Cell invasion during competitive growth of polycrystalline solidification patterns.

Spatially extended cellular and dendritic array structures forming during solidification processes such as casting, welding, or additive manufacturing are generally polycrystalline. Both the array structure within each grain and the larger scale grain structure determine the performance of many structural alloys. How those two structures coevolve during solidification remains poorly understood. By in situ observations of microgravity alloy solidification experiments onboard the International Space Station, we have discovered that individual cells from one grain can unexpectedly invade a nearby grain of different misorientation, either as a solitary cell or as rows of cells. This invasion process causes grains to interpenetrate each other and hence grain boundaries to adopt highly convoluted shapes. Those observations are reproduced by phase-field simulations further demonstrating that invasion occurs for a wide range of misorientations. Those results fundamentally change the traditional conceptualization of grains as distinct regions embedded in three-dimensional space.

Analysis of gravity effects during binary alloy directional solidification by comparison of microgravity and Earth experiments with in situ observation.

Under terrestrial conditions, solidification processes are influenced to a large degree by the gravity effects such as natural convection or buoyancy force, which can dramatically modify the final characteristics of the grown solid. In the last decades, the coupling of in situ observation of growth from the melt, that enables the study of microstructure formation dynamics, and microgravity experimentation, that allows to approach diffusive conditions, has been implemented for both transparent and metallic materials. The results of these investigations enable to test the validity of advanced solidification theories, to validate or develop numerical models and sometimes to reveal unexpected phenomena. The aim of this paper is to present a selection of conclusive experiments obtained with this combined approach in our group to highlight the gravity effects by a comparative study of experiments carried out on earth and in microgravity conditions.

Oscillatory-nonoscillatory transitions for inclined cellular patterns in three-dimensional directional solidification.

The oscillatory behavior of cellular patterns produced by directional solidification of a transparent alloy under microgravity conditions was recently observed to depend on the misorientation of the main crystal axis with respect to the direction of the imposed thermal gradient [Pereda et al., Phys. Rev. E 95, 012803 (2017)2470-004510.1103/PhysRevE.95.012803]. To characterize the oscillatory-nonoscillatory transition resulting from the variations of the crystal misorientation, new experiments performed in DECLIC-DSI onboard the International Space Station and phase-field simulations are analyzed and combined in the present study. Experimental results are extracted from movies showing regions that extend on both sides of a boundary between two grains with respective misorientations of roughly 3 and 7 degrees. A set of tools are developed to analyze the experimental data and the same analysis is reproduced for the numerical data. A number of points are addressed in the simulations, like the effects of the system dimensions. The oscillatory state is found to be favored by the increase of the geometrical degrees of freedom. In bulk samples, a good agreement is found between the experimental and the numerical oscillatory-nonoscillatory threshold given by the ratio of the drift time to the oscillation period at the transition. The existence and the origin of bursts of localized groups of oscillating cells within a globally nonoscillatory pattern are characterized. A qualitative description of the physical mechanism that governs the oscillatory-nonoscillatory transition is provided.

Temperature and solvent effects in the solubility of some pharmaceutical compounds: Measurements and modeling.

In this work, pure solvent solubilities of drugs, such as paracetamol, allopurinol, furosemide and budesonide, measured in the temperature range between 298.2-315.2K are presented. The solvents under study were water, ethanol, acetone, ethyl acetate, carbon tetrachloride and n-hexane. Measurements were performed using the shake-flask method for generating the saturated solutions followed by compositional analysis by HPLC. Previous literature values on the solubilities of paracetamol were used to assess the experimental methodology employed in this work. No literature data was found for any of the other drugs studied in this assay. Melting properties of the pure drugs were also determined by differential scanning calorimetry (DSC) to provide a broader knowledge about the solubilization process and also for modeling purposes. The solubility data as a function of temperature were used to determine the thermodynamic properties of dissolution like, Gibbs energy, enthalpy and entropy. Theoretical work was essentially focused on the evaluation of the Nonrandom Two-Liquid Segment Activity Coefficient (NRTL-SAC) model, which has been referred as a simple and practical thermodynamic framework for drug solubility estimation. A satisfactory agreement was found between experimental and calculated values: the absolute average deviation was 68% for the correlation in the organic solvents and 38% for the prediction in water, where the best results in prediction could be related to the selected solvents.

Solubilities of biologically active phenolic compounds: measurements and modeling.

Aqueous solubilities of natural phenolic compounds from different families (hydroxyphenyl, polyphenol, hydroxybenzoic, and phenylpropenoic) were experimentally obtained. Measurements were performed on tyrosol and ellagic, protocatechuic, syringic, and o-coumaric acids, at five different temperatures (from 288.2 to 323.2 K), using the standard shake-flask method, followed by compositional analysis using UV spectrophotometry. To verify the accuracy of the spectrophotometric method, some data points were measured by gravimetry, and in general, the values obtained with the two methods are in good agreement (deviations lower than 11%). To adequately understand the solubilization process, melting properties of the pure phenolics were obtained by differential scanning calorimetry (DSC), and apparent acid dissociation constants were measured by potentiometry titration. The aqueous solubilities followed the expected general exponential trend. The melting temperatures did not follow the same solubility tendency, and for tyrosol and ellagic acid, not only the size and extent of hydrogen bonding, but also the energy associated with their crystal structures, determine the solubility. For these binary systems, acid dissociation is not important. Approaches for modeling the measured data were evaluated. These included an excess Gibbs energy equation, the modified UNIQUAC model, and the cubic-plus-association (CPA) equation of state. Particularly for the CPA approach, a new methodology that explicitly takes into account the number and nature of the associating sites and the prediction of the pure-component parameters from molecular structure is proposed. The results indicate that these are appropriate tools for representing the water solubilities of these molecules.