Direct Contact Angle Measurements on Polished Sections of Coal

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Abstract
The wettability of three bituminous Canadian coals was determined by direct contact angle measurements. Advancing contact angles of water, glycerol and ethylene glycol were measured at room temperature on segments of coal with five different surface finishes. It was found that the contact angles measured with water varied only moderately, but randomly, with the roughness of the coal surface; contact angles of organic liquids increased with decreasing surface roughness. These findings are at variance with results normally obtained with more homogeneous solids, such as Teflon, under similar experimental conditions. The irregular behaviour of coal is interpreted on the basis of both the chemical and mechanical heterogeneity of coal. The solid/vapour surface tensions, γsv, of the coal samples, calculated from the contact angles, were compared with surface tensions obtained from other techniques using powders of the same coals. This comparison suggests that the information obtained from the contact angles is less reliable than that from the indirect techniques.

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