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Indices for Characterizing Soil-Water Repellency Based upon Contact Angle-Surface Tension Relationships¹

ABSTRACT

The relation between apparent liquid-solid contact angle and surface tension was established for three water repellent materials using both a series of aqueous ethanol solutions and organic liquids. The media included two artificially treated sands and one naturally occurring soil. With one exception there was an approximate linear relationship between the cosine of the contact angle and the surface tension. The relationship for the forest soil tended to be curvilinear when using ethanol solutions of high surface tension. The repellency of the materials was best differentiated by using their intercepts with cos = 0. A simple drop test using the same materials and ethanol solutions placed the materials in the same order of repellency. The drop test was further developed to allow classification of porous media on the basis of (i) 90° surface tension (_n), and (ii) stability to water interaction. The tests can be readily performed both in the laboratory and the field. Two indices were then proposed to describe water-repellent characteristies of soils. These indices will be valuable in assessing erosion and runoff problems, and also the feasibility of surfactants to promote infiltration.

¹ Contribution of the Dep. of Soils and Plant Nutrition, Univ. of California, Riverside. Research supported by matching fund project B-072-CAL of the Office of Water Resources Research, U.S. Department of the Interior as authorized under the Water Resources Act of 1964 and implemented by the Univ. of California, Water Resources Center, as project W-206.

² Research Assistant and Professor of Soil Physics, respectively.

Received for publication April 6, 1970. Accepted for publication July 16, 1970.