Collapse Time Research Articles

Early Evolution of Spin Direction in Dark Matter Halos and the Effect of the Surrounding Large-scale Tidal Field* *Thesis work conducted at Facultad de Matemática, Astronomía, Física y Computación, Universidad Nacional de Córdoba, Córdoba, Argentina. † † PhD Thesis directed by Manuel Merchán; PhD Degree awarded 2023 July 31.

It is usually assumed that the angular momentum (AM) of dark matter halos arises during the linear stages of structure formation, as a consequence of the coupling between the proto-haloes’ shape and the tidal field produced by their surrounding density perturbations. This approach, known as linear tidal torque theory (TTT), has been shown to make fairly good predictions about the mean evolution of both the AM amplitude and orientation up to approximately the time when the proto-haloes collapse. After this point, proto-haloes are increasingly affected by nonlinear processes that are not taken into account by the model. However, it has been seen in numerical simulations that, even at very early stages, the AM of proto-haloes is systematically reoriented toward perpendicularity with respect to the forming cosmic filaments, in contradiction with the fixed direction expected from the TTT. In this work we present a novel analytical approach that introduces an anisotropic scaling factor to the standard TTT equations, which allows the AM orientation to change in time, even during the linear regime. The amplitude and direction of this shift depend on the large scale tidal field around the forming proto-haloes. Our results significantly improve the predictions for the AM direction up to the time of protohalo collapse and, in some cases, even further in time.

Study on the Influence of a Rigid Wall on Cavitation in Underwater Explosions Near the Free Surface

A two-fluid, phase transition-based multiphase flow model is employed to simulate the dynamics of phase transition between liquid and vapor phases during shock wave and rarefaction wave propagation in underwater explosions. The aim is to understand the influence of a rigid wall on the cavitation evolution process and the cavitation collapse load, considering various charge quantities and water depths. The evolution of crucial physical qualities, such as the density, pressure, and the cavitation domain, within the flow field are analyzed and summarized. The presence of a rigid wall is found to significantly impact the cavitation evolution process in underwater explosions. It affects the shape, size, and dynamics of the cavitation domain, as well as the interaction between the explosion and the surrounding fluid. Specifically, the reflected wave on the wall influences the cavitation collapse load, leading to notable differences in the collapse time and collapse pressure compared to free-field conditions. Under different operating conditions, the size and position of the cavitation domain exhibit distinct changes. The proximity of the rigid wall results in unique patterns of cavitation domain evolution, which in turn lead to variations in the pressure distribution and the emergence of new cavitation regions. The findings of this study provide valuable insights into the behavior of cavitation and atomization induced by underwater explosions near the free surface. The understanding gained from these investigations can contribute to the development of effective safety measures and protective strategies in marine and underwater engineering applications. By accurately predicting and mitigating the effects of cavitation, it is possible to enhance the design and operation of underwater structures, ensuring their integrity and minimizing the potential risks associated with underwater explosions.

Cavitation bubble dynamics inside a droplet suspended in a different host fluid

In this paper, we present a theoretical, experimental and numerical study of the dynamics of cavitation bubbles inside a droplet suspended in another host fluid. On the theoretical side, we provided a modified Rayleigh collapse time and natural frequency for spherical bubbles in our particular context, characterized by the density ratio between the two liquids and the bubble-to-droplet size ratio. Regarding the experimental aspect, experiments were carried out for laser-induced cavitation bubbles inside oil-in-water (O/W) or water-in-oil (W/O) droplets. Two distinct fluid-mixing mechanisms were unveiled in the two systems, respectively. In the case of O/W droplets, a liquid jet emerges around the end of the bubble collapse phase, effectively penetrating the droplet interface. We offer a detailed analysis of the criteria governing jet penetration, involving the standoff parameter and impact velocity of the bubble jet on the droplet surface. Conversely, in the scenario involving W/O droplets, the bubble traverses the droplet interior, inducing global motion and eventually leading to droplet pinch-off when the local Weber number exceeds a critical value. This phenomenon is elucidated through the equilibrium between interfacial and kinetic energies. Lastly, our boundary integral model faithfully reproduces the essential physics of the non-spherical bubble dynamics observed in the experiments. We conduct a parametric study spanning a wide parameter space to investigate bubble–droplet interactions. The insights from this study could serve as a valuable reference for practical applications in the field of ultrasonic emulsification, pharmacy, etc.

