Molecular dynamics simulation of electro-osmotic flows in rough wall nanochannels
- 9 May 2006
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review E
- Vol. 73 (5), 051203
- https://doi.org/10.1103/physreve.73.051203
Abstract
We performed equilibrium and nonequilibrium molecular dynamics simulation to study electro-osmotic flows inside charged nanochannels with different types of surface roughness. We modeled surface roughness as a sequence of two-dimensional subnanoscale grooves and ridges (step function-type roughness) along the flow direction. The amplitude, spatial period, and symmetry of surface roughness were varied. The amplitude of surface roughness was on the order of the Debye length. The walls have uniform negative charges at the interface with fluids. We included only positive ions (counterions) for simplicity of computation. For the smooth wall, we compared our molecular dynamics simulation results to the well-known Poisson-Boltzmann theory. The density profiles of water molecules showed “layering” near the wall. For the rough walls, the density profiles measured from the wall are similar to those for the smooth wall except near where the steps are located. Because of the layering of water molecules and the finite size effect of ions and the walls, the ionic distribution departs from the Boltzmann distribution. To further understand the structure of water molecules and ions, we computed the polarization density. Near the wall, its z component dominates the other components, indicating the preferred orientation (“ordering”) of water molecules. Especially, inside the groove for the rough walls, its maximum is 10% higher (stronger ordering) than for the smooth wall. The dielectric constant, computed with a Clausius-Mosotti-type equation, confirmed the ordering near the wall and the enhanced ordering inside the groove. The residence time and the diffusion coefficient, computed using the velocity autocorrelation function, showed that the diffusion of water and ions along the direction normal to the wall is significantly reduced near the wall and further decreases inside the groove. Along the flow direction, the diffusion of water and ions inside the groove is significantly lowered while it is similar to the bulk value elsewhere. We performed nonequilibrium molecular dynamics simulation to compute electro-osmotic velocities and flow rates. The velocity profiles correspond to those for overlapped electric double layers. For the rough walls, velocity inside the groove is close to zero, meaning that the channel height is effectively reduced. The flow rate was found to decrease as the period of surface roughness decreases or the amplitude of surface roughness increases. We defined the ζ potential as the electrostatic potential at the location of a slip plane. We computed the electrostatic potential with the ionic distribution and the dielectric constant both from our molecular dynamics simulation. We estimated the slip plane from the velocity profile. The ζ potential showed the same trend as the flow rate: it decreases with an increasing amplitude and a decreasing period of surface roughness.Keywords
This publication has 36 references indexed in Scilit:
- Atomistic simulation of KCl transport in charged silicon nanochannels: Interfacial effectsColloids and Surfaces A: Physicochemical and Engineering Aspects, 2005
- Hydrodynamic interaction of rough spheresGranular Matter, 2005
- Overview of the lattice Boltzmann method for nano- and microscale fluid dynamics in materials science and engineeringModelling and Simulation in Materials Science and Engineering, 2004
- Charge Inversion and Flow Reversal in a Nanochannel Electro-osmotic FlowPhysical Review Letters, 2004
- Electrokinetic Transport through Rough MicrochannelsAnalytical Chemistry, 2003
- Constant-Wall-Temperature Nusselt Number in Micro and Nano-Channels1Journal of Heat Transfer, 2001
- Nanochannel fabrication for chemical sensorsJournal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1997
- Molecular Dynamics Study of a Membrane-Water InterfaceThe Journal of Physical Chemistry, 1995
- Continuum Deductions from Molecular HydrodynamicsAnnual Review of Fluid Mechanics, 1995
- Molecular dynamics with coupling to an external bathThe Journal of Chemical Physics, 1984