High Packing Fraction Research Articles

Micron-Sized Silica-PNIPAM Core-Shell Microgels with Tunable Shell-To-Core Ratio.

Micron-sized hard core-soft shell hybrid microgels are promising model systems for studies of soft matter as they enable in-situ optical investigations and their structures/morphologies can be engineered with a great variety. Yet, protocols that yield micron-sized core-shell microgels with a tailorable shell-to-core size ratio are rarely available. In this work, we report on the one-pot synthesis protocol for micron-sized silica-poly(N-isopropylacrylamide) core-shell microgels that has excellent control over the shell-to-core ratio. Small-angle light scattering and microscopy of 2- and 3-dimensional assemblies of the synthesized microgels confirm that the produced microgels are monodisperse and suitable for optical investigation even at high packing fractions.

Powder Injection Molding an Alternative Method in the Manufacturing of Parts for Vehicles

Powder injection molding (PIM) is one of the most versatile methods for manufacturing small complex shaped components from metal, ceramic powders for the use in many applications. Interesting applications of PIM include the automotive and aerospace industries. PIM can be used to combine multiple parts into a single part with complex geometry, and parts are lighter since porosity remains in the final part while the fatigue resistance increases. Therefore, it can be said that PIM can help reduce the weight of moving vehicles, which is a major concern from the environmental and economic point of view. One of the main limitations of current PIM technology is the long debinding time, which is one of the steps within the process. This has been partially solved by introducing binder systems that undergo catalytic sublimation. However, current catalytic binders have the main problem of high viscosity. In this study, two ways to decrease the viscosity of PIM feedstock materials with polyoxymethylene were investigated. The first way was to reduce the average molecular weight of the binder and the second one to select a polydisperse particle size distribution with high maximum packing fraction.

Front-End Design for SiPM-Based Monolithic Neutron Double Scatter Imagers.

Neutron double scatter imaging exploits the kinematics of neutron elastic scattering to enable emission imaging of neutron sources. Due to the relatively low coincidence detection efficiency of fast neutrons in organic scintillator arrays, imaging efficiency for double scatter cameras can also be low. One method to realize significant gains in neutron coincidence detection efficiency is to develop neutron double scatter detectors which employ monolithic blocks of organic scintillator, instrumented with photosensor arrays on multiple faces to enable 3D position and multi-interaction time pickoff. Silicon photomultipliers (SiPMs) have several advantageous characteristics for this approach, including high photon detection efficiency (PDE), good single photon time resolution (SPTR), high gain that translates to single photon counting capabilities, and ability to be tiled into large arrays with high packing fraction and photosensitive area fill factor. However, they also have a tradeoff in high uncorrelated and correlated noise rates (dark counts from thermionic emissions and optical photon crosstalk generated during avalanche) which may complicate event positioning algorithms. We have evaluated the noise characteristics and SPTR of Hamamatsu S13360-6075 SiPMs with low noise, fast electronic readout for integration into a monolithic neutron scatter camera prototype. The sensors and electronic readout were implemented in a small-scale prototype detector in order to estimate expected noise performance for a monolithic neutron scatter camera and perform proof-of-concept measurements for scintillation photon counting and three-dimensional event positioning.

Multifluid model simulations of gravitational instabilities in fluidized binary granular materials

AbstractPredicting granular flow using continuum approaches is fundamental to advance granular physics and industrial applications. Previous studies have demonstrated that continuum modeling can capture some hydrodynamic instability analogs in monodisperse granular materials. Recently, a family of gravitational instabilities was found between two types of grains, including a Rayleigh–Taylor analog, a granular bubble rising, a granular droplet splitting and particle segregation. The high packing fraction and bi‐disperse nature of these flows allow for new challenges to continuum models. Herein, we applied multifluid modeling (MFM) that treats the gas phase and two separate granular phases as fully interpenetrating continua to simulate these instabilities and compared them with newly conducted experiments. Simulation results reproduced all of the four instabilities and demonstrated that frictional particle stress and solids–solids drag are needed to capture critical qualitative flow features. This study can serve as a basis for challenging future particle stress and solids–solids drag models.