Learning and Living in Times of Collapse: Interview with Margaret Wheatley

Interview with Margaret Wheatley on ways to learn and live in times of collapse.

Ultra-high-speed dynamics of acoustic droplet vaporization in soft biomaterials: Effects of viscoelasticity, frequency, and bulk boiling point

Phase-shift droplets are a highly adaptable platform for biomedical applications of ultrasound. The spatiotemporal response of phase-shift droplets to focused ultrasound above a certain pressure threshold, termed acoustic droplet vaporization (ADV), is influenced by intrinsic features (e.g., bulk boiling point) and extrinsic factors (e.g., driving frequency and surrounding media). A deep understanding of ADV dynamics is critical to ensure the robustness and repeatability of an ADV-assisted application. Here, we integrated ultra-high-speed imaging, at 10 million frames per second, and confocal microscopy for a full-scale (i.e., from nanoseconds to seconds) characterization of ADV. Experiments were conducted in fibrin-based hydrogels to mimic soft tissue environments. Effects of fibrin concentration (0.2 to 8 % (w/v)), excitation frequency (1, 2.5, and 9.4 MHz), and perfluorocarbon core (perfluoropentane, perfluorohexane, and perfluorooctane) on ADV dynamics were studied. Several fundamental parameters related to ADV dynamics, such as expansion ratio, expansion velocity, collapse radius, collapse time, radius of secondary rebound, resting radius, and equilibrium radius of the generated bubbles were extracted from the radius vs time curves. Diffusion-driven ADV-bubble growth was fit to a modified Epstein-Plesset equation, adding a material stress term, to estimate the growth rate. Our results indicated that ADV dynamics were significantly impacted by fibrin concentration, frequency, and perfluorocarbon liquid core. This is the first study to combine ultra-high-speed and confocal microscopy techniques to provide insights into ADV bubble dynamics in tissue-mimicking hydrogels.

Effect of seawater temperature on the corrosion and cavitation erosion-corrosion resistance of Al10Cr28Co28Ni34 high-entropy alloy coating

The influence of temperature on the corrosion and cavitation erosion-corrosion (CE-C) behavior of Al10Cr28Co28Ni34 high-entropy alloy (HEA) coating in seawater was investigated. Interestingly, the corrosion resistance of the coating progressively diminished with an increase in seawater temperature, but the CE-C resistance demonstrated a trend of initial increase followed by a decrease. At higher seawater temperature, the resistance of the coating surface and the destructive force of the bubbles were respectively enhanced and weakened due to the increase in oxide film thickness, maximum bubble size, and collapse time, together leading to a reduction in the damage degree to the coating.

Estimating viscoelastic, soft material properties using a modified Rayleigh cavitation bubble collapse time

Accurate determination of high strain rate (>103 1/s) constitutive properties of soft materials remains a formidable challenge. Albeit recent advancements among experimental techniques, in particular inertial microcavitation rheometry (IMR), the intrinsic requirement to visualize the bubble cavitation dynamics has limited its application to nominally transparent materials. Here, in an effort to address this challenge and to expand the experimental capability of IMR to optically opaque materials, we investigated whether one could use the acoustic signature of the time interval between the bubble's maximum radius and first collapse time point, characterized as the bubble collapse time, to infer the viscoelastic material properties without being able to image the bubble directly in the tissue. By introducing a modified Rayleigh collapse time for soft materials, which is strongly dependent on the stiffness of the material at hand, we show that, in principle, one can obtain an order of magnitude or better estimate of the viscoelastic material properties of the soft material under investigation. Using a newly developed energy-based theoretical framework, we show that for materials stiffer than 10 kPa the bubble collapse time during a single bubble cavitation event can provide quantitative and meaningful information about the constitutive properties of the material at hand. For very soft materials (i.e., shear modulus less than 10 kPa), our theory shows that unless the collapse time measurement has very high precision and low uncertainties, the material property estimates based on the bubble collapse time only will not be accurate and require visual resolution of the full cavitation kinematics.

Gravitationally-induced wave function collapse time for molecules.

