Polymer induced changes of the crystallization scenario in suspensions of hard sphere like microgel particles
We investigated the crystallization scenario of highly cross linked polystyrene particles dispersed in the good solvent 2-ethylnaphtalene and their mixtures with non-adsorbing low molecular weight polysterene polymer using time resolved static light scattering. The samples were prepared slightly below the melting volume fraction of the polymer free system. For the polymer free samples, we obtained polycrystalline solids via crystallization scenario known from hard sphere suspensions with little competition of wall crystal formation. Addition of non-adsorbing low molecular weight polystyrene polymer leads to a considerably slowing down of the bulk crystallization kinetics. We observed a dela…
Crystallization in charged two-component suspensions
We report on the crystallization of colloidal crystals comprising of charged particles with different size ratio dispersed in thoroughly deionized water. Single components were characterized carefully and their nucleation behavior was investigated before the preparation of mixtures. Mixtures investigated at constant particle number densities showed body centred cubic structure, conductivity, and shear moduli comply with the assumption of a randomly substituted crystal. Most importantly, for the first time we obtain the dependence of the nucleation rate densities in dependence on the composition and (for one fixed composition) the particle number density. The process of nucleation in random …
Nucleation kinetics in deionized charged colloidal model systems: A quantitative study by means of classical nucleation theory
We have studied the nucleation kinetics of charged colloidal model systems under salt free conditions crystallizing in bcc structure covering a wide range of particle number densities $18\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}{\mathrm{m}}^{\ensuremath{-}3}\ensuremath{\le}n\ensuremath{\le}66.3\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}{\mathrm{m}}^{\ensuremath{-}3}$. We employed direct video-microscopic observation of individual nucleation events to obtain time resolved nucleation rate densities. Polarization microscopy and static light scattering on the resulting solids in combination with Avrami theory is used to determine the steady state nucleation rate at high unde…
Crystallization of hard-sphere colloids -- deviations from classical nucleation theory
The creation of three-dimensional ordered colloidal crystals, for application in a range of nanotechnologies, has been a goal for many researchers in the past few years. The main difficulty in creating macroscopic sized crystals of densely packed colloidal particles is that colloidal particles always have a range of particle sizes - ie, they are polydisperse. This paper studied the crystallization kinetics of a hard-sphere colloid with a well defined Gaussian polydispersity. The authors find that crystallization occurs in two stages, and does not follow the simple classical nucleation picture. The paper discusses the implications of these results for research into colloidal crystals as poss…
A comparative study on the phase behaviour of highly charged colloidal spheres in a confining wedge geometry
We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 × 10−6 mol l−1. Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0<S<50 µm and observed with video microscopy. Irrespective of the initial particle density n we reproducibly observe the particles to migrate to the narrow wedge side on the timescale of a few days. Thereby an interface between a crystalline structure and a near particle free region is formed, which propagates slowly until the dilute region is exhausted of particles. While the origin of this separation is still unclear, the final extension of t…
Fast microscopic method for large scale determination of structure, morphology, and quality of thin colloidal crystals.
We present a novel fast microscopic method to analyze the crystal structures of air-dried or suspended colloidal multilayer systems. Once typical lattice spacings of such films are in the range of visible light, characteristic Bragg scattering patterns are observed. If in microscopic observations these are excluded from image construction, a unique color coding for regions of different structures, morphologies, and layer numbers results. Incoherently scattering defect structures, however, may not be excluded from image construction and thus remain visible with high resolution.
