Explosive transitions in epidemic dynamics

In recent studies, it has been established that higher-order interactions in coupled oscillators can induce a process from continuous to explosive phase transition. In this study, we identify a phase transition, termed the stepwise explosive phase transition, characterized by the emergence of multiple critical phase plateaus in a globally frequency-weighted coupled pendulum model. This transition bridges the continuous and explosive phase transitions, arising from a delicate balance between attractive higher-order interactions and repulsive pairwise interactions. Specifically, the stepwise explosive phase transition occurs when the higher-order coupling is moderate, neither large nor small, while the pairwise interactions remain repulsive. Our analysis shows that stronger attractive higher-order interactions necessitate weaker repulsive pairwise interactions, leading to partial frequency locking among oscillators and triggering the stepwise transition. We construct an analytical framework using self-consistent equationsto provide an approximation of the steady-state behavior. This study uncovers an alternative pathway to the desynchronization, and it provides additional insights into the phase transitions in coupled dynamical networks.

The ɛ-susceptible-infected-susceptible (SIS) epidemic model on a graph adds an independent, Poisson self-infection process with rate ɛ to the "classical" Markovian SIS process. The steady state in the classical SIS process (with ɛ=0) on any finite graph is the absorbing or overall-healthy state, in which the virus is eradicated from the network. We report that there always exists a phase transition around τ_{c}^{ɛ}=O(ɛ^{-1/N-1}) in the ɛ-SIS process on the complete graph K_{N} with N nodes, above which the effective infection rate τ>τ_{c}^{ɛ} causes the average steady-state fraction of infected nodes to approach that of the mean-field approximation, no matter how small, but not zero, the self-infection rate ɛ is. For τ<τ_{c}^{ɛ} and small ɛ, the network is almost overall healthy. The observation was found by mathematical analysis on the complete graph K_{N}, but we claim that the phase transition of explosive type may also occur in any other finite graph. We thus conclude that the overall-healthy state of the classical Markovian SIS model is unstable in the ɛ-SIS process and, hence, unlikely to exist in reality, where "background" infection ɛ>0 is imminent.

Across natural and human-made systems, transition points mark sudden changes of order and are thus key to understanding overarching system features. Motivated by recent experimental observations, we here uncover an intriguing class of transitions in coupled oscillators, extreme synchronization transitions, from asynchronous disordered states to synchronous states with almost completely ordered phases. Whereas such a transition appears like discontinuous or explosive phase transitions, it exhibits markedly distinct features. First, the transition occurs already in finite systems of N units and so constitutes an intriguing bifurcation of multi-dimensional systems rather than a genuine phase transition that emerges in the thermodynamic limit N → ∞ only. Second, the synchronization order parameter jumps from moderate values of the order of N−1/2 to values extremely close to 1, its theoretical maximum, immediately upon crossing a critical coupling strength. We analytically explain the mechanisms underlying such extreme transitions in coupled complexified Kuramoto oscillators. Extreme transitions may similarly occur across other systems of coupled oscillators as well as in certain percolation processes. In applications, their occurrence impacts our ability of ensuring or preventing strong forms of ordering, for instance in biological and engineered systems.

We present a comprehensive structural and thermomechanical investigation of N-salicylideneaniline, a Schiff base derivative that exhibits remarkable thermosalient phase transition behavior. By combining variable-temperature X-ray powder diffraction (VT-XRPD), differential scanning calorimetry (DSC), hot-stage microscopy, and Hirshfeld surface analysis, we reveal two distinct thermosalient mechanisms operating in different polymorphic forms. Form I displays pronounced anisotropic thermal expansion with negative strain along a principal axis, culminating in a sudden and explosive phase transition into Form IV. In contrast, Form III transforms more gradually through a microstrain accumulation mechanism. Fingerprint plots and contact evolution from Hirshfeld surface analysis further support this dual-mechanism model. These insights highlight the importance of integrating macro- and microscale structural descriptors to fully capture the mechanical behavior of responsive molecular solids. The findings not only enhance the fundamental understanding of thermosalience but also inform the rational design of functional materials for actuating and sensing applications.

The hypothesis of a Superfluid Space-Time (granular quantum) is consistent with knowledge acquired in Cosmology and, moreover, conform to the Standard Model. We show, here, that the relationships arising from this hypothesis can also allow to go further in predicting the numbered evolution of the Universe. They lead to the accreditation of a cyclical evolution incorporating a rebound (Big Bounce), identical to the Big Bang, through an explosive phase transition.

