Sudden drops of the total water content require another explanation, though. At constant environmental conditions, the evaporation rate is proportional to the water-air surface area. Therefore, if the water in a pore evaporates to a point where the pore diameter suddenly increases, the evaporation rate may increase. A closer look at the locations of the water loss after 12 hours reveals that the drop in the water volume is mainly due to accelerated loss at two sites, as indicated in Fig. 5. The examination of the sites indicates at another mechanism leading to a sudden acceleration of the evaporation: It seems that a water lid blocks the pore heading to the outside of the sample and that the evaporation of the inner water volume accelerates after the disappearance of the lid. The water lid would function in analogy to the volatile lids proposed for preventing the evaporation of solvent in chipbased nanovials (Litborn and Roeraade, 2000