Low Packing Fractions Research Articles

Diffusionless rotator-crystal transitions in colloidal truncated cubes.

Upon osmotic compression, rotationally symmetric faceted colloidal particles can form translationally ordered, orientationally disordered rotator mesophases. This study explores the mechanism of rotator-to-crystal phase transitions where orientational order is gained in a translationally ordered phase, using rotator-phase forming truncated cubes as a testbed. Monte Carlo simulations were conducted for two selected truncations (s), one for s = 0.527 where the rotator and crystal lattices are dissimilar and one for s = 0.572 where the two phases have identical lattices. These differences set the stage for a qualitative difference in their rotator-crystal transitions, highlighting the effect of lattice distortion on phase transition kinetics. Our simulations reveal that significant lattice deviatoric effects could hinder the rotator-to-crystal transition and favor arrangements of lower packing fraction instead. Indeed, upon compression, it is found that for s = 0.527, the rotator phase does not spontaneously transition into the stable, densely packed crystal due to the high lattice strains involved but instead transitions into a metastable solid phase to be colloquially referred to as "orientational salt" for short, which has a similar lattice as the rotator phase and exhibits two distinct particle orientations having substitutional order, alternating regularly throughout the system. This study paves the way for further analysis of diffusionless transformations in nanoparticle systems and how lattice-distortion could influence crystallization kinetics.

Comparison between tritium breeders with pebble bed and Gyroid block configurations using numerical investigation: Flow behavior and heat transfer performance

This study proposed using a Gyroid structure, a type of triply periodic minimal surface (TPMS), to replace conventional pebble bed configurations and address drawbacks such as inherently low packing fractions, extensive stress concentrations, high flow resistance, and low thermal conductivity. The feasibility of the Gyroid block configuration was evaluated by comparing it with a pebble bed configuration regarding the flow behaviors and heat transfer performance. The flow behavior of the porous block configuration was superior to that of the pebble bed configuration. At the same inlet velocity of 0.1–0.2 m/s, the porous block configuration had a ∼56 % higher average flow velocity and only half the pressure drop. The convective heat transfer coefficient and Nusselt numbers in the porous block configuration were 56–84 % and 2.6–3.1 times higher, respectively, than those in the pebble bed configuration. The effective thermal conductivity of the porous block configuration increased by ∼18 % at an inlet velocity of 0.15 m/s and a temperature of 773–1173 K. This study preliminarily verified the feasibility of the Gyroid structures for tritium breeders in engineering applications and is anticipated to provide new opportunities for the design of tritium breeders with higher lithium densities, remarkable flow behavior, and outstanding thermal performance.

Stereolithography and gelcasting of Li2TiO3 block breeders with a Gyroid structure: Slurry optimization and performance evaluation

This study proposes porous block structures as a viable alternative to conventional pebble bed structures, aiming to overcome the structural drawbacks including low packing fractions, extensive stress concentrations, and low thermal conductivity. To validate the feasibility of the porous block structures, Gyroid Li2TiO3 block breeders were prepared by the hybrid process of stereolithography and gelcasting. The resulting Gyroid Li2TiO3 block breeders, measuring ∼18.8 × 18.8 × 18.8 mm3, achieved a high packing fraction of ∼78.8% through optimized slurry and sintering at 1100 °C for 3 h. These Gyroid Li2TiO3 block breeders exhibited clear contours, integrated channels, a compressive strength of 8.78 MPa, and a low isotropic sintering shrinkage of ∼6%. The hybrid process demonstrated enormous advantages over the reported 3D printing for the preparation of porous block breeders with low shrinkage and large-sized dimensions. This study offers an effective avenue for the manufacture of porous block breeders with tunable tritium-releasing channels and large-sized configurations.

A passive star polymer in a dense active bath: insights from computer simulations.

Using computer simulations in two dimensions (2D), we explore the structure and dynamics of a star polymer with three arms made of passive monomers immersed in a bath of active Brownian particles (ABPs). We analyze the conformational and dynamical changes of the polymer as a function of activity and packing fraction. We also study the process of motility induced phase separation (MIPS) in the presence of a star polymer, which acts as a mobile nucleation center. The presence of the polymer increases the growth rate of the clusters in comparison to a bath without the polymer. In particular, for low packing fraction, both nucleation and cluster growth are affected by the inclusion of the star polymer. Clusters grow in the vicinity of the star polymer, resulting in the star polymer experiencing a caged motion similar to a tagged ABP in the dense phase. Due to the topological constraints of the star polymers and clustering nearby, the conformational changes of the star polymer lead to interesting observations. Inter alia, we observe the shrinking of the arm with increasing activity along with a short-lived hairpin structure of one arm formed. We also see the transient pairing of two arms of the star polymer, while the third is largely separated at high activity. We hope our findings will help in understanding the behavior of active-passive mixtures, including biopolymers of complex topology in dense active suspensions.