3D Colloidal photonic crystal fabrication improved by AC fields for frequency conversion enhancement in photovoltaics
We report on the fabrication of high-quality 3D colloidal photonic crystals, whose finality is to modify the emission properties of frequency converters for photovoltaic applications. The band structure of the photonic crystals can be used to enhance the emission of down- and up-converters materials embedded in the 3D structures. However, in order to achieve an efficient conversion the first step is to obtain highly ordered templates with appropriated dimensions. We analyze here the photonic crystals obtained by the dip-coating or vertical deposition method. Moreover, by using perpendicular AC fields during the assembly of the nanospheres, the quality of the packed structures has been impro…
Construction and stability of a close-packed structure observed in thin colloidal crystals
We have characterized a close-packed structure of confined charged colloidal spheres, which has been recently discovered. Using different microscopy experiments, the vertically arranged hexagonal planes of n - hcp perpendicular are found to continuously evolve from the horizontally oriented stacks of n hexagonal planes (nDelta) following the maximum packing criterion, but discontinuously transform to a stack of n+1 square planes [(n+1)[SHAPE OF A SQUARE]]. Large mechanically stable domains with threefold twin structures are regularly observed in the suspended state at packing fractions between 0.4 and 0.58.
Easy-use and low-cost fiber-based two-color dynamic light-scattering apparatus
We present a small and compact two-color cross-correlation light-scattering setup designed to study the structure and dynamics of colloidal suspensions in the regime of considerable turbidity. Using a homemade concentric four-arm goniometer, combined with fiber-optical illumination and detection devices, allows an easy and long time stable alignment-even under a temperature variation of ∼5 °C. Tests with a nearly multiple scattering free suspension of small Rayleigh scatterers show intercepts in cross correlation near 0.4 over a wide range of scattering angles from 20° to 150°. Measuring slightly turbid samples in cross-correlation mode multiple scattering is sufficiently suppressed allowin…
Crystallization kinetics of polydisperse hard-sphere-like microgel colloids: Ripening dominated crystal growth above melting.
Highly cross-linked polystyrene microgel colloids dispersed in an index and density matching solvent provide a system with hard-sphere-like interactions, where gravity effects are effectively minimized. They are a suitable target for time-resolved observations of solidification in purely repulsive systems. We have investigated the crystallization kinetics at increasing undercooling using time resolved light scattering. Crystallization starts always with the formation of compressed, structurally heterogeneous precursor domains. In the coexistence region the precursors, after being converted into true crystallites, start growing fast by assimilating particles from the melt. The resulting poly…
Enhanced crystal stability in a binary mixture of charged colloidal spheres.
We report on the phase behavior of a binary charged sphere mixture of size ratio $\ensuremath{\Gamma}=0.68$ and charge ratio $\ensuremath{\Lambda}\ensuremath{\approx}1$ as a function of composition $p$ and number density $n$. For $p=0.1--0.3$ we observe freezing at densities well below the freezing densities of the pure components. At all compositions our data indicate the formation of substitutional alloy crystals of body centered cubic structure. No indications for compound formation were observed. Rather, our findings point at the first observation of an upper azeotrope. Measurements of the crystallization kinetics reveal a combined density and composition dependence of growth velocities…
Seed- and wall-induced heterogeneous nucleation in charged colloidal model systems under microgravity
Understanding the process that drives an undercooled fluid to the crystal state is still a challenging issue for condensed matter physics and plays a key role in designing new materials. The crystallization kinetics and the resulting polycrystalline morphology are given by a complex interplay of crystal nucleation, growth, and ripening. A great deal of progress has been made in recent years using colloidal suspensions as model systems in the study of crystallization. Close analogies to atomic systems are observed which can be exploited to address questions not accessible in atomic solidification. Here we present systematic measurements of the crystallization kinetics of a charged colloidal …
Drastic Variation of the Microstructure Formation in a Charged Sphere Colloidal Model System by Adding Merely Tiny Amounts of Larger Particles
We have investigated the changes of the crystallization scenario in charged sphere colloidal suspensions upon adding tiny amounts of a second, larger and higher charged particle species (size ratio 4.3, charge ratio 1.6). For even minute amounts of added particles, less than 1%, the whole crystallization behavior changes dramatically and for this reason the resulting microstructure of the solidified material. Both nucleation and growth of crystals slow down drastically and considerably coarsen the resulting microstructure. We show that this is related to a considerable decrease of the kinetic prefactors of both nucleation and growth, while at the same time the nucleation barrier height show…
Small changes in particle-size distribution dramatically delay and enhance nucleation in hard sphere colloidal suspensions.
