Particle Rotation Research Articles

Measuring the velocity profile of spinning particles and its impact on Cr(VI) sequestration

The evolution and development of magnetic nanoparticles (MNPs) are of great interest to researchers owing to their novel properties and wide-ranging applications. Essentially, MNPs coupled with magnetic fields (MF) have been used in biomedical separation, catalysis, cell separation, magnetic target drug delivery, hyperthermia, nucleic acid separation and wastewater treatment. In that regard, this paper reports the investigation of the sorption phenomena of chromium six (Cr(VI)) to MNPs interaction spun under the influence of alternating MF utilizing this unique particle shadow velocimetry technique. Under an alternating MF, the velocity field of spinning particles was observed to increase with magnetic field intensification. The determined mean velocity field for the spinning particles in the Cr(VI) solution increases from 0.0075-0.0125 m/s at a MF range of 16.54-25.54 mT. The rotating MF was observed to improve the magnetic force and torque induced on the particles, with particle aggregates reorienting with the magnetic moment of the field. The variability observed at the liquid-particle interface led to a fast mixing, with chaotic flow behaviour in the channel wall induced by the magnetic force observed. The induced cluster particle rotation and broad magnetic velocity distributions also led to cluster particle collisions hence the enhanced sorption process.

Chaotic dynamics of small-sized charged Yukawa dust clusters.

In a recent work [Maity et al., Phys. Rev. E 102(2), 023213 (2020)] the equilibrium of a cluster of charged dust particles mutually interacting with screened Coulomb force and radially confined by an externally applied electric field in a two-dimensional configuration was studied. It was shown that the particles arranged themselves on discrete radial rings forming a lattice structure. In some cases with a specific number of particles, no static equilibrium was observed. Instead, angular rotation of particles positioned at various rings was observed. In a two-ringed structure, it was shown that the direction of rotation of the particles positioned in different rings was opposite. The direction of rotation was also observed to change apparently at random time intervals. A detailed characterization of the dynamics of small-sized Yukawa clusters, with a varying number of particles and different strengths of the confining force, has been carried out. The correlation dimension and the largest Lyapunov index for the dynamical state have been evaluated to demonstrate that the dynamics is chaotic. This is interesting considering that the charged microparticles have many applications in a variety of industrial processes.

Topological acoustic tweezer and pseudo-spin states of acoustic topological insulators

Topological protection wave engineering in artificially structured media is at the forefront of metamaterials research. Acoustic analogs of electronic topological insulators have brought a lot of new opportunities for manipulating sound propagation and have attracted the attention of many scholars. A variety of artificial acoustic systems hosting topological edge states have been proposed. However, the pseudo-spin edge states at a boundary have not been clearly and intuitively characterized. In this article, we provide experimental verification of pseudo-spin states by the method of acoustic particle manipulation by using the rotation of the particle to describe the pseudo-spin sound field characteristics at the boundary of the acoustic topological insulator in detail. It indicates the existence of pseudo-spin–orbit coupling effects and pseudo-spin-related acoustic unidirectional transmission even if the defect exists. Furthermore, the pseudo-spin sound field can be used to control the rotation of multiple particles and droplets. This kind of topological acoustic tweezer with defect immunity has great application potential in the fields of biomedicine and materials science.

Study on Meso-Material Parameters of Submarine Weathered Granite Based on Parallel Bond Model.

In order to study the mechanical properties of submarine weathered granite under marine geological conditions, uniaxial compression tests were carried out on the original medium weathered granite of the seafloor of an offshore area in Pingtan, Fujian Province by using triaxial experimental apparatus to analyze the fracture characteristics, stress–strain characteristics, and compressive strength indexes. Based on the theory of discontinuous medium, the uniaxial compression and uniaxial tensile tests of rocks were simulated, and the microscopic mechanical parameters of discrete elements of granite samples were determined based on the indoor macroscopic mechanical tests: effective modulus Et, compressive elastic modulus Ec, macro Poisson’s ratio μ, and uniaxial compressive strength σc. The results show that the parallel bond model has good simulation results for the uniaxial compression test, but the tensile strength and tensile–compression ratio were quite different from the experimental values. When the confining pressure is large, the calibrated parameter adaptability by uniaxial compression is poor. The reason for certain errors is a large resistance of the parallel bond model to particle rotation and the influence of normal stress on shear strength is not considered. The cementation model can be modified by adding coefficients based on laboratory test results.

