Electroosmotic Flow Hysteresis for Fluids with Dissimilar pH and Ionic Species

Area of Science:

• Microfluidics and Nanofluidics
• Electrokinetics
• Solution Chemistry

Background:

• Electroosmotic flow (EOF) is crucial for micro-/nanofluidic applications involving fluid displacement.
• Existing research on EOF hysteresis lacks studies on solution pairs with significant pH differences.
• Dissimilar ionic species with varying pH exhibit complex hysteretic phenomena in EOF.

Purpose of the Study:

• To investigate electroosmotic flow (EOF) hysteresis in a sodium bicarbonate (NaHCO3) and sodium chloride (NaCl) solution pair.
• To analyze the impact of dissimilar ionic species and substantial pH differences on EOF-driven fluid displacement.
• To develop and validate a slip velocity model with a modified wall condition using finite element simulations.

Main Methods:

• Current monitoring technique for experimental analysis of EOF.
• Finite element simulations employing a developed slip velocity model with a modified wall condition.
• Quantitative comparison of experimental and simulation results.

Main Results:

• Concentration evolutions of NaHCO3-NaCl systems follow patterns of dissimilar anion species.
• When NaCl displaces NaHCO3, reduced EOF occurs due to the displacement of high-pH NaHCO3, hindering complete NaCl displacement.
• When NaHCO3 displaces NaCl, slow EOF caused by low-pH NaCl impedes NaHCO3 displacement.
• Complete displacement is favored by lowering NaCl concentration (increasing zeta potential), while NaHCO3 concentration has minimal impact.

Conclusions:

• The study elucidates complex EOF hysteresis in microfluidic systems with solutions of dissimilar pH and ionic species.
• Understanding these pH-dependent displacement behaviors is critical for optimizing micro-/nanofluidic device performance.
• Findings contribute to the fundamental knowledge of electrokinetic phenomena in multi-component fluid systems.