Bath Of Particles Research Articles

Simulation study of passive rod diffusion in active bath: Nonmonotonic length dependence and abnormal translation-rotation coupling

Diffusion of tracer particles in active bath has attracted extensive attention in recent years. So far, most studies have considered isotropic spherical tracer particles, while the diffusion of anisotropic particles has rarely been involved. Here we investigate the diffusion dynamics of a rigid rod tracer in a bath of active particles by using Langevin dynamics simulations in a two-dimensional space. Particular attention is paid to how the translation (rotation) diffusion coefficient DT (DR) change with the length of rod L and active strength Fa. In all cases, we find that rod exhibits superdiffusion behavior in a short time scale and returns to normal diffusion in the long time limit. Both DT and DR increase with Fa, but interestingly, a nonmonotonic dependence of DT (DR) on the rod length has been observed. We have also studied the translation-rotation coupling of rod, and interestingly, a negative translation-rotation coupling is observed, indicating that rod diffuses more slowly in the parallel direction compared to that in the perpendicular direction, a counterintuitive phenomenon that would not exist in an equilibrium counterpart system. Moreover, this anomalous (diffusion) behavior is reentrant with the increase of Fa, suggesting two competitive roles played by the active feature of bath particles.

Transport of finite chains with free ends through thin channels

We describe the transport of a finite chain of [Formula: see text] identical particles in a thermal bath, through thin channels that forbid any crossing with a conceptually and technically simple method, that is neither restricted to the thermodynamic limit (infinite systems with finite density) nor to overdamped systems. We obtain analytically the mean squared displacement of each particle. Regardless of the damping, we identify a correlated regime for which chain transport is dominated by the correlations between individual particles. At large damping, the mean squared displacement evidences the typical single file behavior, with a time dependence that scales as [Formula: see text]. At small damping, the correlated regime is rather described by a diffusion-like behavior, with a diffusivity which is neither the individual particle diffusivity nor the Fickian diffusivity of the chain as a whole. We emphasize that, for a chain with free ends, the fluctuations of the chain ends are larger by a factor two than the fluctuations of its center. This effect is observed whatever the damping [Formula: see text], but the duration of this fluctuations enhancement is found to scale as [Formula: see text] for low damping and as [Formula: see text] for high damping. We discuss the relevance of this model to the transport of actual systems in confined geometries.

Study of fabrication and wear properties of Ni–SiC composite coatings on A356 aluminum alloy

This study produces Ni–SiC composite coatings by electrodeposition with A356 aluminum alloy as the substrate. The concentration of SiC particles in the plating bath and the current density are varied to produce composite coatings with particle concentrations from 2.5 to 12.3 vol%, and hardness from 439 to 538 HV. Dry friction and lubrication friction experiments use a ball-on-disk to determine the frictional behavior of the Ni–SiC composite coatings. The coefficient of friction and wear resistance of a Ni–SiC composite coating is determined for different load and lubrication environments.The results show that in a dry friction environment, a composite coating with a SiC particle content of 8.0–10.0 vol% and a hardness of 500–540HV has a lower coefficient of friction and abrasion rate due to the high hardness and elastic modulus ratio (H/E). In a lubricating environment, load resistance is 150 N if the coating contains 8.0–10.0 vol% particles. When the load is increased to 200 N, a large number of SiC particles are removed due to friction so the Ni-12.3 vol%SiC many oil storage pores are created in the composite coating. This hydraulic lubrication produces a coating with better wear performance.

Emergence of scale invariance in the dynamics of an ellipsoidal particle on a granular magnetic bath

Recently, scale invariance in the position time series of particles constituting a non-vibrating magnetic granular system was determined, employing Fourier analysis (FA) and detrended fluctuation analysis (DFA). Such system consists of steel balls confined inside a circular wall that surrounds a glass plate, where kinetic energy is provided to the particles by means of the application of an external vertical time-dependent magnetic field. Now, in the present contribution, the magnetic particles are considered as an agitated granular bath where a single non-magnetic ellipsoidal particle lies. In this work, we carry out experiments considering two different sizes of the ellipsoidal particle, and, in addition to the oscillating magnetic field, we also consider a constant magnetic field. In this new scenario, we found that the Fourier power spectra of the time series of the ellipsoidal particle positions in x-direction, and the angular time series constructed from rotational movements, follow a power law, P(f)∝1∕fβ with β≈2, indicating Brownian-type motion. Additionally, we observed that this scale invariance property is not true for the position time series of the particles in the granular bath. These results are confirmed by applying DFA. In this way, by employing time series analysis techniques, a characterization of the emergence of Brownian paths in this model of active matter is achieved.

