Electro-osmotic flow of a model electrolyte

Area of Science:

• Computational physics
• Physical chemistry
• Fluid dynamics

Background:

• Continuum theories like Poisson-Boltzmann and hydrodynamic theories are used to model fluid behavior.
• Electro-osmotic flow (EOF) is crucial in microfluidic devices and separation sciences.
• Discrepancies exist between theoretical predictions and simulation results for EOF in confined geometries.

Purpose of the Study:

• To investigate electro-osmotic flow using nonequilibrium molecular dynamics simulations.
• To elucidate factors affecting the velocity profile in a model channel system.
• To compare simulation results with existing continuum theories, specifically Poisson-Boltzmann and hydrodynamic theories.

Main Methods:

• Nonequilibrium molecular dynamics (NEMD) simulations were employed.
• A model system with spherical ions, solvent, and charged walls was utilized.
• The channel height was set to 20 particle diameters.

Main Results:

• Hydrodynamic theory accurately describes pressure-driven (Poiseuille) flow.
• Poisson-Boltzmann theory fails to predict ion distribution near channel walls due to reduced ion solvation.
• Corrected ion distribution in hydrodynamic theory restores agreement with simulation results.
• Analytic theory shows wall repulsion increases flow rate, while attraction decreases it.

Conclusions:

• Poisson-Boltzmann theory requires refinement for accurate ion distribution near charged walls in confined systems.
• Hydrodynamic theory, when supplied with a corrected ion distribution, effectively models electro-osmotic flow.
• Ion-wall interactions significantly influence electro-osmotic flow rates.