Percolation of diffusionally evolved two-phase systems

Evolution of Polymer Colloid Structure During Precipitation and Phase Separation

Polymer colloids arise in a variety of contexts ranging from synthetic to natural systems. The structure of polymeric colloids is crucial to their function and application. Hence, understanding the mechanism of structure formation in polymer colloids is important to enabling advances in their production and subsequent use as enabling materials in new technologies. Here, we demonstrate how the specific pathway from precipitation to vitrification dictates the resulting morphology of colloids fabricated from polymer blends. Through continuum simulations, free energy calculations, and experiments, we reveal how colloid structure changes with the trajectory taken through the phase diagram. We demonstrate that during solvent exchange, polymer-solvent phase separation of a homogeneous condensate can precede polymer-polymer phase separation for blends of polymers that possess some degree of miscibility. For less-miscible, higher-molecular-weight blends, phase separation and kinetic arrest compete to determine the final morphology. Such an understanding of the pathways from precipitation to vitrification is critical to designing functional structured polymer colloids.

Amyloid-β Interactions with Lipid Rafts in Biomimetic Systems: A Review of Laboratory Methods

Biomimetic lipid bilayer systems are a useful tool for modeling specific properties of cellular membranes in order to answer key questions about their structure and functions. This approach has prompted scientists from all over the world to create more and more sophisticated model systems in order to decipher the complex lateral and transverse organization of cellular plasma membranes. Among a variety of existing biomembrane domains, lipid rafts are defined as small, dynamic, and ordered assemblies of lipids and proteins, enriched in cholesterol and sphingolipids. Lipid rafts appear to be involved in the development of Alzheimer's disease (AD) by affecting the aggregation of the amyloid-β (Aβ) peptide at neuronal membranes thereby forming toxic oligomeric species. In this review, we summarize the laboratory methods which allow to study the interaction of Aβ with lipid rafts. We describe step by step protocols to form giant (GUVs) and large unilamellar vesicles (LUVs) containing raft-mimicking domains surrounded by membrane nonraft regions. Using fluorescence microscopy GUV imaging protocols, one can design experiments to visualize micron-scale raft-like domains, to determine the micron-scale demixing temperature of a given lipid mixture, construct phase diagram, and photogenerate domains in order to assess the dynamics of raft formation and raft size distribution. LUV fluorescence spectroscopy protocols with proper data analysis can be used to measure molecular packing of raft/nonraft regions of the membrane, to report on nanoscale raft formation and determine nanoscale demixing temperature. Because handling of the Aβ requires dedicated laboratory experience, we present illustrated protocols for Aβ-stock aliquoting, Aβ aqueous solubilization, oligomer preparation, determination of the Aβ concentration before and after filtration. Thioflavin binding, dynamic light scattering, and transmission electron microscopy protocols are described as complementary methods to detect Aβ aggregation kinetics, aggregate sizes, and morphologies of observed aggregates.

Datasets for the microstructure of nanoscale metal network structures and for its evolution during coarsening

The datasets in this work are files containing atom position coordinates of volume elements approximating nanoporous gold made by dealloying and annealing. The material is represented in an as-prepared state and in various stages of coarsening, as described in Phys. Rev. Mater, 3 (2019) 076001. Realistic initial structures of different solid fractions have been constructed by the leveled-wave algorithm, approximating mixtures at the end of early-stage spinodal decomposition. The microstructural evolution during coarsening by surface diffusion was approximated by on-lattice kinetic Monte-Carlo simulation. The data sets refer to solid fractions from 0.22 to 0.50, providing for different initial connectivity of the bicontinuous structures. Coarsening at two temperatures, 900 K and 1800 K, explores two different degrees of surface energy anisotropy - more faceted at 900 K and more rough at 1800 K. Each structure takes the form of a face-centred cubic lattice with approximately 32 million sites. A site can be occupied by either void or atom. 3D periodic boundary conditions are satisfied. Tables list each structure's properties, and specifically the specific surface area, two different measures for the ligament size, the net topological genus as well as the scaled genus. The atom coordinate files may serve as the basis for geometry analysis and for atomistic as well as finite element simulation studies of nanoporous as well as spinodally decomposed materials. The data sets are accessible via the TORE repository at http://hdl.handle.net/11420/3253.