Production of thermal axions across the electroweak phase transition

Light axions can potentially leave a cosmic background, just like neutrinos. We complete the study of thermal axion production across the electroweak scale by providing a smooth and continuous treatment through the two phases. Focusing on both flavor conserving and violating couplings to third generation quarks, we compute the amount of axions produced via scatterings and decays of thermal bath particles. We perform a model independent analysis in terms of axion effective couplings, and we also make predictions for specific microscopic QCD axion scenarios. This observable effect, parameterized as it is conventional by an effective number of additional neutrinos, is above the 1σ sensitivity of future CMB-S4 surveys. Moreover, if one assumes no large hierarchies among dimensionless axion couplings to standard model particles, future axion helioscopes will provide a complementary probe for the parameter region we study.

Brownian motion in a bath affected by an external harmonic potential

The Brownian motion is studied within a modification of the Zwanzig-Caldeira-Legget model when both the Brownian particle and the surrounding bath particles respond to an external harmonic field. It is shown for the derived generalized Langevin equation that the second fluctuation-dissipation theorem remains valid but, as distinct from the usual consideration, with the thermal random force depending on the strength of the confinement potential. An approximate method to evaluate the memory function, the apparent stiffness of the external potential, and the time correlation functions used in the description of the Brownian motion is proposed and applied for the frequency distribution of the bath oscillators, which in the absence of external forces corresponds to the exponentially decaying memory function.

Controlling Entropic Uncertainty and Quantum Correlation of a Bath of Spins

We investigate the dynamical behaviors of quantum-memory-assisted entropic uncertainty relation (QMA-EUR) and quantum correlation (both quantum entanglement and quantum discord) for a two-qubit system coupled to a spin environment by a z–x-type interaction. Our purpose is to find out how the QMA-EUR and quantum correlations are affected by the pre-designed factors of the environment, including initial spin environmental states and the coupling constants. The numerical results show that the initial environmental state has significant effect on the dynamics of quantum correlations of the central spin system. We find that the environment should be prepared in a block entangled state with Bell state. This option is beneficial to reducing entropic uncertainty and inhibits the damping of quantum correlation. We also find that the high frequency particles in a spin bath can significantly enhance the robustness of quantum correlation of the system.

Quantum Brownian motion: Drude and Ohmic baths as continuum limits of the Rubin model.

The motion of a free quantum particle in a thermal environment is usually described by the quantum Langevin equation, where the effect of the bath is encoded through a dissipative and a noise term, related to each other via the fluctuation dissipation theorem. The quantum Langevin equation can be derived starting from a microscopic model of the thermal bath as an infinite collection of harmonic oscillators prepared in an initial equilibrium state. The spectral properties of the bath oscillators and their coupling to the particle determine the specific form of the dissipation and noise. Here we investigate in detail the well-known Rubin bath model, which consists of a one-dimensional harmonic chain with the boundary bath particle coupled to the Brownian particle. We show how in the limit of infinite bath bandwidth, we get the Drude model, and a second limit of infinite system-bath coupling gives the Ohmic model. A detailed analysis of relevant equilibrium correlation functions, such as the mean squared displacement, velocity autocorrelation functions, and response function are presented, with the aim of understanding the various temporal regimes. In particular, we discuss the quantum-to-classical crossover time scales where the mean square displacement changes from a ∼lnt to a ∼t dependence. We relate our study to recent work using linear response theory to understand quantum Brownian motion.

Confinement-induced alternating interactions between inclusions in an active fluid.

