Nanodroplets Impacting on Graphene
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Nanodroplets Impacting on Graphene Ygor M. Jaques, Gustavo Brunetto and Douglas S. Galvão Applied Physics Department, University of Campinas, Campinas, SP 13081-970, Brazil ABSTRACT The unique and remarkable properties of graphene can be exploited as the basis to a wide range of applications. However, in spite of years of investigations there are some important graphene properties that are not still fully understood, as for example, its wettability. There are controversial reported results whether graphene is really hydrophobic or hydrophilic. In order to address this problem we have carried out classical molecular dynamics simulations of water nanodroplets shot against graphene surface. Our results show that the contact angle values between the nanodroplets and graphene surfaces depend on the initial droplet velocity value and these angles can change from 86º (hydrophobic) to 35º (hydrophilic). Our preliminary results indicate that the graphene wettability can be dependent on spreading liquid dynamics and which can explain some of the apparent inconsistencies reported in the literature. INTRODUCTION Graphene is one of the most studied nanomaterials [1–4] due to its unique structural, electronic, thermodynamic and mechanical properties [5–9], which can be exploited in many different applications. In spite of the large number of theoretical and experimental works on graphene, there are still some aspects that are not fully understood. One example is its wettability [10–12]. Many experimental [13–15] and theoretical [16–18] works have been carried out to determine if indeed graphene is a hydrophobic or a hydrophilic material. Recently [19], it was reported that graphene is hydrophobic based on the observed low adhesion work between graphene and some liquids. More recent works claimed that the observed hydrophobicity behavior is in fact due to contamination by hydrocarbons present in the interface liquid/graphene and that for clean graphene surfaces the expected hydrophilic behavior is recovered [14,17]. Graphene wettability has been a hotly debated issue, been even object of a recent Nature Materials editorial [20]. Thus, more works are necessary to help clarifying this important issue. One point to be considered is whether the wettability behavior could be dependent on some experimental/model set up conditions, e.g., droplet features (shape, size and impact velocity values, etc.). In order to address some of these aspects we have carried out classical molecular dynamics (MD) simulations of water nanodroplets shot against graphene targets at different conditions. We then measure the contact angle value of the spread droplet to determine whether a relationship between the graphene wettability and nanodroplet velocity values exist. THEORY
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The MD simulations were carried out using classical force fields, as implemented in LAMMPS package [21]. Our systems consist of graphene membranes with dimensions of 200 x 200 Å2 and water droplets containing 5000 molecules, initially placed at 27 Å above the center of the membr
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