Interparticle Distance Research Articles

Numerical investigation on the heat transfer of particle arrays in supercritical water

The heat transfer characteristics of particles in supercritical water (SCW) are the research foundation for the development of the technology of coal gasification in SCW. The study encounters two challenging problems: the complexity of particle spatial distribution and the special variation of SCW properties. Therefore, this work focuses on contrasting the particle interaction characteristics between in SCW and conventional fluid, instead of the direct study of particle clusters in SCW. Research is conducted by the particle-resolved numerical simulation method. In the simulations, the SCW inflow is 973 K and 25 MPa which is a usual operating condition, and all particle surface temperatures are constant and uniform with the values of 300 K, 672 K and 973 K which are respectively below, at and beyond the pseudocritical temperature zone. The side-by-side and streamwise distributed arrays, which are simple but representative, are adopted as the research objects firstly. The effects of particle number and inter-particle distance on heat transfer are studied. Results indicate that the effects of particle interaction on heat transfer under all heat transfer states are similar in the side-by-side array; while only the effects under the pseudocritical and supercritical states are similar in the streamwise array. Then the structured packed array is adopted, and the effects of void fraction (ε) under different heat transfer states are studied. Moreover, a new correlation dependent on ε and Re is built for predicting the ratio of the Nu of a particle in the structured packed array and that of a single particle.

A locally corrected multiblob method with hydrodynamically matched grids for the Stokes mobility problem

Inexpensive numerical methods are key to enabling simulations of systems of a large number of particles of different shapes in Stokes flow and several approximate methods have been introduced for this purpose. We study the accuracy of the multiblob method for solving the Stokes mobility problem in free space, where the 3D geometry of a particle surface is discretised with spherical blobs and the pair-wise interaction between blobs is described by the RPY-tensor. The paper aims to investigate and improve on the magnitude of the error in the solution velocities of the Stokes mobility problem using a combination of two different techniques: an optimally chosen grid of blobs and a pair-correction inspired by Stokesian dynamics. Different optimisation strategies to determine a grid with a given number of blobs are presented with the aim of matching the hydrodynamic response of a single accurately described ideal particle, alone in the fluid. It is essential to obtain small errors in this self-interaction, as they determine the basic error level in a system of well-separated particles. With an optimised grid, reasonable accuracy can be obtained even with coarse blob-resolutions of the particle surfaces. The error in the self-interaction is however sensitive to the exact choice of grid parameters and simply hand-picking a suitable geometry of blobs can lead to errors several orders of magnitude larger in size. The pair-correction is local and cheap to apply, and reduces the error for moderately separated particles and particles in close proximity. Two different types of geometries are considered: spheres and axisymmetric rods with smooth caps. The error in solutions to mobility problems is quantified for particles of varying inter-particle distances for systems containing a few particles, comparing to an accurate solution based on a second kind BIE-formulation where the quadrature error is controlled by employing quadrature by expansion (QBX).

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Functionally Graded AA7075 Components Produced via Hot Stamping: A Novel Process Design Inspired from Analysis of Microstructure and Mechanical Properties

Herein, functionally graded AA7075 components manufactured via hot stamping are investigated by focusing on the effect of different process variables on localized microstructure evolution. To realize gradation through stamping, an active tool is designed and applied. The design of experiments allows to assess the impact of transfer time from the furnace to the tool, quenching time in the tool, and final quenching media. Related characteristics of mechanical properties throughout the hat‐shaped profile are assessed via hardness and tensile tests. As expected, the sections of the samples formed in the cooled part of the tool are characterized by higher mechanical strength following subsequent aging, while sections formed in the heated part exhibit higher ductility. Moreover, the microstructural analysis reveals that fine precipitates with minimum interparticle distances only form in the cooled section of the samples. Increasing the tool temperature at the heated side to 350 °C results in the formation of coarse precipitates in the grain interior and along the grain boundaries. A sharp gradient in terms of microstructural and mechanical properties is found between these conditions. After reducing the transfer time, an increased volume fraction of fine precipitates leads to further improvements in hardness and mechanical strengths.

