Cage Dynamics Research Articles

Slipping in cylindrical roller bearings at the simultaneous rotation of inner and outer rings

Slipping in cylindrical roller bearings is investigated at the simultaneous rotation of inner and outer rings. A 6-DOF dynamic model is developed to analyze roller, cage, and ring dynamics, assessing driven and resistance moments, cage rotating speed, and roller-raceway sliding velocities. The results show that if the inner and outer rings rotate inversely, the motion of rollers is relatively stable, which is beneficial for preventing the slipping in the bearing. However, if the inner and outer rings rotate in the same direction, the churning moment of rollers is high. Meanwhile, more rollers have a negative value of the sum of the churning moment and traction moment. This leads to an increase in the slipping of the bearing.

Comparative assessment of the performance of density functionals and dispersion correction on different properties of dicationic ionic liquids - an ab initio molecular dynamics (AIMD) study.

In this study, we investigated the effect of DFT density functionals and dispersion correction on an imidazolium-based dicationic ionic liquid (DIL) using ab initio molecular dynamics simulations. To achieve this purpose, the electronic structures, as well as the structural and dynamical properties of [C3(mim)2][NTF2]2 DIL, were obtained using the BLYP and PBE functionals, both with and without D3-correction, and the results were compared with experimental values. Radial distribution functions and structure factors revealed that applying D3-correction increases the interaction between the anion and hydrogen atoms of the rings and side chains. The simulation of the studied DIL with the BLYP-D3 functional depicted lower structural heterogeneity compared to the other functionals. Analysis of Voronoi tessellation and linkage chain conformations showed a reduction in the aggregation of the linkage alkyl chains in the presence of D3-correction, which is more pronounced in the BLYP functional than in PBE. Additionally, it was observed that the probability of forming a hydrogen-bond network depends on both the type of used density functionals and applying dispersion correction. The results of dynamical properties, such as the self-diffusion coefficients, velocity autocorrelation function, and van Hove correlation function, as well as ion pair, ion cage, and hydrogen bond dynamics, indicated that applying D3-correction in both density functionals leads to an increase in the dynamics of the studied DIL. Additionally, the ratio of self-diffusion coefficients of the anion to the cation in the BLYP functional is closer to experimental values compared to the PBE functional. Furthermore, the electronic structure, including dipole moment distribution, and also infrared (IR) and power spectra were studied. Applying D3-correction and the type of density functionals have a significant effect on the dipole moment distribution of ions. Moreover, the results of IR and power spectra demonstrated that only in the BLYP functional, by applying D3-correction, the hydrogen bonding between the anion and the hydrogen atoms of the cation is strengthened at high wavenumbers. Thus, we conclude that applying D3 correction to both the BLYP and PBE density functionals improves the accuracy in describing the various properties of the studied system. Overall, the evaluation of different structural, dynamical, and vibrational properties of [C3(mim)2][NTF2]2 DIL suggests that the BLYP-D3 density functional may be the best choice among the studied density functionals.

Assessing OPLS-based force fields for investigating the characteristics of imidazolium-based dicationic ionic liquids: A comparative study with AIMD simulations and experimental findings.

In this study, we extended the optimized potentials for liquid simulation-ionic-liquid virtual site (OPLS-VSIL) force field (FF) to imidazolium-based dicationic ionic liquids (DILs) and evaluated the ability of different OPLS-based FFs (i.e., OPLS-2009IL, 0.8*OPLS-2009IL, and OPLS-VSIL) in predicting different properties of the studied DIL by comparing their results with abinitio molecular dynamics (AIMD) simulation and experimental results. To achieve this purpose, MD simulations with three different OPLS-based FFs as well as AIMD simulation were performed for [C3(mim)2][NTF2]2 DIL and its structural, dynamical, vibrational, and volumetric properties were analyzed. Structural properties of the studied DIL, i.e., radial distribution functions (RDFs), structure factor, and hydrogen-bond network, showed that compared to 0.8*OPLS-2009IL FF, there is a much better agreement between the results of both OPLS-2009IL and OPLS-VSIL FFs with the AIMD simulation. On the other hand, the results of dynamical properties, such as mean square displacements, van Hove correlation functions as well as hydrogen bond, ion pair, and ion cage dynamics, depicted that in both 0.8*OPLS-2009IL and OPLS-VSIL FFs, the dynamics of the system is almost similar, and compared to OPLS-2009IL FF, they have better agreements with experimental results where they exist. So, it can be seen that although reducing the total charge of studied DIL by 20% leads to an increase in the dynamics of the system, the type distribution of partial charges on each atom does not significantly affect the system's dynamics. The calculated infrared (IR) and power spectra showed that the vibrational features of studied DIL in three OPLS-based FFs are mostly the same and reducing total charge and different type distribution of partial charges have no significant effect on the studied system. Furthermore, in volumetric properties, OPLS-VSIL FF shows somehow better agreement with experimental results. Overall, the evaluation of different structural, dynamical, vibrational, and volumetric properties of [C3(mim)2][NTF2]2 DIL shows that the OPLS-VSIL FF may be the best choice among the different studied OPLS FFs.