The cavitation phenomenon has been extensively studied by numerous researchers because the collapse of the cavitation bubbles is responsible for a number of phenomena of interest in the fields of science and engineering, such as: Luminescence, sonochemistry, cavitation damage, ultrasonic cleaning, etc. The most common methods to produce cavitation bubbles are: pulsed lasers, electroforming, sonophoresis, radiofrequency, and Venturi effect. In this paper we are interested in a method called thermocavitation, which is induced by low power, CW laser radiation in a highly-absorbing solution of copper nitrate (CuNO4) dissolved in water. The bubble formation occurs when an overheated region (~300°C) is created followed by explosive phase transition and the formation of vapor-gas bubbles, which expand and later collapse very rapidly emitting intense acoustic shockwaves. The characteristic effects of bubble dynamics, in particular the formation, growth, collapse and a high-speed liquid jet at the moment of collapse are recorded using a high speed video camera (Phantom V7, Version 9.1) with frame rates of up to one hundred thousand frames/s at different laser powers (62 to 200 mW). The shockwaves are sufficiently energetic that they may be employed to generate deep lesions in biological tissue models, such as agar gel and chicken breast samples as a preamble to future studies on thermocavitation for tissue ablations. This approach of achieving thermocavitation is attractive due to the fact that it is generated with low power CW lasers which decrease cost and complexity relative to other approaches.

We generalize the original majority-vote model by incorporating inertia into the microscopic dynamics of the spin flipping, where the spin-flip probability of any individual depends not only on the states of its neighbors, but also on its own state. Surprisingly, the order-disorder phase transition is changed from a usual continuous or second-order type to a discontinuous or first-order one when the inertia is above an appropriate level. A central feature of such an explosive transition is a strong hysteresis behavior as noise intensity goes forward and backward. Within the hysteresis region, a disordered phase and two symmetric ordered phases are coexisting and transition rates between these phases are numerically calculated by a rare-event sampling method. A mean-field theory is developed to analytically reveal the property of this phase transition.

Magnetocaloric effect (MCE) receives an increasing interest for its promising applicability in energy efficient refrigerators. Among the different magnetocaloric materials having potential applications for cooling devices that operate near room temperature, those exhibiting a first-order phase transition (FOPT) usually show larger responses than those exhibiting a second-order phase transition (SOPT), but the intrinsic hysteresis could be deleterious for cyclic operations [1]. Some FOPT materials such as Heusler alloys can also exhibit the Curie transition of their magnetic phases, thus leading to alloys exhibiting both direct (positive isothermal entropy change, ΔSiso) and inverse MCE (negative ΔSiso). However, the close proximity of the two types of transitions can produce a convoluted and compensated global response due to the different nature, direct and inverse, of their MCEs [2]. In order to evaluate the potential of the FOPT for magnetocaloric applications and, therefore, ascertain the usefulness of tuning the Curie temperature of the alloy by compositional modifications, it is necessary to be able to deconvolute its responses.It has previously been shown that concurrent SOPTs can be deconvoluted by applying the scaling laws of the MCE [3]. In this work, we propose an extension of the methodology to deconvolute concurrent FOPT and SOPT. For this purpose, a Heusler alloy with stoichiometry Ni48.1Mn36.5In15.4 has been used as a model case. Upon heating, this sample exhibits a FOPT from a low magnetization martensite to an austenite with higher magnetization. This magneto-structural transition produces an inverse MCE that is immediately followed by the Curie transition of the austenite, with its corresponding conventional MCE. With increasing the magnetic field, two different behaviors are observed: 1) the onset of the FOPT shifts to lower temperatures, with a peak ΔSiso value that increases abruptly at the beginning, reaches a maximum for ~5 T and subsequently decreases, and 2) a Curie transition whose response increases gradually in height and width, with a peak position that remains almost field independent. Due to the proximity of these two competing transitions, the overlapping of their responses increases with increasing field. The application of the scaling laws for the SOPT magnetocaloric response allows us to deconvolute the contributions of the two transitions. Top panel of Fig. 1 shows the experimental ΔSiso of the sample together with its reconstructed response. The separated contributions of the two phase transitions are presented in the bottom panel of Fig. 1. The good agreement between experimental and reconstructed curves allows us to reach the following conclusions:- Universal scaling can be used to deconvolute overlapping first and second order phase transitions.- The broadening of the SOPT with increasing field causes a larger overlap between transitions despite the shift of the FOPT to lower temperatures.- The experimental decrease of the inverse MCE peak above ~5T is due to the competing contribution of the SOPT and is not intrinsic to the FOPT.This deconvolution methodology allows to extract the information of the different phase transitions in the material without having to experimentally synthesize new alloys with different compositions.Work supported by AEI/FEDER-UE (grant PID2019-105720RB-I00), US/JUNTA/FEDER-UE (grant US-1260179), Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (grant P18-RT-746), and Army Research Laboratory under Cooperative Agreement Number W911NF-19-2-0212. **