Microwave absorption properties of Ni0.6Zn0.4Fe2O4 composites with nonmagnetic nanoferrite percentages

Microwave absorption materials need to be thin and lightweight and possess strong wave absorption ability and a wide absorption frequency band. To satisfy these conditions, we changed the microstructure of the composite by mixing ferrite material with different particle sizes. Specifically, we mixed nanosized nonmagnetic ZnFe2O4 powder into Ni0.6Zn0.4Fe2O4 powder, investigated the microwave absorption properties depending on the packing fraction. The crystal structure of the synthesized ferrite powders was analyzed through XRD, and the particle size was analyzed using a PSA and SEM. The density of the powders, which is required to measure the packing fraction, was determined via the gas disposition method, and the magnetic properties of the composites were analyzed using a VSM. The reflection loss represents the electromagnetic wave absorption characteristics, and it was calculated by substituting the measured permittivity and permeability values, into the equation based on the transmission line theory. The Ni0.6Zn0.4Fe2O4/ZnFe2O4 composite showed 99.9% absorption with a high packing fraction, and the absorption peak shifted to high frequencies. These characteristics suggest that the absorption ability and frequency range of the electromagnetic-wave-shielding composite can be easily controlled. Because of the high-absorption characteristic, absorption frequency control, and cost effectiveness, this composite can be applied to products such as thin electromagnetic-wave-shielding sheets.

Oxidative stability of Pickering emulsions.

In recent years, Pickering emulsions have been the focus of growing interest because of their possible role as alternatives to conventional emulsions. Some reviews have investigated the physical stability of Pickering emulsions, but the oxidative stability of these emulsions remains largely unexplored. In this review, the oxidation mechanism and factors affecting lipid oxidation rates in Pickering emulsions are discussed. Then, different food-grade solid particles are evaluated for their ability to stabilize Pickering emulsions. Finally, several strategies are reviewed for improving the oxidative stability of Pickering emulsions. These strategies are based on efforts to manipulate the physical and chemical properties of the interfacial layer, increase the concentration of antioxidants at the interfacial layer through incorporating them into solid particles, cause oil droplets to crowd at high packing fractions, trap oil droplets in a gel network and increase the viscosity of the continuous phase.

High volume packing fraction TRISO-based fuel in light water reactors

For a decade, fully ceramic microencapsulated (FCM) fuel, containing tri-structural isotropic (TRISO) fuel particles in a silicon carbide (SiC) matrix, has been investigated as an accident-tolerant fuel for light water reactors (LWRs). Other examples exist of TRISO-based concepts for LWR fuels with different matrix materials. Previous studies assumed TRISO particle volume packing of approximately 0.44 in SiC (or another) matrix, the highest realistic packing fractions possible with conventional manufacturing. Recent advances in advanced manufacturing have yielded the development and demonstration of a fuel form that consists of conventionally manufactured TRISO particles in a 3D-printed SiC matrix with significantly higher possible TRISO packing fractions (0.5–0.7). This increased uranium loading enhances the viability of using TRISO-based particle fuel forms in LWRs. The viability of high-packing-fraction TRISO-based particle fuel forms in LWRs is assessed from the perspective of fuel cycle length, achievable fuel burnup, reactivity coefficients, and fuel cycle performance. Higher-packing-fraction TRISO-based fuel enables either longer cycle lengths (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant enrichment or decreased enrichments (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant cycle length. Studies of different fuel kernel types (uranium nitride, uranium oxycarbide, and uranium carbide) yield similar results, although the cycle length of uranium oxycarbide is shorter than for uranium nitride or uranium carbide (due to the lower density of uranium oxide). This work also characterized the production of 14C resulting from neutron absorption in 14N during operation for uranium mononitride fuel kernels; the ratio of 14C/N was 1–2 at. % at discharge. For the fuel cycle evaluation, the activity of spent nuclear fuel and high-level waste at 100 and 100,000 years was lower for high-packing-fraction fuels than for conventional LWR fuel. Environmental impact metrics were similar overall, but higher on the front end of the fuel cycle and lower on the back end of the fuel cycle. Reactivity coefficients of higher-packing-fraction TRISO-based fuel were reasonable compared with those of conventional fuels.