Visualizing Heterogeneous Protein Conformations with Multi-Tilt Nanoparticle-Aided Cryo-Electron Microscopy Sampling.

Obtaining the heterogeneous conformation of small proteins is important for understanding their biological role, but it is still challenging. Here, we developed a multi-tilt nanoparticle-aided cryo-electron microscopy sampling (MT-NACS) technique that enables the observation of heterogeneous conformations of small proteins and applied it to calmodulin. By imaging the proteins labeled by two gold nanoparticles at multiple tilt angles and analyzing the projected positions of the nanoparticles, the distributions of 3D interparticle distances were obtained. From the measured distance distributions, the conformational changes associated with Ca2+ binding and salt concentration were determined. MT-NACS was also used to track the structural change accompanied by the interaction between amyloid-beta and calmodulin, which has never been observed experimentally. This work offers an alternative platform for studying the functional flexibility of small proteins.

Robust Rules for Optimal Colorimetric Sensing Based on Gold Nanoparticle Aggregation.

Spurred by outstanding optical properties, chemical stability, and facile bioconjugation, plasmonic metals have become the first-choice materials for optical signal transducers in biosensing. While the design rules for surface-based plasmonic sensors are well-established and commercialized, there is limited knowledge of the design of sensors based on nanoparticle aggregation. The reason is the lack of control over the interparticle distances, number of nanoparticles per cluster, or multiple mutual orientations during aggregation events, blurring the threshold between positive and negative readout. Here we identify the geometrical parameters (size, shape, and interparticle distance) that allow for maximizing the color difference upon nanoparticle clustering. Finding the optimal structural parameters will provide a fast and reliable means of readout, including unaided eye inspection or computer vision.

One-dimensional array of nanoparticles using segmented polyurethane nanofibers as a template for enhanced catalytic efficiency

Metal nanoparticles are used to catalyze chemical reactions. Among them, noble metal nanoparticle catalysts need to be used in small quantities. Some reports reveal catalytic activity is further improved by controlling nanoparticle arrangement and distribution. Much research has been directed toward the formation of one-dimensional arrays by compositing metal nanoparticles with template materials. However, previously reported methods form arrays that lack linearity or suitable interparticle distances, which is ascribable to array crossover and particle aggregation, in addition their fabrication procedures are expensive and not suitable for large-scale practical use. Here we show that one-dimensional arrays of platinum nanoparticles (PtNPs) were formed by using electrospun polyurethane nanofibers as a template. PtNPs adsorbed on each polyurethane nanofiber form a one-dimensional array over longer distances. The catalytic H2O2 decomposition performance of the prepared one-dimensional PtNP arrays was 45.6% decomposition in 15 min, which revealed a decomposition rate more than twice that obtained using the same number of PtNPs randomly distributed on the template or dispersed in a liquid. Although this method is a very simple method for one-dimensional arrangement of metal nanoparticles, thereby improving catalytic efficiency per metal nanoparticle, which help to reduce the amount of metal nanoparticles used during catalysis and contributes the cost of catalysis products cost.

Machine Learning Color Feature Analysis of a High Throughput Nanoparticle Conjugate Sensing Assay.

