Capillary rise
The combination of adhesion forces, between water molecules and solid wall, and cohesion forces, between water molecules, causes water to rise in capillary tubes and soil pores above the free water level as shown in Fig. 3.4. The adhesion forces cause the rise in capillary, and the cohesion brings all the water molecules together to follow the upward pull. The analyses by several researchers show that the wall of the capillary tube exerts an upward force on the water through the surface free energy difference (TSV - TSW ), where TSV and TSW are the interfacial surface tension between solid-vapor and solid-water respectively. As mentioned before, the concave curvature indicates the presence of pressure difference across the meniscus. The pressure below the meniscus will be smaller than the atmospheric pressure, above the meniscus as shown in Fig. 3.4. This is because the water in the capillary tube is suspended from the meniscus, which in turn is attached to the walls by hydrogen bonds. Therefore, the water is under tension, which is defined as “negative pressure”. According to several other researchers, the pressure difference across the meniscus in the capillary is responsible for the rise of water in capillary tube. Capillary rise is also explained in terms of the surface forces around the periphery of the meniscus. The straight-wall capillary due to upward force from the meniscus πdT cos θ is balanced by the weight of the water column. The height of the capillary tube,
hc , can be expressed from this force equilibrium as
.....................................................................................(3.2)
which can also be derived after substituting hc ρg in Eq. (1) for the gauge water pressure. The contact length between the top of the water column and the tube is proportional to the diameter of the tube, while the weight of the liquid column is proportional to the square of the tube's diameter. Thus, a narrow tube will draw a liquid column higher than a wider capillary tube as shown in Fig. 3.5
(for picture showing the capillary rise experiments, visit: http://web.mit.edu/nnf/education/wettability/capillaryrise.JPG).
The capillary water can rise up to several meters above the free water level when the capillary diameter is very small, which is typical in clay soils where the capillary rise extends several tens of meters above the water table.