Collisional Interactions Research Articles

The supermassive black hole merger-driven evolution of high-redshift red nuggets into present-day cored early-type galaxies

ABSTRACT Very compact ($R_\mathrm{e}\lesssim 1$ kpc) massive quiescent galaxies (red nuggets) are more abundant in the high-redshift Universe ($z\sim 2$–3) than today. Their size evolution can be explained by collisionless dynamical processes in galaxy mergers which, however, fail to reproduce the diffuse low-density central cores in the local massive early-type galaxies (ETGs). We use sequences of major and minor merger N-body simulations starting with compact spherical and disc-like progenitor models to investigate the impact of supermassive black holes (SMBHs) on the evolution of the galaxies. With the ketju code we accurately follow the collisional interaction of the SMBHs with the nearby stellar population and the collisionless evolution of the galaxies and their dark matter haloes. We show that only models including SMBHs can simultaneously explain the formation of low-density cores up to sizes of $R_\mathrm{b} \sim 1.3$ kpc with mass deficits in the observed range and the rapid half-mass size evolution. In addition, the orbital structure in the core region (tangentially biased orbits) is consistent with observation-based results for local cored ETGs. The displacement of stars by the SMBHs boost the half-mass size evolution by up to a factor of 2 and even fast rotating progenitors (compact quiescent discs) lose their rotational support after 6–8 mergers. We conclude that the presence of SMBHs is required for merger-driven evolution models of high-redshift red nuggets into local ETGs.

Kinetics of monolayer MoS2-encapsulated nanobubbles on hexagonal boron nitride substrates

Understanding the kinetics of nanobubbles encapsulated by ultrathin two-dimensional (2D) layered van der Waals crystal membranes on atomically flat substrates is important to the applications of 2D materials and the pursuit of 2D nanobubble technologies. Here, we investigate the controlled motion of monolayer molybdenum disulfide (MoS2)-encapsulated nanobubbles on flat hexagonal boron nitride substrates using atomic force microscopy (AFM). Our study reveals a distinct transition from standstill bubble deformations to stable, stepwise bubble translations on flat substrates. The membrane tension-dominated 2D nanobubble behaves like an elastic soft body in its collision interaction with the AFM tip. This delicate motion-control technique enables neighboring 2D nanobubbles to move closer and eventually coalesce into larger nanobubbles. These findings pave the way for high-precision manipulation of nanobubbles and facilitate the exploration of their emerging applications.

Effect of surface pore on bubble–solid surface collisional interaction and adhesion process

ABSTRACT In low-rank and oxidized coal flotation, the pore structure on particle surfaces can significantly affect the bubble-particle collisional adhesion process, which in turn affects the flotation performance. In this study, to understand its mechanism, the collisional interaction process between bubbles and substrates with a single pore was observed throjugh a self-made system, and the interaction force was measured during the adhesion and detachment process. The results proved that the pore accelerated the kinetic energy dissipation of the bubbles after collision and largely shortened the induction time during the adhhesion process. The induction time on the perforated substrate was an order of magnitude shorter than that on the smooth surface. The interaction force measurement results showed that bubble interaction behavior on smooth and perforated surfaces was essentially the same, which suggests that the single pore on hydrophobic surface can accelerate bubble adhesion without affecting adhesion stability, thus strengthening bubble – particle adhesion.

Experimental study of the collision behavior between moving and sessile droplets on curved surfaces

In this study, we used pure water droplets to investigate the collision interactions of two droplets on curved surfaces. We captured the collision dynamics at different droplet impact velocities with the use of a high-speed camera. We observed three main modes during droplet collisions: deformation, coalescence, and rebound. We also plotted a collision regime map for different surfaces using the Weber number We and presented the critical We value for coalescence and rebound. With the increases of curvature radius, the critical We variates around 6.7 to 12.3 and presenting the tendency of increases first and then decreases. Furthermore, we quantitatively analyzed the transformation laws of droplet rebound height and compression value. Our main findings describe how the surface curvature radius influences droplet interactions, thus affecting the contact time, compression value and droplet deformation factor. The outcomes of droplet collisions on curved surfaces contribute to the development of technologies related to droplet manipulation and their applications in chemical and industrial processes.

