Length Scales Research Articles

Transient Absorption Microscopy Maps Spatial Heterogeneity and Distinct Chemical Environments in Photocatalytic Carbon Nitride Particles.

Limitations in solar energy conversion by photocatalysis typically stem from poor underlying charge carrier properties. Transient Absorption (TA) reveals insights on key photocatalytic properties such as charge carrier lifetimes and trapping. However, on the microsecond timescale, these measurements use relatively large probe sizes ranging in millimetres to centimetres which averages the effect of spatial heterogeneity at smaller length scales. A home-built Transient Absorption Microscopy (TAM) setup is reported and used to study single particles of carbon nitride (CNx), an emerging photocatalyst. For the first time, to the best of the authors' knowledge, µs-s timescales are explored within individual particles to gain a more complete understanding of their photophysics. The dynamics of trapped charges are monitored, enabling measurement and quantification of heterogeneity in the transient absorptance signal of individual CNx particles and within them. Particle-to-particle heterogeneity in the trapped charge density is observed, while spatial heterogeneity in lifetimes within a particle is revealed using a smaller probe beam with a ≈5µm diameter. Overall, the observations suggest that contributions from different local environments independently influence charge trapping at different timescales. TAM on the micron and microsecond spatiotemporal resolution will aid in tackling design questions about optimal chemical environments for the promotion of photoactivity.

Electrified Enhanced Recovery of Lithium from Unconventional Sources.

Demand for lithium is expected to quadruple by the end of the decade. Without new sources of production, the supply-demand curve is expected to invert. Traditional geological reserves will not be able to meet the anticipated gap, thus unconventional sources of lithium will need to be utilized, setting the stage for fierce competition for perhaps the most critical of mineral resources required for the energy transition. Direct Lithium Extraction refers to the umbrella of technologies being developed to access lithium from unconventional sources. Electrochemical extraction offers significant promise for its selectivity and low operating cost when coupled with renewable energy. This review aims to describe materials and process design considerations for electrochemical extraction of lithium from aqueous sources with a specific emphasis on ζ-V2O5 designed in our research group as an insertion host. We point to specific strategies for improving capacity and selectivity for electrochemical lithium extraction based on materials design across length scales. Strategies range from site-selective modification of insertion hosts to controlled tortuosity of ion diffusion pathways in porous electrode architectures. Electrochemical lithium extraction from unconventional sources stands poised to be a linchpin of a sustainable economy when coupled with cleaning of wastewater, hydrogen generation, and recovery of ancillary critical metals.

Overcoming Inaccuracies in Machine Learning Interatomic Potential Implementation for Ionic Vacancy Simulations.

Machine learning interatomic potentials, particularly ones based on deep neural networks, have taken significant strides in accelerating first-principles simulations, expanding the length and time scales of the simulations with accuracies akin to first-principles simulations. Notwithstanding their success in accurately describing the physical properties of pristine ionic systems with multiple oxidation states, herein we show that an implementation of deep neural network potentials (DNPs) yield vacancy formation energies in MgO with a significant ∼3 eV error. In contrast, we show that moment tensor potentials can accurately describe all properties of the oxide, including vacancy formation energies. We show that the vacancy formation energy errors in DNPs correlate with the strength of ionic interactions in the system as evidenced by contrasting MgO with the less ionic systems CuxOy and AgxOy. Our findings suggest that descriptors employed in the DNP may be inadequate and cannot accurately describe vacancies in ionic systems.

Dependencies between effective parameters in coarse-grained models for phase separation of DNA-based fluids.

DNA is now firmly established as a versatile and robust platform for achieving synthetic nanostructures. While the folding of single molecules into complex structures is routinely achieved through engineering basepair sequences, very little is known about the emergence of structure on larger scales in DNA fluids. The fact that polymeric DNA fluids can undergo phase separation into dense fluid and dilute gas opens avenues to design hierachical and multifarious assemblies. Here, we investigate to which extent the phase behavior of single-stranded DNA fluids can be captured by a minimal model of semiflexible charged homopolymers while neglecting specific hybridization interactions. We first characterize the single-polymer behavior and then perform direct coexistence simulations to test the model against experimental data. While low-resolution models show great promise to bridge the gap to relevant length and time scales, obtaining consistent and transferable parameters is challenging. In particular, we conclude that counterions not only determine the effective range of direct electrostatic interactions but also contribute to the effective attractions.