Theoretical framework to predict the acoustophoresis of axisymmetric irregular objects above an ultrasound transducer array.

Acoustic radiation force and torque arising from wave scattering are able to translate and rotate matter without contact. However, the existing research mainly focused on manipulating simple symmetrical geometries, neglecting the significance of geometric features. For the nonspherical geometries, the shape of the object strongly affects its scattering properties, and thus the radiation force and torque as well as the acoustophoretic process. Here, we develop an analytical framework to calculate the radiation force and torque exerted on the sound-hard or the sound-soft, axisymmetric particles excited by a user-customized transducer array based on a conformal transformation approach, capturing the significance of the geometric features. The derivation framework is established under the computation coordinate system (CCS), whereas the particle is assumed to be static. For the dynamic processes, the rotation of particle is converted as the opposite rotation of transducer array, achieved by employing a rotation transformation to tune the incident driving field in the CCS. Later, the obtained radiation force and torque in the CCS should be transformed back to the observation coordinate system for force and torque analysis. The radiation force and torque exerted on particles with different orientations are validated by comparing the full three-dimensional numerical solution in different phase distributions. It is found that the proposed method presents superior computational accuracy, high geometric adaptivity, and good robustness to various geometric features, while the computational efficiency is more than 100 times higher than that of the full numerical method. Furthermore, it is found that the dynamic trajectories of particles with different geometric features are completely different, indicating that the geometric features can be a potential degree of freedom to tune acoustophoretic process. The ability to predict the acoustophoretic process of nonspherical particles above a user-customized transducer array has improved our understanding of the effect of shape asymmetry, which can also be used to verify the effectiveness of acoustic tweezers in manipulating nonspherical objects.

Effects of particle morphology on the shear response of granular soils by discrete element method and 3D printing technology

AbstractThe development of quantitative correlation between particle morphology and shear responses of granular soils is an ongoing challenge. Herein, 3D printing technology, combined with the experiments and discrete element simulations were conducted to study the effects of different particle shapes on the particle migration, the development of shear zone, and the anisotropy of particle contact during shearing. Both numerical simulations and experimental tests confirmed that the increase of modified shape parameterShstrengthens the interlock between particles, which in turn limits the shear deformation in the direction of shearing. As the shear load increases further, the rotation of particles begins to develop from both sides of the shear box boundary, which gradually expands to the center, and finally forms a stable disturbance region in the center of the specimen; meanwhile, the positions of the maximum particle rotation appear at the center and boundary of the shear zone. Moreover, particle shape does not affect the distribution of fabric anisotropy before loading, however, the change of particle shape parameterShmakes the fabric anisotropy become prominent. In addition, particle shape is found to improve the shear resistance by changing the number and distribution of contacts between particles, rather than the contact force among particles.

Electro-rotation of spheroids in fluids under linear-polarized AC electric field: A dynamic model

Under a linear-polarized AC electric field, a spheroid suspended in fluids typically aligns one of its axes with the field. The time-averaged torque model is widely used to predict the orientation of the spheroid with respect to the field. Different from the AC case, the dynamical behaviors including stable orientation, stable spinning, and the limit cycle of spheroids under the DC electric field are much richer. An inconsistency exists between the two cases, and a criterion for the validity of the time-average torque model is also missing. In this article, the dynamic model for the DC electric field was adapted to its AC counterpart and the full dynamics of spheroids under the AC field were studied. We bridged the DC and AC dynamics of spheroidal particles and widened the frequency range for applying the time-averaged torque model. It was found that the phase diagram at the DC limit is a very instructive guiding map for predicting the dynamical behavior at the AC field and ωτη ∼ 1 (ω: angular frequency of the electric field, τη: characteristic time of particle rotation) appears to be a universal criterion for the time-averaged model to be effective. The flipping of particle orientation was explained with bifurcations of the periodic solutions and the irregular dynamics at low frequencies were uncovered with the Poincaré map and the power spectrum analysis. Our study sheds light on even richer dynamical behaviors of the particles under the AC electric field and may help realize other unconventional dynamical behaviors of particles in the future.

