Here is a particular case of a capillary that has (a) porous walls with open pores and (b) whose wall atoms are hydrophilic. The surface area of the wall will be much greater because of the pores. The quantity of water adsorbed in the wall will be greater. The area of the walls of this capillary are not uniform because the pores vary on their transversal section (they get thicker or thinner) or even because some pores end and others begin. This is one of the causes of capillary rise: the continuous variation in surface area, on a micrometer scale, produces pressure and velocity variations of water inside the porous system and can cause ascension against gravitational pull, as is the case in the ceramic brick walls. Another factor that helps the climbing process, as already mentioned, is evaporation followed by condensation of water on the capillary walls.
Here the temperature of the water influences the process. If the surface of the brick is colder than the center of the brick, the water will migrate to the surface. If the base of the wall of a house is warmer than the top of the wall (as it is in the winter in southeast Brazil, which is cold and dry), water will climb against gravitational pull (See Figure 2.3.4 below).