Detection of Energetic Low Dimensional Subspaces in Spatio-Temporal Space in Turbulent Pipe Flow

AbstractLow dimensional subspaces are extracted out of highly complex turbulent pipe flow at $$Re_{\tau }=181$$ R e τ = 181 using a Characteristic Dynamic Mode Decomposition (CDMD). Having lower degrees of freedom, the subspaces provide a more clear basis to detect events which meet our understanding of large-scale coherent structures. To this end, a temporal sequence of state vectors from direct numerical simulations are rotated in space-time such that persistent dynamical modes on a hyper-surface are found travelling along its normal in space-time, which serves as the new time-like coordinate. The main flow features are captured with a minimal number of modes on a moving frame of reference whose velocity matches that of the most energetic scale. Reconstruction of the candidate modes in physical space gives the low rank model of the flow. The structures living in this subspace have long lifetimes, posses wide range of length-scales and travel at group velocities close to that of the moving frame of reference. The modes within this subspace are highly aligned, but are separated from the remaining modes by larger angles. We are able to capture the essential features of the flow like the spectral energy distribution and Reynolds stresses with a subspace consisting of about 10 modes. The remaining modes are collected in two further subspaces, which distinguish themselves by their axial length scale and degree of isotropy.

Characterization of the SCR-1 Stellarator Physics: Investigating Plasma Discharge, MHD Equilibrium Calculations, and O-X-B Mode Conversion Feasibility

Abstract The Stellarator de Costa Rica 1 (SCR-1) is a small modular stellarator that serves as a valuable research and training tool for plasma magnetic confinement. This study aimed to analyze the characteristics of SCR-1, including its peripheral systems, technical plasma discharge processes, and advancements in plasma characterization. In addition, this study explored a new heating mechanism and the factors that influence it. The current state of the device and plasma discharge are initially presented. Subsequently, the measurement process was utilized to determine the electronic density and plasma temperature using a single Langmuir probe and the results were compared with theoretical predictions based on the particle and energy balance. Additionally, the VMEC code was employed to calculate magnetic flux surfaces with characteristics such as a low aspect ratio, low beta parameter, negative magnetic shear, and decreasing rotational transform along magnetic flux surfaces. The Mercier criterion was employed to conduct a linear stability analysis, which identified a magnetic well that played a crucial role in the linear stability of the majority of magnetic flux surfaces. Feasibility studies of electron Bernstein waves were conducted using the IPF-FMDC full-wave code. The results obtained from the IPF-FMDC code revealed that the O-X conversion percentage reached a maximum of 63 % when considering radiation reflection in the vacuum vessel. Significant effects of plasma curvature on the O-X wave conversion and normalized electron density scale length were observed, while the change in the SCR-1 heating position did not produce a significant impact. Three damping mechanisms affecting O-X conversion were studied, and one of the principal effects was the SX-FX conversion due to steep electron density gradient. Additionally, stochastic electron heating showed a low electron field amplitude, which is important for limiting the electron Bernstein wave propagation.

MRI on ion exchange resins at different length scales

AbstractIon exchange resins were studied on different length scales by magnetic resonance imaging (MRI) with the focus on their interactions with nanoparticles (NP) and molecular clusters. On the length scale of resin beds (bed diameters <20 mm), the behavior of NP and of molecular clusters was shown to depend on the kind of ion exchange resin and nanoscale moiety. The kinetics of absorption and penetration into the resin beads was quantified on a smaller length scale of a stack of resin beads (sample with an outer diameter of 1.7 mm). Finally, using an MRI μ‐coil (3D spatial resolution ≥8 μm), adsorption of superparamagnetic NP on individual resin beads was observed via the magnetic field disturbance characteristic for magnetic dipoles. As a result, this allows the detection of NP (diameter ≤100 nm) by MRI on much larger length scales of several micrometers.

Dielectric properties of nanoconfined water.

