0000000000009502
AUTHOR
Andreas Stipp
Crystal growth kinetics in deionised two-component colloidal suspensions
We report first measurements of the crystal growth velocity in deionised two-component aqueous suspensions of charged polystyrene latex spheres. The size ratio was 1:1.3 and mixing ratios up to 18% of the larger particle were investigated. For the pure components limiting growth velocities of (V ∞,120 = 4.8 µm s−1 and V ∞,156 = 2.9 µm s−1) were observed. In the mixture v drops with increasing mixing ratio even below (V ∞,156) of the large minority component. Careful monitoring of the deionisation procedure excludes the explanation of enhanced ionic contamination. Alternatives based on the kinetics of particle attachment and the position of the phase boundary are discussed.
Optical experiments on a crystallizing hard-sphere–polymer mixture at coexistence
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 cry…
Heterogeneous nucleation of colloidal melts under the influence of shearing fields
Large, oriented single crystals may be obtained from shear melts of colloidal particles after nucleation at the container walls. We are here interested in the processes occurring during the initial phase of their formation. Using different microscopic and scattering techniques we here studied highly charged suspensions of spherical particles, dispersed in low salt or deionized water, in single and double wall confinement, during and after cessation of shear. While the equilibrium phase of our colloidal solids is body centred cubic, the shear induced precursors of heterogeneous nuclei consist of wall based, oriented, registered or freely sliding layers with in plane hexagonal symmetry. Cessa…
Growth and anisotropic ripening in twinned colloidal crystals
The heterogeneous crystallization of colloidal suspensions of Yukawa particles on a wall was observed by Bragg microscopy. The growth velocity ν 110 of the fluid-crystal interface can be very well described by a Wilson-Frenkel growth law, while the limiting velocity is found to be lower than in previous investigations. Twinned bcc domains in lateral direction (parallel to the wall) and their ripening behavior are for the first time investigated systematically. A strong anisotropic growth behavior is found: in the direction of the shear flow, which shear melts the suspension prior to the crystallization process, the extension of the domains is much larger than perpendicular to that. The incr…
Unusual crystallization kinetics in a hard sphere colloid-polymer mixture.
We investigated the crystallization kinetics of a hard sphere colloid-polymer mixture at conditions where about 95% of solid coexists with about 5% of fluid. From time resolved Bragg and small angle light scattering, we find that the crystallite size increases with a power law of exponent alpha approximately 1/3 during both the conversion and the coarsening stage. This observation points to a single conserved order parameter for both stages and cannot be explained if the mixture is regarded as an effective one-component system. We alternatively suggest that--based on local geometric demixing--the polymer density takes the role of the conserved order parameter.
Micro-structure evolution of wall based crystals after casting of model suspensions as obtained from Bragg microscopy.
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 coars…
Crystallizing polystyrene microgel colloids
Spherical microgel particles of sufficiently high degree of internal cross-linking and swollen in a good solvent in many respects behave quite similarly to hard-sphere colloids. Due to solvent uptake they can be refractive-index-matched and density-matched in suitable organic solvents. We present preliminary measurements of the crystallization kinetics of 1:10 cross-linked polystyrene microgel particles. We measured Bragg and small-angle light scattering of the solidifying shear melt. Two different scattering patterns, a set of Debye-Scherrer rings and a second ring pattern at small angles could be observed. We check for similarities and differences compared to previously investigated collo…
Crystal growth kinetics in binary mixtures of model charged sphere colloids
From the formation of crystalline materials from their melts is at the heat of many important industrial processes. A rigorous and comprehensive understanding of the crystallization process, however, is still lacking, and its experimental and theoretical investigation has remained a considerable challenge. We studied the crystal growth velocities v of random composition body centred cubic (rc-bcc) colloidal crystals formed from a shear molten binary mixture of charged colloidal spheres suspended in deionized water. We used Bragg microscopy at constant particle number density n and as a function of the number fraction of large particles p. With increasing p the growth velocity v first drops …
Observation of Oriented Close-Packed Lattice Planes in Polycrystalline Hard-Sphere Solids.
We report time-resolved Bragg scattering experiments on solidifying colloidal suspensions of hard spheres. The polar angle-averaged, integrated intensity of the (111) and (311) reflections show a transient, two-step behavior below melting, which depends in a complex way on the volume fraction and is not present for (200) or (220). Detailed analysis of the full two-dimensional scattering pattern reveals intensity maxima of sixfold symmetry close to the position of the (111) and (311) Debye-Scherrer rings. These can be explained assuming oriented crystals with close-packed planes parallel to the container walls. We show that the observed temporal behavior is due to competing homogeneous and h…
Solidification kinetics of hard-sphere colloidal suspensions
We investigate the solidification dynamics of hard-sphere colloidal suspensions applying simultaneously small-angle and Bragg light scattering. These experiments allow a consistent picture of nucleation and crystal growth on the level of large-scale density fluctuations and of density fluctuations on the level of individual crystallites. We observe a temporally almost constant nucleation rate after an induction time that decreases with supersaturation. The classical expectation for the nucleation rate density as a function of supersaturation is in accordance with our data. We investigate the validity of the Wilson—Frenkel growth law for hard-sphere systems, which also fits our data satisfac…