Micro-structure evolution of wall based crystals after casting of model suspensions as obtained from Bragg microscopy

Abstract

Nucleation and crystal growth in a suspension of charged colloidal silica spheres with bi-modal size distribution studied by time-resolved ultra-small-angle X-ray scattering The Journal of Chemical Physics 141, 214906 (2014) Growth of heterogeneously nucleated, wall based crystals plays a major role in determining the micro-structure during melt casting. This issue is here addressed using a model system of charged colloidal spheres in deionized aqueous suspension observed by Bragg microscopy which is a combination of light scattering and microscopy. We examine the evolution of the three-dimensional size, shape, and orientation of twin domains in monolithic crystals growing from two opposing planar walls into a meta-stable (shear-) melt. At each wall crystal orientation and twinning emerges during nucleation with small domains. During growth these widen and merge. From image analysis we observe the lateral coarsening velocities to follow a power law behaviour L XY ∝ t 1/2 as long as the vertical growth continues at constant speed. Lateral coarsening terminates upon intersection of the two solids and hardly any further ripening is seen. Initial lateral coarsening velocities show a Wilson Frenkel type dependence on the melt meta-stability.

In situ characterization of crystallization and melting of soft, thermoresponsive microgels by small-angle X-ray scattering

“…In addition, the swelling would lead to entanglement of the outer chains of the PNIPAM shell, and thus lead to an early separation into two states, while also explaining the delayed melting of the crystallites even below ϕ f . However, real space investigations, such as polarization microscopy, 38,57 will be needed to confirm this melting process.…”

“…The similarities in behaviour of the “green” and “red” crystallites indicate that these crystals might be two crystalline domains formed on the capillary walls, as observed in other charged particle systems. 57 A temperature gradient close to the walls induced by the short cooling to T = 20 °C of the sample before the heating measurement will lead to increased swelling of the CS microgel shell ( R h (20 °C) = 228.9 nm) and could explain the larger lattice spacing in these crystals close to the wall. In addition, the swelling would lead to entanglement of the outer chains of the PNIPAM shell, and thus lead to an early separation into two states, while also explaining the delayed melting of the crystallites even below ϕ f .…”

Crystallization kinetics of colloidal model suspensions: recent achievements and new perspectives

“…In addition, colour coding due to loss of Bragg scattered light allows fast large scale scans of the structure in (thin film) samples [195]. Illumination under Bragg condition allows to selectively monitor nucleation and growth of individual crystals or selected twin orientations [196,115,197]. Microscopy may further be combined with optical tweezers, e.g.…”

Confined colloidal crystals in and out of equilibrium

“…Unidirectional shear leads to shear induced oriented precursor crystals in form of hexagonal layers at the cell wall ((111)-fcc parallel to the wall) and further quasi-epitaxial growth of bcc crystals on this precursor crystal [56,57]. The precursor acts as structured substrate reducing the nucleation barrier height to a minimum.…”