Plasmonic nanoparticles are finding applications within the single molecule sensing field in a "dimer" format, where interaction of the target with hairpin DNA causes a decrease in the interparticle distance, leading to a localized surface plasmon resonance shift. While this shift may be detected using spectroscopy, achieving statistical relevance requires the measurement of thousands of nanoparticle dimers and the timescales required for spectroscopic analysis are incompatible with point-of-care devices. However, using dark-field imaging of the dimer structures, simultaneous digital analysis of the plasmonic resonance shift after target interaction of thousands of dimer structures may be achieved in minutes. The main challenge of this digital analysis on the single-molecule scale was the occurrence of false signals caused by non-specifically bound clusters of nanoparticles. This effect may be reduced by digitally separating dimers from other nanoconjugate types. Variation in image intensity was observed to have a discernible impact on the color analysis of the nanoconjugate constructs and thus the accuracy of the digital separation. Color spaces wherein intensity may be uncoupled from the color information (hue, saturation, and value (HSV) and luminance, a* vector, and b* vector (LAB) were contrasted to a color space which cannot uncouple intensity (RGB) to train a classifier algorithm. Each classifier algorithm was validated to determine which color space produced the most accurate digital separation of the nanoconjugate types. The LAB-based learning classifier demonstrated the highest accuracy for digitally separating nanoparticles. Using this classifier, nanoparticle conjugates were monitored for their plasmonic color shift after interaction with a synthetic RNA target, resulting in a platform with a highly accurate yes/no response with a true positive rate of 88% and a true negative rate of 100%. The sensor response of tested single stranded RNA (ssRNA) samples was well above control responses for target concentrations in the range of 10 aM-1 pM.

Isotropic and Anisotropic Monolayer Structures in RF Discharge Plasma

We present the results of an experimental and analytical study of the structural and dynamic properties of a monolayer consisting of dust grains in an electrostatic trap in an RF discharge plasma. The possibility of forming a monolayer with an isotropic distribution for interparticle distance and kinetic energy of particles in the structure has been experimentally shown. Isotropy has crucial importance for the study of various processes in such systems, including the kinetics of phase transitions, the formation of directed flows, wave propagation, and others.

Effect of Hydrophobic Moieties on the Assembly of Silica Particles into Colloidal Crystals

To boost the implementation of colloidal crystals (CCs) in separation science, the effects of the most common chromatographic reversed phases, that is, butyl and octadecyl, on the assembly of silica particles into CCs and on the optical properties of CCs are investigated. Interestingly, particle surface modification can cause phase separation during sedimentation because the assembly is highly sensitive to minute changes in surface characteristics. Solvent-induced surface charge generation through acid-base interactions of acidic residual silanol groups with the solvent is enough to promote colloidal crystallization of modified silica particles. In addition, solvation forces at small interparticle distances are also involved in colloidal assembly. The characterization of CCs formed during sedimentation or via evaporative assembly revealed that C4 particles can form CCs more easily than C18 particles because of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran when C18 chains with a high bonding density have extra hydroxyl side groups. These groups can only be hydrolyzed from trifunctional octadecyl silane but not from a monofunctional one. Moreover, after evaporative assembly, CCs formed from particles with different surface moieties exhibit different lattice spacings because their surface hydrophobicity and chemical heterogeneity can modulate interparticle interactions during the two main stages of assembly: the wet stage of crystal growth and the late stage of nano dewetting (evaporation of interparticle solvent bridges). Finally, short, alkyl-modified CCs were effectively assembled inside silica capillaries with a 100 μm inner diameter, laying the foundation for future chromatographic separation using capillary columns.

The underlying mechanism trade-off between particle proximity effect and low-Pt loading for oxygen reduction and methanol oxidation reaction activity

Highly efficient and stable Pt-based electrocatalysts have broad application prospects in fuel cells, among which, Pt loading accompanied with particle size, interparticle distance (ipd), and interaction with support are identified as key contributors for high activity and selectivity. Herein, we have developed the nitrogen-doped graphite-like carbon sheets supporting low-Pt loading (Pt@N-GC) with uniform size and controllable ipd, and disentangled the main effect of ipd on oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activity. The optimal 5.0 wt% low-Pt loading results in more abundant Pt sites and lower ipd, which leads to the higher surface reactivity by proximity effect, and 2e− pathway can be hindered by strongly resorbing and reducing H2O2 to promote 4e− pathway towards ORR. Simultaneously, the 5.0 wt% Pt@N-GC can effectively improve the MOR activity, enhance the adsorption of -OH group under the appropriate ipd, so as to promote the instantaneous oxidation of CO and enhance the tolerance, and accelerate the oxidative dehydrogenation of methanol. Benefiting by the synergistic effect of Pt nanoparticles and N-GC support, all Pt@N-GCs exhibit bifunctional electrocatalytic activity and durability. We believe the gained trade-off of the loading and proximity effect of Pt, and its influence on catalytic activity and selectivity contributes to design more promising low-Pt loading bifunctional catalysts for fuel cell.

