Electroviscous Dissipation in Aqueous Electrolyte Films with Overlapping Electric Double Layers

Area of Science:

• Colloid and surface science
• Nanomechanics
• Physical chemistry

Background:

• Understanding nanoscale fluid behavior is crucial for various applications.
• The interplay between surface charge, electrolyte concentration, and fluid dynamics at interfaces is complex.
• Classical models of hydrodynamic damping may not fully capture phenomena in confined geometries.

Purpose of the Study:

• To investigate the forces and energy dissipation during the squeezing of aqueous electrolyte films between an AFM tip and silica substrates.
• To explore the influence of pH and salt concentration on these forces.
• To compare experimental results with continuum theory models for electroviscous effects.

Main Methods:

• Dynamic atomic force microscopy (AFM) was employed to probe tip-sample interactions.
• Amplitude and phase signals from AFM were analyzed to determine conservative and dissipative forces.
• Surface charge density was calculated using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with charge regulation.
• Experimental data was compared against electroviscous dissipation and visco-electric enhancement models.

Main Results:

• Dissipation was enhanced by up to a factor of 5 near the Debye length, exceeding classical hydrodynamic predictions.
• The observed viscosity enhancement correlated positively with increasing surface charge density.
• The visco-electric model qualitatively matched experimental trends, but quantitative agreement was limited.

Conclusions:

• Nanoscale confinement and surface charge significantly alter fluid dissipation compared to bulk behavior.
• Electrokinetic phenomena, particularly visco-electric effects, play a key role in the enhanced dissipation.
• Further theoretical development incorporating microscopic ion behavior is needed for a complete quantitative description.