We present hard-sphere crystallization kinetics for three samples with small differences in polydispersity. We show that an increase in polydispersity of 1% is sufficient to cause dramatic changes in the crystallization kinetics: Crystallization is delayed by almost one decade in time and quantitative and qualitative changes in the crystallization scenario are observed. Surprisingly the nucleation rate density is enhanced by almost a factor of 10. We interpret these results in terms of polydispersity limited growth, where local fractionation processes lead to a delayed but faster nucleation.
Transient Moiré rotation patterns in thin colloidal crystals
A high resolution optical microscopy study on low salt charged sphere suspensions settling under gravity reveals the formation of AB-stacked triangular layers via different transient structures. In particular, metastable triangular Moire rotation patterns (formally corresponding to 1 × 1 Rα super structures) with rotation angles α = 27.8° and α = 38.2° are identified.
Experimental visualization of inoculation using a charged colloidal model system
Inoculation is widely used to tune the microstructure of a polycrystalline solid from the melt and thus its material properties. We present here a systematic time resolved experimental study of inoculation in a charged colloidal model system investigating the changes of the crystallization scenario upon adding spherical seeds and show that the nucleation kinetics solely determines the resulting microstructure.
Solidification Experiments in Single-Component and Binary Colloidal Melts
Regular Horizontal Patterning on Colloidal Crystals Produced by Vertical Deposition
Colloidal particles have proved to be a suitable precursor to the formation of nanoscaled materials. More explicitly, they are a suitable way to create photonic band gap materials in 3D. Several methods have been developed to assemble colloidal multilayer systems, and have yielded various levels of success. The vertical deposition method has shown itself to be one of the best in terms of time, control of the final product, crystal size and homogeneity. Despite this, the resulting crystals often present point defects, dislocations, cracks and polycrystallinity, as well as a horizontal modulation of film thickness. These defects compromise the possible utilities of the crystals. The study of …
Non-equilibrium melting of colloidal crystals in confinement
International audience; We report on a novel and flexible experiment to investigate the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration complex morphologic evolution is observed. Crystal-melt interface positions and the melting velocity are obtained quantitatively from time resolved Bragg- and polarization microscopic measurements. A simple theoretical model pr…
Consistence of the Mean Field Description of Charged Colloidal Crystal Properties
The Debye-Huckel-Potential in combination with an effective or renormalized charge is a widely and often successfully used concept to describe the interaction in charged colloidal model systems and the resulting suspension properties. In particular the phase behaviour can be described in dependence of the parameters particle number density, salt concentration and effective charge. We performed simultaneous measurements of the phase behaviour, the shear modulus and the low frequency conductivity of deionised aqueous suspensions of highly charged colloidal spheres. From the shear modulus the interaction potential at the nearest neighbour distance in terms of a Debye-Huckel potential can be de…
Microscopic investigations of homogeneous nucleation in charged sphere suspensions.
We studied the homogeneous nucleation kinetics of an aqueous suspension of charged colloidal spheres under de-ionized conditions. Samples of equilibrium crystalline structure were shear molten and the metastable melt left to solidify after cessation of shear. At low particle number densities n, corresponding to low metastability of the melt, nucleation was monitored directly via video microscopy. We determined the nucleation rates gamma(t) by counting the number of newly appearing crystals in the observation volume per unit time. Using a suitable discrete adaptation of Avrami's [J. Chem. Phys. 7, 1003 (1939); ibid.8, 212 (1940); ibid.9, 177 (1941)] model for solidification via homogeneous n…
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…
From nuclei to micro-structure in colloidal crystallization: Investigating intermediate length scales by small angle laser light scattering.
