Molecular Dynamics Investigation Of Nanoscale Electroosmosis

With the development of micro-electro-mechanical system, micropump technology becomes the leading solution to driving and controlling the fluid flow in microchannels or even nanochannels. Due to no moving parts and ease of control, electroosmotic micropumps are suitable for microelectronic cooling applications and miniaturized total analysis systems and thus attract more and more attentions. Continuum theory based on Poisson-Boltzmann equation and the Navies-Stokes equation has been popularly used to understand electroosmotic flow in microscale channels, however, a fundamental issue that needs to be discussed is whether or not continuum theories can be used to describe electroosmotic flow in nanoscale channels.Molecular dynamics simulation of electroosmotic flow in a 2.9nm wide channel has been carried out in this dissertation. Simulation results indicate that the concentration of the coion exceeds that of the counterion in a certain region of the electric double layer. It is also found that the electroosmotic flow is in the opposite direction to that predicted by the classical continuum theory. Both charge inversion and flow reversal can't be accounted for in continuum theory. Different ion- wall and ion-charge interactions are dominating causes to the charge inversion, while flow reversal is mainly induced by the dehydration of sodion and charge inversion.Through applying two different types of surface charge, the shear flow in the nanochannel where the upper portion of fluid moves in the opposite direction to the lower portion is simulated. Electrostatic potential and velocity of water molecules across the channel are also calculated in continuum theory for comparison. The missing interaction and the improper continuum assumptions in nanoscale system lead to the significant differences.Moreover, an improved model is used to investigate how surface charges influence ion distribution in the electric double layer. As the increase of surface charge density, the amount of counterion increases linearly and that of coion almost remains steady, meanwhile, the region where charge inversion occurs expands and the number of net charge in the system increases.