Use of sodium fluorescein dye to visualize the vaporization plane within porous media

摘要

The vaporization plane in porous media separates the region of capillary flow from the dry surface layer, where the water transports only in its gas phase. Knowledge of the depth and geometry of the vaporization plane is critical for estimating water flux in the soil-atmosphere interphase, for understanding evaporating processes in general, and for prediction of locations of damaging salt crystallization, etc. However, detection of the vaporization plane is a challenging task. This paper explores the use of sodium fluorescein dye (uranine), a popular hydrological tracer, to visualize the vaporization plane in porous media. Uranine was used in the forms of solution or powder on sand, sandstone, and autoclaved aerated concrete, in both laboratory and field experiments. The property of uranine solution to change its color according to its concentration can be used to: i) visualize the vaporization plane by forming a distinctive dark-orange zone where the pore water evaporates, and ii) to distinguish the zone of vapor flow from the zone where capillary flow is present. Similarly, uranine powder, when applied onto a porous material, clearly visualizes the dry surface layer and the capillary zone, divided by the vaporization plane. This technique can also visualize a complex-shaped vaporization plane in hydrophobic materials. In comparison with other techniques, such as sensible heat balance or heat pulse methods, the use of uranine is accurate, cost-effective, and straightforward.