Unraveling the structural and dynamic heterogeneities of hydroxyl-functionalized di-cationic ionic liquids (HFDILs): An integrated ab initio and molecular dynamics approach

This study comprehensively investigates the effects of hydroxyl groups in side chains and cation symmetry on intra- and inter-molecular structures, dynamics, and thermodynamic properties of two hydroxyl-functionalized di-cationic ionic liquids (HFDILs). Using a combination of molecular dynamics (MD) simulations and quantum chemistry (QC) calculations, we compare the influence of these effects on various properties with corresponding non-hydroxyl DILs reported in previous works. The structure of HFDILs is explored using angle distribution, radial distribution, spatial distribution, and combined distribution functions. Also, the averaged noncovalent interaction (aNCI) analysis is used to characterize the weak noncovalent interactions in fluctuating environment of HFDILs. Results showed that the oxygen atoms of anion have the strongest interaction with the hydrogen atom of hydroxyl group, followed by the hydrogen atom at the top of the imidazolium ring. Domain analysis and Voronoi calculations are performed to investigate the number of polar and non-polar subunits, revealing that symmetric HFDILs with two hydroxyl groups in their side chains have greater structural heterogeneity than asymmetric ones with only one hydroxyl group due to the larger accumulation of non-polar subunits. To investigate dynamical heterogeneity, we calculate the non-Gaussian parameter, van Hove correlation, hydrogen bond, ion pair, ion cage, and reorientation dynamics. Results indicated that symmetric HFDILs have slower dynamics, greater dynamic heterogeneity, and more stability than asymmetric ones. Finally, the structure and strength of ion triplet interactions are examined using quantum mechanical methods, revealing that symmetric HFDILs are more stable than asymmetric ones. This study provides valuable information for understanding DILs and their potential applications in various fields.

A dynamic model for the planetary bearings in a double planetary gear set

Double planetary gear sets (DPGSs) are widely applied to high power and high torque mechanical transmission systems due to the high loading capacity, steady transmission, and high transmission efficiency. Most previous works studied the planetary bearing vibration characteristics without the gear excitations. This work establishes a dynamic model for a DPGS containing all components (sun gear, ring gear, carrier, inner planet, outer planet, planetary bearing roller, and planetary bearing cage). The components’ excitations in this work are more comprehensive than the reported references. The proposed model considers the planetary bearing roller and cage dynamics. Moreover, the gear interactions, planetary gear-bearing-carrier interactions, and roller-cage interactions are contained in the proposed model. The vibrations of planetary bearing roller, planet, carrier, and planetary bearing cage are analyzed. The effect of sun gear rotation speed on the planetary bearing contact force, planetary bearing roller-cage impact force, and vibrations of planetary bearing parts are studied. The simulated and experimental results are compared to prove the correctness of proposed model. Moreover, the results from proposed model and model without the cage are compared to show the advantage of proposed model.