Water permeability of sunlit lunar highlands regolith using LHS-1 simulant

The regolith on the Earth's Moon has been attracting attention from the perspective of understanding the planetary surface processes and procuring in-situ resources. However, very few studies have investigated the hydraulic properties of the lunar regolith, which should pose one of the most significant physical properties for understanding the lunar surface processes and evaluating regolith utilizations. In this paper, we study the water permeability of the surficial regolith layer on sunlit lunar highlands using the lunar highlands regolith simulant LHS-1. We measure the water permeability of LHS-1 at packing fractions of 0.49–0.60 by simulating lunar gravitational environments and evaluating the influences of freeze-thaw, which is expected to occur on the sunlit lunar highland surfaces. Our results indicate that the water permeability of lunar highland regolith is one to two orders of magnitudes lower than that of fine silica sands and monodisperse spherical glass beads at the given packing fraction. The low water permeability of lunar highland regolith attributes to the regolith particles having irregular and angular shapes with sharp edges and micron-sized chunks due to the least weathering, unlike soil particles on the Earth. The horizontal water permeability of lunar highland regolith simulant before freeze-thaw is 1.6 × 10−13 m2 at a packing fraction of 0.6. From the porosity-depth profile available from previous studies, we also obtained the water permeability-depth curve, representing the vertical water permeabilities of 6 × 10−13–2 × 10−12 m2 and 7 × 10−14–3 × 10−13 m2 at the lunar surface and the depth of 5 m within the surficial lunar regolith layer, respectively. We also found that the water permeability of lunar highland regolith increased after one freeze-thaw cycle, achieving 6.4 times higher than the permeability before the freeze, and the increase was more significant with lower packing fractions. We suggest that the pore water in the solid phase due to the freeze encourages the rearrangement of the regolith particles changing the total contact area between the particles. This study will help us better understand the abundance and distribution of molecular water on the lunar surface, lunar surface processes, and regolith utilizations.

Collective vortical motion and vorticity reversals of self-propelled particles on circularly patterned substrates.

The collective behavior of self-propelled particles (SPPs) under the combined effects of a circularly patterned substrate and circular confinement is investigated through coarse-grained molecular dynamics simulations of polarized and disjoint ring polymers. The study is performed over a wide range of values of the SPPs packing fraction ϕ[over ¯], motility force F_{D}, and area fraction of the patterned region. At low packing fractions, the SPPs are excluded from the system's center and exhibit a vortical motion that is dominated by the substrate at intermediate values of F_{D}. This exclusion zone is due to the coupling between the driving force and torque induced by the substrate, which induces an outward spiral motion of the SPPs. For high values of F_{D}, the SPPs exclusion from the center is dominated by the confining boundary. At high values of ϕ[over ¯], the substrate pattern leads to reversals in the vorticity, which become quasiperiodic with increasing ϕ[over ¯]. We also found that the substrate pattern is able to separate SPPs based on their motilities.

The interplay between chemo-phoretic interactions and crowding in active colloids.

Many motile microorganisms communicate with each other and their environments via chemical signaling which leads to long-range interactions mediated by self-generated chemical gradients. However, consequences of the interplay between crowding and chemotactic interactions on their collective behavior remain poorly understood. In this work, we use Brownian dynamics simulations to investigate the effect of packing fraction on the formation of non-equilibrium structures in a monolayer of diffusiophoretic self-propelled colloids as a model for chemically active particles. Focusing on the case when a chemical field induces attractive positional and repulsive orientational interactions, we explore dynamical steady-states of active colloids of varying packing fractions and degrees of motility. In addition to collapsed, active gas, and dynamical clustering steady-states reported earlier for low packing fractions, a new phase-separated state emerges. The phase separation results from a competition between long-range diffusiophoretic interactions and motility and is observed at moderate activities and a wide range of packing fractions. Our analysis suggests that the fraction of particles in the largest cluster is a suitable order parameter for capturing the transition from an active gas and dynamical clustering states to a phase-separated state.