In a system of colloidal inclusions suspended in an equilibrium bath of smaller particles, the particulate bath engenders effective, short-ranged, primarily attractive interactions between the inclusions, known as depletion interactions, that originate from the steric depletion of bath particles from the immediate vicinity of the inclusions. In a bath of active (self-propelled) particles, the nature of such bath-mediated interactions can qualitatively change from attraction to repulsion, and they become stronger in magnitude and range of action as compared with typical equilibrium depletion interactions, especially as the bath activity (particle self-propulsion) is increased. We study effective interactions mediated by a bath of active Brownian particles between two fixed, impenetrable, and disk-shaped inclusions in a planar (channel) confinement in two dimensions. Confinement is found to strongly influence the effective interaction between the inclusions, specifically by producing alternating interaction profiles with possible attractive and repulsive regions in sufficiently narrow channels. We study the dependence of the ensuing interactions on different system parameters and the orientational (parallel versus perpendicular) configuration of the inclusion pair relative to the channel walls. The confinement effects are largely regulated by the layering of active particles next to the surface boundaries, both of the inclusions and the channel walls that counteract one another in accumulating the active particles in their own proximities. In narrow channels, the combined effects due to the channel walls and the inclusions lead to peculiar structuring of active particles (reminiscent of wavelike interference patterns) within the channel.

Dark matter bound-state formation at higher order: a non-equilibrium quantum field theory approach

The formation of meta-stable dark matter bound states in coannihilating scenarios could efficiently occur through the scattering with a variety of Standard Model bath particles, where light bosons during the electroweak cross over or even massless photons and gluons are exchanged in the t-channel. The amplitudes for those higher-order processes, however, are divergent in the collinear direction of the in- and out-going bath particles if the mediator is massless. To address the issue of collinear divergences, we derive the bound-state formation collision term in the framework of non-equilibrium quantum field theory. The main result is an expression for a more general cross section, which allows to compute higher-order bound-state formation processes inside the primordial plasma background in a comprehensive manner. Based on this result, we show that next-to-leading order contributions, including the bath-particle scattering, are i) collinear finite and ii) generically dominate over the on-shell emission for temperatures larger than the absolute value of the binding energy. Based on a simplified model, we demonstrate that the impact of these new effects on the thermal relic abundance is significant enough to make it worthwhile to study more realistic coannihilation scenarios.

Brownian motion in a gas of charged particles under the influence of a magnetic field

The Brownian motion of a particle immersed in a gas of other charged particles is considered when the system is placed in a constant magnetic field. The Zwanzig–Caldeira–Legget particle-bath model is modified so that not only the charged Brownian particle (BP) but also the surrounding it particles respond to the external field. For systems conditioned to be stationary, two non-Markovian equations of motion for the BP across the field are derived. They are of the type of generalized Langevin equations with two different memory functions. As distinct from all the previous theories, the random thermal force is found to depend on the field magnitude. Its time correlation function is connected with one of the found memory functions through the familiar Kubo’s second fluctuation–dissipation theorem. The other memory function disappears when the magnetic force does not affect the bath particles. Analytical expressions can be obtained for the velocity correlation functions and other relevant quantities such as the mean square displacement and the diffusion coefficient of the BP for different distributions of the eigenfrequencies of the bath oscillators. Assuming the Drude distribution of the frequencies, it is found that the motion of the particle at long times is sub-diffusive, with the exponent 1/2. The case of the bath frequencies corresponding to the fractional thermal noise is also analyzed.

Effect of the early kinetic decoupling in a fermionic dark matter model

We study the effect of the early kinetic decoupling in a model of fermionic dark matter (DM) that interacts with the standard model particles only by exchanging the Higgs boson. There are two DM-Higgs couplings, namely CP-conserving and CP-violating couplings. If the mass of the DM is slightly below half of the Higgs boson mass, then the couplings are suppressed to obtain the measured value of the DM energy density by the freeze-out mechanism. In addition, the scattering processes of DM off particles in the thermal bath are suppressed by the small momentum transfer if the CP-violating DM-Higgs coupling is larger than the CP-conserving one. Due to the suppression, the temperature of the DM can differ from the temperature of the thermal bath. By solving coupled equations for the number density and temperature of the DM, we calculate the DM-Higgs couplings that reproduce the right amount of the DM relic abundance. We find that the couplings have to be larger than the one obtained without taking into account the difference in the temperatures. A consequence of the enhancement of the DM-Higgs couplings is the enhancement of the Higgs invisible decay branching ratio. The enhancement is testable at current and future collider experiments.