Dynamics of a pile-moored fish cages in current and waves: A numerical study

We propose a numerical study of the dynamics of a double-constrained cylindrical pile-moored fish-farm cage, to be possibly installed at the foundations of dismissed offshore structures. Our model, derived from the net-truss model of Kristiansen and Faltinsen (2012), takes into account the elasticity of the net material, the real size of the net and is applied with and without mesh-grouping approaches, this allowing us to evaluate the range of validity of mesh-grouping techniques. First, cage dynamics have been studied with small-scale tests, in which the numerical model accurately reproduces the behaviour of the cage in both sea currents (velocity in the range 0.5−1m/s) and waves of small amplitude (with height of 1 m and period in the range 4−32 s). Use of the net real size for the computation grid of small-scale tests allows for identification of the main phenomena influencing the load distribution and the volume loss, which are: the dominant sail-shape deformation in the streamwise direction, the flattened deformations in the cross flow direction, and high-frequency oscillations in the vertical direction in long waves. Subsequently, a prototype-scale test is run to simulate the dynamics of a realistic installation of a double-constrained cylindrical pile-moored fish-farm cage in the Adriatic sea (considering both 1m/s sea current and storm sea state characterized by a peak period Tp=4 s and significant wave height Hs=1.5 m), showing that the major loads are a compression load on the pile up to 1 kN and a bending moment at the pile base up to 9.1 kNm. The proposed configuration always showed a small net relative volume loss VL<5%.

Molecular Simulation-Guided Spectroscopy of Imidazolium-Based Ionic Liquids and Effects of Methylation on Ion-Cage and -Pair Dynamics.

Classical molecular dynamics simulations were performed to assess an atomistic interpretation of the ion-probe structural interactions in two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm][NTf2] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BDimIm][NTf2] through computational ultrafast spectroscopy. The nitrile stretching vibrations of the thiocyanate anion, [SCN]-, serve as the local mode of the ultrafast system dynamics within the imidazolium-based ionic liquid environment. The wavelet transform of classical trajectories determines the time-varying fluctuating frequencies and the stretch spectral signatures of SCN- in the normalized distribution. However, computational modeling of the two-dimensional (2D) spectra from the wavelet-derived vibrational frequencies yields time evolution of the local molecular structure along with the varied time-dependent dynamics of the spectral diffusion process. We calculated the frequency-frequency correlation functions (FFCFs), time correlations associated with the ion-pair and -cage dynamics, and mean square displacements as a function of time, depicting diffusive dynamics. The calculated results based on the pair correlation functions and the distribution of atomic density suggest that the hydrogen and methylated carbon at the two-position of the imidazolium ring of [BMIm] and [BDimIm] cations, respectively, strongly interact with the probe through the N of the thiocyanate anion rather than the S atom. The center-of-mass center-of-mass (COM-COM) cation-probe radial distribution functions (RDFs) in conjunction with the site-specific structural analysis further reveal well-structured interactions of the thiocyanate ion and [BMIm]+ cation rather than the [BDimIm] cation. In contrast, the anion-probe COM-COM RDFs depict weak interactive associations within the vibrational probe [SCN]- and [NTf2]- ions. Methylation at the two-position of the imidazolium ring predicts slower structural reorganization and breaking and reformation dynamics of the ion pairs and cages within the ionic liquid framework.

A combined QTAIM, DFT and molecular dynamics study on the nanoscale dynamical and structural organization of imidazolium-based dicationic ionic liquids

In this work, the effects of linkage alkyl chain length on the dynamical and structural micro-heterogeneities of some imidazolium-based dicationic ionic liquids (DILs) i.e. [Cn(mim)2][BF4]2, (n = 3, 6, 9, and 12) were studied via quantum mechanical calculations and also classical molecular dynamics (MD) simulations. Hydrogen bondings and also the interaction energies between the studied ions have been calculated using the density functional theory (DFT) calculations and also the quantum theory of atoms in molecules (QTAIM) method. The results showed that with increasing the linkage alkyl chain length of DILs, the interaction energies and electron densities are decreased, because the electrostatic interactions in DILs are weakened with increasing the –CH2 groups in the linkage chain. Different kinds of combined distribution functions (CDFs) were computed to better understand the orientations of ions in the studied DILs. Although, two rings of cation are almost perpendicular with respect to each other in all studied DILs, the orientation of linkage chain with respect to the rings is different in short- and long-linkage chain DILs. To understand the arrangement of the nearest neighboring ions, neighborhood analysis and spatial distribution functions (SDFs) of ions were analyzed. Furthermore, the mean square displacement (MSD), non-Gaussian parameter, van Hove correlation of cation and anions and also the ion pair, ion cage, hydrogen bond and reorientation dynamics were computed. DILs with longer alkyl chain length have more deviation in non-Gaussian parameter and Gaussian behavior of distribution of particle displacements, probably due to weakening the electrostatic interactions. Also, they have higher ion cage/ion pair stabilities than DILs with shorter linkage chain. Altogether, the linkage chain length in DILs has significant influences on their organizations in such a way that with increasing linkage chain length, their dynamical and structural micro-heterogeneities increase.

Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N-H Vibrational Probe from FPMD Simulations.

We employed density functional theory (DFT)-based molecular dynamics simulations to explore the structure, dynamics, and spectral properties of the protic ionic entity trimethylammonium chloride (TMACl). Structural investigations include calculating the site-site radial distribution functions (RDFs), the distribution of constituent cations and anions in three-dimensional space, and combined distribution functions of the hydrogen-bonded pair RDF versus angle, revealing the structural characteristics of the ionic solvation and the intermolecular interactions within ions. Further, we determined the instantaneous vibrational stretching frequencies of the intrinsic N-H stretch probe modes by applying the time-series wavelet method. The associated ionic dynamics within the protic ionic compound were investigated by calculating the time-evolution of the fluctuating frequencies and the frequency-time correlation functions (FFCFs). The time scale related to the local structural relaxation process and the average hydrogen bond lifetime, ion cage dynamics, and mean squared displacement were investigated. The faster decay component of the FFCFs, depicting the intermolecular motion of intact hydrogen bonds in TMACl, is 0.07 ps for the Perdew-Burke-Ernzerhof (PBE)-based simulation and 0.06 ps for the PBE-D2 representation. The slower time scale of the longer picosecond decay time component of PBE and PBE-D2 representations are 3.13 and 2.87 ps, respectively. These picosecond time scales represent more significant fluctuations of the hydrogen-bonding partners in the ionic entity and hydrogen-bond jump events accompanied by large angular jumps. The longest picosecond time scales represent structural relaxation, including large angular jumps and ion-pair dynamics. Also, ion cage lifetimes correlate with the slowest time scale of the associated dynamics of vibrational spectral diffusion despite the type of DFT functional. This study benchmarks DFT treatments of the exchange-correlation functional with and without the van der Waals (vdW) dispersion correction scheme. The inclusion of vdW interactions to the PBE functional represents a less structured state of the ionic entity and faster dynamics of the molecular motions relative to the one predicted by the PBE system. All the results illustrate the necessity of accurately describing the Coulomb interactions, vdW dispersive interactive forces, and localized hydrogen bonds required to sustain the energetic balance in this ionic salt.

Drag-free design based on Embedded Model Control for TianQin project

With the ability to detect low-frequency gravitational waves (GWs), space-borne detectors will play an important role in exploring the universe in the future. The TianQin project proposed in China participates in this challenge by aiming to detect millihertz GWs. The TianQin GW detector consists of three spacecraft forming a triangular constellation. Each spacecraft carries a pair of test masses (TM) which, inside shielding cages, are left to free fall along the local geodesic. In this way, a pair of TMs on two separate spacecraft can be each other aligned to become the gravitational references of the GW detector, made by an inter-satellite laser interferometer. Each free-falling TM is limited by parasitic forces to be appropriately bounded. The relevant acceleration bound has been fixed to 10−15 m/s2/Hz within the TianQin measurement bandwidth (MBW) ranging from 0.1mHz to 0.1Hz. In turn, TM to cage fluctuations must be kept below 4nm/Hz to limit the stiffness coupling with the spacecraft displacement caused by non-gravitational disturbances, such as solar radiations and the thruster noise. Suppression of such disturbances calls for a challenging drag-free control technology, since the center of mass (CoM) of a single spacecraft cannot track the separated CoMs of two TMs simultaneously, and consequently tracking must be limited to three degrees of freedom (DoF): two non-orthogonal sensitive axes (one for each TM) and the perpendicular direction to their plane. Control design and simulated tests of this paper will be restricted to the drag-free control along the sensitive axes. The remaining nine DoFs of the two TMs (position and attitude) are controlled by electrostatic suspensions, not to be treated here, but accounted for in the simulated trials. The design of the two-DoF drag-free control relies on a model-based control methodology, the Embedded Model Control (EMC), capable of predicting and suppressing unknown disturbances within the required bandwidth and of decoupling the TM dynamics along the non-orthogonal sensitive axes. The paper starts with the nonlinear model of the TM to cage dynamics, followed by the relevant EMC design, restricted to the sensitive axes. Numerical simulations are employed to validate closed-loop performance and robust stability. Simulated results show that the residual TM to cage fluctuations can be kept below 3nm/Hz, which leaves a margin within the required spectral bound. The EMC methodology discussed in the paper can provide a reference for future developments and implementations.

