Diffusing wave microrheology of highly scattering concentrated monodisperse emulsions

Area of Science:

• Soft Matter Physics
• Colloidal Science
• Rheology

Background:

• Diffusing wave spectroscopy (DWS) is used to study soft matter, but collective scattering effects in dense emulsions are often overlooked.
• In DWS microrheology experiments, emulsion droplets act as both probes and the source of elasticity.
• Prior DWS studies have not fully accounted for how droplet interactions influence measurements.

Purpose of the Study:

• To investigate the impact of collective scattering on DWS mean-square displacements (MSDs) in jammed emulsions.
• To accurately calculate low-frequency plateau elastic shear moduli ([Formula: see text]) using corrected DWS data.
• To compare DWS microrheology results with mechanical rheometry and a theoretical model.

Main Methods:

• Measured scattering mean free path as a function of droplet volume fraction (φ).
• Calculated a φ-dependent average structure factor to correct DWS MSDs.
• Employed the generalized Stokes-Einstein relation to determine elastic shear moduli.
• Utilized mechanical rheometry for comparison.

Main Results:

• Demonstrated that collective scattering significantly affects DWS MSDs in dense colloidal emulsions.
• Corrected DWS MSDs by up to a factor of 4 using measured structure factors.
• Achieved good agreement between DWS-microrheological and mechanically measured [Formula: see text] over three orders of magnitude for weakly deformed, jammed droplets.
• Found consistency between experimental results and the entropic-electrostatic-interfacial (EEI) model.

Conclusions:

• Collective scattering corrections are crucial for accurate DWS microrheology in dense emulsions.
• The study provides a self-consistent understanding of weakly jammed emulsions through DWS, mechanical tests, and the EEI model.
• The developed approach can enhance DWS microrheology for other highly scattering, jammed colloidal systems.