According to a new study, graphene coatings may offer the ability to control the water evaporation process from various surfaces. Carried out by a team from the Chinese Academy of Sciences and the Collaborative Innovation Center of Quantum Matter in Beijing, China, the study looked at the interactions of water molecules with various graphene-covered surfaces. The team reports its findings in a paper in 2D Materials.
"Water droplet evaporation is a ubiquitous and complicated phenomenon, and plays a pivotal role in nature and industry," said lead author Yongfeng Huang from the Chinese Academy of Sciences. "Understanding its mechanism at the atomic scale, and controlling evaporation rate rationally is important for applications including heat transfer and body-temperature control. However, it remains a significant challenge."
"Our results are an important discovery on graphene-mediated evaporation, and also point to new ways to rationally control evaporation process, for realistic applications in heat transfer, printing and related areas."Yongfeng Huang, Chinese Academy of Sciences
The team's experiments showed that a graphene coating controls water evaporation by suppressing the evaporation rate on hydrophilic surfaces and accelerating evaporation on hydrophobic ones. "More importantly, we found graphene is 'transparent' for evaporation," said Huang. "When a hydrophilic surface is coated with graphene, the contact line of the water droplet is dramatically shortened or elongated, because of adjustment in wetting angles. This leads to changes in the evaporation rate."
The researchers wanted to understand the 'transparency' in graphene-mediated evaporation and uncover its underlying structure at the atomic scale. To do this, they conducted molecular dynamics simulations of water droplet evaporation, on surfaces with and without a graphene coating. This allowed them to identify, for the first time, the atomic-scale mechanism for substrate-induced evaporation events. The mechanism turned out to involve a water molecule forming a precursor state at the contact line before it evaporates.
"Further analysis showed water density in evaporation transition states is largest at the contact line, then decreases exponentially as it goes away from the substrate," explained Huang. "Single water desorption at the contact line dominates the droplet evaporation process. Since the graphene does not alter the binding energy of a single water molecule, it has negligible effects on evaporation of per contact line.
"Our results are an important discovery on graphene-mediated evaporation, and also point to new ways to rationally control evaporation process, for realistic applications in heat transfer, printing and related areas."
"Using experiments supplemented with molecular dynamics simulations, Dr. Huang and co-workers have provided fascinating insights into the molecular mechanisms governing the evaporation of water droplets on technologically relevant graphene-coated substrates," said James Sprittles from the University of Warwick, UK, who was not involved in the study. "Their research shows that wettability is solely responsible for evaporation rate changes, and simultaneously opens up several interesting topics for future research, such as how molecular effects (e.g. precursor nanofilms and thermal fluctuations) can be incorporated into macroscopic modelling."
This story is adapted from material from IOP Publishing, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.