Investigating Plasmonic Catalysis Kinetics on Hot-Spot Engineered Nanoantennae.

Strong hot-spots can facilitate photocatalytic reactions potentially providing effective solar-to-chemical energy conversion pathways. Although it is well-known that the local electromagnetic field in plasmonic nanocavities increases as the cavity size reduces, the influence of hot-spots on photocatalytic reactions remains elusive. Herein, we explored hot-spot dependent catalytic behaviors on a highly controlled platform with varying interparticle distances. Plasmon-meditated dehalogenation of 4-iodothiophenol was employed to observe time-resolved catalytic behaviors via in situ surface-enhanced Raman spectroscopy on dimers with 5, 10, 20, and 30 nm interparticle distances. As a result, we show that by reducing the gap from 20 to 10 nm, the reaction rate can be sped up more than 2 times. Further reduction in the interparticle distance did not improve reaction rate significantly although the maximum local-field was ∼2.3-fold stronger. Our combined experimental and theoretical study provides valuable insights in designing novel plasmonic photocatalytic platforms.

Topology-Directed Self-Locking of Colloidal Suprastructures

The robustness and stability of self-assembled colloidal suprastructures are pivotal and challenging in the field of soft condensed matter and nanotechnology. Herein, we develop a novel approach to “self-locking” the colloidal suprastructures to achieve excellent stability by taking advantage of the topology of polymer brushes on nanoparticles. Compared to linear polymer brushes, looped polymer brushes can significantly improve the structural stability of the suprastructures. A combination of experiments and simulations reveals that the suprastructures achieve their significant stabilities through a “self-locking” mechanism by forming folded entanglement between looped polymer molecules. The stabilized suprastructures can stay intact in various organic solvent environments or under heating and mechanical oscillations. Furthermore, the interparticle spacing of suprastructures can be modulated by controlling polymer conformations in an aqueous solution through varying salt concentrations. This new approach of “self-locking” provides an easy-to-operate strategy to stabilize suprastructures and modulate interparticle distances without adding chemicals, which allows the potentially wide applications of colloidal suprastructures under harsh environments.

Elucidating the particle concentration-dependent flow behavior of aqueous TiO2 suspensions based on polymer dispersant structures and interparticle surface distances

Flowing behavior of concentrated aqueous TiO2 suspensions was evaluated in relation to the structure of polymeric dispersants, their adlayer thickness on particle surface, and particle concentrations. Comb-type polymeric dispersants comprising polyethylene glycol-based side chains with different combinations of unit number and side chain length were investigated. Based on particle concentration–apparent viscosity curves of the suspensions with TiO2 nanoparticles saturated with dispersants, maximum packing fraction and polymer adlayer thickness were estimated through the Krieger–Dougherty model. When the steric repulsive force of polymer dispersant dominated van der Waals attractive forces, the flow curves of the suspension exhibited hysteresis characteristics as the average particle surface distance approached the adlayer thickness of polymer dispersants. Additionally, by introducing the concept of relative surface distance, which is the average particle surface distance divided by the adlayer thickness, the particle concentration dependency of the suspension viscosities with different dispersants followed a unified trend.

