Control of surface wettability via strain engineering
W Xiong and JZ Liu and ZL Zhang and QS Zheng, ACTA MECHANICA SINICA, 29, 543-549 (2013).
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 theta of water droplets on a graphene vary from 72.5A degrees to 106A degrees 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 theta upon the applied strains is more sensitive, i.e., from 0A degrees to 74.8A degrees. 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 theta as a linear function of the adsorption energy E (ads) 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 E (ads) 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.
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