The dielectric function of a dipolar liquid exhibits a strong wavenumber dependence in the bulk homogeneous state. Such a behavior seems to suggest the possibility of a strong system size dependence of the dielectric constant (DC) of a nanoconfined liquid, although details have been revealed only recently. The dielectric properties of nanoconfined water, indeed, show a marked sensitivity not only to the size and shape (dielectric boundaries) of confinement but also to the nature of surface-water interactions. For geometries widely studied, namely, water confined in a narrow slit, nanocylinder, and nanospherical cavity, the asymptotic approach to the bulk value of the DC with the increase in confinement size is found to be surprisingly slow. This seems to imply the appearance of a dipolar cross correlation length, much larger than the molecular length-scale of water. In narrow slits and narrow cylinders, the dielectric function becomes both inhomogeneous and anisotropic, and the longitudinal and transverse components display markedly different system size dependencies. This sensitivity can be traced back to the dependence of the DC on the ratio of the mean square dipole moment fluctuation to the volume of the system. The observed sensitivity of collective dipole moment fluctuations to the length scale of confinement points to the possibility of using DC to estimate the orientational correlation length scale, which has been an elusive quantity. Furthermore, the determination of volume also requires special consideration when the system size is in nanoscale. We discuss these and several other interesting issues along with several applications that have emerged in recent years.

Observation of an Inverse Turbulent-Wave Cascade in a Driven Quantum Gas

We observe an inverse turbulent-wave cascade, from small to large length scales, in a driven homogeneous 2D Bose gas. Starting with an equilibrium condensate, we drive the gas isotropically on a length scale much smaller than its size, and observe a nonthermal population of modes with wavelengths larger than the drive one. At long drive times, the gas exhibits a steady nonthermal momentum distribution. At length scales increasing from the drive one to the system size, this distribution features in turn: (i) a power-law spectrum with an exponent close to the analytical result for a particle cascade in weak-wave turbulence, and (ii) a spectrum reminiscent of a nonthermal fixed point associated with universal coarsening in an isolated 2D gas. In further experiments, based on anisotropic driving, we reveal the complete qualitative picture of how the steady-state cascade forms. Published by the American Physical Society 2024

Covalent Organic Frameworks for Photocatalysis.

The global energy crisis and environmental concerns are driving research into renewable energy and sustainable energy conversion and storage technologies. Solar energy, as an ideal sustainable resource, has significant potential to contribute to the goal of net-zero carbon emissions if effectively harnessed and converted into a reliable and storable form of energy. Photocatalysts have the potential to convert sunlight into chemical energy carriers. In this respect, covalent organic frameworks (COFs) have shown great promise due to their tunable structure on different length scales, high surface areas, and beneficial optical properties such as broad visible light absorption. This review offers a comprehensive overview of the key developments in COF-based photocatalysts for various applications, including water splitting, hydrogen peroxide generation, organic transformations, and carbon dioxide and nitrogen reduction. The underlying mechanisms, essential principles for material design, and structure-function relationships of COFs in various photocatalytic applications are discussed. The challenges faced by COF-based photocatalysts are also summarized and various strategies to enhance their performance are explained, such as improving crystallinity, regulating molecular structures, tailoring linkages, and incorporating cocatalysts. Finally, critical strategies are proposed for the utilization of photocatalytically generated chemicals into value-added products.

State-dependency of dynamic and thermodynamic contributions to effective precipitation changes

Abstract Reliable projections of the future hydrological cycle are needed for designing adaptation and mitigation measures under global warming. However, uncertainties in the projected sign and magnitude of effective precipitation changes (precipitation minus evaporation, P − E) remain high. Here, we examine the state-dependency of circulation, temperature, and relative humidity contributions to P − E changes in simulations of the Last Glacial Maximum (LGM), mid-Holocene, and abrupt quadrupling of the atmospheric carbon dioxide concentration. To this purpose, we apply a moisture budget decomposition and a thermodynamic scaling approximation to CMIP6/PMIP4 simulations with the Earth system model MPI-ESM1.2. We find that the importance of thermodynamic and dynamic contributions to P − E changes and the patterns of dynamic contributions depend strongly on the underlying forcing. Greenhouse gas forcing leads to a stronger thermodynamic than dynamic response. The LGM ice sheets yield a large dynamic contribution with zonally heterogeneous patterns. Orbital forcing induces a predominantly dynamic response with a hemispherically anti-symmetric structure. We also identify state invariant features: the importance of temperature and relative humidity contributions to specific humidity changes is consistent across states, and the wet-get-wetter-dry-get-drier paradigm proposed for global warming holds in almost all regions dominated by thermodynamic contributions. By definition, the P − E budget and the respective thermodynamic, dynamic, and transient eddy contributions vanish in the global mean. Moreover, we find that for increasing length scales, the spatial variability of these contributions decays with similar rates. We suggest repeating our analysis for more models and states which could help constraining hydroclimate projections.