Electrodeposition of Photocatalytic Sn–Ni Matrix Composite Coatings Embedded with Doped TiO2 Particles

Direct current electrodeposited Sn–Ni/TiO2 nanostructured coatings were produced by embedding two different doped types of TiO2 particles within the alloy matrix, a commercially available doped carbon-based and doped N,S-TiO2 particles. The structural characteristics of the composite coatings have been correlated with the effect of loading, type of particles in the electrolytic bath, and the applied current density. Regardless of the type of doped particles TiO2, increasing values of applied current density resulted in a reduction of the co-deposition percentage of TiO2 particles and an increase of Tin content into the alloy matrix. The application of low current density values accompanied by a high load of particles in the bath led to the highest codeposition percentage (~3.25 wt.%) achieved in the case of embedding N,S-TiO2 particles. X-ray diffraction data demonstrated that in composite coatings the incorporation of the different types of TiO2 particles in the alloy metal matrix modified significantly the nano-crystalline structure in comparison with the pure coatings. The best photocatalytic behavior under visible irradiation was revealed for the composite coatings with the highest co-deposition percentage of doped N,S-TiO2 particles, that also exhibited enhanced wear resistance and slightly reduced microhardness compared to pure ones.

Brownian motion of ellipsoidal particles on a granular magnetic bath.

We study the Brownian motion of ellipsoidal particles lying on an agitated granular bath composed of magnetic particles. We quantify the mobility of different floating ellipsoidal particles using the mean-square displacement and the mean-square angular displacement, and relate the diffusion coefficients to the bath particle motion. In terms of the particle major radius R, we find the translational diffusion coefficient scales roughly as 1/R^{2} and the rotational diffusion coefficient scales as roughly 1/R^{4}; this is consistent with the assumption that diffusion arises from random kicks of the bath particles underneath the floating particle. By varying the magnetic forcing, the bath particles' diffusivity changes by a factor of ten; over this range, the translational and rotational diffusion of the floating particles change by a factor of 50. However, the ratio of the two diffusion constants for the floating particles is forcing-independent. Unusual aspects of the floating particle motion include non-Gaussian statistics for their displacements.

Corrosion and Wear Properties of Zn-Based Composite Coatings

Zn electroplated coatings are usually applied to protect the surface of steel components against corrosion. However, to increase their service life, improved mechanical properties of the coatings are required. The present study aimed to investigate the corrosion and sliding wear behavior of Zn electroplated composite coatings deposited onto the surface of a carbon steel substrate. The coatings were obtained by adding diamond and aluminum oxide suspensions in the base deposition bath. The morphology and microstructure of the coatings were characterized by confocal laser scanning microscopy and scanning electron microscopy, and the chemical composition was determined by energy-dispersive x-ray spectroscopy. The corrosion behavior was assessed by electrochemical measurements, and the sliding wear resistance was evaluated by pin on disk method. The results revealed that the addition of diamond and alumina particles in the zinc deposition bath had a positive influence on the tribological behavior of the coatings.

Brownian motion in a bath responding to external electric fields

The Brownian motion of a charged particle immersed in a medium of charged particles is considered when the system is placed in electric fields. Coming from the Zwanzig-Caldeira-Legget particle-bath model, we modify it so that not only the Brownian particle (BP) but also the bath particles respond to the external fields. The generalized Langevin equation is derived for stationary systems. Arbitrarily time-dependent electric fields do not affect the memory function, the thermal noise force, and the BP velocity correlation function. Despite the effect of the external field on the bath particles, Kubo's second fluctuation-dissipation theorem remains unchanged. A general equation for the mean velocity of the BP, which is affected by the bath particles' response to the electric field, is obtained and solved for the Drude model of the frequency distribution of the bath oscillators.