The Diósi-Penrose model states that the wave function collapse ending a quantum superposition occurs due to the instability of coexisting gravitational potentials created by distinct geometric conformations of the system in different states. The Heisenberg time-energy principle can be invoked to estimate the collapse time for the energy associated with this instability, the gravitational self-energy. This paper develops atomistic models to calculate the Diósi-Penrose collapse time. It applies them to a range of systems, from small molecules to large biological structures and macroscopic systems. An experiment is suggested to test the Diósi-Penrose hypothesis, and we critically examine the model, highlighting challenges from an atomistic perspective, such as gravitational self-energy saturation and limited extensivity.

Interacting Supernovae from Wide Mass-transferring Binaries

The light curves and spectra of many Type I and Type II supernovae (SNe) are heavily influenced by the interaction of the SN ejecta with circumstellar material (CSM) surrounding the progenitor star. The observed diversity shows that many progenitors have undergone some level of stripping and polluted their CSM shortly before the explosion. The presence of a binary companion and the mass transfer that can ensue offers a mechanism that can give rise to this diversity. We present a set of detailed massive evolutionary models in which the donor star, a Red Supergiant (RSG) is in a wide orbit around a main-sequence companion, and undergoes stable or unstable mass transfer in the later stages of evolution, up to the moment of core collapse. We also discuss some significant physics of these systems that may impact our results, from the presence of pulsations and extended atmospheres in RSGs, to the initial eccentricity of the orbit. The resulting SN types range from Type IIP to H-deficient IIb and H-free Ib. In models undergoing stable mass transfer, the material lost during this process is expected to form a dense CSM surrounding the system by the time of core collapse and give rise to significant interaction effects in the SN light curve and spectra. In the systems with unstable mass transfer mass transfer, the SN may occur during common-envelope evolution. In this case, the progenitor may show significant variability in the last few thousand years before core collapse, and the following SN will likely exhibit strong interaction effects.

Platform Regulation in Times of Environmental Collapse: The Digital Services Act and the Climate Emergency

Platform Regulation in Times of Environmental Collapse: The Digital Services Act and the Climate Emergency

Large-scale molecular dynamics simulations of bubble collapse in water: Effects of system size, water model, and nitrogen

Molecular dynamics simulations in the microcanonical ensemble are performed to study the collapse of a bubble in liquid water using the single-site mW and the four-site TIP4P/2005 water models. To study system size effects, simulations for pure water systems are performed using periodically replicated simulation boxes with linear dimensions, L, ranging from 32 to 512 nm with the largest systems containing 8.7 × 106 and 4.5 × 109 molecules for the TIP4P/2005 and mW water models, respectively. The computationally more efficient mW water model allows us to reach converging behavior when the bubble dynamics results are plotted in reduced units, and the limiting behavior can be obtained through linear extrapolation in L−1. Qualitative differences are observed between simulations with the mW and TIP4P/2005 water models, but they can be explained by the models’ differences in predicted viscosity and surface tension. Although bubble collapse occurs on time scales of only hundreds of picoseconds, the system sizes used here are sufficiently large to obtain bubble dynamics consistent with the Rayleigh–Plesset equation when using the models’ thermophysical properties as input. For the conditions explored here, extreme heating of the interfacial water molecules near the time of collapse is observed for the larger mW water systems (but the model underpredicts the viscosity), whereas heating is less pronounced for the TIP4P/2005 water systems because its larger viscosity contribution slows the collapse dynamics. The presence of nitrogen within the bubble only starts to affect bubble dynamics near the very end of the initial collapse, leading to an incomplete collapse and strong rebound for the mW water model. Although nitrogen is non-condensable at 300 K, it becomes highly compressed and reaches a liquid-like density near the collapse point. We find that the dissolution of nitrogen is much slower than the movement of the collapsing water front, and the re-expansion of the dense nitrogen droplet gives rise to bubble rebound. The incompatibility of the collapse and dissolution time scales should be considered for continuum-scale modeling of bubble dynamics. We also confirm that the diffusion coefficient for dissolved nitrogen is insensitive to pressure as the liquid transitions from a compressed to a stretched state.

