Modelling Liquid Flow Through Carbon Nanotubes

In this paper, the structure of a liquid and character of its flow in carbon nanotube is investigated. A review of the literature and the results of experiments show that the simulation of fluid flow for nanoscale systems should be based on the continuum hypothesis taking into account the quantized character of the liquid in the length scale of intermolecular distances. Consideration of the flow characteristics allowed construction of the analogy of behavior of the liquid in a nanotube with a flow of a viscoplastic Bingham fluid. A model of mass transfer of liquid in a nanotube, based on the possibility of forming an empty interlayer between the moving fluid particles and the particles of the wall of the nanotube, is presented. DOI: 10.4018/ijcce.2012070102 16 International Journal of Chemoinformatics and Chemical Engineering, 2(2), 15-27, July-December 2012 Copyright © 2012, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. tion of the wall is completely absent (complete slippage). In the framework of classical continuum fluid dynamics, in order to describe the experimental data Navier already in 1823 wrote a boundary condition, which considers slippage of fluid along a solid surface. The results of molecular dynamics simulation for nanosystems with liquid, with characteristic dimensions of the order of the size of the fluid particles, show that a large slippage lengths (of the order of microns) should occur in the carbon nanotubes of nanometer diameter and, consequently, can increase the flow rate by three orders of Kalra et al. (2003) and Holt et al. (2006). Thus, the flow with slippage is becoming more and more important for hydrodynamic systems of small size. The results of molecular dynamics simulation of unsteady flow of mixtures of water water vapor, water and nitrogen in a carbon nanotube are presented in the Kotsalis et al. (2004) work. A flow of water through carbon nanotubes with different diameters at temperature of 300 K were considered in the work. The effects of slippage of various liquids on the surface of the nanotube were studied in detail. It was found out that as the diameter decreases, the speed of slippage of particles on the wall of nanotube also decreases. The authors attribute this to the increase of the surface friction. Experiments with various pressure drops in nanotubes demonstrated slippage of fluid in micro-and nanosystems. The most remarkable were the two recent experiments, which were conducted to improve the flow characteristics of carbon nanotubes with the diameters of 2 and 7 nm, respectively (Holt et al., 2006; Majumder et al., 2006). In the membranes in which the carbon nanotubes were arranged in parallel, there was a slip of the liquid in the micrometre range. This led to a significant increase in flow rate up to three four orders of magnitude. In the experiments for the water moving in microchannels on smooth hydrophobic surfaces, there are slidings at about 20 nm (Lauga et al., 2006; Cottin-Bizonne et al., 2005). If the wall of the channel is not smooth but twisty or rough, and at the same time, hydrophobic, such a structure would lead to an accumulation of air in the cavities and become superhydrophobic (with contact angle greater than 160 ). It is believed that this leads to creation of contiguous areas with high and low slippage, which can be described as “effective slip length” (Stone, 2003; Cottin-Bizonne et al., 2004). This effective length of the slip occuring on the rough surface can be several tens of microns, which was indeed experimentally confirmed (CottinBizonne et al., 2004; Ou et al., 2004; Ou & Perot, 2005). It should be noted that for practical use of advantages of nanotubes with slippage is necessary to solve many more problems. For example, as Churaev et al. (2002) have already shown, hydrophobic surfaces tend to form bubbles. On the other hand, the surfaces used by Churaev et al. (2002) were rough, and the use of smooth surfaces could have generally reduced the formation of bubbles. Another possible problem filling of the hydrophobic systems with liquid. Filling of micron size hydrophobic capillaries is not a big problem, because pressure of less than 1 atm is sufficient. However, capillary pressure is inversely proportional to the diameter of the channel, and filling for nanochannels can be very difficult. Liquid Layer Density near the Wall of Carbon Nanotube In his article Kotsalis et al. (2004) showed radial density profiles of oxygen averaged in time and hydrogen atoms in the “zigzag” carbon nanotube. The distribution of molecules in the area near the wall of the carbon nanotube indicated a high density layer near the wall of the carbon nanotube. Such a pattern indicates the presence of structural heterogeneity of the liquid in the flow of the nanotube. In his work Chen et al. (2008) using molecular dynamic simulations obtained similar results for the flow of the water in the carbon nanotubes. It is mentioned that the area avail11 more pages are available in the full version of this document, which may be purchased using the "Add to Cart" button on the product's webpage: www.igi-global.com/article/modelling-liquid-flow-throughcarbon/68018?camid=4v1 This title is available in InfoSci-Journals, InfoSci-Journal Disciplines Engineering, Natural, and Physical Science. 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