Hard sphere suspensions are well recognized model systems of statistical physics and soft condensed matter. We here investigate the temporal evolution of the immediate environment of nucleating and growing crystals and/or their global scale distribution using time resolved Small Angle Light Scattering (SALS). Simultaneously performed Bragg scattering (BS) measurements provide an accurate temporal gauging of the sequence of events. We apply this approach to studies of re-crystallization in several different shear molten hard sphere and attractive hard sphere samples with the focus being on the diversity of observable signal shapes and their change in time. We demonstrate that depending on th…
Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition
We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to…
Experimental determination of effective charges in aqueous suspensions of colloidal spheres
Abstract We determined the low frequency conductivity σ , the phase behaviour and the shear modulus G of colloidal fluids, respectively solids prepared from deionised aqueous suspensions of highly charged spherical particles. Conductivity measures the number of freely moving small ions Z * σ and thus relates to the ion condensation process in the electric double layer under conditions of finite macro-ion concentrations. Phase behaviour and elasticity data are consistently described by a Debye–Huckel pair potential assuming pair-wise additive macro-ion interactions. Like Z * σ , also the effective charges Z * G derived from the elasticity data scales with the ratio of macro-ion radius to Bje…
Correlation between dynamical and structural heterogeneities in colloidal hard-sphere suspensions
Experiments combining dynamic and static light scattering have probed a colloidal hard-sphere system for the formation of dynamical and structural heterogeneities, which play a role in both forms of solidification: crystallization and vitrification.
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…
Drude-type conductivity of charged sphere colloidal crystals: Density and temperature dependence
We report on extensive measurements in the low-frequency limit of the ac conductivity of colloidal fluids and crystals formed from charged colloidal spheres suspended in de-ionized water. Temperature was varied in a range of 5 degrees CTheta35 degrees C and the particle number density n between 0.2 and 25 microm(-3) for the larger, respectively, 2.75 and 210 microm(-3) for the smaller of two investigated species. At fixed Theta the conductivity increased linearly with increasing n without any significant change at the fluid-solid phase boundary. At fixed n it increased with increasing Theta and the increase was more pronounced for larger n. Lacking a rigorous electrohydrodynamic treatment f…
Solidification of a colloidal hard sphere like model system approaching and crossing the glass transition
We investigated the process of vitrification and crystallization in a model system of colloidal hard spheres. The kinetics of the solidification process was measured using time resolved static light scattering, while the time evolution of the dynamic properties was determined using time resolved dynamic light scattering. By performing further analysis we confirm that solidification of hard sphere colloids is mediated by precursors. Analyzing the dynamic properties we can show that the long time dynamics and thus the shear rigidity of the metastable melt is highly correlated with the number density of solid clusters (precursors) nucleated. In crystallization these objects convert into highly…
Heterogeneous and homogeneous crystal nucleation in colloidal hard-sphere like microgels at low metastabilities
We studied the crystallization kinetics of the homogeneous bulk as well as of the heterogeneous wall crystallization in a colloidal hard sphere model system within the fluid crystal coexistence region. Using time resolved static light scattering and Bragg microscopy we determined several parameters characterizing the crystallization process (induction times, growth coefficients, nucleation rate densities,…). By performing further analysis according to classical nucleation theory we can show that wall crystallization is favored by a vanishing nucleation barrier height leading to the conclusion that a hard sphere crystal completely wets a structureless hard wall.
Exotic crystal superstructures of colloidal crystals in confinement.
Colloidal model systems have been used for over three decades for investigating liquids, crystals, and glasses. Colloidal crystal superstructures have been observed in binary systems of repulsive spheres as well as oppositely charged sphere systems showing structures well known from atomic solids. In this work we study the structural transition of colloidal crystals under confinement. In addition to the known sequence of crystalline structures, crystal superstructures with dodecagonal and hexagonal symmetry are observed in one component systems. These structures have no atomic counterpart.