Gravitational collapse of Bose-Einstein condensate dark matter halos with logarithmic nonlinearity

"If dark matter is composed of massive bosons, a Bose-Einstein Condensation process must have occurred during the cosmological evolution. Therefore galactic dark matter may be in a form of a condensate, characterized by a strong self-interaction. One of the interesting forms of the self-interaction potential of the condensate dark matter is the logarithmic form. In the present work we investigate one of the astrophysical implications of the condensate dark matter with logarithmic self-interaction, namely, its gravitational collapse. To describe the condensate dark matter we use the Gross-Pitaevskii equation, and the Thomas-Fermi approximation. By using the hydrodynamic representation of the Gross-Pitaevskii equation we obtain the equation of state of the condensate, which has the form of the ideal gas equation of state, with the pressure proportional to the dark matter density. In the Thomas-Fermi approximation, the evolution equations of the condensate reduce to the classical continuity, and Euler equations of fluid dynamics. We obtain the equations of motion of the condensate radius in spherical symmetry, by assuming certain particular forms for the velocity and density of the condensate. The collapse time required for the formation of a stable macroscopic astrophysical object is obtained in an integral form, and explicit numerical estimations for the formation of astrophysical objects with masses ranging from 106M⊙ to 1012M⊙ are presented."

Модельные исследования взаимодействия перекрестно-балочного фундамента и наклонного основания над карстовой полостью

The article presents the results of field experiments carried out on the joint work of a cross-beam foundation and an inclined foundation when it is limited by a rigid vertical retaining structure at the time of a sudden collapse of the karst cavity arch. The regularities of the transformation of the sinkhole prism are determined depending on the angle of inclination of the base and the location of the cavity relative to the foundation structure.

Dynamics of skyrmion contraction and expansion in a magnetic film

Contraction and expansion of skyrmions in ferromagnetic films are investigated. In centrosymmetric systems, the dynamics of a collapsing skyrmion is driven by dissipation. The collapse time has a minimum on the damping constant. In systems with broken inversion symmetry, the evolution of skyrmions toward equilibrium size is driven by the Dzyaloshinskii–Moriya interaction. Expressions describing the time dependence of the skyrmion size are derived and their implications for skyrmion-based information processing are discussed.

Manipulation of bubble collapse patterns near the wall of an adherent gas layer

This paper aims to apply experimental methods to investigate the effect of the thickness of gas layers on the wall on the collapse direction of spark-induced bubbles. In the experiment, two high-speed cameras synchronously record the time evolution of the bubbles and the corresponding parameters such as the normalized collapse position and bubble collapse time. Experiments yielded results for individual bubbles over a range of normalized distances from 0 to 4.0 for different air layer thicknesses. Based on the morphology of the bubbles, the experimental jets were visualized into six different modes, namely, forward jet (FJ), merging jet (MJ), bidirectional jet (BJ), reversing jet (RJ), forward followed by reversing jet (FRJ), and non-directional jet (NDJ). The height of the air layer on the wall is affected by the fluctuation of the bubble volume and shows the opposite trend to the change of the bubble volume. The air film reaches its maximum height when the bubble collapses, which affects the final jet pattern. In addition, as the thickness of the air layer increases, the center of the bubble gradually migrates away from the wall. The different collapse modes and the migration of the bubble centers have positive significance for reducing cavitation erosion in engineering.

On the mechanics of thermal collapse of a vapor bubble translating in an isothermal subcooled liquid

In this work, we numerically simulate bubble radius and velocity in a thermal collapse of a vapor bubble translating in an isothermal subcooled liquid based on solving the equations of energy and motion simultaneously. The mechanics of the collapse in the thermal regime plays a significant role in the thermal convection from the vapor-liquid interface of a translating bubble. In our mathematical model, the equation of motion considers the added mass force due to phase change and shape change, while the equation of energy includes thermal conduction and convection. The simulations include both spherical and non-spherical bubbles, the effect of initial bubble diameter and degree of subcooling. The simulations have been performed in three modes of heat transfer (conduction, convection and combined conduction-convection) to observe their effect on the bubble dynamics. The numerical results have been compared with the two classes of thermal collapse experiments (Type A and B) and CFD analysis reported in literature. They agree well with the experiments and CFD-analysis. We have seen a remarkable effect of the initial shape of the bubble in terms of aspect ratio on the time scale and modes of the heat transfer and the collapse time. An interesting concern is that the mechanism of the collapse and the modes of heat transfer depends on how the collapse is initiated in the experiments. Type A and B thermal collapses are mainly classified based on the bubble motion and the modes of thermal transport from the interface.

