Reversible Dewetting of a Molecularly Thin Fluid Water Film in a Soft Graphene–Mica Slit Pore

Abstract

The behavior of water and other molecular liquids confined to the nanoscale is of fundamental importance, e.g., in biology, material science, nanofluidics, and tribology. Direct microscopic imaging of wetting dynamics in subnanometer pores is however challenging. We will show in the following that a molecularly thin water film confined between mica and graphene is fluid. Ambient humidity allows to control the wetting and dewetting of the film. We follow these processes in space and time using scanning force microscopy imaging of the graphene conforming to the film. At sufficiently high humidity a continuous molecularly thin water film wets the interface between the graphene and mica. At lower humidities the film dewets with fractal depressions exhibiting dimensions around 1.7 and depths comparable to the size of a water molecule. The soft graphene cover offers a previously unexplored semihydrophilic slit pore of self-adjustable size, which enables high-resolution imaging of confined molecularly thin fluid films, and bears the potential for the fabrication of novel nanofluidic devices.

Evidence of Stranski–Krastanov growth at the initial stage of atmospheric water condensation

“…There are many water plateaus on the MoS 2 sample, and with sample-to-sample variations such island-like plateaus can also be observed in graphene samples even via tape-free mechanical exfoliation ( Supplementary Fig. 3), in agreement with previous reports 11,14,16 . The plateaus show no obvious difference in phase contrast with the surrounding elevated areas ( Supplementary Fig.…”

“…The quantitative water coverage, y, defined as y¼1 À ðA f =A 0 Þ, where A f is the fractal and A 0 the whole scanning area, was calculated and is plotted in Fig. 2b. An extremely similar result was reported previously, where the ambient RH was controlled to reversibly dewet the water film on the mica surface covered by graphene 14 . It means that increasing the temperature has a similar effect as reducing RH, and thus also leads to dewet the water film confined between graphene and mica referring to the diffusion-limited aggregation growth mechanism (three-dimensional (3D) image and illustrated model in Supplementary Fig.…”

“…4), which indicates that these plateau structures occurred initially under both graphene and MoS 2 sheets, which further confirmed the sandwich arrangement as shown in Fig. 1b 11,14 .…”

“…1a). Under ambient conditions (25°C; relative humidity (RH) B35%), freshly cleaved mica substrate is hydrophilic and thus will inevitably absorb a heterogeneous water film (whole layer or islands of water molecules) with a thickness depending on the ambient RH 14,20 . Given this default water layer on the mica surface, graphene and MoS 2 sheets were then carefully transferred onto mica through mechanical exfoliation (Fig.…”

“…Different non-contact AFM and scanning polarization force microscopy methods have been developed to overcome many of these problems and have provided important results, but a disadvantage is that the large distance between the tip and the sample results in lower spatial resolution. Recently, graphene, albeit only one-atom thick, was found to not only prevent the evaporation of molecularly thin adlayers but also to act as an ideal template for studying the structures of ultrathin liquid adlayers on solid support under ambient conditions [10][11][12][13][14][15] . With the assistance of graphene, Xu et al 11 pioneered work on the observation of the first two ice-like water adlayers, increasing the humidity to 90%.…”