Quantum simulations of thermally activated delayed fluorescence in an all-organic emitter.

We investigate the prototypical NAI-DMAC thermally activated delayed fluorescence (TADF) emitter in the gas phase- and high-packing fraction limits at finite temperature, by combining first principles molecular dynamics with a quantum thermostat to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔEST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔEST vanishes in the perfect crystal, it is of the order of ∼0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between the HOMOs and LUMOs has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results on ΔEST and oscillator strengths, together with our estimates of direct and reverse intersystem crossing rates, show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds. Our findings highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC.

Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid): A Calorimetric Study.

Stimuli-responsive microgels have recently attracted great attention in fundamental research as their soft particles can be deformed and compressed at high packing fractions resulting in singular phase behaviours. Moreover, they are also well suited for a wide variety of applications such as drug delivery, tissue engineering, organ-on-chip devices, microlenses fabrication and cultural heritage. Here, thermoresponsive and pH-sensitive cross-linked microgels, composed of interpenetrating polymer networks of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc), are synthesized by a precipitation polymerization method in water and investigated through differential scanning calorimetry in a temperature range across the volume phase transition temperature of PNIPAM microgels. The phase behaviour is studied as a function of heating/cooling rate, concentration, pH and PAAc content. At low concentrations and PAAc contents, the network interpenetration does not affect the transition temperature typical of PNIPAM microgel in agreement with previous studies; on the contrary, we show that it induces a marked decrease at higher concentrations. DSC analysis also reveals an increase of the overall calorimetric enthalpy with increasing concentration and a decrease with increasing PAAc content. These findings are discussed and explained as related to emerging aggregation processes that can be finely controlled by properly changing concentration, PAAc content an pH. A deep analysis of the thermodynamic parameters allows to draw a temperature–concentration state diagram in the investigated concentration range.

Investigation on thermo-mechanical performance of fully ceramic microencapsulated fuel

• A finite element method (FEM) model for FCM fuel thermo-mechanical performances analysis was presented. • The thermo-mechanical behaviors, coupled with irradiation behaviors were developed. • A 3D modeling of FCM fuel pellet with typical packing fractions was realized by an optimized stochastic strategy. • Effects of thermal boundaries, burnup, particle spacing, no fuel zone width and other key parameters were well investigated. Fully ceramic microencapsulated (FCM) fuel is a promising ATF concept with extraordinary fission product inclusion capacity, excellent oxidation and corrosion resistance, and high thermal conductivity. The thermo-mechanical performances are of great significance when evaluating the safety and applicability of a nuclear fuel. In this study, a finite element method (FEM) model for FCM thermo-mechanical performances analysis was presented. The thermo-mechanical behaviors, coupled with irradiation behaviors (including irradiation-induced dimensional change (IIDC), heat generation, conduction, thermal expansion, creep, burnup, and gap/plenum pressure, etc.), were developed to investigate the thermal and mechanical characteristics of TRISO-based FCM Fuel. A simplified model of TRISO fuel particles was established and validated. After that, a full 3D modeling of FCM fuel pellet with typical packing fractions was realized by an optimized stochastic strategy. A numerical method was validated and successfully applied to realize the simulations of a FCM fuel pellet. Two different thermal boundaries, namely fixed particle power and fixed pellet power, were performed to investigate the irradiation behavior of FCM fuel. Moreover, the effects of burnup, particle spacing, no fuel zone width and other key parameters were well studied and investigated. The performed calculation showed that temperature and stress in matrix and TRISO are both positively correlated with the packing fraction. It's worth noting that compared with the cases of fixed particle power, cases of fixed pellet power may lead to higher fuel temperature and higher stress. The results also implied that from the perspective of probability evaluation, most of the TRISO particles in FCM fuel can maintain the structural integrity under different packing fractions, but some of the particles with higher stress may crack or damage, especially in the cases of high burnup and high packing fraction.

