Research Overview: Graphene Cleans, Drives, and Repels Water
Water is essential for life. In humans, water forms about 60% of body volume, acting both as a solvent for nutrients and as the delivering mechanism of those nutrients to cells. The human body generally needs replenishment from a clean water source on a daily basis, which causes problems in remote and poor locations. More generally, water serves a variety of functions in different living organisms, like for example in self-hygiene of a lotus leaf.
Engineers and scientists are now studying how water interacts with surfaces and chemicals, and how we can engineer those interactions for the benefit of human health and world energy. Naturally, with its strong hydrophobic nature, graphene has found its own uses in water engineering.
In recent research performed at the Rensselaer Polytechnic Institute, scientists have shown that covering a rough surface with a layer of graphene reduces the sticking of water droplets on that surface. As opposed to bare rough surfaces on which water tends to pin to the crevices, a graphene-coated surface is smooth and hydrophobic, allowing water droplets to glide away from the material, similar to the lotus leaf effect found in nature. For their research, published in the journal ACS Nano, the researchers used a monolayer graphene sheet grown by chemical vapor deposition (CVD) and transferred to the target rough surface using a polymer film.
Interestingly, also recently, scientists at the US Naval Research Lab (NRL) have shown that they can guide water droplets in a desired direction on a graphene surface. This new achievement offers potential applications ranging from electronics to mechanical resonators to bio/chemical sensors. For example, the ability to guide small amounts of liquids to a desired location on a chip is the motivation of the burgeoning technology of microfluidics.
Microfluidic chips are being used as biosensors for various diseases, and can be used as sensors for dangerous chemicals. In the work performed at NRL, also published in ACS Nano, the graphene surface was modified with a chemical gradient, induced by a clever modification of plasma-doping. The chemical gradient also creates a potential gradient which pulls the droplets in a pre-specified direction. It will be interesting to see whether this work can be extended to dynamical gradients through, for example, electrostatic gating. Such research would open up the way to real-time control of the motion of liquids on a surface.
Both mentioned works used high-speed cameras to record the motion of water on graphene, and ACS Nano offers open access to the “supplementary information” section where you can see the cool movies.
Graphene, as part of a composite nanomaterial that also includes carbon nanotubes and iron oxide, is also good for cleansing water of the dangerous chemical arsenic. The work of the Graphene Research Center in KAIST in Korea was just published in Environmental Science and Technology (also an ACS publication). The advanced material has a carefully engineered structure, consisting of a graphene layer, covered with upright carbon nanotubes, which are topped by iron oxide molecules. The iron oxide has a magnetic function, but apart from that also shows a high affinity for arsenic. A solution containing the nanomaterial was shown to efficiently extract arsenic from contaminated water. Arsenic contamination of groundwater is a critical problem that affects millions of people across the world and results in severe diseases such as skin or lung cancer and bladder cancer.
Graphene is indeed entering the world of water manipulation and water filtration, which we see as a welcome addition to the many outstanding applications that we expect from graphene. -- by Marko Spasenovic, © Graphenea (manufacturer of graphene, welcomes collaboration with researchers)