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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Origins of anisotropic transport in the electrically switchable antiferromagnet Fe 1/3 NbS 2

Recent experiments on the antiferromagnetic intercalated transition metal dichalcogenide Fe 1/3 NbS 2 have demonstrated reversible resistivity switching by application of orthogonal current pulses below its magnetic ordering temperature, making Fe 1/3 NbS 2 promising for spintronics applications. Here, we perform density functional theory calculations with Hubbard U corrections of the magnetic order, electronic structure, and transport properties of crystalline Fe 1/3 NbS 2 , clarifying the origin of the different resistance states. The two experimentally proposed antiferromagnetic ground states, corresponding to in-plane stripe and zigzag ordering, are computed to be nearly degenerate. In-plane cross sections of the calculated Fermi surfaces are anisotropic for both magnetic orderings, with the degree of anisotropy sensitive to the Hubbard U value. The in-plane resistance, computed within the Kubo linear response formalism using a constant relaxation time approximation, is also anisotropic, supporting a hypothesis that the current-induced resistance changes are due to a repopulating of antiferromagnetic domains. Our calculations indicate that the transport anisotropy of Fe 1/3 NbS 2 in the zigzag phase is reduced relative to stripe, consistent with the relative magnitudes of resistivity changes in experiment. Finally, our calculations reveal the likely directionality of the current-domain response, specifically, which domains are energetically stabilized for a given current direction.

36 MATERIALS SCIENCE↗
DOE OSTI2022

On the Dynamics of the Carbon–Bromine Bond Dissociation in the 1-Bromo-2-Methylnaphthalene Radical Anion

This paper studies the mechanism of electrochemically induced carbon–bromine dissociation in 1-Br-2-methylnaphalene in the reduction regime. In particular, the bond dissociation of the relevant radical anion is disassembled at a molecular level, exploiting quantum mechanical calculations including steady-state, equilibrium and dissociation dynamics via dynamic reaction coordinate (DRC) calculations. DRC is a molecular-dynamic-based calculation relying on an ab initio potential surface. This is to achieve a detailed picture of the dissociation process in an elementary molecular detail. From a thermodynamic point of view, all the reaction paths examined are energetically feasible. The obtained results suggest that the carbon halogen bond dissociates following the first electron uptake follow a stepwise mechanism. Indeed, the formation of the bromide anion and an organic radical occurs. The latter reacts to form a binaphthalene intrinsically chiral dimer. This paper is respectfully dedicated to Professors Anny Jutand and Christian Amatore for their outstanding contribution in the field of electrochemical catalysis and electrosynthesis.

1-bromo-2-methylnaphthalene↗
DOE OSTI2024

Ligand-modified nanoparticle surfaces influence CO electroreduction selectivity

Improving the kinetics and selectivity of CO 2 /CO electroreduction to valuable multi-carbon products is a challenge for science and is a requirement for practical relevance. Here we develop a thiol-modified surface ligand strategy that promotes electrochemical CO-to-acetate. We explore a picture wherein nucleophilic interaction between the lone pairs of sulfur and the empty orbitals of reaction intermediates contributes to making the acetate pathway more energetically accessible. Density functional theory calculations and Raman spectroscopy suggest a mechanism where the nucleophilic interaction increases the sp 2 hybridization of CO (ad) , facilitating the rate-determining step, CO* to (CHO)*. We find that the ligands stabilize the (HOOC–CH 2 )* intermediate, a key intermediate in the acetate pathway. In-situ Raman spectroscopy shows shifts in C–O, Cu–C, and C–S vibrational frequencies that agree with a picture of surface ligand-intermediate interactions. A Faradaic efficiency of 70% is obtained on optimized thiol-capped Cu catalysts, with onset potentials 100 mV lower than in the case of reference Cu catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Hybrid-Supported Bilayers Formed with Mixed-Charge Surfactants on C 18 -Functionalized Silica Surfaces

