A microscopic interpretation of hysteresis in the water retention curve of sand

Abstract

The hysteresis observed in a water retention curve was experimentally studied through the use of X-ray micro-computed tomography (μCT). The CT images were acquired during the main drying and wetting processes of a water retention test for uniform sand. The porosity and degree of saturation (Sr) of pore-scale local subsets were calculated using images segmented into soil, water and air phases. The simultaneous distribution of the local porosity and Sr was also analysed to investigate the relationship between the pore size and retained water volume. At similar suction levels, only small pore spaces retained water during wetting, while relatively large pore spaces retained water during drying, demonstrating the ink-bottle effect. At similar saturation levels, the simultaneous distribution was independent of the wetting and drying processes, indicating no significant difference in the water retention state between the two processes. Another image analysis was performed to calculate the principal curvatures of the air–water interface. At similar saturation levels, the calculated curvature of the drying process was greater than that of the wetting process, indicating contact angle hysteresis. The contact angle between the soil particles and water is different at the microscopic level, whereas the water retention states of the two processes are similar.