Composite materials on the manufacturing of metallic surface alloys

This study focused on the influence of composite materials on electrodeposition process. The exploit of composite materials due to their exceptional characteristics such as, aesthetic beauty, mechanical performance or electrochemical stability can never be over emphasized. While electrodeposition is seen a unique method for surface improvement development of right bath framework and particle infringement are major concern. Therefore, there is a need to adequately highlight the uniqueness of composite material development, their application using deposition route.

Dynamics and friction of a large colloidal particle in a bath of hard spheres: Langevin dynamics simulations and hydrodynamic description.

The analysis of the dynamics of tracer particles in a complex bath can provide valuable information about the microscopic behavior of the bath. In this work, we study the dynamics of a forced tracer in a colloidal bath by means of Langevin dynamics simulations and a theory model within continuum mechanics. In the simulations, the bath is comprised of quasihard spheres with a volume fraction of 50% immersed in a featureless quiescent solvent, and the tracer is pulled with a constant small force (within the linear regime). The theoretical analysis is based on the Navier-Stokes equation, where a term proportional to the velocity arises from coarse-graining the friction of the colloidal particles with the solvent. As a result, the final equation is similar to the Brinkman model, although the interpretation is different. A length scale appears in the model, k_{0}^{-1}, where the transverse momentum transport crosses over to friction with the solvent. The effective friction coefficient experienced by the tracer grows with the tracer size faster than the prediction from Stokes's law. Additionally, the velocity profiles in the bath decay faster than in a Newtonian fluid. The comparison between simulations and theory points to a boundary condition of effective partial slip at the tracer surface. We also study the fluctuations in the tracer position, showing that it reaches diffusion at long times, with a subdiffusive regime at intermediate times. The diffusion coefficient, obtained from the long-time slope of the mean-squared displacement, fulfills the Stokes-Einstein relation with the friction coefficient calculated from the steady tracer velocity, confirming the validity of the linear response formalism.

Constraint Dependence of Active Depletion Forces on Passive Particles.

Using simulations and experiments, we demonstrate that the effective interaction between passive particles in an active bath substantially depends on an external constraint suffered by the passive particles. Particularly, the effective interaction between two free passive particles, which is directly measured in simulation, is qualitatively different from the one between two fixed particles. Moreover, we find that the friction experienced by the passive particles-a kinematic constraint-similarly influences the effective interaction. These remarkable features are in significant contrast to the equilibrium cases, and mainly arise from the accumulation of the active particles near the concave gap formed by the passive spheres. This constraint dependence not only deepens our understanding of the "active depletion force," but also provides an additional tool to tune the effective interactions in an active bath.

Preparation of Fe3O4@TiO2 blended PVDF membrane by magnetic coagulation bath and its permeability and pollution resistance

Fe3O4@TiO2 magnetic photocatalytic particles were synthesized and characterized. A composite PVDF separation membrane blended with Fe3O4@TiO2 particles was prepared by the phase inversion method. The main purpose was to explore the effect of magnetic fields on the performance of membranes during coagulation bath. The structure characterization, permeability, interception ability, and anti-pollution performance of inorganic particle blend membranes were then tested. Results showed that magnetic field coagulation baths have little effect on the pore structure and hydrophilicity of the membrane surface. When the liquid membranes was condensed under a magnetic field, the magnetic photocatalytic particles could migrate at a greater rate to the surface of the membrane. Through performance tests of the PVDF modified membranes, it was found that a magnetic coagulation bath could promote more Fe3O4@TiO2 to aggregate on the surface of the membrane, which led to the blend membrane formed under the magnetic coagulation bath had better antifouling performance against humic acid solution under ultraviolet light irradiation. Membranes with 0.5wt% magnetic photocatalytic particles in a magnetic field coagulation bath performed best. When filtered under the condition of ultraviolet light, its pure water flux was 1752L/(m2h), and the rejection rate of 20mg/L humic acid solution was 67.17%. The flux attenuation before and after filtration was 38.84%, the flux recovery rate was 91.10%, and the irreversible pollution index was only 8.90%.