Transport of thin water films: From thermally activated random walks to hydrodynamics

Abstract

Under ambient atmospheric conditions, a thin film of water wets many solid surfaces, including insulators, ice, and salt. The film thickness as well as its transport behavior sensitively depend on the surrounding humidity. Understanding this intricate interplay is of highest relevance for water transport through porous media, particularly in the context of soil salinization induced by evaporation. Here, we use molecular simulations to evaluate the transport properties of thin water films on prototypical salt and soil interfaces, namely NaCl and silica solid surfaces. Our results showtwo distinct regimes for water transport: at low water coverage, the film permeance scales linearly with the adsorbed amount, in agreement with the activated random walk model.For thicker water films, the permeance scales as the adsorbed amount to the power of 3, in line with the Stokes equation. By comparing results obtained for silica and NaCl surfaces, we find that, at low water coverage, water permeance at the silica surface is considerably lower than at the NaCl surface, which we attribute to difference in hydrogen bonding. We also investigate the effect of atomic surface defects on the transport properties. Finally, in the context of water transport through porous material, we determine the humidity-dependent crossover between a vapor dominated and a thin film dominated transport regimes depending on the pore size.

NMR Investigation of Water in Salt Crusts: Insights from Experiments and Molecular Simulations

“…Density profiles computed along the z -axis normal to the pore surface using MDAnalysis together with the MAICoS Python toolkit show that the maximum density within the first molecular layer is almost twice as large in the case of NaCl as compared to Na 2 SO 4 (Figure a). The apparent weaker structuring of the water in the case of the Na 2 SO 4 as compared to NaCl could be a consequence of the different atomic roughness of the two solid surfaces: the NaCl block remains perfectly crystalline and its surface along the (100)-plane remains atomically smooth in contact with water (as has been previously observed for example in refs and ), while the atoms of the Na 2 SO 4 block slightly move away from their perfect crystalline reference and form surfaces that are atomically rough (Figure S3).…”

NMR Investigation of Water in Salt Crusts: Insights from Experiments and Molecular Simulations

“…The apparent weaker structuring of the water in the case of the Na 2 SO 4 as compared to NaCl could be the consequence of the different atomic roughness of the two solid surfaces: the NaCl block remains perfectly crystalline and its surface along the (100)-plane remains atomically smooth in contact with water (as has been previously observed for example in Refs. [42,43]), while the Na 2 SO 4 block is slightly disorganized and its surface is atomically rough in contact with water (Fig. S3).…”

Fluids meet solids

“…43 Gravelle et al study by MD the transport properties of thin water films on salt and solid surfaces (NaCl and silica), which provides insight into soil salinization by evaporation. 44 These articles highlight that the properties of both fluids and surfaces change when they are brought together. Thus, it is not surprising that interactions between surfaces change by the presence of a fluid.…”