Using Machine Learning Methods for Predicting Cage Performance Criteria in an Angular Contact Ball Bearing

Rolling bearings have to meet the highest requirements in terms of guidance accuracy, energy efficiency, and dynamics. An important factor influencing these performance criteria is the cage, which has different effects on the bearing dynamics depending on the cage’s geometry and bearing load. Dynamics simulations can be used to calculate cage dynamics, which exhibit high agreement with the real cage motion, but are time-consuming and complex. In this paper, machine learning algorithms were used for the first time to predict physical cage related performance criteria in an angular contact ball bearing. The time-efficient prediction of the machine learning algorithms enables an estimation of the dynamic behavior of a cage for a given load condition of the bearing within a short time. To create a database for machine learning, a simulation study consisting of 2000 calculations was performed to calculate the dynamics of different cages in a ball bearing for several load conditions. Performance criteria for assessing the cage dynamics and frictional behavior of the bearing were derived from the calculation results. These performance criteria were predicted by machine learning algorithms considering bearing load and cage geometry. The predictions for a total of 10 target variables reached a coefficient of determination of R2≈0.94 for the randomly selected test data sets, demonstrating high accuracy of the models.

Experimental Investigation of Cage Dynamics and Ball-Cage Contact Forces in an Angular Contact Ball Bearing

The objectives of this investigation were to experimentally examine the cage motion and ball-cage contact forces for an angular contact ball bearing (ACBB) operating under various load and speed combinations. In order to achieve the objectives, a Counter Rotating Angular Contact Ball Bearing Test Rig (CRACTR) was designed and developed such that both the races of an ACBB can be rotated simultaneously in opposite directions, providing valuable insights into the ball-cage interaction. Five proximity probes were used concurrently to measure the in-plane (radial) and out-of-plane (axial) motion for three commercially available cage designs. Two load cells were used to measure the contact forces between the balls and cage pocket. The experimental results demonstrate that high inner race speeds and high axial loads develop the smallest and most circular cage whirl motions. Additionally, cage radius, cage pocket clearance, cage pocket area and cage inertia were the prominent factors affecting the magnitude of cage whirl. Experimental results also indicate that contact forces between the ball and cage pocket increase linearly with the increased race speeds. Ball-cage contact force was shown to be different for the three cages and was primarily governed by the cage pocket area and the cage radius. Results from this investigation can be utilized to predict the correlation between the ball-cage contact forces and the race speeds, and identify the key parameters governing the stable cage motion during bearing operation.

Trapping of protein cargo molecules inside DNA origami nanocages.

The development of the DNA origami technique has directly inspired the idea of using three-dimensional DNA cages for the encapsulation and targeted delivery of drug or cargo molecules. The cages would be filled with molecules that would be released at a site of interest upon cage opening triggered by an external stimulus. Though different cage variants have been developed, efficient loading of DNA cages with freely-diffusing cargo molecules that are not attached to the DNA nanostructure and their efficient retention within the cages has not been presented. Here we address these challenges using DNA origami nanotubes formed by a double-layer of DNA helices that can be sealed with tight DNA lids at their ends. In a first step we attach DNA-conjugated cargo proteins to complementary target strands inside the DNA tubes. After tube sealing, the cargo molecules are released inside the cavity using toehold-mediated strand displacement by externally added invader strands. We show that DNA invaders are rapidly entering the cages through their DNA walls. Retention of ∼70 kDa protein cargo molecules inside the cages was, however, poor. Guided by coarse-grained simulations of the DNA cage dynamics, a tighter sealing of the DNA tubes was developed which greatly reduced the undesired escape of cargo proteins. These improved DNA nanocages allow for efficient encapsulation of medium-sized cargo molecules while remaining accessible to small molecules that can be used to trigger reactions, including a controlled release of the cargo via nanocage opening.