Mixtures of cationic-anionic surfactants have been demonstrated to spontaneously form ordered monolayers at hydrophobic-hydrophilic boundaries including air-water and oil-water interfaces. Here, confocal Raman microscopy is used to investigate the structure of hybrid-supported surfactant bilayers (HSSBs) formed by deposition of a distal leaflet of mixed cationic-anionic surfactants onto a proximal leaflet of n-alkane (C 18 ) chains on the interior surfaces of chromatographic silica particles. The surface coverage of the two surfactants in a hybrid bilayer was determined from carbon analysis and the relative Raman scattering of their respective head-groups. Within the measurement uncertainty, the stoichiometric ratio of the two surfactants is one-to-one, equivalent to mixed-charge-surfactant monolayers at air-water and oil-water interfaces and consistent with the role of the head-group electrostatic interactions in their formation. When self-assembled on the hydrophobic surface, pairs of oppositely-charged n-alkyl-chain surfactants resemble a phospholipid (phosphati¬dylcholine) molecule, with its zwitterionic head group and two hydrophobic acyl-chain tails. Indeed, the structure of these hybrid-supported surfactant bilayers on C 18 -modified silica surfaces is similar to that of hybrid-supported lipid bilayers (HSLBs) on the same supports, but with denser and more-ordered n-alkyl chains. Hybrid-supported surfactant bilayers exhibit a melting phase transition (gel to liquid-crystalline phase) with structural and energetic characteristics similar to hybrid-supported bilayers prepared from a zwitterionic phospholipid of the same alkyl chain length. These mixed-charge surfactants on n-alkane modified silica are stable in water over time (months), results that suggest the potential use of these hybrid bilayers for generating supported lipid bilayer-like surfaces or for separation applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Reaction Mechanism of Vapor-Phase Formic Acid Decomposition over Platinum Catalysts: DFT, Reaction Kinetics Experiments, and Microkinetic Modeling

A combination of periodic density functional theory (DFT, PW91-GGA) calculations, reaction kinetics experiments, and mean-field microkinetic modeling is used to derive insights on the reaction mechanism and determine the nature of the active site under reaction conditions for the vapor-phase decomposition of formic acid (FA, HCOOH) over Pt/C catalysts. Microkinetic models formulated using DFT energetics derived on the clean Pt(100) and Pt(111) required large parameter adjustments to reproduce the experimentally measured apparent activation energies and reaction orders. Further, these models predicted high surface coverage of adsorbed carbon monoxide (CO*), inconsistent with the environment of the active site in the DFT calculations on the clean surfaces. Consequently, we reperformed DFT calculations for the entire reaction network on partially CO*-covered (4/9 monolayer, ML) Pt(111) and Pt(100). The resultant microkinetic models, with thermochemistry and kinetics explicitly dependent on CO* coverage, were able to reproduce the experimentally determined activation energies and reaction orders, in addition to being self-consistent in CO* coverage. Our results suggest that Pt(100) is likely poisoned by CO* under typical reaction conditions and does not contribute significantly to the experimentally observed reactivity. Instead, we find that Pt(111) better represents the active site for FA decomposition reaction on Pt/C catalysts. The optimized model on 4/9 ML CO*-covered Pt(111) suggests that the reaction occurs via the carboxyl (COOH*) intermediate and that the spectator CO*-assisted pathways play a significant role under reaction conditions. Lastly, this study underscores the importance of spectator species on the energetics and the mechanism of a catalytic reaction and their key role in developing a model that better addresses the nature of the active site under realistic catalytic reaction conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Inverse design of pore wall chemistry and topology through active learning of surface group interactions

Design of next-generation membranes requires a nanoscopic understanding of the effect of biologically inspired heterogeneous surface chemistries and topologies (roughness) on local water and solute behavior. In particular, the rejection of small, neutral solutes, such as boric acid, poses a heretofore unsolved challenge. In prior work, a computational inverse design technique using an evolutionary optimization successfully uncovered new surface design strategies for optimized transport of water over solutes in smooth, model pores consisting of two surface chemistries. However, extending such an approach to more complex (and realistic) scenarios involving many surface chemistries as well as surface roughness is challenging due to the expanded design space. In this work, we develop a new approach that uses active learning to optimize in a reduced feature space of surface group interactions, finding parameters that lead to their assembly into ordered, optimal patterns. This approach rapidly identifies novel surface functionalizations that maximize the difference in water and boric acid transport through the nanopore. Moreover, we find that the roughness of the nanopore wall, independent of its chemistry, can be leveraged to enhance transport selectivity: oscillations in the pore wall diameter optimally inhibit boric acid transport by creating energetic wells from which the solute must escape to transport down the pore. Furthermore, this proof-of-concept demonstrates the potential for active learning strategies, in concert with molecular simulations, to rapidly navigate complex design spaces of aqueous interfaces and is promising as a tool for engineering water-mediated surface interactions for a broad range of applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Fast Correction of Errors in the DFT‐Calculated Energies of Gaseous Nitrogen‐Containing Species

