Modeling slip and flow enhancement of water in carbon nanotubes
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troduction Nanofluidics is the scientific investigation and technological application of fluid flow in and around nano-sized materials with at least one dimension below a few hundred nanometers.1,2 It is interdisciplinary to many branches of science, including physics, chemistry, biology, materials, and geology, and it is an integral part of “nanotechnology.” Many devices, such as micro- or nanoelectromechanical systems (MEMS/ NEMS), nanobiosensors, and lab-on-a-chip devices involve fluids at the nanoscale with potential applications such as water desalination, drug discovery and delivery, molecular sensing, energy production and storage, and many more. Numerous challenges remain despite the enormous growth in this area over the past two decades. To make devices inexpensively and for their applications to become a commercial reality, we need to understand the science of their fundamental behavior and overcome experimental limitations in controlling and probing fluids confined to the nanoscale. For confinement widths less than a few nanometers, the well-established field of classical hydrodynamics based on the continuum hypothesis may fail. Carbon nanotubes (CNTs) represent one of the smallest cylindrical nanopores (10,000× smaller than a single human hair) available today. Graphene is a two-dimensional ultrathin single layer of carbon atoms. These novel carbon-based materials
have unusually high mechanical strength, excellent thermal and electric conducting properties, ultrasmooth hydrophobic surfaces, and high-aspect ratio. Water, of course, is the most ubiquitous molecule and one of the most vital elements of life. Even though it is the most studied substance on earth, we are still trying to understand its anomalous bulk properties, and we continue to discover new and fascinating properties of water in confinement. In fluidics applications, both graphene and CNTs have shown super-lubricity, almost frictionless ultrafast mass transport, and high heat conduction. Amplifying the flow rates by reducing interfacial friction at the water–nanotube (or water– graphene) interface could have enormous economic advantages in applications, such as water desalination, by reducing energy usage. Water flow in CNTs has become the subject of intense research from the early 2000s.3–6 Various properties of water in CNTs—structural, thermodynamic, and transport— have been studied extensively using experimental,7–16 computational,6,11,17–24 and theoretical25,26 methods. In this article, we focus on molecular dynamics (MD) studies of water flow in CNTs, which have specifically reported on slip length and flow enhancement. Experimental, continuum, multiscale, and MD studies with subnanometer diameter (single chain water) are discussed in articles by Calabrò, Corry, Kannam et al., Majumder et al., Min et al., and Borg and Reese in this issue.
Sridhar Kumar Kannam, IBM Research–Australia, Australia; [email protected] Peter J. Daivis, RMIT University, Australia; [email protected] B.D. Todd, Swinburne University of Technology, Australia; btodd@swi
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