Heterogeneous and homogeneous crystal nucleation in a colloidal model system of charged spheres at low metastabilities
We have studied the nucleation kinetics in the bulk as well as at the container walls of a charged sphere colloidal model system under salt-free conditions close to the fluid–crystal phase boundary. We determined time resolved nucleation rate densities for heterogeneous nucleation on the wall, the corresponding nucleation rate densities for homogeneous nucleation and compare the resulting nucleation barrier heights. The homogeneous barrier height decreases exponentially with increasing metastability, while the heterogeneous barrier displays a sharp transition from a constant value close to the phase boundary to zero at higher metastability. As a consequence the microstructure evolution is d…
Precursor-mediated crystallization process in suspensions of hard spheres.
We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real and reciprocal space data, a picture of a two-step crystallization process is supported: First dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that ha…
How narrow is the linewidth of parametric X-ray radiation?
Parametric x-ray or quasi-Cherenkov radiation is produced by the passage of an electron through a crystal. A critical absorber technique has been employed to investigate its linewidth. Experiments have been performed with the 855MeV electron beam from the Mainz Microtron MAMI. Thin absorber foils were mounted in front of a CCD camera serving as a position sensitive photon detector. Upper limits of the linewidth of 1.2 and 3.5eV were determined for the (111) and (022) reflections of silicon at photon energies of 4966 and 8332eV. These limits originate from geometrical line broadening effects that can be optimized to reach the ultimate limit given by the finite length of the wave train. {copy…
Ripening-dominated crystallization in polydisperse hard-sphere-like colloids
We report on the crystal growth scenario in gravity-matched, polydisperse hard-sphere-like colloids at increasing particle concentration. In the fluid-crystal coexistence region, the crystal size as a function of time shows two separate regimes corresponding to crystal growth and crystal ripening. At higher supersaturation the crystal size grows according to the same power law through the whole experimental window of a few days: crystal growth and ripening merge together. We show that our observations cannot be explained by considering the slowing down of single-particle dynamics due to increasing volume fraction. We suggest that size fractionation occurring at the crystal-fluid interface i…
Effective charges along the melting line of colloidal crystals
The shear modulus G of charged colloidal crystals was measured at several constant particle densities n and varying salt concentrations c up to the melting salt concentration cM using torsional resonance spectroscopy. Far from the phase boundary the samples are polycrystalline and the shear modulus stays roughly constant as a function of c. Upon approaching the melting transition an increasing amount of wall based crystal material is formed surrounding a shrinking polycrystalline core and G drops nearly linearly. When the transition is complete G again stays constant. The morphologic transitions may be scaled upon a single master curve. For the polycrystalline morphology, the elastic data a…
Competition between heterogeneous and homogeneous nucleation near a flat wall
We studied the competition between heterogeneous and homogeneous nucleation of an aqueous suspension of charged colloidal spheres close to the container walls. Samples of equilibrium crystalline structure were shear-melted and the metastable melt left to solidify after the cessation of shear. The crystallization kinetics was monitored using time-resolved scattering techniques: at low particle number densities n we applied an improved static light scattering method while at large particle concentrations ultra-small-angle x-ray scattering was applied for the first time. Our results show some unexpected behavior: the heterogeneous nucleation at the container walls is delayed in comparison to t…
Two-Step Crystallization Kinetics in Colloidal Hard-Sphere Systems
The crystallization kinetics of colloidal hard spheres was studied using a special Bragg spectrometer with high sensitivity. In contrast with the classical scenario we observe a two-step nucleation process: the number of crystallites increases slowly at early times, followed by a dramatic reduction at intermediate times, prior to undergoing a rapid increase at late times. We explain these results in terms of a polydispersity limited growth of crystallites, where the crystallization at early times is governed by local fractionation processes, leading to a long delay prior to final crystallization.