Fractal Phenomena in Soil and Wettability Research

This paper reviewed and introduced some fractal phenomena in soil and wettability research. Soil is a typical porous material. Soil surface is irregular and is self-similar fractal one. Its fractal dimension represents extent of its irregularity and has close relationship with its mineral constitution. Different soils have different dimensions. The fractal dimension of kaolinite is higher than those of feldspar and quartz. Fractal dimension of soil surface can be measured by adsorption methods (BET or adsorption from solution),image analysis and small-angle X-ray-scattering. The adsorption method is valid for measuring molecular fractal dimension of soil surface.The outline, the crack curve and the crack distribution of clods formed with rotary tillage are self-similar fractals. The fractal dimensions of the outline and the crack curve can be obtained by measuring their apparent length at different scales. The dimension of the crack distribution can be obtained by measuring the probability density of the crack area distribution.The spatial distribution of soil properties, such as chemical composition, pH value, clay contents and density, has fractal phenomena and its fractal dimension can be estimated by semivariance. The fractal dimension of the spatial distribution of soil properties is more than 1. 5 for most of the soils. This indicates their spatial variation is complicated.Wetting behaviour of water to solid surfaces affects adhesion of soil to them. Wettability of water to self-similar surfaces of solids has close relationship with their properties and fractal dimensoins. If solid surfaces are of high surface energy,the apparent contact angles for water on them are less than their inherent angles and depressed as their fractal dimensions increase. Whereas, if solid surfaces are of low surface energy, for example, polytetrafluoroethylene (PTEE) surface and soil animals' skin, the apparent contact angles are more than their inherent contact angles and elevated as their fractal dimensions increase. Therefore, increase of fractal dimensions of low energy surfaces always enhances their hydrophobic property. This is favourable for reducing soil adhesion and sliding resistance against their surfaces.