Single charged Higgs pair production in exclusive processes at the LHC

The production of a single charged Higgs boson pair by photon–photon interactions in pp collisions at the LHC is investigated in this exploratory study. We focus on the exclusive production, which is characterized by intact protons and two-rapidity gaps in the final state, and assume the type-I two-Higgs-doublet model, which still allows a light charged Higgs. Assuming the leptonic H±→[τντ]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H^{\\pm }\\rightarrow [\ au \ u _{\ au }]$$\\end{document} decay mode, we derive predictions for the transverse momentum, rapidity and invariant mass distributions of the τ+τ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au ^+ \ au ^-$$\\end{document} pair for different values of the charged Higgs mass. The contributions of different background processes are also estimated. Our results indicate that the contribution of the exclusive H+H-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H^+ H^-$$\\end{document} production for the [τ+ντ][τ-ντ]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[\ au ^{+}\ u _{\ au }][\ au ^{-} \ u _{\ au }]$$\\end{document} final state is non-negligible and can, in principle, be used to searching for a light charged Higgs.

The difficult path to coalescence: massive black hole dynamics in merging low-mass dark matter haloes and galaxies

ABSTRACT We present a high-resolution numerical study of the sinking and merging of massive black holes (MBHs) with masses in the range of $10^3 - 10^7 \, \mathrm{M}_\odot$ in multiple minor mergers of low-mass dark matter haloes without and with galaxies ($4\times 10^8 \, \mathrm{M}_\odot \lesssim {M}_{\mathrm{halo}} \lesssim 2\times 10^{10} \, \mathrm{M}_\odot)$. The ketju simulation code, a combination of the gadget tree solver with accurate regularized integration, uses unsoftened forces between the star/dark matter components and the MBHs for an accurate treatment of dynamical friction and scattering of dark matter/stars by MBH binaries or multiples. Post-Newtonian corrections up to order 3.5 for MBH interactions allow for coalescence by gravitational wave emission and gravitational recoil kicks. Low-mass MBHs ($\lesssim 10^5 \, \mathrm{M}_\odot$) hardly sink to the centre or merge. Sinking MBHs have various complex evolution paths – binaries, triplets, free-floating MBHs, and dynamically or recoil ejected MBHs. Collisional interactions with dark matter alone can drive MBHs to coalescence. The highest mass MBHs of $\gtrsim 10^6 \, \rm M_\odot$ mostly sink to the centre and trigger the scouring of dark matter and stellar cores. The scouring can transform a centrally baryon-dominated system into a dark-matter-dominated system. Our idealized high-resolution study highlights the difficulty to bring in and keep low-mass MBHs in the centres of low-mass haloes/galaxies – a remaining challenge for merger assisted MBH seed growth mechanisms.

Spin–spin interaction and magnetic Feshbach resonances in collisions of high-spin atoms with closed-shell atoms

We investigate interactions and ultracold collisions of high-spin but spherical atoms with closed-shell atoms, focusing on the example of Cr with Yb. Such a combination has only one potential energy curve but gives rise to a substantial intra-atomic spin–spin interaction, which causes Feshbach resonances due to rotationally excited states. We find such resonances are guaranteed below 250 G and can reach widths of tens of G in favorable circumstances. We study the effect of hyperfine structure, which can create additional wide resonances at low fields. Finally, we consider isotopic substitution and show that for any realistic potential it is very likely that at least one combination of commonly used isotopes will have resonances with experimentally favorable properties. Thus, this is a promising system for both molecule formation and studying quantum mixtures including dipolar species. Published by the American Physical Society 2024

Highly controllable stabilization and switching of multiple colliding soliton sequences with generic Ginzburg–Landau gain–loss

We investigate propagation of J soliton sequences in a nonlinear optical waveguide array with generic weak Ginzburg–Landau (GL) gain–loss and nearest-neighbor (NN) interaction. The propagation is described by a system of J perturbed coupled nonlinear Schrödinger (NLS) equations. The NN interaction property leads to the elimination of collisional three-pulse interaction effects, which prevented the observation of stable multisequence soliton propagation with J>2 sequences in the presence of generic GL gain–loss in all previous studies. We show that the dynamics of soliton amplitudes can be described by a generalized J-dimensional Lotka–Volterra (LV) model. Stability and bifurcation analysis for the equilibrium points of the LV model, which is augmented by an application of the Lyapunov function method, is used to develop setups that lead to robust and scalable transmission stabilization and switching for a general J value. The predictions of the LV model are confirmed by extensive numerical simulations with the perturbed coupled-NLS model with J=3, 4, and 5 soliton sequences. Furthermore, soliton stability and the agreement between the LV model’s predictions and the simulations are not reduced with increasing value of J. Therefore, our study provides the first demonstration of robust control of multiple colliding sequences of NLS solitons in the presence of generic weak GL gain–loss with an arbitrary number of sequences. Due to the robustness and scalability of the results, they can have important applications in stabilization and switching of broadband soliton-based optical waveguide transmission.

