Thermodynamic properties of subsurface colloids are important information for understanding a variety of phenomena, such as adhesion, flotation, mobility, fate, and transport of various colloids in the unsaturated subsurface environments. The goal of this research was to study contact angles of subsurface colloids and to elucidate the mechanism of colloid mobilization in unsaturated porous media. The main objectives were as follows: 1. To investigate the impact of water content during infiltration in porous media on colloid release patterns, rates, and quantity, and to elucidate the dominant colloid release mechanisms. 2. To compare different experimental methods to determine contact angles of subsurface colloids. 3. To investigate the effects of cations and humidity on contact angles and surface tensions of subsurface colloids. 4. To clarify the role of a moving air-water interface on colloid mobilization in porous media. We conducted experiments on in situ colloid mobilization under transient conditions using columns repacked with sediments. The cumulative amount of colloids released was proportional to the column water content established after steady-state flow rates were achieved. According to DLVO theory and the Young-Laplace equation, the electrostatic, van der Waals, hydrodynamic, and capillary forces exerted on colloids were calculated and used to analyze the experimental results. Five different methods, static sessile drop, dynamic sessile drop, Wilhelmy plate, thin-layer wicking, and column wicking, were used to measure the contact angles of subsurface colloids. The colloids were deposited on glass slides to make thin films. The colloidal films can be categorized into three types: (1) films without pores and with polar-liquid interactions (smectite), (2) films with pores and with polar-liquid interactions (kaolinite, illite, goethite), and (3) films without pores and no polar-liquid interactions (hematite). Based on our results, we could recommend specific methods for different colloids. The effects of surface and interlayer cations, and relative humidity on contact angle measurements of colloids were investigated using the sessile drop method. Surface tension components and parameters of the colloids were calculated from contact angles using the Lewis acid-base approach. The effects of cations and relative humidity were reflected in the surface tension components of the colloids. To quantify the forces acting on a solid particle when an air-water interface passes over the particle, we used a tensiometer to measure the forces between the particle and the air-water interface. Theoretical calculations using the Young-Laplace equation were used to support the experimental data and conclusions. The theoretical forces as a function of [zeta]-potential, particle radius, and contact angle were calculated to unveil the mechanistic principle of colloid mobilization in the vadose zone
