Self-Driven Fluid Imbibition of Salt Solutions into Mesoporous Films

Fluid water imbibition into nanoscale porous materials is important in nature and technology and has been shown to follow the Lucas-Washburn behavior. Looking at drop imbibition into nanoscale porous films, the imbibition reaches a steady state governed by capillary imbibition and evaporation, which is reflected in a constant imbibition length. This imbibition length in a steady state depends on the porous structure as well as on the material's wettability and the fluid evaporation rate, for example. As the presence of salts affects these parameters, the imbibition length is expected to be influenced by the solutes present in the drop. Here, we show the influence of salt presence, salt concentration, and ion type on the fluid imbibition of an aqueous salt solution into mesoporous silica films. We observe that salts affect the Lucas-Washburn imbibition occurring directly after drop positioning on mesoporous films. Interestingly, the presence of salts also causes a continuous increase of the characteristic imbibition length at longer time scales, preventing the formation of a steady state after the initial Lucas-Washburn-like imbibition. We identified a self-amplified fluid pumping mechanism using mesoporous silica films. We attribute this continuous fluid pumping and, thus, the continuous increase of the imbibition length to the increasing osmotic pressure within mesoporous materials upon constant fluid flow and fluid evaporation. Understanding aqueous salt solution imbibition into mesoporous films is an important aspect, e.g., in the context of material exchange between drops through mesoporous films.