(173c) Water Flow in Nano Channels of Quartz, Titanium Dioxide and Carbon Nanotubes

The flow of liquid through narrow pores is a key component in many natural and industrial processes. For example, the erosion of rock in the subsurface can be initiated as flow through nanopores, while the separation of mixtures in membranes by molecular sieving exploits the different flow rates of molecules through pores as a result of both attractive and steric interactions between the molecules and the pore material. Flow in such nanopores is driven by a pressure drop and/or chemical potential gradient. In order to understand such processes at the molecular level requires the use of non-equilibrium molecular dynamics (NEMD) simulation methods that allow the application of an external field [1,2]. In this work, we consider NEMD simulations of water flow in the mineral nanochannels. In order to model a pressure drop across the nanopore, we add a force to each water atom directly so as to induce the flow in the channels [3]. The application of this external force on the system requires that viscous heat generation must be removed in order to reach a steady state; this is achieved by applying a thermostat on the wall atoms [3].

Three types of channel wall were chosen for study: quartz slit nanopores, titanium dioxide slit nanopores and carbon nanotubes. The former two channels are hydrophilic while the last one is hydrophobic. We find that the pressure-drop-induced flow in nano channels does not alter in any appreciable way the structure and dynamic properties of the water inside the nanopore. The primary reason for this is that even a large pressure drop, such as 300 MPa, translates into an additional force per atom that is no more than 2% of the forces on the atoms in the absence of a pressure drop (i.,e., at eqiulibrium). Additionally, the consequent streaming velocity of flow is only 10% of the average speed of each water atoms. The flux in hydrophilic channels yields a velocity profile consistent with the classical continuum-theory Hagen-Poiseuille equation. In contrast, those in the hydrophobic nanotube pore do not, since we find that the no-slip boundary condition used to derive the Hagen-Poiseuille equation is not valid.

(1) Predota, M.; Cummings, P. T.; Wesolowski, D. J. The Journal of Physical Chemistry C 2007, 111, 3071.
(2) Joseph, S.; Aluru, N. R. Nano Letters 2008, 8, 452.
(3) Todd, B. D.; Evans, D. J.; Daivis, P. J. Physical Review E 1995, 52, 1627.