Study on kinetic parameters of pebble bed reactor with TRISO duplex fuel

Thorium, in this case, 232Th has a higher thermal neutron capture cross-section than 238U, which means that more fertile isotopes can be transmuted and could lead to higher fissile isotope 233U. In addition, 233U has a good performance in the thermal spectrum. Theoretically, a nuclear reactor using thorium fuel can also last longer than one using uranium fuel. The use of TRISO duplex fuel is predicted to produce better neutronic behavior in a pebble bed reactor. This work aims to study the kinetic parameters of a pebble bed reactor with TRISO duplex fuel. The configuration of the TRISO duplex fuel pebble consists of an inner region filled with UO2 TRISO particles and an outer region filled with ThO2 TRISO particles surrounded by a graphite matrix of fuel pebble. Three configurations with volume fraction of UO2-ThO2 were considered in this study: 80-20 %, 75-25 %, and 70-30 %. The HTR-10 reactor was chosen as a reactor model because its geometry and material specifications are known. A series of calculations were conducted using the Monte Carlo transport code MCNP6 and ENDF/B-VII.1 nuclear data library. The calculation results were then analyzed to investigate the effect of UO2 and ThO2 compositions in TRISO duplex fuel on the kinetic parameters of the pebble bed reactor with various TRISO packing fractions of 1-50 %. It can be concluded that the utilization of TRISO duplex fuel in a pebble bed reactor could significantly affect the core multiplication factor and kinetic parameters caused by an increase in Th content. On the other hand, the TRISO packing fraction is taking part in neutron moderation since a lower packing fraction means higher moderation for fueled pebble.

Diffusion and Velocity Correlations of the Phase Transitions in a System of Macroscopic Rolling Spheres.

We study an air-fluidized granular monolayer composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never lose contact with the grid and so that the dynamics may be regarded as pseudo two dimensional (or two dimensional, if the effects of the sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all the cases, we found the coexisting phases display a strong energy non-equipartition. In the first transition, at a weak fluidization, a glass phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at a very low packing fraction and for a wide range of granular temperatures. Both phases are also characterized by strong anticorrelated velocities upon a collision. Thus, the dynamics observed for this quasi two-dimensional system unveil phase transitions with peculiar properties, very different from the predicted behavior in well-know theories for their equilibrium counterparts.

Granular piston-probing in microgravity: powder compression, from densification to jamming

The macroscopic response of granular solids is determined by the microscopic fabric of force chains, which, in turn, is intimately linked to the history of the solid. To query the influence of gravity on powder flow behavior, a granular material is subjected to compression by a piston in a closed container, on-ground and in microgravity (on parabolic flights). Results show that piston-probing densifies the packing, eventually leading to jamming of the material compressed by the piston, regardless of the gravitational environment. The onset of jamming is found to appear at lower packing fraction in microgravity ({varphi }_{J}^{mu -{{{rm{g}}}}}=0.567pm 0.014) than on-ground ({varphi }_{J}^{{{{rm{gnd}}}}}=0.579pm 0.014). We interpret these findings as the manifestation of a granular fabric altered by the gravitational force field: in absence of a secondary load (due to gravitational acceleration) to stimulate reorganization in a different direction to the major compression stress, the particles’ configuration becomes stable at lower density, as the particles have no external drive to promote reorganization into a denser packing. This is coupled with a change in interparticular force balance which takes place under low gravity, as cohesive interactions become predominant. We propose a combination of microscopic and continuum arguments to rationalize our results.

Monitoring SARS-CoV-2 Surrogate TGEV Individual Virions Structure Survival under Harsh Physicochemical Environments.

Effective airborne transmission of coronaviruses via liquid microdroplets requires a virion structure that must withstand harsh environmental conditions. Due to the demanding biosafety requirements for the study of human respiratory viruses, it is important to develop surrogate models to facilitate their investigation. Here we explore the mechanical properties and nanostructure of transmissible gastroenteritis virus (TGEV) virions in liquid milieu and their response to different chemical agents commonly used as biocides. Our data provide two-fold results on virus stability: First, while particles with larger size and lower packing fraction kept their morphology intact after successive mechanical aggressions, smaller viruses with higher packing fraction showed conspicuous evidence of structural damage and content release. Second, monitoring the structure of single TGEV particles in the presence of detergent and alcohol in real time revealed the stages of gradual degradation of the virus structure in situ. These data suggest that detergent is three orders of magnitude more efficient than alcohol in destabilizing TGEV virus particles, paving the way for optimizing hygienic protocols for viruses with similar structure, such as SARS-CoV-2.

