Enhancement of capillary flow via precursor film thickening in graphene nanochannels

The wicking dynamics of total wetting liquids in a nanocapillary differ from those of partial wetting liquids due to the presence of the precursor film. In this work, the wetting behavior of total wetting liquids (isopropyl alcohol and dimethylformamide) on graphene sheets, as well as the imbibition dynamics in graphene-based nanoslits, are investigated using molecular dynamics. The spontaneous spreading dynamics can be described by two power laws, with the long-term behavior conforming to the viscosity-dominated Tanner's law. The imbibition behavior within nanoslits shows a deviation from Washburn's equation, displaying a unique two-stage pattern. This pattern is marked by a turning point that is related to the type of liquid and is independent of the channel width. Notably, the imbibition rate in the second stage exceeds that in the first. The advancing rate of the precursor film is insensitive to changes in channel width. After the precursor film reaches the end of the channel, the second stage commences, followed by the re-thickening of the film. This re-thickening reduces the curvature radius of the meniscus and increases the driving Laplace pressure, thereby enhancing the capillary flow in the second stage.