Microstructural diversity, nucleation paths, and phase behavior in binary mixtures of charged colloidal spheres.

Area of Science:

• Colloid Science
• Materials Science
• Soft Matter Physics

Background:

• Binary colloidal suspensions are model systems for studying crystallization and phase behavior.
• Understanding crystallization pathways is crucial for designing materials with specific properties.

Purpose of the Study:

• To compare crystallization processes in bulk versus confined geometries.
• To investigate the influence of gradients on phase formation and microstructure diversity.
• To characterize the stability and transformation of different colloidal phases.

Main Methods:

• Preparation of low-salt, binary aqueous suspensions of charged colloidal spheres.
• Solidification from a homogeneous shear-melt for bulk samples.
• Deionization in commercial slit cells to induce gradients.
• Qualitative characterization using imaging and optical microscopy.

Main Results:

• Bulk samples typically form body-centered cubic substitutional alloys, stable in gas-tight vials.
• Confinement in slit cells leads to non-volume-filling alloy formation and diverse microstructures due to gradients.
• Observation of alpha (α) and beta (β) phases with low solubility, alongside homogeneous nucleation routes.
• Crystals melt upon salt concentration increase, with wall-based β-phase and facetted α-crystals melting last.

Conclusions:

• Substitutional alloys formed by homogeneous nucleation in bulk are mechanically stable but thermodynamically metastable.
• Gradients in confined systems promote diverse crystallization and transformation pathways, leading to complex microstructures.
• The study highlights the critical role of confinement and gradients in colloidal crystallization and phase stability.