Adhesion Percolation Determines Global Deformation Behavior in Biomimetic Emulsions

Characterizing the mechanical properties of tissues is key for the understanding of fundamental biological processes such as morphogenesis or tumor progression. In particular, the intercellular adhesion forces, mediated by transmembrane proteins like cadherins, are expected to control the topology and viscoelastic behavior of tissues under mechanical stress. In order to understand the influence of adhesion in tissues, we use biomimetic emulsions in which droplets mimic cells and adhere to each other through specific bonds. Here, we tune both the binding energy of the adhesive inter-droplets contacts as well as the fraction of contacts that are adhesive, thereby defining a so-called adhesiveness. Our experimental results show that adhesion prevents the emergence of local order in emulsions even at high packing fractions by preventing energetically costly droplet rearrangements. By studying the deformation of droplets within packings with different average adhesiveness values, we reveal the existence of a threshold value of adhesiveness above which all droplets in a packing are deformed as adhesive ones irrespective of their local adhesive properties. We show that this critical adhesiveness coincides with the threshold for percolation of adhesive structures throughout the tissue. From a biological point of view, this indicates that only a fraction of adhesive cells would be sufficient to tune the global mechanical properties of a tissue, which would be critical during processes such as morphogenesis.

An Effective Thermal Model of Coated Particle Dispersed Fuel with High Packing Fraction

Accurate prediction of the temperature distribution in nuclear fuel is an important step for the design and screening of multi-coating particle dispersed nuclear fuel. This study aims to establish an effective thermal model and numerical solutions for macro scale heat transfer analysis by investigating the effective thermal properties of multi-coating particles and the dispersed fuel bulk. The effects of particle arrangement, sizes, clustering on temperature distribution inside nuclear fuel element was studied with the established effective model. This study helps clarify the micro- and macro- scale heat transfer mechanism of multi-coating particle dispersed fuel, and provides a guidance for the design, optimization and safety analysis of this type of fuel.

High-resolution time-of-flight PET detector with 100 ps coincidence time resolution using a side-coupled phoswich configuration

Photon time-of-flight (TOF) capability in positron emission tomography (PET) enables reconstructed image signal-to-noise ratio (SNR) improvement. With the coincidence time resolution (CTR) of 100 picosecond (ps), a five-fold SNR improvement can be achieved with a 40 cm diameter imaging subject, relative to a system without TOF capability. This 100 ps CTR can be achieved for a clinically relevant detector design (crystal element length ≥20 mm with reasonably high crystal packing fraction) using a side-readout PET detector configuration that enables 511 keV photon interaction depth-independent light collection efficiency and lower variance in scintillation photon transit time to the silicon photomultiplier (SiPM). In this study, we propose a new concept of TOF-PET detector to achieve high (<2 mm) resolution, using a ‘side-coupled phoswich’ configuration, where two crystals with different decay times (τ d ) are coupled in a side-readout configuration to a common row of photosensors. The proposed design was validated and optimized with GATE Monte Carlo simulation studies to determine an efficient detector design. Based on the simulation results, a proof-of-concept side-coupled phoswich detector design was developed comprising two LSO crystals with the size of 1.9 × 1.9 × 10 mm3 with decay times of 34.39 and 43.07 ns, respectively. The phoswich crystals were side-coupled to the same three 4 × 4 mm2 SiPMs and detector performances were evaluated. As a result of the experimental evaluation, the side-coupled phoswich configuration achieved CTR of 107 ± 3 ps, energy resolution of 10.5% ± 1.21% at 511 keV and >95% accuracy in identifying interactions in the two adjacent 1.9 × 1.9 × 10 mm3 crystal elements using the time-over-threshold technique. Based on our results, we can achieve excellent spatial and energy resolution in addition to ∼100 ps CTR with this novel detector design.