Bespoke particle shapes in granular matter

Among granular matter, one type of particle has special properties. Upon being assembled in disordered configurations, these particles interlock, hook, almost braid, and – surprisingly, considering their relatively low packing fractions – show exceptional shear strength.Such is the case of non-convex particles. They have been used in the shapes of tetrapods, ‘L’, ‘Z’, stars, and many others, to protect coasts or build self-standing structures requiring no binders or external supports. Although these structures are often designed without a comprehensive mechanical characterization, they have already demonstrated great potential as highly resistant construction materials. Nevertheless, it is natural to attempt to find the most appropriate non-convex shapes for any given application. Can a particle shape be tuned to obtain a desired mechanical behavior? Although this question cannot be answered yet, current technological, simulation, and experimental developments strongly suggest that it can be resolved in the next decade. A clear understanding of the relationships between particle shapes, mechanical response, and packing properties will be key to providing insights into the behavior of these materials. Such work should stand on 1) robust and general shape descriptors that encode the complexity of non-convex shapes (i.e., the number of arms, the symmetries, and asymmetries of the bodies, the presence of holes, etc.), 2) the analysis of the response of assemblies under different loading conditions, and 3) the disposition and reliability of non-convex shapes to ensure durability. The manufacturing process and an efficient use of resources are additional elements that could further help to optimize particle shape. In the quest of designing bespoke non-convex particles, this paper consolidates the challenges that remain unresolved. It also outlines some routes to explore based on the latest developments in technology and research.

Universal features of the stick-slip dynamics of an intruder moving through a confined granular medium.

Experiments and simulations of an intruder dragged by a spring through a two-dimensional annulus of granular material exhibit robust force fluctuations. At low packing fractions (ϕ<ϕ_{0}), the intruder clears an open channel. Above ϕ_{0}, stick-slip dynamics develop, with an average energy release that is independent of the particle-particle and particle-base friction coefficients but does depend on the width W of the annulus and the diameter D of the intruder. A simple model predicts the dependence of ϕ_{0} on W and D, allowing for a data collapse for the average energy release as a function of ϕ/ϕ_{0}. These results pose challenges for theories of mechanical failure in amorphous materials.

Simple hexagonal structured gold with eight-coordination formed with ordered structural vacancies

Phase engineering of nanomaterials attracts increasing research interest, as crystal structures of metals often determine their physical and chemical properties. Metals tend to form close-packed crystal structures to maximize their space filling and atomic coordination numbers due to the isotropic metallic bonding. Hitherto no metals yet crystallize with the simple hexagonal packing which has low coordination number and packing fraction. Here, we fabricate a novel simple hexagonal structured gold through in situ selective etching on ordered Cu-Au intermetallic compound, where the new crystal structure has only 8 nearest neighbors for each atom and a low packing fraction of 0.60. The simple hexagonal structured Au demonstrates an electrical conductivity of 389 S/m, approximately 105 times lower than that of face-centered cubic structured gold, according to first principles calculation, associated with localized valence electrons. The discovery of simple hexagonal structure suggests that metals can form more diverse crystal structures by changing effective atomic volume, which provides a pathway to engineer new structures of metals for novel physical properties and applications.

Effect of Internal Architecture on the Assembly of Soft Particles at Fluid Interfaces.

Monolayers of soft colloidal particles confined at fluid interfaces are at the core of a broad range of technological processes, from the stabilization of responsive foams and emulsions to advanced lithographic techniques. However, establishing a fundamental relation between their internal architecture, which is controlled during synthesis, and their structural and mechanical properties upon interfacial confinement remains an elusive task. To address this open issue, which defines the monolayer’s properties, we synthesize core–shell microgels, whose soft core can be chemically degraded in a controlled fashion. This strategy allows us to obtain a series of particles ranging from analogues of standard batch-synthesized microgels to completely hollow ones after total core removal. Combined experimental and numerical results show that our hollow particles have a thin and deformable shell, leading to a temperature-responsive collapse of the internal cavity and a complete flattening after adsorption at a fluid interface. Mechanical characterization shows that a critical degree of core removal is required to obtain soft disk-like particles at an oil–water interface, which present a distinct response to compression. At low packing fractions, the mechanical response of the monolayer is dominated by the outer polymer chains forming a corona surrounding the particles within the interfacial plane, regardless of the presence of a core. By contrast, at high compression, the absence of a core enables the particles to deform in the direction orthogonal to the interface and to be continuously compressed without altering the monolayer structure. These findings show how fine, single-particle architectural control during synthesis can be engineered to determine the interfacial behavior of microgels, enabling one to link particle conformation with the resulting material properties.

