation of contact angle occurs over the entire range, and is simply more pronounced at high angles. A linear relationship should exist for systems of identical pore geometry; however, the gradient is anticipated to change as a function of different pore geometries. The gradient of the line will dictate the actual contact angle that corresponds to the point where the apparent liquid penetration angle equals 90°, and will therefore dictate the real contact angle above which it will be impossible to achieve spontaneous penetration of liquid into the bed. © VSP 1993.