Institute of Physics, Chinese Academy of Sciences found transparency of graphene induced evaporation

Evaporation of water droplets on a solid surface is a widespread phenomenon. Evaporation plays a very important role in both life activities and industrial production because it can remove heat to maintain a proper temperature on the solid surface. However, how to effectively regulate evaporation is a very challenging topic.

Recently, Dr. Huang Yongfeng, Lu Jun (State Key Laboratory of Magnetics) and researcher Meng Sheng of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Center for Condensed Matter Physics have studied water molecules on the basis of their previous work. The interaction with the graphene-coated substrate was found (Fig. 1): a single layer of graphene with only one atomic layer can effectively change the evaporation rate of water droplets, and the maximum change can reach nearly 20%. They found that the root cause was that graphene changed the contact angle of the water droplets on the substrate, thereby changing the length of the three-phase line in contact with the substrate. For example, on a hydrophilic substrate, graphene increases the contact angle of water droplets and thus reduces the length of the three-phase line, and the evaporation rate is suppressed. On the hydrophobic substrate, graphene reduces the contact angle and thus increases The length of the three-phase line, the evaporation rate of the water droplets is accelerated.

Surprisingly, however, the average evaporation rate per unit three-phase line, with or without the presence of graphene, hardly changes (the rate of change is less than 5%), so graphene is "transparent" to the evaporation process. The average evaporation rate remains essentially constant due to the highest rate of evaporation at the three-phase line (compared to the surface of the water droplets). Through experimental research and molecular dynamics simulation (Fig. 2, Fig. 3), they found that before evaporation, the water molecules are attracted by the solid surface, diffused from the three-phase line along the substrate to the periphery, and then they are monohydrated. Forms are separated from the solid surface to complete the evaporation process, rather than evaporating directly from the surface of the water droplets. Because of its single atomic layer thickness, graphene has little change in the adsorption energy of single water molecules, thus showing the transparency of the water evaporation process. In the direction perpendicular to the substrate, the number of water molecules decreases exponentially, corresponding to an evaporation rate that exhibits an exponential decay from the three-phase line to the surface of the water droplet.

This work uses the atomic-scale interaction to achieve effective regulation of macroscopic evaporation behavior, and at the molecular level reveals the physical image of evaporation at the three-phase line, which is of great significance for regulating evaporation behavior under different conditions. Related results are published in 2D Materials 5, 041001 (2018).