Understanding Capillary Film-Induced Transportation and Subsequent Salt Precipitation in a Highly Constrained Single Vertical Layered Porous Medium.

Capillary liquid films play an important role in evaporation-driven transport processes by maintaining a hydraulic connectivity between the bulk liquid and the exposed evaporating surface. In this study, we experimentally investigated the role of capillary films in coupled evaporation and salt precipitation in a highly constrained quasi-1D porous medium under external heating. Glass spheres of 2.00 mm and 0.40-0.60 mm diameter were packed in a capillary tube (internal diameter of 2.10 mm) and saturated with 1 M NaCl solution. The temporal evolution of the liquid level, film morphology, and salt precipitation was monitored using a digital microscope without measuring the top surface temperature. The experiments showed that liquid-film connectivity critically governs salt precipitation behavior. For the 2.00 mm case, weaker capillary forces limited film continuity, resulting in reduced salt deposition and no vertical salt growth. In contrast, the 0.40-0.60 mm case sustained film connectivity from deeper regions, leading to enhanced salt precipitation and pronounced vertical salt growth on the top surface. Distinct receding dynamics were observed: collar-like liquid films formed around the equatorial region in the 2.00 mm case, whereas pendular liquid clusters formed around smaller spheres in contact with the capillary wall. Despite the presence of foreign particles, the liquid level decreases linearly during most of the experimental duration, similar to that of a capillary tube alone. In rod-based systems, near-zero contact radii acted as efficient capillary wicks for salt transport from the bulk reservoir. Overall, this study shows that salt precipitation is governed not only by evaporation but also by the evolution of capillary liquid films, geometric confinement, and transport pathways.