Detection of Vapor Released from Sublimating Materials Encased in Porous Medium

Detection of materials encased in porous media is important in many applications and can be attempted via sensing vapor sublimating from them. Development of such sensors requires evaluation of vapor concentration expected at the free surface from materials encased at different depths. In the present work experiments are conducted with a model material, naphthalene, which is relatively easy to detect. One of the main aims of these experiments is, however, in the development of a comprehensive theory (also in the present work) which is applicable to any sublimating material. Accordingly, the data acquired using the model experimental material (naphthalene buried in sand or clay) is used here for verification of the theory. An additional aim is related to the question whether it is possible to amplify the vapor flux reaching the free surface by means of suppression of its partial adsorption in the bulk. Using sand as a porous medium of interest, it is shown that it is possible to achieve this goal by dip coating of sand in polymer solution with a subsequent drying. In a modified sand with a dramatically reduced surface area of the grains smoothened by the polymer coating, vapor adsorption is dramatically suppressed, while vapor flux toward the free surface is dramatically increased, thus increasing the chances of detection.