Molecular dynamics simulation of electroosmotic & pressure driven flows in nanochannels

by 1982- Miao, Miao

By Miao Miao, MS Washington State University August 2007 Chair: Prashanta Dutta In order to deepen our understanding of pressure driven flow and electroosmotic flow in nano channels, a molecular dynamics simulation program was developed. Pressure driven flow happens due to a pressure gradient in the channel, whereas ions’ movement due to an external electric field induces electroosmotic flow. For pressure driven flow, Lennard-Jones potential is simply used to simulate the van der Waals interaction. In addition to that, 3D Ewald Summation was employed to simulate the electrostatic interaction in electroosmotic flow. Our system consists of 168 spherical rigid wall molecules and 2048 spherical liquid molecules. Channel length, width and height (Lx, Ly, Lz) are 4.588 nm, 4.588nm and 3.614nm respectively. Water molecules are sandwiched between top and bottom walls with a FCC structure as the initial configuration. In electroosmotic flow, there are 32 ions out of 2048 liquid molecules and 32 wall molecules are assigned charges in order to reach an electrostatic neutral condition. A periodical boundary was added to simulate bulk flow in x and z direction. A Berendsen thermostat is also used to control the system temperature. Verlet neighbor hood list helps to reduce the computation time when calculating the pair interaction of molecules. iv Six studies were conducted in this reearch: A comparison test of pressure driven flow with existing continuum theories; a parametric study of effect of external force in pressure driven flow; a limiting case study where the continuum theory completely breaks down in nano channels; a comparison study between simulation and continuum results of electroosmotic flow; a parametric study of electroosmotic flow with respect to effects of external electric field. Lastly we investigated the influence of density on electroosmotic flow. From the results we do see small jumps of velocities near the channel wall. Flow velocity magnitude is proportional to the external field. We found that in a channel height of 5.5?, parabolic velocity profile is no longer seen. It was also discovered that Poisson-Boltzmann theory fails to describe the ion distribution. In conclusion, our simulation results do agree with the existing molecular dynamics results from other research groups. v