Time-resolved rheology on complex fluids

The aim of this work was to explore the potential of a combined rheology and X-ray scattering approach on complex fluids. Shear rates between 0.9·10$^5$ s$^{−1}$ and 5.6·10$^5$ s$^{−1}$, which are magnitudes higher than found in classical rheometry studies, were applied to a suspension of colloidal silica nanoparticles by a microfluidic jet device. Characteristic structure formation was studied along and across the flow direction with small angle X-ray scattering. The anisotropy of the diffraction patterns was evaluated by X-ray cross-correlation analysis. Furthermore, the decay of the shear-induced ordering after the cessation of the shear was quantified. With particle sizes of r=15 nm-76.5 nm Péclet numbers of 1 to 1162 were investigated, a dynamic regime where diffusive motion is dominated by shear-dominated dynamics. For different Rayleigh nozzle sizes and geometries characteristic decay times between 25 $\mu$s and 495 $\mu$s were measured and correlated with the Péclet number of the system.The influence of electro-static forces was investigated by adding salt to the colloidal suspension, which reduced the overall ordering. The impact of the particle charge on the effective screening of the particles and the ionic strength of the suspension were explored. By modeling string-like particle distributions and comparison with the corresponding diffraction patterns and the measured shape asymmetry, it was possible to determine a variation of the volume fraction over the azimuthal angle of $\approx\pm$5% for the maximum ordered state in the jet. This interpretation was in good agreement with rescaled mean spherical approximation modeling

Plasmonic Supercrystals with a Layered Structure Studied by a Combined TEM-SAXS-XCCA Approach

Supercrystals composed of plasmonic nanoparticles constitute a promising material platform for tailored light‐matter interactions. The optimization of their optical properties requires precise syntheses and a detailed structural characterization that addresses not only the basic geometrical parameters but also the degree of order. Herein, plasmonic supercrystals with a well‐defined layered structure are studied by a combined transmission electron microscopy, small‐angle X‐ray scattering and X‐ray cross‐correlation analysis (TEM‐SAXS‐XCCA) approach. It is demonstrated that scanning small‐angle x‐ray scattering (SAXS) data can unambiguously be assigned to the number of crystalline layers by comparison with complementary transmission electron microscopy (TEM) experiments on the same regions of interest. A small but significant increase of the lattice constant with increasing number of layers and a high degree of orientational order irrespective of the number of layers is found. This points to specifics of the supercrystal formation mechanism that could be utilized to improve the control of self‐assembly for supercrystal geometries with subnanometer precision

Kinetics of pressure-induced nanocrystal superlattice formation

Colloidal nanocrystals (NC) are known to self-organize into superlattices that promise many applications ranging from medicine to optoelectronics. Recently, the formation of high-quality PEGylated gold NC was reported at high hydrostatic pressure and high salt concentrations. Here, we study the formation kinetics of these superlattices after pressure jumps beyond their crystallisation pressure by means of small-angle X-ray scattering with few ms experimental resolution. The timescale of NC formation was found to be reduced the larger the width of the pressure jump. This is connected to an increase of crystal quality, i.e., the faster the NC superlattice forms, the better the crystal quality. In contrast to the formation kinetics, the melting of the NC superlattice is approximately one order of magnitude slower and shows linear kinetics

Three-step colloidal gelation revealed by time-resolved X-ray photon correlation spectroscopy

The gelation of PEGylated gold nanoparticles dispersed in a glycerol–water mixture is probed in situ by x-ray photon correlation spectroscopy. Following the evolution of structure and dynamics over 10$^4$ s, a three-step gelation process is found. First, a simultaneous increase of the Ornstein–Zernike length $ξ$ and slowdown of dynamics is characterized by an anomalous q-dependence of the relaxation times of $τ ∝ q^{−6}$ and strongly stretched intermediate scattering functions. After the structure of the gel network has been established, evidenced by a constant $ξ$, the dynamics show aging during the second gelation step accompanied by a change toward ballistic dynamics with $τ ∝ q^{−1}$ and compressed correlation functions. In the third step, aging continues after the arrest of particle motion. Our observations further suggest that gelation is characterized by stress release as evidenced by anisotropic dynamics once gelation sets in

Supercrystal Formation of Gold Nanorods by High Pressure Stimulation

We demonstrate the pressure-induced formation of supercrystalsmade from PEGylated gold nanorods (NRs) in aqueous suspension.Utilizing the combined effect of hydrostatic pressure and salt on the solubilityof the organic poly(ethylene glycol) (PEG) shell that passivates the NRs, thereversible formation of two-dimensional hexagonal supercrystals has beenobserved by means of small-angle X-ray scattering. The pressure dependenceof the crystal lattice’s structural parameters is determined. By time-resolvedmeasurements performed after a pressure jump, the growth process of thecrystals is found to be completed already after a few seconds. The presented results demonstrate that by PEGylatingnanoparticles, pressure-induced homogeneous supercrystals can be formed for different particle shapes, in particular, anisotropicNRs, which determine the resulting lattice type

Shear-induced ordering in liquid microjets seen by x-ray cross correlation analysis

We applied shear to a silica nanoparticle dispersion in a microfluidic jet device and observed direction-dependent structure along and across the flow direction. The asymmetries of the diffraction patterns were evaluated by x-ray cross correlation analysis. For different Rayleigh nozzle sizes and shapes, we measured the decay of the shear-induced ordering after the cessation of the shear. At large tube sizes and small shear rates, the characteristic times of the decay become longer, but Péclet-weighted times do not scale linearly with Péclet numbers. By modeling particle distributions with the corresponding diffraction patterns and comparing measured shape asymmetry to simulations, we determined the variation of volume fraction over the azimuthal angle for the maximum ordered state in the jet

Dynamics and Timescales of Higher Order Correlations in Hard Sphere Systems in Supercooled Colloidal Systems

The dynamics and time scales of higher-order correlations are studied in supercooled colloidal systems. A combination of X-ray photon correlation spectroscopy (XPCS) and X-ray cross-correlation analysis (XCCA) shows the typical slowing of the dynamics of a hard sphere system when approaching the glass transition. The time scales of higher-order correlations are probed using a novel time correlation function g$_C$, tracking the time evolution of cross-correlation function C. With an increasing volume fraction, the ratio of relaxation times of g$_C$ to the standard individual particle relaxation time obtained by XPCS increases from ∼0.4 to ∼0.9. While a value of ∼0.5 is expected for free diffusion, the increasing values suggest that the local orders within the sample are becoming more long-lived for larger volume fractions. Furthermore, the dynamics of local order is more heterogeneous than the individual particle dynamics. These results indicate that not only the presence but also the lifetime of locally favored structures increases close to the glass transition