Abstract Modeling adsorption phenomena on surfaces by DFT calculations often involves substantial errors, resulting in inaccurate predictions of catalytic activities. Such errors partly stem from the inaccurate description of the energetics of free molecules. Herein, we use a semiempirical group‐additivity method to correct the DFT‐calculated heats of formation of 106 carbon‐ and nitrogen‐containing gaseous compounds belonging to 15 different chemical families. PBE, PW91, RPBE and BEEF‐vdW initially yield mean absolute errors (MAEs) with respect to experiments in the range of 0.32–0.75 eV. After correcting the systematic errors, the overall MAEs decrease to ∼0.05 eV. Additionally, upon applying the corrections to three types of reaction enthalpies, the resulting MAEs are below 0.10 eV. These functional‐group corrections can be used in (electro)catalysis to correct the gas‐phase references necessary to evaluate equilibrium potentials and adsorption energies, predict error cancellation, and assess conflicting experimental data.

Urrego‐Ortiz, Ricardo↗
DOE OSTI2026

Ammonolysis with N 2 -diluted NH 3 suppresses Ta( IV ) defects in BaTaO 2 N and enhances photocatalytic water oxidation

BaTaO 2 N stands out among oxynitride photocatalysts because of its ability to capture visible light and to drive the photoelectrochemical water oxidation reaction. However, its solar energy conversion performance is limited by electron–hole recombination at Ta( IV ) defects in the material. These defects are formed by overreduction of the Ta(v) oxide precursor by excess ammonia under the high temperature conditions during ammonolysis. Here we show for the first time that Ta( IV ) defect concentrations can be lowered by conducting the ammonolysis reaction in mixed NH 3 /N 2 gas. The obtained BaTaO 2 N samples crystallize in the cubic CaTiO 3 structure type and form 200–300 nm faceted nanocrystals, based on X-ray diffraction, scanning electron microscopy, and HRTEM. Electron paramagnetic resonance spectra observe the Ta( IV ) defects at g = 1.999 and confirm an 11-fold reduction for the product synthesized in mixed (0.13 : 1.0 vol) NH 3 /N 2 gas, equivalent to 1.14 × 10 16 cm −3 Ta( IV ) ions. This optimized BaTaO 2 N has nearly twice the photocatalytic oxygen evolution activity (AQE of 6.78% at 400 nm) of a reference material made with 1.0 atm ammonia and 78% higher photoelectrochemical water oxidation photocurrent (0.9 mA cm −2 at 1.23 V vs. RHE) under simulated sunlight. According to X-ray photoelectron spectroscopy, remaining Ta( IV ) defects are concentrated in the surface region of the BaTaO 2 N particles, where >50% of all Ta ions are found in the +4 oxidation state. This surface Ta( IV ) population can be directly observed in Vibrating Kelvin Probe Surface Photovoltage Spectra (VK-SPV) via its 1.2–1.4 eV photovoltage onset. Here, it suggests that the surface Ta( IV ) ions contribute empty d-states 0.5–0.7 eV below the BaTaO 2 N conduction band edge. These findings highlight how the energetics and concentrations of Ta( IV ) defects influence the photoelectrochemical water oxidation ability of BaTaO 2 N. Additionally, the work establishes ammonolysis with diluted NH 3 as a new tool to minimize defects in BaTaO 2 N and to raise its solar energy conversion efficiency toward its theoretical limit. Because of its simplicity, the reduced ammonia pressure strategy will likely be applicable to other oxynitrides, which generally suffer from overreduction problems during ammonolysis.