Various nonlinear characteristics of breather/rogue waves and controllable interaction phenomena for a new KdV equation with variable coeffcients

In this paper, we investigate and analyze various nonlinear phenomena of a new (2+1)-dimensional KdV equation with variable coefficients, and successfully obtain breather/rogue wave solutions and interaction solutions of the KdV equation by using the bilinear neural network method and symmetry transformation. Subsequently, we analyze the dynamical characteristics and evolution process of these obtained solutions through the 3-D animations, and find a series of interesting nonlinear phenomena concerning breather/rogue waves, such as fission, regeneration, annihilation, collision, and controllable interaction phenomena on nonzero backgrounds. This paper provides a more intuitive understanding for the nonlinear phenomena of these obtained solutions, and these nonlinear phenomena have potential application value in fluid dynamics, elastic mechanics and other fields of nonlinear science.

Hyperfine and Zeeman interactions in ultracold collisions of molecular hydrogen with atomic lithium.

We present a rigorous quantum scattering study of the effects of hyperfine and Zeeman interactions on cold Li-H2 collisions in the presence of an external magnetic field using a recent abinitio potential energy surface. We find that the low-field-seeking states of H2 predominantly undergo elastic collisions: the ratio of elastic-to-inelastic cross sections exceeds 100 for collision energies below 100 mK. Furthermore, we demonstrate that most inelastic collisions conserve the space-fixed projection of the nuclear spin. We show that the anisotropic hyperfine interaction between the nuclear spin of H2 and the electron spin of Li can have a significant effect on inelastic scattering in the ultracold regime, as it mediates two processes: the electron spin relaxation in lithium and the nuclear spin-electron spin exchange. Given the predominance of elastic collisions and the propensity of inelastic collisions to retain H2 in its low-field-seeking states, our results open up the possibility of sympathetic cooling of molecular hydrogen by atomic lithium, paving the way for future exploration of ultracold collisions and high-precision spectroscopy of H2 molecules.

Spin- and Momentum-Correlated Atom Pairs Mediated by Photon Exchange and Seeded by Vacuum Fluctuations.

Engineering pairs of massive particles that are simultaneously correlated in their external and internal degrees of freedom is a major challenge, yet essential for advancing fundamental tests of physics and quantum technologies. In this Letter, we experimentally demonstrate a mechanism for generating pairs of atoms in well-defined spin and momentum modes. This mechanism couples atoms from a degenerate Bose gas via a superradiant photon-exchange process in an optical cavity, producing pairs via a single channel or two discernible channels. The scheme is independent of collisional interactions, fast, and tunable. We observe a collectively enhanced production of pairs and probe interspin correlations in momentum space. We characterize the emergent pair statistics and find that the observed dynamics is consistent with being primarily seeded by vacuum fluctuations in the corresponding atomic modes. Together with our observations of coherent many-body oscillations involving well-defined momentum modes, our results offer promising prospects for quantum-enhanced interferometry and quantum simulation experiments using entangled matter waves.

A CFD-DEM Simulation of Droplets in an Airless Spray Coating Process of a Square Duct

The purpose of this paper is to provide a numerical simulation, taking into account the collisional interactions of droplets in an airless rotary spray coating process. The hydrodynamics of gas and droplets are simulated using the CFD-discrete element method (DEM) with the JKR contact model in an airless rotary spray coating process of a horizontal square duct. The surface energy parameter used in the JKR model is calibrated using a virtual accumulation angle test in the funnel device. Based on the distribution of accumulation angles, a suitable surface energy for wall droplets is proposed. A rational gas RNG k-ε model is suggested in accordance with the comparisons of velocities, standard deviations, and the skewness of droplet number fractions from three turbulence models. The simulations of droplet film thicknesses agree with measurements from the literature regarding the film thickness along a vertical panel. The correlations of the exit gas and droplet velocities of sprayer holes are proposed with a discharge coefficient of 0.85 for gas and 5.87 for droplets. A number index of droplets is introduced in order to measure the uniformity of droplet distributions. A low droplet number index is found at low rotational speeds, representing a more uniform distribution of droplets as the rotation speeds reduce within the square duct. The normal force between the droplet and the wall is approximately an order of magnitude larger than the droplet–wall tangential force of collisions.

An Alternative Calibration Method for Measuring N2O5 with an Iodide-Chemical Ionization Mass Spectrometer and Influencing Factors.