Slow axisymmetric rotation of a soft sphere in a circular cylinder

A semi-analytical study of the creeping flow of an incompressible Newtonian fluid around a soft spherical particle (a hard core covered by a permeable porous layer) rotating about a diameter lying on the axis of a long circular cylinder is presented. To solve the Stokes and Brinkman equations, a solution comprising of the general solutions in spherical and cylindrical coordinates is utilized. The boundary conditions are applied first at the confining cylinder wall through the Fourier transform and then at the particle and hard-core surfaces via a collocation method. Accurate results of the hydrodynamic torque acting on the particle can be obtained for various values of the particle-to-cylinder radius ratio, core-to-particle radius ratio, and ratio of the particle radius to the porous layer’s flow penetration length. The boundary effect of the cylinder on the rotation of the soft particle is significant. The hydrodynamic torque exerted on the confined soft sphere increases with an increase in the particle-to-cylinder radius ratio, and in general remains finite even as the soft particle fills the cylinder. This torque is less than that on an equal-sized hard sphere (or soft sphere having a smaller thickness or lower permeability of its porous layer).

Ferroelectric nematic liquid crystal thermomotor.

A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become electrically charged in the fluid. These observations provide a fascinating example of the unique nature of fluid ferroelectric liquid crystals.

Translation and rotation of particles in different flow pattern areas of a silo

The present paper reports the results obtained for translational and rotational velocity profiles of spherical particles for the mixed flow in a conical silo. The discrete element method (DEM) based on Hertz-Mindlin (no slip) with RVD rolling friction contact model is used for simulations. Opposite correlations are found between translational and rotational velocities in different flow areas of the silo. In particular, the abrasion caused by rotation is dominant in the funnel flow area. In addition, increase of the mass flow rate of silo can effectively reduce the abrasion induced by rotation. This highlights that understanding of dynamic characteristics of particles is helpful for optimization of silos and reduction of granular material abrasion.

Turbulent channel flow of a binary mixture of neutrally buoyant ellipsoidal particles

A turbulent channel flow of a binary mixture of finite-size neutrally buoyant ellipsoidal particles is studied by using a parallel direct-forcing fictitious domain method at a friction Reynolds number of 180 and the particle aspect ratios of 1/3 (oblate particle) and 4 (prolate particle), respectively. The total particle volume fraction is fixed at 14.16% and 21.24% and the relative fraction of prolate and oblate particles are varied. The mean velocity profile normalized by the bulk velocity shows a strong difference between the single phase flow and particulate flow, while showing a small difference between mono-disperse and bi-disperse particulate flow cases. The lower fluid and particle Reynolds stress contributions in a high oblate particle volume fraction ratio cases lead to the drag reduction. Under moderate particle volume concentration, the oblate and prolate particle results show self-similarity of velocity profiles of solid phase, including particle velocity, concentration, orientation, and rotation. Both kinds of ellipsoidal particles tend to align their long axes with the streamwise direction in the whole channel.

Interior $$\epsilon$$-regularity theory for the solutions of the magneto-micropolar equations with a perturbation term

We develop here a particular version of the partial regularity theory for the magneto-micropolar equations (MMP) where a perturbation term is added. These equations are used in some special cases, such as in the study of the evolution of liquid crystals or polymers, where the classical Navier–Stokes equations are not an accurate enough model. The incompressible magneto-micropolar system is composed of three coupled equations: the first one is based in the Navier–Stokes system, the second one considers mainly the magnetic field while the last equation introduces the microrotation field representing the angular velocity of the rotation of the fluid particles. External forces are considered and a specific perturbation term is added as it is quite useful in some applications.

Critical Comparison of Phase-Field, Peridynamics, and Crack Band Model M7 in Light of Gap Test and Classical Fracture Tests

Abstract The recently conceived gap test and its simulation revealed that the fracture energy Gf (or Kc, Jcr) of concrete, plastic-hardening metals, composites, and probably most materials can change by ±100%, depending on the crack-parallel stresses σxx, σzz, and their history. Therefore, one must consider not only a finite length but also a finite width of the fracture process zone, along with its tensorial damage behavior. The data from this test, along with ten other classical tests important for fracture problems (nine on concrete, one on sandstone), are optimally fitted to evaluate the performance of the state-of-art phase-field, peridynamic, and crack band models. Thanks to its realistic boundary and crack-face conditions as well as its tensorial nature, the crack band model, combined with the microplane damage constitutive law in its latest version M7, is found to fit all data well. On the contrary, the phase-field models perform poorly. Peridynamic models (both bond based and state based) perform even worse. The recent correction in the bond-associated deformation gradient helps to improve the predictions in some experiments, but not all. This confirms the previous strictly theoretical critique (JAM 2016), which showed that peridynamics of all kinds suffers from several conceptual faults: (1) It implies a lattice microstructure; (2) its particle–skipping interactions are a fiction; (4) it ignores shear-resisted particle rotations (which are what lends the lattice discrete particle model (LDPM) its superior performance); (3) its representation of the boundaries, especially the crack and fracture process zone faces, is physically unrealistic; and (5) it cannot reproduce the transitional size effect—a quintessential characteristic of quasibrittleness. The misleading practice of “verifying” a model with only one or two simple tests matchable by many different models, or showcasing an ad hoc improvement for one type of test while ignoring misfits of others, is pointed out. In closing, the ubiquity of crack-parallel stresses in practical problems of concrete, shale, fiber composites, plastic-hardening metals, and materials on submicrometer scale is emphasized.