Continuum Percolation in Stochastic Homogenization and the Effective Viscosity Problem

This contribution is concerned with the effective viscosity problem, that is, the homogenization of the steady Stokes system with a random array of rigid particles, for which the main difficulty is the treatment of close particles. Standard approaches in the literature have addressed this issue by making moment assumptions on interparticle distances. Such assumptions, however, prevent clustering of particles, which is not compatible with physically-relevant particle distributions. In this contribution, we take a different perspective and consider moment bounds on the size of clusters of close particles. On the one hand, assuming such bounds, we construct correctors and prove homogenization. On the other hand, based on subcritical percolation techniques, these bounds are shown to hold for various mixing particle distributions with nontrivial clustering. As a by-product of the analysis, we also obtain similar homogenization results for compressible and incompressible linear elasticity with unbounded random stiffness.

Enhancement and modulation of three-body near-field radiative heat transfer via anisotropic hyperbolic polaritons

Anisotropic hyperbolic polaritons (HPs) provide novel opportunities to control light at nanoscale because of their promising ability of subdiffractional confinement, low losses, and polarization sensitivities. However, it has not been explored in the modulation of many-body near-field radiative heat transfer (NFRHT). In this work, we investigate the NFRHT in an α-phase molybdenum trioxide three-body nanoparticles (NPs) system. The results show that the near-field radiative conductance (NFRHC) can be significantly enhanced by adding the third NP between two NPs due to the excitation of the localized hyperbolic polaritons (LHPs). The enhancement effect can be actively modulated by rotating the intermediate NP, which is attributed to the high directionality of the LHPs. In addition, the effects of interparticle distance and vertical displacement on the NFRHC are also studied. We achieved an enhancement ratio of over 20,000 when the interparticle distance and vertical displacement is 4 times and 1.2 times the radius, 38 times higher than that of previous works. This work demonstrates the potential of anisotropic HPs in NFRHT modulation within the many-body system.

Topological phonon polariton enhanced radiative heat transfer in bichromatic nanoparticle arrays mimicking Aubry-André-Harper model

Topological phonon polaritons (TPhPs) are promising optical modes relevant in long-range radiative heat transfer, information processing and infrared sensing, whose topological protection is expected to enable their robust existence and transport. In this work we show that TPhPs can be supported in one-dimensional (1D) bichromatic silicon carbide nanoparticle (NP) chains, and demonstrate that they can considerably enhance radiative heat transfer for an array much longer than the wavelength of radiation. By introducing incommensurate or commensurate modulations on the interparticle distances, the NP chain can be regarded as an extension of the off-diagonal Aubry-Andr\'e-Harper (AAH) model. By calculating the eigenstate spectra with respect to the modulation phase that creates a synthetic dimension, we demonstrate that under this type of modulation the chain supports nontrivial topological modes localized over the boundaries, since the present system inherits the topological property of two-dimensional integer quantum Hall systems. In this circumstance the gap-labeling theorem and corresponding Chern number can be used to characterize the features of band gaps and topological edge modes. Based on many-body radiative heat transfer theory for a set of dipoles, we theoretically show the presence of topological gaps and midgap TPhPs can substantially enhance radiative heat transfer for an array much longer than the wavelength of radiation. We show how the modulation phase that acts as the synthetic dimension can tailor the radiative heat transfer rate by inducing or annihilating topological modes. We also discuss the role of dissipation in the enhancement of radiative heat transfer. These findings therefore provide a fascinating route for tailoring near-field radiative heat transfer based on the concept of topological physics.

Combined effects of matrix molecular weight and crystallinity on the impact toughness of PP/EPR blends: The role of chain entanglement

