The nature of wetting on urban soil samples: wetting kinetics and evaporation assessed from sessile drop shape

The understanding of soil water repellency in its complexity requires knowledge of the mechanisms leading to changes in surface characteristics. Wetting kinetics may serve as means to investigate the origin of soil water repellency, but have been scarcely investigated yet. We observe the wetting kinetics of soil samples from two locations via the time-dependent shape of sessile drop (TISED) at three temperatures. We show that drop penetration may be exceeded by evaporation for high water drop penetration time (WDPT). The time-dependent change of drop shape and apparent contact angle is explained by surface hydrophilisation including the change from Cassie-Baxter into Wenzel's state. We identify principal differences in the nature of water repellency between the two investigated locations: Only the samples from the former sewage field, Buch, lose most initial differences in wettability upon air drying and storage. Wetting of these samples requires an activation energy of 65-94 kJ mol(-1), indicating chemical reactions as rate-limiting step. In contrast, wetting of the samples from the inner city park, Tiergarten, requires an activation energy of 42 kJ mol(-1) for the repellent and 8-20 kJ mol(-1) for the wettable samples, which suggests physico-chemical and physical processes as rate-limiting steps. Our study shows for the first time that the process of soil wetting can be monitored by TISED assessment, and that assessment of the temperature dependence of the wetting kinetics allows distinguishing between different natures of wetting and soil water repellency. It therefore represents a novel approach to investigate wetting processes. Combination of such approaches with spectroscopic investigations will help to deepen our understanding on possible causes of water repellency. They further help to understand the great variety of suggested causes of repellency and indicate locational material -specific effects rather than one general cause for water repellency. Copyright (c) 2007 John Wiley & Sons, Ltd.