Relative effects of polymer composition and sample preparation on glass dynamics

Modern design of common adhesives, composites and polymeric parts makes use of polymer glasses that are stiff enough to maintain their shape under a high stress while still having a ductile behavior after the yield point. Typically, material compositions are tuned with co-monomers, polymer blends, plasticizers, or other additives to arrive at a tradeoff between the elastic modulus and toughness. In contrast, strong changes to the mechanics of a glass are possible by changing only the molecular packing during vitrification or even deep in the glassy state. For example, physical aging or processing techniques such as physical vapor deposition increase the density, embrittle the material, and increase elastic modulus. Here, we use molecular simulations, validated by positron annihilation lifetime spectroscopy (PALS) and quasi-elastic neutron scattering, to understand the free volume distribution and the resulting dynamics of glassy co-polymers where the composition is systemically varied between polar 5-norbornene-2-methanol (NBOH) and non-polar ethylidene norbornene (ENB) monomers. In these polymer glasses, we analyze the structural features of the unoccupied volume using clustering analysis, where the clustering is parameterized to reproduce experimental measurements of the same features from PALS. Further, we analyze the dynamics, quantified by the Debye-Waller factor, and compare the results with softer, lower density states. Our findings indicate that faster structural relaxations and potentially improved ductility are possible through changes to the geometric structure and fraction of the free volume, and that the resulting changes to the glass dynamics are comparable to large changes in the monomer composition.

Impact of polydispersity and confinement on diffusion in hydrodynamically interacting colloidal suspensions

We present a computational study of the equilibrium dynamics of a polydisperse hard-sphere colloidal dispersion confined in a spherical cavity. We account for many-body hydrodynamic and lubrication interactions between particles and with the confining cavity utilizing our confined Stokesian dynamics model, expanded here for size polydispersity. We find that, even though the tendency of polydispersity to homogenize structure in a suspension is still present in confinement, strong correlations induced by the cavity resist homogenization. Although seemingly opposite, these two effects have a common driver, which is to maximize configurational entropy of particles in the cavity interior. These structural effects couple with the hydrodynamics to change the particle dynamics: polydispersity weakens lubrication effects near the cavity wall, allowing small (large) particles to diffuse faster (slower) than in a monodisperse suspension. As a small (large) particle gets farther from the wall, polydispersity weakens many-body hydrodynamic couplings, driving diffusivity up (down). While the local cage dynamics dominates short-time self-diffusion, long-time dynamics is also affected. In the concentrated regime, polydispersity and confinement combine to induce radial de-mixing into size-segregated populations. The cavity becomes the most influential ‘nearest neighbour’, setting the length scale of and dynamics within these radial domains. This intermediate length-scale caging makes the angular dynamics insensitive to polydispersity but leads to radial long-time mean-square displacement that changes qualitatively with volume composition. These results hold promise for explaining colloidal-scale physics implicated in the functioning of biological cells, and the engineering of non-living confined colloids where size de-mixing could be useful in the design of encapsulated micro-reactors and therapeutic vesicles.

Prediction of Rolling Bearing Cage Dynamics Using Dynamic Simulations and Machine Learning Algorithms

Cage instability or highly dynamic cage movement can have a strong influence on the performance of rolling bearings. In addition to very loud and disturbing noises (“squeal”), bearing failure due to cage fracture can occur. This article deals with two topics: the general classification of cage motions and the prediction of application-dependent cage motions to prevent cage instability during operation. The dependencies of the unstable cage movement on the bearing’s load and geometric characteristics of the cage are analyzed using a large number of sophisticated simulations, based on multibody dynamics. To evaluate the cage movements, first a key figure called the “cage dynamics indicator” (CDI) is introduced, which is used to classify the simulation results by means of quadratic discriminant analysis into three types “unstable,” “stable,” and “circling” (= classification of cage motion). Second, a machine learning algorithm trained and tested on the basis of more than 4,000 simulation results enables a time-efficient prediction of the physical correlations between bearing load and cage properties and the resulting cage dynamics (= prediction of cage motion). A comparison of the calculated cage dynamics with the results of an optical measurement of the cage dynamics rounds off this article. This comparison illustrates the high quality of the simulation models and the training data used for machine learning.

