Optical Experiments on a Crystallizing Hard Sphere - Polymer Mixture at Coexistence

TL;DR

This study investigates the crystallization process in a colloidal hard sphere-polymer mixture using scattering and microscopy, revealing detailed kinetics, grain boundary properties, and the evolution of crystalline structures over time.

Contribution

It provides new insights into the crystallization kinetics, grain boundary characteristics, and the late-stage scattering behavior in a colloidal system with short-range attractions.

Findings

Crystals coexist with melt at grain boundaries after crystallization.

Furukawa scaling describes the SALS signal at all times.

Grain boundaries exhibit reduced Bragg scattering but increased refractive index.

Abstract

We report on the crystallization kinetics in an entropically attractive colloidal system using a combination of time resolved scattering methods and microscopy. Hard sphere particles are polystyrene microgels swollen in a good solvent (radius a=380nm, starting volume fraction 0.534) with the short ranged attractions induced by the presence of short polymer chains (radius of gyration rg = 3nm, starting volume fraction 0.0224). After crystallization, stacking faulted face centred cubic crystals coexist with about 5% of melt remaining in the grain boundaries. From the Bragg scattering signal we infer the amount of crystalline material, the average crystallite size and the number density of crystals as a function of time. This allows to discriminate an early stage of conversion, followed by an extended coarsening stage. The small angle scattering (SALS) appears only long after completed conversion and exhibits Furukawa scaling for all times. Additional microscopic experiments reveal that the grain boundaries have a reduced Bragg scattering power but possess an increased refractive index. Fits of the Furukawa function indicate that the dimensionality of the scatterers decreases from 2.25 at short times to 1.65 at late times and the characteristic length scale is slightly larger than the average crystallite size. Together this suggests the SALS signal is due scattering from a foam like grain boundary network as a whole.