Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance

Modes of Nanodroplet Formation and Growth on an Ultrathin Water Film

Using nanobubbles as geometrical confinements, we create a thin water film (∼10 nm) in a graphene liquid cell and investigate the evolution of its instability at the nanoscale under transmission electron microscopy. The breakdown of the water films, resulting in the subsequent formation and growth of nanodroplets, is visualized and generalized into different modes. We identified distinct droplet formation and growth modes by analyzing the dynamic processes involving 61 droplets and 110 liquid bridges within 31 Graphene Liquid Cells (GLCs). Droplet formation is influenced by their positions in GLCs, taking on a semicircular shape at the edge and a circular shape in the middle. Growth modes include liquid mass transfer driven by Plateau-Rayleigh instability and merging processes in and out-of-plane of the graphene interface. Droplet growth can lead to the formation of liquid bridges for which we obtain multiview projections. Data analysis reveals the general dynamics of liquid bridges, including drawing liquids from neighboring residual water films, merging with surrounding droplets, and merging with other liquid bridges. Our experimental observations provide insights into fluid transport at the nanoscale.

Nanoscale Contact Line Pinning Boosted by Ångström-Scale Surface Heterogeneity

The pinning effect plays an important role in many fluidic systems but remains poorly understood, especially at the nanoscale. In this study, we measured the contact angles of glycerol nanodroplets on three different substrates using atomic force microscopy. By comparison of the shapes of the three-dimensional images of droplets, we found that a possible origin of the long-discussed deviation of the contact angles of nanodroplets from the macroscopic value is the pinning force induced by ångström-scale surface heterogeneity. It was also revealed that the pinning forces acting on glycerol nanodroplets on a silicon dioxide surface are up to twice as large as those acting on macroscale droplets. On a substrate where the effect of pinning was strong, an unexpected irreversible change from an irregularly shaped droplet to an atomically flat liquid film occurred. This was explained by the transition of the dominant force from liquid/gas interfacial tension to an adsorption force.

Observation of Interfacial Instability of an Ultrathin Water Film

We observed the instability of a few-nanometer-thick water film encapsulated inside a graphene nanoscroll using transmission electron microscopy. The film, that was left after recession of a meniscus, formed ripples along the length of the nanoscroll with a distance only 20%-44% of that predicted by the classical Plateau-Rayleigh instability theory. The results were explained by a theoretical analysis that incorporates the effect of the van der Waals interactions between the water film and the graphene layers. We derived important insights into the behavior of liquid under nanoscale confinement and in nanofluidic systems.

Nanoscale Bubble Dynamics Induced by Damage of Graphene Liquid Cells

Graphene liquid cells provide the highest possible spatial resolution for liquid-phase transmission electron microscopy. Here, in graphene liquid cells (GLCs), we studied the nanoscale dynamics of bubbles induced by controllable damage in graphene. The extent of damage depended on the electron dose rate and the presence of bubbles in the cell. After graphene was damaged, air leaked from the bubbles into the water. We also observed the unexpected directional nucleation of new bubbles, which is beyond the explanation of conventional diffusion theory. We attributed this to the effect of nanoscale confinement. These findings provide new insights into complex fluid phenomena under nanoscale confinement.

Water-Assisted Permeation of Gases in Carbon Nanomembranes

Planar nanomaterials finished with transverse ducts represent an intriguing avenue for exploring interfacial phenomena. Due to their small thickness, the kinetics of molecular diffusion across the channels is likely to be dominated by entrance events. Therefore, measuring transport rates in freestanding films can yield valuable information on surface processes. In this work, we study permeation of gases in carbon nanomembranes (CNMs) when accompanied by saturated water vapor. The experimental data reveal a manifold increase in transmembrane fluxes compared to dry conditions. Gas molecules are found to be trapped in adsorbed water, which enhances their translocation likelihood. We demonstrate that the permeance correlates with the vapor relative pressure and discuss the observed crossing mechanism in terms of water condensation and Henry's law. Our findings provide guidance for designing gas separation membranes upon two-dimensional materials.