Meta-Structure Surface Based on Dielectric Tensor Rotation Induced by AC Electric Field for Insulation Improvement

Electric field distribution along gas–solid interfaces determines the reliability of insulating components. However, the dielectric gradient insulating component prepared by the conventional method is considered to only control the internal electric field, but is not beneficial to the surface insulation, especially when the electrode is at an acute angle from the dielectric surface. The aim of this study is to clarify that the meta-structure surface based on dielectric tensor rotation constructed by the electric field-induced assembly (EIA) method could improve surface insulation. The self-assembly and orientation axis rotation of filler particles near the interface were observed by in-situ optical observation, and the relationship between the dielectric tensor rotation and electric field refraction was revealed. Simulation of meta-structure surface induced by the EIA method on the basin insulator was conducted. The meta-structure surface exhibits negative and high reduced permittivity, confining electric field path and transferring the electrical stress from gas to insulator, which offers theoretical support to optimize the surface electric field. Flashover tests on the basin insulator proved the insulation improvement by meta-structure surface.

Effect of Ballast Degradation on Track Dynamic Behavior Using Discrete Element Modeling

Ballast degradation refers to the traffic use of in-service track when aggregate particles become smaller in size and fines are generated because of particle breakage and abrasion to cause geometry defects and influence the serviceability of ballasted track affecting train operations. To evaluate effects of ballast degradation on track substructure maintenance needs, single crosstie models were developed to simulate new and degraded ballast behavior under train loading. Particle size distributions and shape properties of the new and degraded ballast materials studied were adopted from previous laboratory tests conducted at the University of Illinois, U.S. Crosstie vibrations, ballast particle contact force distributions and particle rotations, and localized contact force chain networks were compared among different single crosstie models incorporating new and degraded ballast. The following key observations were made based on the simulation results: (1) crosstie on degraded ballast exhibits larger vibrations and more inconsistent vibration patterns; (2) degraded ballast shows a higher number of localized force chains with more particle contacts while the characteristic trend observed for new ballast is the presence of more symmetric force chain networks, which might be attributed to the new ballast layer’s uniform gradation and good interlock; (3) average normal contact forces are distributed more uniformly in new ballast in both horizontal and vertical directions as opposed to these forces concentrated more in vertical direction in degraded ballast; and (4) more ballast particles experience large rotations in degraded ballast and the related impact zone, which encloses all particles with large rotations, is also broader and deeper.

Amplification of light within aerosol particles accelerates in-particle photochemistry.

Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive particles. Single iron(III)-citrate particles were probed using the iron oxidation state as a photochemical marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.

Granular surface flows confined between flat, frictional walls. Part 1. Kinematics

We report and analyse the results of extensive discrete element method simulations of three-dimensional gravity driven flows of cohesionless granular media over an erodible bed, the whole being confined between two flat and frictional sidewalls. We focus on the role of sidewalls by performing simulations for different gap widths ( $W$ ) between the two confining sidewalls: from $5$ to $30$ grain sizes ( $d$ ). Our results indicate the existence of two distinct regimes: regime I for flow angles smaller than the critical angle $\theta_c\approx 40^\circ$ and regime II at flow angles larger than $\theta_c$ . Regime I corresponds to dense flows whereas flows belonging to regime II exhibit a strong variation of the volume fraction through the depth. Three relevant lengths are identified in the system: $W$ the gap between sidewalls, $l$ the length characterizing the vertical variation of the volume fraction and $h$ a characteristic length associated with the vertical variation of the streamwise velocity. Using these lengths we can rescale the profiles of various flow properties (e.g. streamwise velocity, granular temperature, particle rotation…). In regime II, in contrast to regime I, $l$ and $h$ have a similar behaviour. As a consequence, the rescaled profiles in regime II only involve $h$ (or equivalently $l$ ) and $W$ . Other dissimilarities exist between regimes I and II. In particular, the scaling of the flow rate with $h$ (at fixed $W$ ) differs in the two regimes, although they display a similar scaling with $W$ (at fixed flow angle).

