Probing Static Structure of Colloid-Polymer Suspensions with Multiply Scattered Light

Area of Science:

• Colloidal Science
• Soft Matter Physics
• Polymer Science

Background:

• Dense colloidal dispersions exhibit complex particle correlations influenced by interparticle forces.
• Understanding these interactions is crucial for controlling material properties and predicting phase behavior.
• Depletion interactions, induced by non-adsorbing polymers, are a key factor in colloidal self-assembly.

Purpose of the Study:

• To investigate the influence of depletion interactions on particle correlations in dense polystyrene dispersions.
• To assess the impact of polyethyleneoxide (PEO) molecular weight and concentration on colloidal ordering.
• To explore time-dependent light propagation as a method for characterizing interaction potentials in multiply scattering systems.

Main Methods:

• Time-dependent measurements of light propagation in aqueous polystyrene dispersions (523 nm diameter) at varying volume fractions (0.1-0.4).
• Addition of soluble polyethyleneoxide (PEO) with varying molecular weights to induce depletion interactions.
• Comparison of experimental results with predictions from Mie scattering theory, Percus-Yevick (P-Y) model, and mean sphere approximation (MSA).

Main Results:

• Depletion forces from PEO increased the transport scattering length beyond hard-sphere predictions, aligning with MSA.
• Onset of flocculation was observed with increased PEO addition, correlating with PEO molecular weight.
• Phase separation was indicated by a plateau in transport scattering length, except for high molecular weight PEO-induced flocculation.

Conclusions:

• Time-dependent light propagation is a viable alternative to scattering techniques for studying interaction potentials in dense, multiply scattering colloidal systems.
• Depletion interactions significantly alter the spatial ordering and scattering properties of colloidal dispersions.
• The study highlights the sensitivity of light propagation measurements to subtle changes in colloidal interactions and phase behavior.