Alternative formulation of the induced surface and curvature tensions approach

We develop a novel method to analyze the excluded volume of the multicomponent mixtures of classical hard spheres in the grand canonical ensemble. The method is based on the Laplace–Fourier transform technique and allows one to account for the fluctuations of the particle number density for the induced surface and curvature tensions equation of state. As a result one can go beyond the Van der Waals (VdW) approximation by obtaining the suppression of the induced surface and curvature tensions coefficients at moderate and high packing fractions. In contrast to the standard induced surface and curvature tensions equation of state the suppression of these coefficients is not the exponential, but a power-like one. The obtained alternative equation of state is further generalized to account for higher virial coefficients. This result is straightforwardly generalized to the case of quantum statistics.

Replication of the tensile behavior of knee ligaments using architected acrylic yarn

Knee ligament injury diagnosis is achieved by a comparison between the laxity levels sensed by a clinician in the injured and healthy limb. This is a difficult-to-learn task that requires hands-on practice to achieve proficiency. The inclusion of a physical knee simulator with biomechanically realistic passive components such as knee ligaments could provide consistent training for medical students and lead to improved care for knee injury patients. In this study, we developed a material construct that is both adaptable to a physical knee model and capable of replicating the non-linear mechanical behavior of knee ligaments with the use of helically arranged acrylic yarn. The microstructure of four different types of acrylic yarn were measured and then tested under uniaxial tension. While the fiber twist angle was similar amongst the four yarn types (range = 17.9–18.8°), one yarn was distinct with a low ply twist angle (15.2 ± 1.6°) and high packing fraction (Φ=0.32±0.08). These microstructural differences yielded a lower toe length and higher stiffness and best corresponded to ligament mechanical behavior. We then made looped-yarn constructs to modulate the sample's toe length and stiffness. We found that the load-displacement curve of the construct can be tuned by changing the loop length and loop number of the looped-yarn constructs, matching the load-displacement curve of specific knee ligaments. This study shows how spun yarn can be used to replicate the mechanical behavior of knee ligaments, creating synthetic ligament constructs that could enable the construction of biomechanically realistic joints.

Synthesis of Rough Colloidal SU-8 Rods and Bananas via Nanoprecipitation

Surface roughness plays an important role in determining the mechanical properties, wettability, and self-assembly in colloidal systems. In this work, we develop a simple and fast method to produce rough colloidal SU-8 rods, bananas, and spheres, via the nanoprecipitation of SU-8 in water. During this process, SU-8 nanospheres are absorbed onto the surface of the colloidal SU-8 particles and then cross-linked using UV-light. The size of the spherical asperities and the asperity density are controlled by the concentration of SU-8 used during the nanoprecipitation reaction. Fluorescent labeling of the rough SU-8 colloidal particles allows for their confocal imaging, which demonstrates their stability at high packing fractions. With these newly developed rough particles, we provide a colloidal model system that allows for studies addressing the impact of surface roughness on materials composed of anisotropic particles.

Modelling the compaction of plastic particle packings

Soft particle materials such as some pharmaceutical and food products are composed of particles that can undergo large deformations under low confining pressures without rupture. The rheological and textural properties of these materials are thus governed by both particle rearrangements and particle shape changes. For the simulation of soft particle materials, we present a numerical technique based on the material point method, allowing for large elasto-plastic particle deformations. Coupling the latter with the contact dynamics method makes it possible to deal with contact interactions between particles. We investigate the compaction of assemblies of elastic and plastic particles. For plastic deformations, it is observed that the applied stress needed to achieve high packing fraction is lower when plastic hardening is small. Moreover, predictive models, relating stress and packing fraction, are proposed for the compaction of elastic and plastic particles. These models fit well our simulation results. Furthermore, it is found that the evolution of the coordination number follows a power law as a function of the packing fraction beyond jamming point of hard particle packings.