Phase transitions and structures in the rare earth hexafluorides Cs 2NaHoF 6, Rb 2NaHoF 6, Cs 2KHoF 6 and Cs 2RbHoF 6 at 12 < T < 300 K

The strategy of adding solid particles to fluids for improving thermal conductivity has been pursued for more than one century. Here, a novel concept of using liquid nanodroplets for enhancing thermal performance has been developed and demonstrated in polyalphaolefin nanoemulsion fluids with dispersed ethanol nanodroplets. The ethanol/polyalphaolefin nanoemulsion fluids are spontaneously generated by self-assembly, and are thermodynamically stable. Their thermophysical properties, including thermal conductivity and viscosity, and impact on convective heat transfer are investigated experimentally. The thermal conductivity enhancement in these fluids is found to be moderate, but increases rapidly with increasing temperature in the measured temperature range from 35 oC to 75 oC. A very remarkable increase in convective heat transfer coefficient occurs in the nanoemulsion fluids due to the explosive vaporization of the ethanol nanodroplets at the superheat limit (i.e., spinodal states, about 122 oC higher than the atmospheric boiling point for ethanol). The explosive liquid-vapor phase transition is monitored using high speed camera. The fluid heat transfer could be augmented through the heat of vaporization (which intuitively raises the base fluid specific heat capacity) and the fluid mixing induced by the sound waves. The development of such phase-changeable nanoemulsion fluids would open a new direction for thermal fluids studies.

Raman frequency shifts of an internal mode near the tricritical and second order phase transitions in NH 4Cl

By critical analyses of the order parameter of symmetry breaking, we have researched the phase transitions at high density in and Gross-Neveu (GN) model and shown that the gap equation obeyed by the dynamical fermion mass has the same effectivenesss as the effective potentials for such analyses of all the second order and some special first order phase transitions. In the meantime we also further ironed out a theoretical divergence and proven that in model a first order phase transition does occur in the case of zero temperature and finite chemical potential.

Despite intensive studies on epidemic spreading in metapopulation networks, the infection process is normally assumed to be a Markovian process without considering time-dependent infectivity. In this paper, a reaction-diffusion SIRS epidemic model with non-Markovian infection processes is constructed on metapopulation networks, where the infection rate function associated with the sojourn time in the infected state is obtained by constructing the generation time distribution of the emerging infection process. Meanwhile, the recurrent mobility patterns are incorporated by the metapopulation model. By Lyapunov stability analysis, the epidemic outbreak condition is derived theoretically. The results show that the non-Markovian infection characteristics can significantly alter the transient behavior of epidemic spreading under the same basic reproduction number. Furthermore, with the increase in the average time for an infected individual to generate secondary cases in the non-Markovian process, the impact of mobility rate on transient epidemic spreading behavior will be reduced. Finally, mobility can have a suppressing effect on epidemic spreading dynamics when the population distribution exhibits significant asymmetry. The results can sharpen our understanding of the role of real-world non-Markovian infection processes in epidemic outbreaks.

The phase transition of a RN–AdS black hole is studied in details using Ehrenfest's equations. There is no discontinuity in entropy which signals a lack of any first order phase transition. We then show that although Ehrenfest's first equation is satisfied, the second is not, so that a true second order phase transition is also ruled out. However this deviation from the second Ehrenfest's equation, for a certain range of the black hole charge, indicates the existence of a new glassy type transition. We finally study the thermodynamic behaviour using state space geometry and find that the scalar curvature diverges exactly at those points where the heat capacity diverges.

Vibrational, DSC and dielectric studies of Cs1.5Li1.5H(SO4)2 crystal