Salmanion, Mahya [University of California, Davis,↗
DOE OSTI2022

Rotational Dynamics and Transition Mechanisms of Surface-Adsorbed Proteins

Assembly of biomolecules at solid-water interfaces requires molecules to traverse complex orientation-dependent energy landscapes through processes that are poorly understood, largely due to the dearth of in-situ single molecule measurements and statistical analyses of the rotational dynamics that define directional selection. Emerging capabilities in high-speed atomic force microscopy and machine learning have allowed us to directly determine the orientational energy landscape and observe and quantify the rotational dynamics for protein nanorods on the surface of muscovite mica under a variety of conditions. Comparisons with kinetic Monte Carlo simulations show that the transition rates between adjacent orientation-specific energetic minima can largely be understood through traditional models of in-plane Brownian rotation across a biased energy landscape, with resulting transition rates that are exponential in the energy-barriers between states. However, transitions between more distant angular states are decoupled from barrier height, with jump-size distributions showing a power-law decay that is characteristic of a non-classical Levy-flight random walk, indicating that large jumps are enabled by alternative modes of motion via activated states. The findings provide new insights into the dynamics of biomolecules at solid-liquid interfaces that lead to self-assembly, epitaxial matching and other orientationally anisotropic outcomes and define a general procedure for exploring such dynamics with implications for hybrid biomolecular-inorganic materials design.

Orientational energy landscapes, Rotational dynami↗
DOE OSTI2024

Wall heating by subcritical energetic electrons generated by the runaway electron avalanche source *

Abstract Subcritical energetic electrons (SEEs) produced by the runaway electron (RE) avalanche source at energies below the runaway threshold are found to be the primary contributor to surface heating of plasma-facing components (PFCs) during final loss events. This finding is supported by theoretical analysis, computational modeling with the Kinetic Orbit Runaway electrons Code (KORC), and qualitative agreement with DIII-D experimental observations. The avalanche source generates significantly more secondary electrons below the runaway threshold, which thermalize rapidly when well-confined. However, during a final loss event, the RE beam impacts the first wall, and SEEs are deconfined before they can thermalize. Additionally, because the energy deposition length decreases faster than energy, the deposited energy density, and thus the maximum PFC surface temperature change, is larger for SEEs than REs. KORC simulations employ an analytic first wall to model particle deconfinement onto a non-axisymmetric wall composed of individual tiles. PFC surface heating is calculated using a 1D model extended to include an energy-dependent deposition length scale. Simulations of DIII-D qualitatively agree with infrared (IR) imaging only when SEEs from the avalanche source are included. These results demonstrate that SEEs are the dominant contributor to PFC surface heating and indicate that the avalanche source plays a critical role in the PFC damage caused during final loss events. The prominence of SEEs also has important implications for interpreting IR imaging, one of the primary diagnostics for RE-wall interaction diagnosis, despite REs dominating the energy and current density. This result improves predictions of wall damage due to post-disruption REs to estimate material lifetime and design RE mitigation systems for ITER and future reactors.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Determining the Adsorption Energetics of 2,3-Butanediol on RuO 2 (110): Coupling First-Principles Calculations With Global Optimizers

As climate change continues to pose a threat to the Earth due to the disrupted carbon cycles and fossil fuel resources remain finite, new sources of sustainable hydrocarbons must be explored. 2,3-butanediol is a potential source to produce butene because of its sustainability as a biomass-derived sugar. Butene is an attractive product because it can be used as a precursor to jet fuel, categorizing this work in the alcohol-to-jet pathway. While studies have explored the conversion of 2,3-butanediol to butene, little is understood about the fundamental reaction itself. We quantify the energetics for three pathways that were reported in the literature in the absence of a catalyst. One of these pathways forms a 1,3-butadiene intermediate, which is a highly exothermic process and thus is unlikely to occur since 2,3-butanediol likely gets thermodynamically trapped at this intermediate. We further determined the corresponding energetics of 2,3-butanediol adsorption on an ensemble of predetermined binding sites when it interacts with a defect-free stoichiometric RuO 2 (110) surface. Within this ensemble of adsorption sites, the most favorable site has 2,3-butanediol covering a Ru 5–coordinated cation. This approach is compared to that obtained using the global optimization algorithm as implemented in the Northwest Potential Energy Surface Search Engine. When using such a global optimization algorithm, we determined a more favorable ground-state structure that was missed during the manual adsorption site testing, with an adsorption energy of -2.61 eV as compared to -2.34 eV when using the ensemble-based approach. We hypothesize that the dehydration reaction requires a stronger chemical bond, which could necessitate the formation of oxygen vacancies. As such, this study has taken the first step toward the utilization of a global optimization algorithm for the rational design of Ru-based catalysts toward the formation of butene from sustainable resources.

