Electrophoretic flow behaviour and mobility of colloidal fluids and crystals

Area of Science:

• Colloid science
• Physical chemistry
• Electrokinetics

Background:

• Charged colloidal spheres exhibit fluid or crystalline order in deionized states.
• Electro-osmotic solvent flow causes parabolic profiles in the fluid state.
• Crystal cohesion leads to complex flow profiles in the crystalline state.

Purpose of the Study:

• To measure electrophoretic mobility (mu) of charged colloidal spheres.
• To investigate the effect of particle concentration on mu.
• To determine if electrokinetic properties are single-particle properties despite structure formation.

Main Methods:

• Measurements of electrophoretic mobility (mu) using a super-heterodyne Doppler velocimeter.
• Analysis of flow profiles in fluid and crystalline states.
• Application of charge renormalization calculations and the Standard Electro-kinetic Model.

Main Results:

• Electrophoretic mobility (mu) decreases logarithmically with increasing particle concentration.
• The decrease in mu is independent of the fluid-to-crystalline phase transition.
• A combination of charge renormalization and single-particle models quantitatively captures the observed behavior.

Conclusions:

• Electrokinetic properties of colloidal spheres can be treated as single-particle properties, even when forming structures.
• Counter-ion condensation significantly reduces effective particle charge at higher concentrations.
• Increased colloid concentration modifies the screening parameter, allowing mapping to single-particle theories.