Diverse densest binary sphere packings and phase diagram

We revisit the densest binary sphere packings (DBSPs) under periodic boundary conditions and present an updated phase diagram, including newly found 12 putative densest structures over the x-α plane, where x is the relative concentration and α is the radius ratio of the small and large spheres. To efficiently explore the DBSPs, we develop an unbiased random search approach based on both the piling-up method to generate initial structures in an unbiased way and the iterative balance method to optimize the volume of a unit cell while keeping the overlap of hard spheres minimized. With those two methods, we have discovered 12 putative DBSPs and thereby the phase diagram is updated, while our results are consistent with those of a previous study [Hopkins et al., Phys. Rev. E 85, 021130 (2012)]PLEEE81539-375510.1103/PhysRevE.85.021130 with a small correction for the case of 12 or fewer spheres in the unit cell. Five of the discovered 12 DBSPs are identified in the small radius range of 0.42≤α≤0.50, where several structures are competitive to each other with respect to packing fraction. Through the exhaustive search, diverse dense packings are discovered and, accordingly, we find that packing structures achieve high packing fractions by introducing distortion and/or combining a few local dense structural units. Furthermore, we investigate the correspondence of the DBSPs with crystals based on the space group. The result shows that many structural units in real crystals, e.g., LaH_{10} and SrGe_{2-δ} being high-pressure phases, can be understood as DBSPs. The correspondence implies that the densest sphere packings can be used effectively as structural prototypes for searching complex crystal structures, especially for high-pressure phases.

AN IMPROVED DISTINCT ELEMENT METHOD FOR HIGH PACKING FRACTION STOCHASTIC MEDIA MODELING

Due to the generality and flexibility of Monte Carlo method in geometric modeling, Monte Carlo method plays an important role in accurate simulation of random media. At present, rand om sequential addition method (RSA) and distinct element method (DEM) are more accurate and mature explicit modeling methods. The former approach has the problem of upper limit of packing fraction, which is suitable for stochastic geometry with lower filling rate. DEM method can fill random medium model with packing fraction higher than 60%, but DEM is not suitable for non-contact dispersed particles based on the interaction between particles. There fore, an improved DEM method is proposed to solve the problem of modeling non-contact p articles dispersed in the stochastic media with high packing fraction. The virtual surfaces are constructed outside of the outer layer of particles to make them in contact with each other. Thus, the particle system is suitable for DEM method. The construction of virtual surface does not affect the neutron transport process. The correctness of the improved DEM is verified by comparing the total filling particle number and calculation results ofkeffwith RSA method. At the same time, according to the distribution of filling particles, the improved DEM method fills the particles more uniformly.

Long-range interactions in model colloidal dispersions with surface charge distributions

The theoretical model, which is based on the density functional approach, has been developed for studying the electrostatic properties of colloidal dispersions (modified colloidal primitive model; MCPM) containing hard-spherical ions with a homogeneous surface charge distribution. The mean-spherical approximation for the multi-component charged ions has been used to account for the electrostatic ion correlation effects. The present study reflects the importance of the charge distribution of macroion in determining the ionic structure and mean forces acting on the macroions. Compared with the MCPM, the colloidal primitive model (CPM), on which the colloidal charge is assumed to be in the center of particle, shows very long-range electrostatic properties and mean force acting on the macroions because of the strong cross correlation between the hard-sphere contribution and the Coulomb interaction. The long-range attractions and repulsions in the charged colloidal dispersions originate from the entropic effects and are found at high packing fractions of the colloidal ions.