Permeation of Water Nanodroplets on Carbon Nanotubes Forests
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Permeation of Water Nanodroplets on Carbon Nanotubes Forests Ygor M. Jaques and Douglas S. Galvao Applied Physics Department, University of Campinas, Campinas, SP 13081-970, Brazil ABSTRACT Fully atomistic molecular dynamics simulations were carried out to investigate how a liquid-like water droplet behaves when into contact with a nanopore formed by carbon nanotube arrays. We have considered different tube arrays, varying the spacing between them, as well as, different chemical functionalizations on the uncapped nanotubes. Our results show that simple functionalizations (for instance, hydrogen ones) allow tuning up the wetting surface properties increasing the permeation of liquid inside the nanopore. For functionalizations that increase the surface hydrophilicity, even when the pore size is significantly increased the droplet remains at the surface without tube permeation. INTRODUCTION The wetting dynamics of surfaces is an important area of fluid dynamics, the wetting behavior many times determines the potential use of a specific material in technological applications [1]. Because of that, the engineering of texturized structures at the micro/nanoscopic level has been used to obtain surfaces with extraordinary properties, like rapid detachment [2], superhydrophilicity and superhydrophobicity [3]. Some of these properties can be very useful to applications concerning anti-icing, antifogging and self-cleaning materials. As an example, engineering patterned surfaces created by placing uniformly spaced pillars, represent an effective and feasible strategy to design materials with such properties [4,5]. In order to understand, at atomic level, how a liquid like water behaves when into contact with such materials, we have investigated through fully atomistic classical molecular dynamics (MD) simulations the permeation behavior of water nanodroplets into contact with patterned carbon nanotubes (CNT) forests (our patterned surface – see Figure 1). More specifically, we have considered different tube arrays, where we varied the spacing between tubes. We have also considered tubes with different edge functional groups (hydrogen and hydroxyl ones) in order to create different degrees of array wettability and/or hydrophilic or hydrophobic behaviors. THEORY The MD simulations were carried out using the LAMMPS code [6]. Our system consisted of 28 (8,0) carbon nanotubes forming a regular pattern to emulate a nanoforest. The length of these nanotubes is 10 nm. All tube-dangling bonds were functionalized (passivated) with hydrogen or hydroxyl groups. It is important to stress that as we are using nanotubes of very small diameters (water molecules cannot enter them), we can investigate the permeation that occur only at the nanopores. To model the behavior of a water nanodroplet near the surface of a nanopore (created by manipulating the nanotube spatial arrays), three different separations were considered at the
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