If you imagine a surface of a solid, or even a liquid in the atomic scale, you can imagine that the top atoms are with less
bonding conections with other neighbour atoms, this unbalanced bounding leads to a positive potencial in the surface, due to this configuration, this surface can atract outside atoms or molecules of other liquids and gases that hit the surface due to polarizing forces or chemical bonds. we will call this phenomenon of adsorption.
In this adsorption process there are many pairs that have large atraction with each other like the case of
silica gel and water. Silica gel is micropourous solid well known for his hidrophilic capabilities, Dubinin studies the properties of this kind of solids, and there are several studies how they can atract or release water molecules due to pressure changes, induced by temperature. Solar
energy can move this kind of cycle in order to create evaporation (cold) to give proper cooling where its needed. Nowadays this kind of system is beeing studied in order to replace convencional€ sistems and also a way of reducing electric consumptions, this sistem can be run only from solar energy (Heat)!!!