Personnel: C.J. Shih, S. Lin, M.S. Strano, and D. Blankschtein
Sponsorship: NSF
Graphene, a sheet consisting of a single-layer sp2 lattice of carbon, combines optical transparency, electrical conductivity, and mechanical strength. The production of large-area graphene using chemical vapor deposition (CVD) has further shed light on engineering the material for real applications. Despite active research to demonstrate the unique electronic and mechanical properties of graphene, only a few reports have studied the interfacial behavior between graphene and liquid, including the wetting properties of graphene. Very recently, Rafiee et al. reported the “wetting transparency” of graphene, which suggests that the van der Waals (vdW) interactions between graphene and any liquid placed on top are negligible, allowing the “transmission” of the substrate contact angle to fluid above graphene. The “wetting transparency” implies that: (i) monolayer graphene itself is a “super-omniphobic” material, where the contact angle, q, of any liquid droplet on a suspended monolayer graphene should approach 180 degrees, and (ii) any surface can become electrically conductive without affecting its inherent wetting behavior by coating it with a monolayer graphene film. Clearly, implications (i) and (ii) above could represent breakthroughs in the field of wetting. However, the experimental evidence presented in Rafiee’s paper is not sufficient to support the complete “transparency” of graphene to wetting, because it only considered systems exhibiting a restricted range of contact angles (30o< q <90o). A more complete study and analysis are required to elucidate the wetting behavior of graphene, including the role of the underlying solid substrates.
In this project, we developed the first theory to properly model the vdW interactions between any liquid and a sheet of N-layer graphene. Since the contact angle of water on graphite ( N approaches infinity ) is known experimentally, we are able to predict the contact angle of water on a suspended monolayer graphene ( N = 1 ), as shown in Figure 1. Note that this predicted contact angle represents the upper limit of contact angles attainable for a water droplet on a graphene-coated surface. To test the theory, we considered water on a sheet of suspended monolayer graphene, and carried out molecular dynamics (MD) simulations to determine the contact angle corresponding to an infinitely-large water droplet, q¥ , as shown in Figure 2. Using the theory, we also obtained an expression for the vdW interactions between liquid and a sheet of N-layer graphene supported by a solid substrate. Through a comparison with the calculated contact angles on bare solid surfaces, as shown in Figure 3, we showed that graphene is not entirely transparent to wetting. Although the wetting behavior of the substrate does get partially transmitted through a sheet of monolayer graphene, the “wetting transparency” breaks down significantly on superhydrophobic and superhydrophilic substrates. Finally, we carried out contact angle measurements of water on CVD-grown graphene supported by hydrophobic and hydrophilic substrates to further validate our theory.
This work has been highlighted in the Cover Story of Physical Review Letters (http://prl.aps.org/toc/PRL/v109/i17) and by MIT News (http://web.mit.edu/newsoffice/2012/how-transparent-is- graphene-1204.html)
Figure 1. Calculated total vdW potential between water and graphene, -FNL, and the corresponding contact angle, q, as a function of N on a suspended N-layer graphene.
Figure 2. Post-equilibrium MD simulation snapshot of a water droplet containing 4,000 molecules on a sheet of flat, fixed, suspended monolayer graphene. Color code: red – oxygen, white – hydrogen, and gray – carbon.
Figure 3. Photographs of water drops and measured values of qS and qGS on an OTS-SiO2, Silica NP, or OP-SiO2 substrate with and without a sheet of monolayer graphene between them.