(30e) Wetting Transparency of Free-Suspended Single-Layer Graphene on Liquid Substrate.

Wetting transparency of single-layer graphene provides immense potential for the development of multi-functional devices in which the wettability of an atomic-thick film can be manipulated by selecting the appropriate supporting substrate. It becomes even more interesting when the substrate is liquid since flowing different liquids beneath the graphene will provide the real-time control of the surface wettability. However, the wetting transparency on liquid substrates has been rarely studied, possibly due to the challenges in performing experiments on liquid substrates. To this end, we have developed a contact angle measurement procedure to determine the surface and interfacial tension of free-suspended single-layer graphene on liquid substrates using Neumann's Triangle Model. By using different testing liquids on top of free-suspended single-layer-graphene or changing liquid substrate underneath single-layer-graphene, our results reveal that single-layer-graphene is nearly complete wetting transparent. This experimental approach can also be applied to other types of 2D materials for similar studies. Our findings provide insight into the underlying mechanism of wetting transparency on single-layer graphene and demonstrate the possibility of controlling the surface wettability of single-layer graphene by engineering the supporting substrate in real time. The results here have important implications in many applications, including water harvesting, microfluidic devices, and liquid-filled single-walled carbon nanotubes.