In recent years, rubber toughened polypropylene (PP) blends have been extensively investigated with special regard to the importance of PP matrix properties (e.g., crystal morphology and structure) in tailoring the toughening efficiency. Unfortunately, although high melt flowability is indispensable to the processing of polymers into complex thin-wall products, it remains a huge challenge to achieve a marvelous balance between stiffness and toughness in the blends with relatively low matrix molecular weight. Herein, taking PP/EPR (ethylene-propylene copolymer) blends as an example, the combined effects of matrix molecular weight and crystallinity on the mechanical properties of PP/EPR blends have been examined in both experiment and theory. The experimental results show that decreasing molecular weight leads to significant deterioration in notched impact toughness, tensile strength and Young's modulus. Impressively, the toughness loss caused by the decrease of molecular weight can be well-compensated by a slight reduction in the matrix crystallinity, without evidently sacrificing the strength and modulus, demonstrating an improved stiffness-toughness balance. Theoretical analysis indicates that the impact toughness of PP/EPR blends is heavily dependent on the chain entanglement density (ve) of PP matrix, and a linear relationship between critical interparticle distance (IDc) and the ve1/3 has been verified from both experiment and theory. Moreover, it is interesting to find that decreasing matrix crystallinity can increase the ve of low-molecular-weight PP matrix, which enables the effective toughening at lower concentration of EPR and thus gives rise to less loss in the strength and modulus. We believe this work not only gives a new insight into the role of chain entanglement in the toughening but also provides a promising guidance for the design of high-performance PP with high melt flowability.

Experimental investigation on the mechanical performance of the Al2O3 and ZrO2 added Al-Mg-Si alloy for structural applications

The present-day structural application requires advanced materials at extreme conditions, and the development of it is not economically feasible. As a result, existing material in the composite form has more visibility to suit the purpose. The material's strengths and qualities must be examined before it can be recommended for use in harsh conditions. In the present study, Al2O3 (3, 6, 9, 12, and 15 wt.%) and ZrO2 particles (2 wt.%) are reinforced with Al-Mg-Si alloy (Al 6061), which possess good isotropic behaviour and better interface bonding. The double stir casting process was adopted for the hybrid composite development, and the samples were cold rolled to reduce grain size and pores in the composites. The developed composites were subjected to mechanical characterisation tests such as hardness, tensile, compression, impact and flexural tests. An increase in reinforcement percentage enhances the mechanical properties. The improvement was attributed to the high thermal mismatch and reduction in the inter-particle distance by cold rolling and the reinforcements. Scanning electron microscope microstructural examination was conducted, and even dispersion of 3 to 15 wt.% Al2O3 reinforcements was observed. Tensile elongation decreases with an increase in reinforcement percentage. High elastic modulus and temperature mismatch between matrix and reinforcements increase the strength of the composites. The fracture behaviour of the composites generally showed dimples which are characteristics of ductile fracture, except at 15 wt.% Al2O3 / 2 wt.% ZrO2 showed a large portion of cleavage planes and grain cracking. The Al-Mg-Si alloy is reinforced with 9 wt%. Al2O3 / 2 wt.% ZrO2 had the best properties suitable for its use in different structural applications.

Large-Area Arrays of Polymer-Tethered Gold Nanorods with Controllable Orientation and Their Application in Nano-Floating-Gate Memory Devices.

In this work, it is reported that large-area (centimeter-scale) arrays of non-close-packed polystyrene-tethered gold nanorod (AuNR@PS) can be prepared through a liquid-liquid interfacial assembly method. Most importantly, the orientation of AuNRs in the arrays can be controlled by changing the intensity and direction of electric field applied in the solvent annealing process. The interparticle distance of AuNR can be tuned by varying the length of polymer ligands. Moreover, the AuNR@PS with short PS ligand are favorited to form orientated arrays with the assistance of electric field, while long PS ligands make the orientation of AuNRs difficult. The orientated AuNR@PS arrays are employed as the nano-floating gate of field-effect transistor memory device. Tunable charge trapping and retention characteristics in the device can be realized by electrical pulse with visible light illumination. The memory device with orientated AuNR@PS array required less illumination time (1 s) at the same onset voltage in programming operation, compared to the control device with disordered AuNR@PS array (illumination time: 3 s). Moreover, the orientated AuNR@PS array-based memory device can maintain the stored data for more than 9000 s, and exhibits stable endurance characteristic without significant degradation in 50 programming/reading/erasing/reading cycles.