Collective diffusion coefficient of a charged colloidal dispersion: interferometric measurements in a drying drop

Area of Science:

• Soft Matter Physics
• Colloidal Science
• Interfacial Phenomena

Background:

• Understanding drying dynamics in colloidal dispersions is crucial for material science and manufacturing.
• Charged colloidal systems exhibit complex behaviors influenced by interparticle forces and confinement.
• Out-of-equilibrium processes like drying present challenges for theoretical modeling.

Purpose of the Study:

• To investigate the drying dynamics of a 2D confined charged colloidal dispersion.
• To accurately measure the colloid concentration field during drying.
• To probe mass transport mechanisms and determine the collective diffusion coefficient.

Main Methods:

• Utilized Mach-Zehnder interferometry for high-accuracy (0.5%) concentration measurements.
• Achieved high temporal (1 frame/s) and spatial (5 μm/pixel) resolution.
• Analyzed drying dynamics in a 2D confined drop geometry.

Main Results:

• Demonstrated that mass transport can be purely diffusive under specific conditions (negligible buoyancy-driven convection).
• Extracted the collective diffusion coefficient D(φ) with high accuracy over a wide concentration range (φ = 0.24–0.5).
• Observed D(φ) values significantly larger (5–12D0) than predicted by the Stokes-Einstein relation.

Conclusions:

• Colloidal interactions play a significant role in enhancing the collective diffusion coefficient during drying.
• The study provides a detailed understanding of mass transport in drying charged colloidal dispersions.
• Experimental results offer valuable data for refining models of colloidal dynamics in non-equilibrium systems.