09 BIOMASS FUELS↗
DOE OSTI2021

Probing basal planes and edge sites in polygranular nuclear graphite by gas adsorption: Estimation of active surface area

Polygranular nuclear graphite, manufactured at 2800–3000 °C from a carbonized filler and binder mix, has high graphitization degree, crystalline structure close to perfect graphite, and about 20% porosity. The pore surfaces expose large regions of rough, defective prismatic edges of graphite crystallites which are the locus of graphite materials surface sites active for oxidation, chemisorption, and electron transfer. However, we show that high-resolution N 2 and Kr first monolayer adsorption on polygranular graphite (P/P 0 < 0.015) occurs in many ways like adsorption on graphitized carbon blacks. This proves the presence of energetically homogeneous basal planes domains in graphite porosity, which was not fully acknowledged before. Using classical analysis methods (Langmuir, Hill-de Boer, adsorption potential distribution) we quantity the basal plane area (BPA) of several polygranular graphite types and correlate it with their microstructure. Finally, we propose that gas adsorption is uniquely positioned to reliably estimate the active surface area of graphite by subtracting BPA from the BET total surface area. Direct estimation based on adsorption is preferable to indirect calculations based on microstructural information.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Synergistic Effects of the Electric Field Induced by Imidazolium Rotation and Hydrogen Bonding in Electrocatalysis of CO 2

The roles of the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO₂ reduction to CO on silver in non-aqueous electrolytes were investigated. The first electron transfer occurs to CO₂ at reduced overpotentials when it is trapped between the planes of [EMIM]⁺ ring and electrode surface due to cation re-orientation as determined from voltammetry, in-situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant faradaic activity, opposing the notion of it as a free proton source. Instead, water acts as a hydrogen bond donor and the proton is sourced from [EMIM]⁺. Furthermore, this study demonstrates that alcohols with varying acidity tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO₂ reduction. Here, the hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]⁺. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in non-aqueous electrolytes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The role of atomic carbon in directing electrochemical CO (2) reduction to multicarbon products

Electrochemical reduction of carbon-dioxide/carbon-monoxide (CO (2) R) to fuels and chemicals presents an attractive approach for sustainable chemical synthesis, but it also poses a serious challenge in catalysis. Understanding the key aspects that guide CO (2) R towards value-added multicarbon (C 2+ ) products is imperative in designing an efficient catalyst. Herein, we identify the critical steps toward C 2 products on copper through a combination of energetics from density functional theory and micro-kinetic modeling. We elucidate the importance of atomic carbon in directing C 2+ selectivity and how it introduces surface structural sensitivity on copper catalysts. Overall, this insight enables us to propose two simple thermodynamic descriptors that effectively identify C 2+ selectivity on metal catalysts beyond copper and hence it defines an intelligible protocol to screen for materials that selectively catalyze CO (2) to C 2+ products.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Improved charge extraction in inverted perovskite solar cells with dual-site-binding ligands

Inverted (pin) perovskite solar cells (PSCs) afford improved operating stability in comparison to their nip counterparts but have lagged in power conversion efficiency (PCE). The energetic losses responsible for this PCE deficit in pin PSCs occur primarily at the interfaces between the perovskite and the charge-transport layers. Additive and surface treatments that use passivating ligands usually bind to a single active binding site: This dense packing of electrically resistive passivants perpendicular to the surface may limit the fill factor in pin PSCs. We identified ligands that bind two neighboring lead(II) ion (Pb 2+ ) defect sites in a planar ligand orientation on the perovskite. We fabricated pin PSCs and report a certified quasi–steady state PCE of 26.15 and 24.74% for 0.05– and 1.04–square centimeter illuminated areas, respectively. The devices retain 95% of their initial PCE after 1200 hours of continuous 1 sun maximum power point operation at 65°C.

