Influence of atomistic physics on electro-osmotic flow: an analysis based on density functional theory

Area of Science:

• Computational physics and chemistry
• Nanofluidics
• Physical chemistry

Background:

• Classical Poisson-Boltzmann (PB) models simplify electric field calculations in electrolytes.
• Accurate modeling of electro-osmotic flows in nanoscale channels is crucial for microfluidic devices.
• Existing models often neglect detailed molecular interactions, leading to potential inaccuracies.

Purpose of the Study:

• To develop and apply a more accurate model for electro-osmotic flows in nanoscale channels.
• To compare the predictions of a density functional theory (DFT) approach with classical PB modeling.
• To investigate the impact of molecular interactions on fluid behavior in confined geometries.

Main Methods:

• Utilized density functional theory (DFT) to determine molecular density profiles and charge distributions.
• Integrated DFT results with continuum Navier-Stokes equations for flow computation.
• Modeled electrolyte ions as charged hard spheres and solvent as neutral hard spheres.
• Incorporated Lennard-Jones, hard sphere repulsion, and electrostatic interactions.

Main Results:

• DFT model predicts higher counterion concentrations near channel walls compared to PB models.
• This leads to a reduced effective Debye layer thickness and lower electro-osmotic velocities.
• Computed zeta potentials and fluid speeds by DFT were 2-3 times smaller than PB results.
• Discrepancies increase with electrolyte concentration, surface charge density, and decreasing channel width.

Conclusions:

• DFT provides a more accurate representation of electro-osmotic flows in nanoscale channels by including molecular interactions.
• The simplified PB model overestimates electro-osmotic velocities due to neglecting these interactions.
• DFT simulations offer comparable accuracy to molecular dynamics but with significantly reduced computational cost.