The discrete element method (DEM) has become a key framework for analyzing granular materials, yet its extension to unsaturated soils remains limited by the complexities of particle morphology, heterogeneity, and anisotropy. Capillary forces govern much of the mechanical behavior in partially saturated soils, but the lack of consensus on a suitable capillary force formulation continues to restrict the reliability of DEM simulations. This review examines DEM-based capillary force models with a focus on the physical mechanisms underlying capillary interaction, the micro-mechanical origins of suction-dependent behavior, and the comparative advantages and limitations of existing modeling approaches. The roles of particle size, particle shape, degree of saturation, void ratio, and rupture distance in shaping capillary forces are systematically assessed, with emphasis on their influence on shear strength, stiffness, and volumetric response. The review identifies persistent challenges related to model fidelity, parameter calibration, and computational demand, and evaluates emerging strategies aimed at improving accuracy and efficiency. It further highlights the need for capillary formulations that provide greater generality and physical realism while remaining computationally tractable for large-scale DEM analyses of unsaturated soils.
Ali et al. (2026) studied this question.