A new hysteresis model of the water retention curve based on pore expansion and contraction

The hysteresis of the water retention curve is significant for evaluating the hydraulic properties of unsaturated porous media. Traditionally, the hysteresis of unsaturated flows is believed to be caused by the contact angle or bottle-neck effect. However, in this paper, a new model of pore expansion and contraction caused by hydraulic loading is established to explain hysteresis. The physical mechanism of expansion and contraction is revealed by a microscopic model based on the fundamental principles of the axis translation technique. Additionally, the pore radius of the porous media (e.g., soil) is redefined to determine the upper and lower boundaries of pore expansion and contraction. Then, the corresponding hysteresis differential equations are developed in combination with the two-parameter (alpha, n) VG equation. The proposed model can predict the experimental data well with only three additional physical parameters (a(d) , eta, lambda). The present work may be helpful for understanding the complex mechanisms of the hysteresis and transport processes in unsaturated porous media.