Structural and dynamical properties of dilute gel networks in colloid-polymer mixtures.

The competition of short-ranged depletion attraction and long-ranged repulsion between colloidal particles in colloid-polymer mixtures leads to the formation of heterogeneous gel-like structures. Our special focus will be on the states where the colloids arrange in thin strands that span the whole system and that we will refer to as dilute gel networks. These states occur at low packing fractions for attractions that are stronger than those at both the binodal line of the equilibrium gas-liquid phase separation and the directed percolation transition line. By using Brownian dynamics simulations, we explore the formation, structure, and aging dynamics of dilute gel networks. The essential connections in a dilute gel network are determined by constructing reduced networks. We compare the observed properties to those of clumpy gels or cluster fluids. Our results demonstrate that both the structure and the (often slow) dynamics of the stable or meta-stable heterogeneous states in colloid-polymer mixtures possess distinct features on various length and time scales and thus are richly diverse.

Packing fraction of clusters formed in free-falling granular streams based on flash x-ray radiography

We study the packing fraction of clusters in free-falling streams of spherical and irregularly shaped particles using flash x-ray radiography. The estimated packing fraction of clusters is low enough to correspond to coordination numbers less than 6. Such coordination numbers in numerical simulations correspond to aggregates that collide and grow without bouncing. Moreover, the streams of irregular particles evolved faster and formed clusters of larger sizes with lower packing fraction. This result on granular streams suggests that particle shape has a significant effect on the agglomeration process of granular materials.

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.

Criticality Study on Different Pebble Arrangement inside HTR Reactor Core

Simplification on modeling of high temperature reactor core (HTR) of pebble bed type has been widely developed before. The last calculation that writer develop on modeling and simplification for HTR modelling has been done on TRISO and single pebble, especially on it is KINF and burnup calculation. From that calculation, it is known that TRISO modeling that using fuel kernels and cover (homogenized buffer, PyC and SiC) gives K-INF values that are not (much) different than the complete TRISO modeling (fuel kernel, buffer, PyC and SiC on each different cell). Then the purpose of this study was to analyze the effect on KEFF and calculation time of combined TRISO particle arrangement modeling in the HTR fuel pebble and this pebble arrangement inside reactor core using MCNPX. The modeling variations that will be carried out include modeling of HTR fuel pebble using simplified TRISO (fuel kernel and homogenized TRISO coating layers) that uniformly dispersed inside pebble using HCP, SC, FCC and BCC lattice, that arranged inside reactor core (also) using HCP, SC, FCC and BCC lattice. For all variations mentioned above, K-EFF calculations has been done for each fuel height variation from 90-190 cm inside reactor core, with 50% fuel pebble to fuel-moderator pebble ratio, in 61% pebble volume fraction ratio except Simple Cubic (SC) lattice (max. 52.36%). Whole modelling gives K-EFF that have not much deviation except all modelling using SC pebble arrangement inside reactor core, because its low packing fraction. SC-type TRISO dispersion mode inside pebble always need more calculation time than other model. The modeling using HCP TRISO unit inside HCP pebble unit inside reactor core gives consistently short calculation time, same as almost all calculation using FCC TRISO unit inside pebble always have shortest calculation time compare to other TRISO unit. FCC and BCC pebble unit inside reactor core using less calculation time compare to HCP at most calculation.

Particle dynamics in two-dimensional point-loaded granular media composed of circular or pentagonal grains

Granular packings exhibit significant changes in rheological and structural properties when the rotational symmetry of spherical or circular particles is broken. Here, we report on experiments exploring the differences in dynamics of a grain-scale intruder driven through a packing of either disks or pentagons, where the presence of edges and vertices on grains introduces the possibility of rotational constraints at edge-edge contacts. We observe that the intruder’s stick-slip dynamics are comparable between the disk packing near the frictional jamming fraction and the pentagonal packing at significantly lower packing fractions. We connect this stark contrast in packing fraction with the average speed and rotation fields of grains during slip events, finding that rotation of pentagons is limited and the flow of pentagonal grains is largely confined in front of the intruder, whereas disks rotate more on average and circulate around the intruder to fill the open channel behind it. Our results indicate that grain-scale rotation constraints significantly modify collective motion of grains on mesoscopic scales and correspondingly enhance resistance to penetration of a local intruder.