Particle/wall electroviscous effects at the micron scale: comparison between experiments, analytical and numerical...

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Nanotechnology

Background:

• Understanding particle-wall interactions is crucial in colloid science.
• The electrical double layer (EDL) significantly influences particle dynamics near charged surfaces.
• Previous studies often simplified EDL effects or focused on 2D models.

Purpose of the Study:

• To experimentally investigate the 3D motion of single particles near functionalized walls under varying ionic strengths.
• To quantify the retardation effect on particle diffusion coefficients at low and moderate salt concentrations.
• To compare experimental results with electroviscous theories and numerical simulations.

Main Methods:

• 3D particle trajectories were tracked using diffracted particle image analysis (point spread function).
• Experiments involved 1 μm particles interacting with bare glass, polyelectrolyte-coated, and lipid monolayer-coated glass surfaces.
• Varying ionic strengths (low salt/pure water to 10 mM salt) were employed.

Main Results:

• A significant retardation effect on short-time diffusion coefficients was observed in pure water, more pronounced in the perpendicular direction.
• This diffusion decrease was independent of the negative surface charge origin.
• Diffusivity recovered classical hydrodynamic behavior upon salt addition (10 mM) or surface charge screening.
• Electroviscous theory with the thin EDL approximation showed good agreement, except at very small particle-wall distances (h).
• 2D numerical solutions of electrokinetic equations qualitatively matched experimental findings.
• Numerical models indicated hydrodynamic and Maxwellian drag components diminish as h approaches 0 due to EDL merging.

Conclusions:

• The study demonstrates a measurable retardation effect on particle diffusion due to EDL interactions at low ionic strengths.
• Electrokinetic theories and simulations provide valuable insights into particle-wall dynamics, particularly the role of EDL merging at close proximity.
• Findings contribute to a deeper understanding of colloidal particle behavior in complex fluid environments.