Effect of Geometry on Electrokinetic Characterization of Solid Surfaces

Area of Science:

• Physical Chemistry
• Electrochemistry
• Fluid Dynamics

Background:

• The classical Helmholtz-Smoluchowski (H-S) equation inaccurately predicts streaming current and potential in complex sample geometries.
• Accurate zeta potential determination is crucial for understanding surface-fluid interactions in various applications.
• Existing models struggle with the geometric complexity inherent in many real-world porous materials.

Purpose of the Study:

• To develop a novel analytical approach for describing pressure-driven streaming current (Istr) and streaming potential (Ustr) in geometrically complex samples.
• To provide a theoretical foundation for more accurate zeta potential measurements, particularly in porous media.
• To address the limitations of the H-S equation in non-ideal sample geometries.

Main Methods:

• Developed an analytical methodology using linear velocity profiles within electrical double layers (EDLs) and approximations for macroscopic flow.
• Derived general expressions for Istr and Ustr generation in arbitrarily shaped samples.
• Applied the methodology to model porous media, including capillaries with nonuniform cross-sections and arrays of cylindrical fibers.

Main Results:

• Obtained analytical expressions for Istr and Ustr generation applicable to complex geometries under specific conditions (thin EDLs, negligible surface conductivity and electroviscous effects).
• Validated the approach using model porous media, analyzing the impact of fiber orientation, porosity, and dimensions.
• Demonstrated the potential for improved zeta potential determination strategies for porous fiber plugs.

Conclusions:

• The new analytical approach offers a more accurate description of electrokinetic phenomena in complex geometries compared to the H-S equation.
• The findings provide valuable insights into factors influencing zeta potential measurements in porous materials.
• This work offers a pathway to resolve the long-standing issue of sample geometry dependence in zeta potential measurements.