In this work, we developed an alternative calibration method for measuring N2O5 with an iodide adduct mass spectrometer (I-CIMS). In this calibration method, N2O5 is heated and then quantified based on the decrease in the amount of NO due to its reaction with the pyrolysis product (NO3). This alternative calibration method was compared with the commonly used method utilizing NOx analyzers equipped with a photolytic converter, which gauge NO2 reduction as a result of its reaction with O3 to quantify N2O5. It is notable that the two methodologies demonstrate favorable consistency in terms of calibrating N2O5, with a variance of less than 10 %. The alternative calibration method is a more reliable way to quantify N2O5 with CIMS, considering the instability of the NO2 conversion efficiency of photolytic converters in NOx analyzers and the loss of N2O5 in the sampling line. The effects of O3 and relative humidity (RH) on the sensitivity toward N2O5 were further examined. There was minimal perturbation of N2O5 quantification upon exposure to O3 even at high concentrations. The N2O5 sensitivity exhibited a nonlinear dependence on RH as it initially rose and then fell. Besides I(N2O5)-, the collisional interaction between I(H2O)- and N2O5 also forms I(HNO3)-, which may interfere with the accurate quantification of HNO3. As a consequence of the pronounced dependence on humidity, it is advisable to implement humidity correction procedures when conducting measurements of N2O5.

Collision-broadening of vibration-inversion-rotation ammonia spectral lines in Q(J)-branch at 6.2 µm by cavity ring-down spectroscopy

In this experimental investigation, collision broadening effects on the Q(J)-branch vibration-inversion-rotation spectral lines of ammonia (NH3) within the fundamental ν4 asymmetric bending vibrational band in the 6.2 µm mid-IR region are reported. A continuous-wave external-cavity quantum cascade laser coupled with a high-sensitive cavity ring-down spectrometer was employed to selectively probe 10 transitions in high-resolution, including few inversion doublets of gaseous NH3. Pressure-broadening coefficients, γNH3-Xi (Xi = He, Ar, N2, O2, zero-air) in cm−1 atm−1, characterizing the collision interaction between NH3 and various external perturbing gases, including helium, argon, nitrogen, oxygen, and zero-air, were determined at room temperature (296 K). Mean collision times and optical collision diameters of each collision partner were explored to gain deeper insight into the perturber-induced collision dynamics. This investigation elucidates the intricate intermolecular interactions and collision phenomena induced by various foreign perturbers with NH3, with potential implications for future spectroscopic and atmospheric research of this polyatomic molecule.

Video Classification of Cloth Simulations: Deep Learning and Position-Based Dynamics for Stiffness Prediction.

In virtual reality, augmented reality, or animation, the goal is to represent the movement of deformable objects in the real world as similar as possible in the virtual world. Therefore, this paper proposed a method to automatically extract cloth stiffness values from video scenes, and then they are applied as material properties for virtual cloth simulation. We propose the use of deep learning (DL) models to tackle this issue. The Transformer model, in combination with pre-trained architectures like DenseNet121, ResNet50, VGG16, and VGG19, stands as a leading choice for video classification tasks. Position-Based Dynamics (PBD) is a computational framework widely used in computer graphics and physics-based simulations for deformable entities, notably cloth. It provides an inherently stable and efficient way to replicate complex dynamic behaviors, such as folding, stretching, and collision interactions. Our proposed model characterizes virtual cloth based on softness-to-stiffness labels and accurately categorizes videos using this labeling. The cloth movement dataset utilized in this research is derived from a meticulously designed stiffness-oriented cloth simulation. Our experimental assessment encompasses an extensive dataset of 3840 videos, contributing to a multi-label video classification dataset. Our results demonstrate that our proposed model achieves an impressive average accuracy of 99.50%. These accuracies significantly outperform alternative models such as RNN, GRU, LSTM, and Transformer.

Another normality is possible. Distributive transformations and emergent Gaussianity

A distributional route to Gaussianity, associated with the concept of Conservative Mixing Transformations (CMT) in ensembles of random vector-valued variables, is proposed. This route is completely different from the additive mechanism characterizing the application of Central Limit Theorem, as it is based on the iteration of a random transformation preserving the ensemble variance. Gaussianity emerges as a “supergeneric” property of ensemble statistics, in the case the energy constraint is quadratic in the norm of the variables. This result puts in a different light the occurrence of equilibrium Gaussian distributions in kinetic variables (velocity, momentum), as it shows mathematically that, in the absence of any other dynamic mechanisms, almost Gaussian distributions stem from the low-velocity approximations of the physical conservation principles. Whenever, the energy constraint is not expressed in terms of quadratic functions non-Gaussian distributions arise. This is case of relativistic collisional interactions where the Jüttner distribution is recovered from CMT.