Influence of freestream turbulence and porosity on porous disk-generated wakes

This study aims to evaluate the effect of freestream turbulence (FST) on wakes produced by disks with different porosity. The wakes are exposed to various freestream turbulence “flavors,” where turbulence intensity and integral length scale are independently varied. The turbulent wakes are interrogated through hot-wire anemometry from 3 to 15 diameters downstream of the disks. It is found that disks with low porosity behave similarly to a solid body, in terms of both entrainment behavior and scaling laws for the centerline mean velocity evolution. Far from the disks, the presence of FST reduces both the wake growth rate and entrainment rate, with a clear effect of both turbulence intensity and integral length scale. As porosity increases, these “solid body” FST effects gradually diminish and are reversed above a critical porosity. The entrainment behavior in disk-generated wakes is significantly influenced by the presence of large-scale coherent structures, which act as a shield between the wake and the surrounding flow, thus impeding mixing in the near wake. We found that higher porosity, turbulence intensity, or integral length scale weakens the energy content of these structures, thereby limiting their influence on wake development to a shorter distance downstream of the disk. This, in turn, potentially reduces the influence of large-scale engulfment on the overall entrainment mechanism to a shorter distance downstream of the disks as well. For low-porosity disks, freestream turbulence intensity initially promotes near-wake growth through the suppression of large-scale structures; however, farther downstream, the wakes grow faster when the background is nonturbulent. Published by the American Physical Society 2024

Cavity-mediated iSWAP oscillations between distant spins

AbstractDirect interactions between quantum particles naturally fall off with distance. However, future quantum computing architectures are likely to require interaction mechanisms between qubits across a range of length scales. In this work, we demonstrate a coherent interaction between two semiconductor spin qubits 250 μm apart using a superconducting resonator. This separation is several orders of magnitude larger than for the commonly used direct interaction mechanisms in this platform. We operate the system in a regime in which the resonator mediates a spin–spin coupling through virtual photons. We report the anti-phase oscillations of the populations of the two spins with controllable frequency. The observations are consistent with iSWAP oscillations of the spin qubits, and suggest that entangling operations are possible in 10 ns. These results hold promise for scalable networks of spin qubit modules on a chip.

A mathematical model for the interaction of anisotropic turbulence with porous surfaces

Leading-edge noise is a complex phenomenon that occurs when a turbulent fluid encounters a solid object, and is a notable concern in various engineering applications. This study enhances a mathematical leading-edge noise model (Hales et al., J. Fluid Mech., vol. 970, 2023, A29) for anisotropic flow and porous boundaries. The model has two key components. First, we adjust the velocity spectrum to account for the possibility of anisotropy in the flow. This paper rigorously introduces a third dimension for the turbulence spectrum that preserves the turbulence kinetic energy and mathematical definitions for integral length scales. Second, we adapt the fully analytical acoustic transfer function to account for different boundaries by implementing convective impedance boundary conditions when formulating the gust-diffraction problem. This problem is then solved using the Wiener–Hopf technique. We discuss important aspects of this method, including the factorisation of a non-trivial scalar kernel function and the application of suitable edge conditions for the problem. Each modification is inspired by experimental leading-edge noise data using a series of different porous leading edges and anisotropic turbulence generated by a cylinder upstream of the edge. Experimental data demonstrate the interplay between anisotropy and leading-edge modifications while achieving the characteristic mid-frequency noise reduction expected from porous leading edges. Our model is adapted to best fit the trends of the data via a tailored impedance function, leading to good agreement with all datasets across an extended frequency range. This tailored function is used to successfully validate the model against other datasets from a different set of experiments.

Ontogenetic change in body shape for white sharks, Carcharodon carcharias, in Australian waters.