Micromechanics of granulated rubber–soil mixtures as a cost-effective substitute for geotechnical fillings

Rubber–soil mixtures (RSM), formed by mixing two types of granules with vastly different mechanical properties, has the potential for serving as a cost-effective substitute for geotechnical fillings. Its micromechanical behavior at a microscopic scale, however, is not yet well understood. In this study, a numerical model of granulated RSM under biaxial compression conditions is established based on the discrete element theory and two-dimensional particle flow code. Microparameters of a particular RSM are studied and calibrated by comparison with triaxial consolidated drained shear tests. The volumetric fraction of sand is assigned as the index of the mixture, and the deviatoric stress–axial strain curves of RSM with different sand fractions are analyzed. Particle rotation, average coordination number, force chain structure, and energy dissipation with axial strains are further investigated to better understand the microscopic mechanisms of RSM. The main findings of the study include: (1) Rubber particles are highly deformable and fill the voids more easily than stiffer sand particles. This contributes to the high friction of sand–rubber interfaces, which delays or even inhibits the relative motion and tumbling of sand particles; (2) The force chains of sand particles are either braced from buckling until large axial strains by rubber particles occupying the intergranular voids or distributed by increased rubber–rubber contacts depending on the rubber content; and (3) RSM with a rubber volumetric content of 30–40% are optimal in that the force chain increases monotonically with axial strain with a stable load-deformation relationship. In addition, the sensitivity analysis of the effect of microparameters on the macroscopic behavior of RSM is presented, which can provide theoretical support for exploring the mechanical properties of similar mixed granular materials subjected to external loading.

Monitoring Particle Movement under Compaction using SmartRock Sensor: A Case Study of Granular Base Layer Compaction

The quality of base course compaction significantly influences pavement’s overall performance. Compared with conventional compaction methods, the intelligent compaction (IC) uses external accelerometers to track pavement responses during compaction and correlates them with pavement layer characteristics to improve the compaction quality. Very few studies have attempted to present interactions of granular materials from the particle level, i.e., how individual particles are responding under the compactive effort. The recent development of a prototype wireless sensor, SmartRock, made tracking the particle movement at meso-scale possible. Accordingly, this paper has the objective of applying the SmartRock sensors to monitor the motion characteristics (i.e., translation and rotation) of granular particles during the compaction process and evaluate the particle response patterns as insights into understanding the compaction mechanism of the unbound aggregate base layer. The experimental site selected was an airport runway test section, designated as the Smart Runway project located in Hill Air Force Base (AFB) in Layton, Utah, United States. SmartRocks were embedded at the bottom of the granular base layer to collect particle motion data during compaction. Analysis results indicated that particle’s motion behavior was closely related to the compactive effort and material properties. Compaction is a process of gaining interlocking among particles and elasticity in materials, and is achieved through the particle’s three-axial angular position arrangement and vertical translation. The particle movement characteristics observed in the granular base were compared to those in asphalt mixtures, which confirmed the unique responses of bound and unbound pavement layer materials. It is suggested that the characteristics of particle rotation behavior during compaction can potentially be used as a control parameter to monitor compaction conditions.

Enhancing near-field optical tweezers by spin-to-orbital angular momentum conversion

Near-field patterns of light provide a way to optically trap, deliver, and sort single nanoscopic particles in a wide variety of applications in nanophotonics, microbiology, and nanotechnology. Using rigorous electromagnetic theory, we investigate the forces and trapping performance of near-field optical tweezers carrying spin and orbital angular momenta. The trapping field is assumed to be generated by a total internal reflection microscope objective at a glass–water interface in conditions where most of the transmitted light is evanescent. We discover aspects of these tweezers, including the possibility to rotate and stably trap nanoscopic beads. More importantly, we show that, under near-field conditions, the contributions of spin and orbital angular momenta to the rotation of small particles are almost equivalent, opening the possibility to cancel each other when they have an opposite sign. We show that these conditions result in optimal optical trapping, giving rise to extremely effective optical tweezers for nanomanipulation, with both circular symmetry and relatively weak rotation.