The Kinematic and Microphysical Characteristics of Extremely Heavy Rainfall in Zhengzhou City on 20 July 2021 Observed with Dual-Polarization Radars and Disdrometers

In this study, we utilized dual-polarization weather radar and disdrometer data to investigate the kinematic and microphysical characteristics of an extreme heavy rainfall event that occurred on 20 July 2021, in Zhengzhou. The results are as follows: FY-2G satellite images showed that extremely heavy rainfall mainly occurred during the merging period of medium- and small-scale convective cloud clusters. The merging of these cloud clusters enhanced the rainfall intensity. The refined three-dimensional wind field, as retrieved by the multi-Doppler radar, revealed a prominent mesoscale vortex and convergence structure at the extreme rainfall stage. This led to echo stagnation, resulting in localized extreme heavy rainfall. We explored the formation mechanism of the notable ZDR arc feature of dual-polarization variables during this phase. It was revealed that during the record-breaking hourly rainfall event in Zhengzhou (20 July 2021, 16:00–17:00 Beijing Time), the warm rain process dominated. Effective collision–coalescence processes, producing a high concentration of medium- to large-sized raindrops, significantly contributed to heavy rainfall at the surface. From an observational perspective, it was revealed that raindrops exhibited significant collision interactions during their descent. Moreover, a conceptual model for the kinematic and microphysical characteristics of this extreme rainfall event was established, aiming to provide technical support for monitoring and early warning of similar extreme rainfall events.

Receptiveness angle: A new surrogate safety measure for monitoring traffic safety

This paper presents a framework for monitoring highway traffic-stream measures using quality trajectory data of mixed (heterogeneous) traffic. The framework includes a new measure that reflects the attentiveness of the follower driver, called receptiveness angle, in the vehicle-following process. This measure is integrated with the traditional measures (distance gap between the leader and follower vehicles and their speeds) to model the probabilistic rear-end collision interactions between the two vehicles. To verify the proposed framework, two road sections in India with mixed traffic conditions, located along the same road, were used. One section has no construction activity (base section) and the other has construction activity. The verification consisted of two tasks. First, to trace the movements of the vehicles, trajectory data over the study sections were developed for three traffic-flow levels, where two flow levels between the two sections were comparable. Second, the trajectory data were used to verify the proposed framework which was evaluated for the traffic streams of the two sections at the three traffic-flow levels. The results showed that smaller vehicles in the traffic stream exhibited a higher receptiveness angle (paid less attention) compared to other vehicle classes. Interestingly, the study revealed variations in safety among the three traffic-flow levels. It was observed that the traffic stream was safer at stop-and-go conditions than at other flow conditions. Furthermore, due to the pre-cautioning measures for the construction section, vehicles in this section were more attentive than those in the base section.

Singular solutions of the r-Camassa–Holm equation

This paper introduces the r-Camassa–Holm (r-CH) equation, which describes a geodesic flow on the manifold of diffeomorphisms acting on the real line induced by the metric. The conserved energy for the problem is given by the full norm. For r = 2, we recover the Camassa–Holm equation. We compute the Lie symmetries for r-CH and study various symmetry reductions. We introduce singular weak solutions of the r-CH equation for and demonstrates their robustness in numerical simulations of their nonlinear interactions in both overtaking and head-on collisions. Several open questions are formulated about the unexplored properties of the r-CH weak singular solutions, including the question of whether they would emerge from smooth initial conditions.

Fluctuation analysis and spatial distribution model of particle concentration in a lab-scale scrubbing-cooling chamber

The motion and distribution of ash from coal gasification during the scrubbing and cooling process are crucial for revealing the fluid flow pattern and improving the efficiency of multiphase separation. Experimental measurements of the local solid holdup (εs) in the liquid pool of a laboratory-scale gas-liquid-solid three-phase scrubbing-cooling chamber were carried out using an optical fiber probe. The fluctuations of εs in time and frequency domains under different operating conditions were discussed. The axial and radial distribution of εs was quantitatively analyzed. The results showed that inter-particle collision and particle-fluid interaction are the key factors leading to a fluctuation in particle concentration. Axial uniformity of particle concentration can be quantified by solid-phase volume fraction, particle Reynolds number, Archimedes number, and Morton number; εs in different regions is expressed according to the degree of influence of bubble wake as a correlation equation consisting only of cross-sectional average solid holdup and dimensionless radial position; a dimensionless expression for εs concerning operating conditions was established.