The analysis of how biological shape changes across ontogeny can provide us with valuable information on how species adapt behaviorally, physiologically, and ecologically. The white shark Carcharodon carcharias is one of the largest and most widely distributed apex predators globally, yet an understanding of ontogenetic changes in body shape and relative scaling of length and weight measures is limited, especially in relation to foraging ecology. Through analysis of a suite of shape-related metrics, we identified ontogenetic patterns of scaling throughout development. Isometric growth was exhibited for most metrics, failing to show a significant deviation from an isometric slope of 1.0 for length-length relationships, and 3.0 for weight-length relationships. The most notable difference from this trend was the negative allometric growth observed for the upper caudal-fin lobe length, trunk length, and the mouth length. The surface area of the fins also presented a strong, positive relationship with precaudal length (PCL) and the girth at the pectoral fin. Negative allometric growth was exhibited for three of the fins (pectoral, upper caudal fin, and lower caudal fin) against PCL, exhibiting a significant deviation from the expected isometric growth of 2.0 for area-length relationships. There were no significant differences in morphometric relationships between geographic regions within Australia that samples were collected from. No differences between the sexes were identified; however, this may be an artifact of the lack of mature animal samples. Conversely, life stage was found to have a significant effect on the girth-length and weight-length relationships. The development of regression equations for morphometric measures allows the assessment of white shark body condition and may serve as an assessment tool to understand the potential impacts of human-induced environmental change on white sharks.

Coordinative Self‐assembly of π‐Electron Magnetic Porphyrins

π‐Electron magnetic compounds on surfaces have emerged as a powerful platform to interrogate spin interactions at the atomic scale, with great potential in spintronics and quantum technologies. A key challenge is organizing these compounds over large length scales, while elucidating their resulting magnetic properties. Herein, we offer a relevant contribution toward this objective, which consists of using on‐surface synthesis coupled with coordination chemistry to promote the self‐assembly of π‐electron magnetic porphyrin species. A porphyrin precursor equipped with carbonitrile moieties in a trans arrangement was prepared by solution synthesis and deposited on a Au(111)/mica. Depending on the specific growth protocol, surface‐promoted reactions led to the transformation of the precursor into non‐magnetic Au‐CN coordinated porphyrin monomers, covalent porphyrin dimers, and one‐dimensional porphyrin polymers (based on porphyrin monomers or covalent porphyrin dimers), as revealed by scanning probe microscopy studies combined with theoretical calculations. Interestingly, the scanning tunneling microscopy tip could convert such closed‐shell porphyrin units into open‐shell species by the removal of some peripheral hydrogen atoms. The magnetic features (i.e., singlet or triplet ground state) of the porphyrin units comprising the polymers were investigated for polymers of different lengths. No magnetic exchange coupling between adjacent units was observed.

Effect of Confinement on the Translational Diffusivity of Small Dye Molecules in Thin Polystyrene Films and Its Connection to Tg-Confinement and Fragility-Confinement Effects.

Using fluorescence, we study the impact of nanoscale confinement on the translational diffusivity (D) of trace levels of a small-molecule dye, 9,10-bis(phenylethynyl)anthracene (BPEA), in supported polystyrene (PS) films via Förster resonance energy transfer (FRET). Reductions in BPEA diffusivity are observed in films thinner than ∼200 nm, with D decreasing by 80-90% in 100 nm-thick films compared to bulk. The activation energy of BPEA diffusivity increases from ∼210 kJ/mol in bulk films to ∼370 kJ/mol in 130 nm-thick films. BPEA exhibits a greater diffusivity-confinement effect than a larger dye, decacyclene, in terms of the length scale at which the effects of confinement become evident and the percentage reduction in diffusivity. For both BPEA and decacyclene, the diffusivity-confinement effect in supported PS films occurs at a length scale much larger than that for the glass transition temperature (Tg)-confinement effect and somewhat larger than that for the fragility-confinement effect. This difference in confinement-effect length scales can be rationalized as follows: small-molecule dye diffusivity relates predominantly to short times in the α-relaxation distribution, whereas Tg relates to long times in the α-relaxation distribution, and fragility reflects the overall breadth of this relaxation time distribution. If confinement results in a narrower relaxation time distribution in PS films with the short-time relaxations being shifted to longer times and the longest-time relaxation regimes being shifted to shorter times, then Tg, diffusivity, and fragility all decrease at sufficient levels of confinement. If the narrowing with confinement begins with the shortest relaxation time regimes, then fragility and small-molecule dye diffusivity are influenced by confinement at larger length scales than Tg.