Mechanisms underlying the influence of skin properties on a single cavitation bubble in low-frequency sonophoresis

As a safe and effective method for systemic transdermal drug delivery (TDD), sonophoresis has drawn much attention from researchers. Despite numerous studies confirming cavitation as the main reason for sonophoresis, the effect skin has on cavitation bubble dynamics remains elusive due to the difficulty of experimental challenges. For a start, we reveal how single cavitation bubble (SCB) dynamics are affected by skin properties, including elasticity, hydrophilicity and texture. We use polydimethylsiloxane (PDMS) to simulate human skin and record the temporary evolution of SCBs with synchronous ultrafast photography. The influences of skin properties on SCBs are concluded: 1) SCBs collapse later near walls with better elasticities and generate microjets with higher speed; 2) SCBs collapse later near hydrophilic walls with slower microjets; and 3) the existence of a texture structure on walls also delays the time of bubble collapse near them and slows the velocities of microjets (v) during collapses.

Halo assembly in cold and warm dark matter during the JWST frontier epoch

ABSTRACT The JWST mission is in the process of probing the galaxy mass function at z > 10, when conceivably any delay in halo assembly due to the presence of a dwarf galaxy-scale power spectrum cutoff may drastically suppress the number of galaxies relative to the cold dark matter (CDM) expectation. We employ N-body simulations of CDM and warm dark matter (WDM) to explore how the difference in halo collapse time between these models scales with z = 0 descendant halo mass. We demonstrate that collapse begins first for the most massive haloes, and the delay in collapse time between CDM and WDM haloes correlates inversely with descendant mass. We thus infer that only present-day dwarf galaxies exhibit any difference in their assembly history between CDM and WDM at z = 10, and therefore support previous studies that have found JWST is unlikely to determine whether our Universe is better described by the CDM cosmology or the WDM cosmology.

The study for optimization strategies on the performance of DCGAN

Since Deep Convolutional Generative Adversarial Networks (DCGAN) was proposed, it has been perceived as a model with difficulty in training due to several factors. To solve this problem, dozens of optimization strategies were presented, but none of them was compared with the others. In this paper, the author chose three representative methods, namely one-label smoothing, the two Time-Scale Update Rule (TTUR), and the Earth-Mover Distance (EMD) or Wasserstein-1 to make a comparison of the optimization effect on the DCGAN model. To be specific, these three approaches were adopted respectively while using MNIST and Fashion-MNIST as datasets. One-side label smoothing was designed to prevent overconfidence in the model by adding a penalty term in the discriminator. TTUR was a simpler update strategy that could help the model find the stationary local Nash equilibrium under mild assumptions. EMD was an alternative loss function that enabled the model to distinguish the difference while the real distribution and generated distribution were not overlapped. Contrast experiments were conducted both vertically and horizontally. The author applied these three methods with the same dataset and the same method with different datasets in order to compare the time of the model collapse, the trend of loss in line graphs, and the impact of different datasets on results. Experimental results indicated that both one-label smoothing and TTUR postponed the model collapse while EMD completely get rid of it. Furthermore, generated images may lose texture information when using more complicated datasets.

Research on the collapse characteristics of single cavitation bubble near solid particle by the VOF method

In this paper, the collapse behavior of a single cavitation bubble at different distances near a solid particle of typical scales is numerically simulated and researched with the volume of fluid (VOF) method. Based on the key parameters analysis of the pressure field, velocity vector, collapse time the tendency of cavitation bubble collapse characteristics at different distances during the change of particle size is studied with the variable of the distance and relative size between the particle and the cavitation bubble. The dimensionless distance parameter ‘γ’ is specifically presented in the simulation process, the cavitation bubble collapse impact is largely directed to the particle when 3>γ > 2, while the wall hardly affects the interaction between the cavitation bubble and the solid particle as γ > 3. The results illustrate that as the solid particle and wall exist, the distance and particle size affect both the peak collapse pressure and the collapse jet velocity of the cavitation bubble, and the influence of solid wall on the cavitation bubble at the same distance is much greater than that of solid particles. When the particle size increases, the particle gradually affects the cavitation bubble in a way similar to the wall. While as the distance decreases or the particle size increases, the influence of particle and wall on the evolution process of the cavitation bubble expands, meanwhile, the collapse pressure and collapse jet velocity of the cavitation bubble are promoted with the optimized distance and particle size, which brings marvelous cavitation effect. The numerical methods and conclusions of this paper provide a valuable reference for cavitation applications of sand-containing fluids.