Investigation of coated hydrophobic granular materials by means of computed tomography and environmental scanning electron microscopy

Abstract

Hydrophobic materials in geotechnical engineering and soil science can have natural or artificial origin. They can be applied, e. g., to waterproof structures in the industry. In this contribution, hydrophobic granular material was manufactured through a cold plasma polymer coating procedure. The monomer used was C4F8 (octafluorocyclobutane) and the material to be coated was Hamburg sand, a coarse grained sand. In this context, computed microtomography and environmental scanning electron microscopy were used to investigate the materials in their unsaturated state. The tools are applied to visualize unsaturated phenomena on the microscale. The hydrophobic and untreated materials were imaged by both techniques at different saturation degrees in order to understand the influence of the coating on the sample’s hydraulic behaviour. The chosen environmental scanning electron microscope is able to provide relative humidity in the sample chamber, and so water drops were condensed on the grain surface, allowing to also observe the initial contact of water and the hydrophobic coating. It was observed how the capillary menisci, their geometry and contact properties evolve at different degrees of saturation. The measurements obtained and respective analyses state qualitatively the influence of the hydrophobic coatings on the pore water dynamics at different saturation degrees, which dictates the material’s hydraulic behaviour. Contact angles were also analysed were it was physically possible.