Impact of phase separation morphology on release mechanism of amorphous solid dispersions

Amorphous solid dispersions (ASDs) can phase separate in the gel phase during dissolution, lowering the chemical potential and thus the driving force for drug release. The purpose of this study is to explore the connection between amorphous phase separation in the hydrated ASD and its resulting release rate. Poorly soluble model compounds - indomethacin (IND) and ritonavir (RTV) - were formulated as ASDs using PVP as carrier. Rotating disk dissolution studies with varying drug loading levels of IND-PVP and RTV-PVP showed that the drug release was fastest at an intermediate drug loading level. This was in part due to faster erosion of the ASD at lower drug loading levels. More interestingly, at low drug loading levels, PVP and the drug co-eroded, while at high drug loading levels, PVP was released preferentially. In the case of RTV-PVP, the loading level corresponding to this transition was correlated with the change in phase separation morphology as probed by confocal fluorescence imaging studies. At low drug loading levels, the hydrophobic domains were discrete domains while at high drug loading levels, hydrophobic domains were continuous. Our results suggest that at low drug loadings, release is mediated by erosion of the polymer along with embedded drug rich droplets, whereas at high drug loadings, formation of a drug-rich domain continuous morphology leads to preferential release of the polymer-rich domains. The transition from hydrophobic discrete to hydrophobic continuous morphology occurs at the percolation threshold. We discuss the two mechanisms of phase separation and its impact on the drug release from ASDs in the context of the ternary phase diagram.

Phase-Field Based Multiscale Modeling of Heterogeneous Solid Electrolytes: Applications to Nanoporous Li3PS4

Modeling the effective ion conductivities of heterogeneous solid electrolytes typically involves the use of a computer-generated microstructure consisting of randomly or uniformly oriented fillers in a matrix. However, the structural features of the filler/matrix interface, which critically determine the interface ion conductivity and the microstructure morphology, have not been considered during the microstructure generation. Using nanoporous β-Li₃PS₄ electrolyte as an example, we develop a phase-field model that enables generating nanoporous microstructures of different porosities and connectivity patterns based on the depth and the energy of the surface (pore/electrolyte interface), both of which are predicted through density functional theory (DFT) calculations. Room-temperature effective ion conductivities of the generated microstructures are then calculated numerically, using DFT-estimated surface Li-ion conductivity (3.14 × 10^-3 S/cm) and experimentally measured bulk Li-ion conductivity (8.93 × 10^-7 S/cm) of β-Li₃PS₄ as the inputs. We also use the generated microstructures to inform effective medium theories to rapidly predict the effective ion conductivity via analytical calculations. When porosity approaches the percolation threshold, both the numerical and analytical methods predict a significantly enhanced Li-ion conductivity (1.74 × 10^-4 S/cm) that is in good agreement with experimental data (1.64 × 10^-4 S/cm). The present phase-field based multiscale model is generally applicable to predict both the microstructure patterns and the effective properties of heterogeneous solid electrolytes.

Observing the morphology of single-layered embedded silicon nanocrystals by using temperature-stable TEM membranes

We use high-temperature-stable silicon nitride membranes to investigate single layers of silicon nanocrystal ensembles by energy filtered transmission electron microscopy. The silicon nanocrystals are prepared from the precipitation of a silicon-rich oxynitride layer sandwiched between two SiO2 diffusion barriers and subjected to a high-temperature annealing. We find that such single layers are very sensitive to the annealing parameters and may lead to a significant loss of excess silicon. In addition, these ultrathin layers suffer from significant electron beam damage that needs to be minimized in order to image the pristine sample morphology. Finally we demonstrate how the silicon nanocrystal size distribution develops from a broad to a narrow log-normal distribution, when the initial precipitation layer thickness and stoichiometry are below a critical value.

Making a tool of an artifact: the application of photoinduced Lo domains in giant unilamellar vesicles to the study of Lo/Ld phase spinodal decomposition and its modulation by the ganglioside GM1

Electroformed giant unilamellar vesicles containing liquid-ordered Lo domains are important tools for the modeling of the physicochemical properties and biological functions of lipid rafts. Lo domains are usually imaged using fluorescence microscopy of differentially phase-partionioning membrane-embedded probes. Recently, it has been shown that these probes also have a photosensitizing effect that leads to lipid chemical modification during the fluorescence microscopy experiments. Moreover, the lipid reaction products are able as such to promote Lo microdomain formation, leading to potential artifacts. We show here that this photoinduced effect can also purposely be used as a new approach to study Lo microdomain formation in giant unilamellar vesicles. Photosensitized lipid modification can promote Lo microdomain appearance and growth uniformly and on a faster time scale, thereby yielding new information on such processes. For instance, in egg phosphatidylcholine/egg sphingomyelin/cholesterol 50:30:20 (mol/mol) giant unilamellar vesicles, photoinduced Lo microdomain formation appears to occur by the rarely observed spinodal decomposition process rather than by the common nucleation process usually observed for Lo domain formation in bilayers. Moreover, temperature and the presence of the ganglioside GM1 have a profound effect on the morphological outcome of the photoinduced phase separation, eventually leading to features such as bicontinuous phases, phase percolation inversions, and patterns evoking double phase separations. GM1 also has the effect of destabilizing Lo microdomains. These properties may have consequences for Lo nanodomains stability and therefore for raft dynamics in biomembranes. Our data show that photoinduced Lo microdomains can be used to obtain new data on fast raft-mimicking processes in giant unilamellar vesicles.