Existence of a maximum flow rate in electro-osmotic systems.

Area of Science:

• Physics
• Physical Chemistry
• Fluid Dynamics

Background:

• Electro-osmotic flow (EOF) is crucial in microfluidic and nanoscale devices.
• Hydrodynamic wall-fluid friction significantly impacts EOF, especially at the nanoscale.
• Understanding the interplay between friction and electrostatic interactions is key to controlling EOF.

Purpose of the Study:

• To investigate the influence of hydrodynamic wall-fluid friction on electro-osmotic flows.
• To determine the conditions for maximum electro-osmotic flow rate in a counterion-only system.
• To validate theoretical predictions with molecular dynamics simulations.

Main Methods:

• Derivation of electro-hydrodynamic equations for EOF velocity profiles.
• Equilibrium molecular dynamics (MD) simulations to determine hydrodynamic slip length.
• Non-equilibrium MD simulations for independent validation.

Main Results:

• A counterion concentration yielding maximum EOF rate was predicted, contingent on wall-fluid friction and electrostatic screening length.
• MD simulations confirmed the predicted maximum EOF rate.
• Standard hydrodynamic theory accurately predicts nanoscale EOF with appropriate slip boundary conditions.

Conclusions:

• Hydrodynamic wall-fluid friction plays a critical role in determining maximum electro-osmotic flow rates.
• The findings provide a more accurate theoretical framework for nanoscale electro-osmotic phenomena.
• Accurate slip boundary conditions are essential for applying hydrodynamic theory to charged nanoscale systems.