Control of surface wettability via strain engineering

Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sample material and molecular dynamic simulations, we demonstrate that strain engineering can serve as an effective way to control the surface wettability. The contact angles θ of water droplets on a graphene vary from 72.5◦ to 106◦ under biaxial strains ranging from −10% to 10% that are applied on the graphene layer. For an intrinsic hydrophilic surface (at zero strain), the variation of θ upon the applied strains is more sensitive, i.e., from 0◦ to 74.8◦. Overall the cosines of the contact angles exhibit a linear relation with respect to the strains. In light of the inherent dependence of the contact angle on liquid-solid interfacial energy, we develop an analytic model to show the cos θ as a linear function of the adsorption energy Eads of a single water molecule over the substrate surface. This model agrees with our molecular dynamic results very well. Together with the linear dependence of Eads on biaxial strains, we can thus understand the effect of strains on the surface wettability. Thanks to the ease of reversibly applying mechanical strains in micro/nano-electromechanical systems, we believe that strain engineering can be a promising means to achieve the reversibly control of surface wettability. The project was supported by the National Natural Science Foundation of China (11172149). W. Xiong · Q.-S. Zheng ( ) Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, 100084 Beijing, China e-mail: [email protected]