Heterogeneity in microstructures and dynamics of dicationic ionic liquids with symmetric and asymmetric cations

In order to understand the role of cation symmetry in intra- and inter-molecular structures and dynamics of dicationic ionic liquids (DILs), we present the MD simulations and Voronoi tessellation analysis of two DILs including [C5(mim)2][NTf2]2 and [C5(tma)(mim)][NTf2]2. In this work, we systematically examined the effect of changing the electrostatic interactions on microheterogeneity (MH) by fixing the strength of the vdW interactions. The qualitative and quantitative analyses of the MH were explored through total/partial structure factors, heterogeneity order parameter (HOP) and domain analysis. Furthermore, radial distribution functions (RDFs), coordination numbers, spatial distribution functions (SDFs), neighborhood analysis of ions and combined distribution functions (CDFs) were calculated to determine the arrangement of the nearest neighboring ions. Local structural heterogeneity leads to local dynamical heterogeneity, which plays an important role in the applications related to ion transportation. To investigate dynamical heterogeneity, the mean square displacement, non-Gaussian parameter, van Hove correlation, and also the hydrogen bond, ion pair, ion cage and reorientation dynamics have been calculated. The results showed that the replacement of one imidazolium head of symmetrical [C5(mim)2]2+ cation with a trimethylammonium group shows significant effects on their structures and dynamics.

Investigations on the rolling bearing cage dynamics with regard to different operating conditions and cage properties

The dynamic behavior of a cage can have a significant influence on the performance and the noise of a rolling bearing. In case of an unstable or high-frequency cage movement, vibrations are transmitted to the adjacent machine elements and the environment, which can influence the operating behavior of the whole system. In addition, a loud noise can often be perceived, which is referred to in the literature as cage rattling or squealing. In this paper, characteristics of different cage movements are investigated using multibody simulations and experimental investigations. For this purpose, essential properties of the fundamentally observable cage movement types “stable”, “unstable” and “circling” are presented. The calculated cage dynamics and the type of cage motion are used to show dependencies between the operating conditions and the resulting cage movement such as inner ring rotational speed, bearing load or cage characteristics such as pocket clearance. Based on the simulations, interactions between the input parameters can also be determined. The results are used to identify operation-critical conditions and cage properties that lead to high cage dynamics. Finally, a comparison between the results of the multibody calculations and optical measurements is made. The optical measurements are performed using high-speed cameras. Reference markers fixed on the cage and digital image correlation allow the evaluation of the kinematics as well as the deformation of the cage. These results are compared with the simulation data to ensure a high quality of dynamics simulation.

Theory of Pressure-Induced Rejuvenation and Strain Hardening in Metallic Glasses.

We theoretically investigate high-pressure effects on the atomic dynamics of metallic glasses. The theory predicts compression-induced rejuvenation and the resulting strain hardening that have been recently observed in metallic glasses. Structural relaxation under pressure is mainly governed by local cage dynamics. The external pressure restricts the dynamical constraints and slows down the atomic mobility. In addition, the compression induces a rejuvenated metastable state (local minimum) at a higher energy in the free-energy landscape. Thus, compressed metallic glasses can rejuvenate and the corresponding relaxation is reversible. This behavior leads to strain hardening in mechanical deformation experiments. Theoretical predictions agree well with experiments.

Dielectric relaxation of water: assessing the impact of localized modes, translational diffusion, and collective dynamics.

A multitude of distinct physical processes and molecular mechanisms have been introduced in the past in an effort to understand the unusual dielectric loss spectrum of water with its pronounced peak at roughly 20 GHz. Our computer simulations including ab initio molecular dynamics provide no evidence for a major impact of cage dynamics or local-diffusive motion on the lineshape below 200 GHz. We also show that the collective motion of hundreds of water molecules and/or their significant diffusive displacements are not required. Instead, the dielectric relaxation of water up to about 200 GHz can be quantitatively described in terms of two unimodal and smoothly decaying spectral contributions.