Science & Technology - Other Topics↗
DOE OSTI2020

Atomistic Studies of Individual Impurities and Impurity Complexes in III-V Semiconductors. Final report

The central hypothesis of our investigation is that reduced symmetries significantly influence the properties of individual impurities and impurity complexes in semiconductors. To disentangle the properties of the local environments of these impurities and impurity complexes from the behavior of an ensemble of impurities, at U. Iowa we performed theoretical calculations of specific local environments and developed the theoretical technology to calculate spin and charge transport through those local environments, in which the contact is a localized probe (scanning tunneling microscopy tip). These calculations were designed to be brought into comparison with experimental measurements made by Prof. Gupta’s group at Ohio State University. The principal environment that we focused on is the (110) surface of GaAs and other III-V semiconductors, because pristine surfaces can be revealed through a natural cleavage plane. We studied how the p-d hybridization of transition-metal dopants is influenced by the energetic location and the local environment’s symmetry-breaking of the d levels, and how the 4f-shell states of rare earth impurities couple to the surroundings. We calculated long-distance properties using tight-binding (linear combination of atomic orbitals, LCAO) Green’s function methods. The spin character of the dopants is often coherent on the time-scale of electronic transport through the region, and so correlations of the spin character of a dopant with that of a spinpolarized STM tip, or another dopant in the vicinity, can influence the measured conductivity from STM. We constructed a robust theory for these effects that can be used to describe quantitatively the measurements made by spin-polarized STM.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Assessing the Role of the Support in Acetic Acid Hydrodeoxygenation Selectivity on Pt/TiO2

To produce drop-in quality biofuels following the thermochemical conversion of biomass via catalytic fast pyrolysis (CFP), hydrodeoxygenation (HDO) reactions can be performed to remove excess oxygen and create a more stable bio-oil product. HDO reactions involve co-feeding the CFP vapor-phase product and H2 gas over bi-functional catalysts, such as noble-metal catalysts supported on reducible metal oxides (e.g., Pt/TiO2). Recent model-compound studies, focusing on important classes of species in the CFP vapor mixture, have sought to determine the role of the various Pt/TiO2 actives sites (i.e., Pt-metal, TiO2-support, Pt-TiO2-interfacial sites) in producing the observed desired deoxygenation and undesired decarboxylation/decarbonylation products. One important class of compounds in the CFP vapor-phase product is carboxylic acids (e.g., acetic acid). Prior experimental work on acetic acid HDO (AA-HDO) found that Pt/C and Pt/TiO2 catalysts favored the formation of undesired C-C and desired C-O bond-dissociation products; however, the fundamental surface chemistry driving this shift in selectivity has not been established. This presentation employs atomic-scale modeling to determine, through comparisons of adsorption and reaction energetics, how Pt-metal, Pt-TiO2-interface, and interfacial-vacancy sites catalyze the competing reaction pathways for AA-HDO. Our analysis indicates that hydroxyl vacancies at the Pt-TiO2 interface are critical for lowering barriers for C-O bond-cleavage steps, such that they become favorable/competitive with respect to C-C bond-dissociation steps.

BIOMASS FUELS,INORGANIC, ORGANIC, PHYSICAL, AND AN↗
DOE OSTI2021

Directing the far-from-equilibrium assembly of nanoparticles in confined liquid crystals by hydrodynamic fields

The assembly of nematic colloids relies on long-range elastic interactions that can be manipulated through external stimuli. Confinement and the presence of a hydrodynamic field alter the defect structures and the energetic interactions between the particles. Here, the assembly landscape of nanoparticles embedded in a nematic liquid crystal confined in a nanochannel under a pressure-driven flow is determined. The dynamics of the liquid crystal tensor alignment field is determined through a Poisson-Bracket framework, namely the Stark–Lubensky equations, coupled with the zero-Reynolds momentum equations and the liquid crystal Landau-de Gennes free energy functional. A second order semi-implicit time integration and a three-dimensional Galerkin finite element method are used to resolve flow and nematic fields under several conditions. In general, the zero Reynolds flow displaces the defects around the particles in the upstream direction and renders the surface anchoring ineffective when the flow strength dominates over the nematic elasticity. More importantly, the potential of mean force for particle assembly is non-monotonic independent of surface anchoring. Our results show that the confinement length scale determines the repulsion/attraction transition between colloids, while the flow strength modifies the static defect structure surrounding the particles and determines the magnitude of the energetic barrier for successful assembly. In the attractive regime, the particles move at different rates through the nematic until one particle eventually catches up with the other. This process occurs against or along the direction of flow depending on the flow strength. Ultimately, these results provide a template for engineering and controlling the transport and assembly of nanoparticles under far-from equilibrium conditions in anisotropic media.

36 MATERIALS SCIENCE↗