Binder-Mediated Supramolecular Polymerization with Controllable Gel-to-Crystal Transformation in Metal-Organic Polyhedra.

Controlling supramolecular polymerization across various length scales through metal-organic polyhedra in aqueous media enables functional nanomaterial fabrication beyond traditional π-chromophoric systems. Herein, we present a straightforward strategy to tune the nano- and microscopic structural evolution of a co-assembled system. Ga-MOC ([Ga8(ImDC)12]12-, ImDC = imidazoledicarboxylate) is introduced as a discrete unit, while the Ni-ethylenediamine complex [Ni(en)3]2+ (Ni-en), served as the binder towards supramolecular polymerization. Comprehensive investigations revealed that adjusting the binder ratio in the bicomponent (Ga-MOC and Ni-en) co-assembly process allows precise control over nanostructure length and evolution by influencing both the kinetics and thermodynamics of the assembly. At higher concentrations, this assembly forms a hydrogel above a critical binder ratio. Furthermore, the binder's ratio significantly influences the viscoelastic strength of the hydrogels by modulating the connectivity between the MOCs through hydrogen (H)-bonding. Intriguingly, the hydrogels gradually transformed into crystals without any external stimuli, with different timescales regulated by the binder ratio. Single crystal structure determination reveals a 3D structure composed of Ga-MOC and Ni-en, extended through charge-assisted H-bonding (CAHB) interactions, resulting through the transformation from a kinetically controlled gel state to a thermodynamically stable crystal product. This study provides an understanding of binder-mediated control over nanostructural evolution in co-assembled MOCs.

Laser-Assisted Surface Modification of TRIP Steels: Chemical and Microstructural Evolution, Residual Stresses, and Micromechanical Properties

AbstractThe use of new lightweight design strategies is implemented in the automotive industry, for applications where high friction forces are present and lubrication systems need to be implemented. Micro-topography modification through laser texturing is an advantageous manufacturing technique that is currently being used by the industry. Combining both ideas, this manuscript aims to analyze the effect of surface modification laser-texturing process on a TRansformation-Induced Plasticity steel. Residual stresses, superficial chemistry, microstructure, and mechanical properties at the submicrometric length scale are investigated. Results show that the intensity and the number of pulses strongly modify the pattern geometry. Also, the pattern distance affects the residual stress state, changing from tensile to compressive stresses for distances higher than 20 µm. Chemical and microstructural changes at the vicinity of the laser may be associated to the fast cooling of the sublimated material around the pattern induced during the laser-assisted surface modification thermal process. Furthermore, the hardness remains stable, and no changes are evident in the bulk material. However, a hardness reduction of around 95 pct in the austenitic value is evident in the pile-up zone around the laser pattern due to (1) the microporosity and defects confined inside the pile-up region and (2) due to the residual chemical composition created during the solidification process, which is rich in iron, chromium, nickel, and manganese and poor in oxygen.

Digitally Programmable Microphase Separation in Polymer Network Generates Microstructure Pattern.

Polymers with microstructure patterns are crucial to many applications, such as separation, optics, electronics, metamaterials, etc. However, introducing microstructure patterns with diverse morphologies and feature sizes ranging from nanometers to micrometers into large-area polymers remains a significant challenge. Here, we design a material system that enables digital programming microphase separation in a polymer network to generate microstructure patterns. The polymer network is engineered to allow digital programming of local modulus, which arrests the length scale of microphase separation to generate various microstructures. These local modulus-regulated microstructures exhibit either bicontinuous or sea-island morphology and have various feature sizes ranging from ∼100 nm to several micrometers. Using household devices of an ink printer and an ultraviolet light lamp, the microstructure patterns can be programmed with fine resolutions (∼100 μm) over large areas (≫100 mm). The locally varied microstructures have different capabilities to scatter light and result in a visible pattern. We further demonstrate this design with a soft anticounterfeiting device. This approach of digital programming microphase separation in polymer networks is applicable to various polymers and provides a platform for designing many other functional devices.