Dynamic effects of nonequilibrium solvation: potential and free energy surfaces for Z/E isomerization in solvent-solute coordinates

A novel definition of a solvent coordinate associated with a given reaction is formulated in terms of molecular-dynamic trajectories of the solvent and is applied to discuss the topography of potential energy and free energy surfaces of model liquid phase Z/E isomerization reactions in solvent-solute coordinates. It is shown that the arrangement of the reactant and product valleys on these surfaces can vary from consecutive to parallel, depending on the strength of the solvent-solute interactions.     

Using light scattering to evaluate the separation of polydisperse nanoparticles

The analysis of natural and otherwise complex samples is challenging and yields uncertainty about the accuracy and precision of measurements. Here we present a practical tool to assess relative accuracy among separation protocols for techniques using light scattering detection. Due to the highly non-linear relationship between particle size and the intensity of scattered light, a few large particles may obfuscate greater numbers of small particles. Therefore, insufficiently separated mixtures may result in an overestimate of the average measured particle size. Complete separation of complex samples is needed to mitigate this challenge. A separation protocol can be considered improved if the average measured size is smaller than a previous separation protocol. Further, the protocol resulting in the smallest average measured particle size yields the best separation among those explored. If the differential in average measured size between protocols is less than the measurement uncertainty, then the selected protocols are of equivalent precision. As a demonstration, this assessment metric is applied to optimization of cross flow (V_x) protocols in asymmetric flow field flow fractionation (AF⁴) separation interfaced with online quasi-elastic light scattering (QELS) detection using mixtures of polystyrene beads spanning a large size range. Using this assessment metric, the V_x parameter was modulated to improve separation until the average measured size of the mixture was in statistical agreement with the calculated average size of particles in the mixture. While we demonstrate this metric by improving AF⁴V_x protocols, it can be applied to any given separation parameters for separation techniques that employ dynamic light scattering detectors.     

ARC-length method for differential equations

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A physico-mechanical approach to modeling of metal forming processes—Part II: damage analysis in processes with plastic flow of metals

A damage analysis is presented for the extrusion of a case-shaped cylindrical part by using a physico-mechanical approach for modeling metal forming processes. Two integral measures related to the hydrostatic and deviatoric parts of the damage tensor are used for the calculation of strain damage. The combined use of two damage measures in contrast to only one allows us to assess not only a risk of macro-fracture of the deformed material but also the stage of formation of large cavernous defects due to coalescence of ellipsoidal voids. Such a refined prediction of the actual quality of the material’s micro-structure is important when producing metalware that is supposed to operate under intense loading and thermal conditions. In case study of this paper the kinetic equations of damage are solved by using mutually consistent fields of stresses, flow velocities, and strains. It is shown that the predicted damage is less than its permissible value since a high hydrostatic pressure in the plastic zone heals the micro-defects, prevents their growth, and, thereby, increases the processing ductility of deformed metals during extrusion.      

Phase Transitions in UHMWPE Fiber Compacts Studied by in situ Synchrotron Microbeam WAXS

Summary: The temperature dependence of the structure of either cross‐linked or non‐cross‐linked ultra‐high‐molecular‐weight polyethylene (UHMWPE) fiber compacts was studied by synchrotron microbeam wide‐angle X‐ray scattering (WAXS), focusing on the fiber‐fiber interface. The phase transition sequence is: melting of the monoclinic phase in the fiber skin, which was completed by 135 °C; melting of the unconstrained orthorhombic phase, by 152 °C; melting of the constrained orthorhombic phase and a orthorhombic‐hexagonal phase transition until 157 °C; and gradual melting of the hexagonal phase, up to 165 °C. Cross‐linking provides additional thermal stabilization.

Histograms of the azimuthally averaged X‐ray intensity as a function of temperature for cross‐linked ultra‐high‐molecular‐weight polyethylene fiber compacts molded at 145 °C.     

High‐Throughput Synthesis and Screening of Combinatorial Heterogeneous Catalyst Libraries

In less than one minute the catalytic activity and selectivity of a single catalyst was measured in combinatorial libraries of ternary Rh‐Pd‐Pt‐Cu alloys. Only slightly more than two hours were needed to complete a library with 136 elements. The elements of the libraries (ca. 2–4 μg of material) are contained in a two‐dimensional array synthesized by a thin‐film technique. The analysis was performed by a scanning mass spectrometer (see picture).