Reversible Transition Process Research Articles

Determining Band Splitting and Spin-Flip Dynamics in Monolayer MoS2.

Femtosecond helicity-resolved pump-probe spectroscopy is performed to study the spin and valley dynamics in monolayer (ML) MoS2. Both the bright to dark intravalley exciton transition (∼50 fs) and the reverse transition process (<50 fs) are directly monitored. It suggests that the bright exciton state of ML MoS2 is lower in energy than the dark one, which is also confirmed by observing the temperature-dependent co-polarized photobleaching dynamics of A and B excitons. Furthermore, the band splitting in the conduction band of ML MoS2 with a value of 15 ± 0.3 meV is determined by fitting the temperature-dependent ratios of the population in bright and dark states using the Boltzmann distribution law. Such minor band splitting allows the phonon-mediated intravalley spin-flip to even occur from the lower to the upper conduction band within tens of femtoseconds, which will have non-negligible effects on the performance of these ML MoS2-based optoelectronic and photonic devices.

Reverse transition of a turbulent spiral Poiseuille flow at

Direct numerical simulations of a turbulent spiral Poiseuille flow (SPF) in a narrow-gap geometry at low Taylor number have been performed to analyse the reverse transition dynamics. The presently investigated SPF results from a Taylor–Couette arrangement with a rotating inner cylinder and a stationary outer one, subject to a time-constant axial pressure gradient. Keeping fixed the Taylor number and reducing the axial Reynolds number, several flow regimes have been obtained until a complete laminarization occurred. In agreement with previous experimental evidence, it has been found that the laminar state is achieved at a Reynolds number significantly smaller than the corresponding non-rotating value. Moreover, the route to turbulence suppression has been shown to differ in the two cases, as confirmed by the increased Reynolds number friction coefficient envelope. The differences occurring in the reverse transition process between SPF and plain Poiseuille flow are attributed to a modification of the isotropy of the Reynolds stress tensor, caused by an alteration of the velocity pressure-strain redistribution mechanisms.

The microscopic transition process from high-density to low-density amorphous state of SnI4

A molecular crystalline SnI4 undergoes pressure-induced solid-state amorphization via molecular dissociation to the high-density amorphous (HDA) state, which we call Am-I. In the present study, we examine the reverse transition process from Am-I to the low-density amorphous (LDA) state, called Am-II. We first measure the structure factor on decompression from 30 GPa down to 1.1 GPa at room temperature, using in situ angle-dispersive synchrotron x-ray measurement and a diamond anvil cell. We then estimate the density, which exhibits an abrupt change between 3.3 and 3.0 GPa, indicating the HDA(Am-I)-to-LDA(Am-II) transition. We use the density and the molecular configuration generated from a molecular dynamics simulation as input to a reverse Monte Carlo fit. The fit vividly visualizes gradual molecular reassociation between 18 and 14 GPa within the Am-I region. The Am-I state can thus be divided into two states: the high-pressure Am-I state containing isolated Sn atoms and the low-pressure Am-I state consisting of deformed molecules connected by metallic I2 bonds. In the latter state, the molecular shape becomes C 3v -like just before the transition to Am-II, in which molecules recover the original T d symmetry. This local symmetry change has been detected on the liquid–liquid transition of SnI4, suggesting the strong coupling between the local symmetry and the global order parameter of density.

Infrared Switching of Self-Heating VO&lt;sub&gt;2&lt;/sub&gt;/ITO Films for Smart Window

The unique metal to insulator transition (MIT) of vanadium dioxide (VO2) makes it receiving extensive attention in the application of smart window. As for VO2-based smart window, the critical transition temperature (Tc) is required to be reduced to near room temperature for practical applications. In this paper, we fabricated VO2 films on ITO glass by hydrothermal method and applied voltage to ITO, therefore, the joule heat generated by ITO triggered the complete MIT of VO2 at room temperature in very short time ~3 s with applied voltage of 12 V. The VO2 film on ITO substrate shows obviously widened hysteresis behavior in the reversible transition process with a thermal hysteresis width of ~33 °C. The widened hysteresis loop makes it possible to stabilize the rutile phase (R) of VO2 at room temperature via applying a low holding voltage of 6 V. The proposed VO2/ITO film exhibits promising application in active smart window, and possesses advantages of simple structure, easy-fabricated and low-cost.

Observation of transition between multimode Q-switching and spatiotemporal mode locking

We report experimental observation of multimode Q -switching and spatiotemporal mode locking in a multimode fiber laser. A typical steady Q -switching state is achieved with a 1.88 μs pulse duration, a 70.14 kHz repetition rate, and a 215.8 mW output power, corresponding to the single pulse energy of 3.08 μJ. We find weak spatial filtering is essential to obtain stable Q -switched pulses, in contrast to the relatively stronger spatial filtering for spatiotemporal mode locking. Furthermore, a reversible transition process, as well as a critical bistable state, between multimode Q -switching and spatiotemporal mode locking, is achieved with specific spatial coupling and waveplates sets. We believe the results will not only contribute to understanding the complicated nonlinear dynamics in multimode, fiber-based platforms, but also benefit the development of promising high-pulse energy lasers.

The effect of rotation on the thermal-solutal capillary-buoyancy flow in a shallow annular pool with various capillary ratios

In order to understand the effect of pool rotation on the thermal-solutal capillary-buoyancy convection at various capillary ratios, a series of three-dimensional simulations are performed in a rotating shallow annular pool filled with silicon-germanium melt and subjected to radial temperature and solute concentration gradients under normal gravity. The capillary ratios are Rσ=−0.5, −0.8, −1.25 and −2, while Taylor number ranges from Ta = 0 to 1200. Three types of basic flows are obtained at different capillary ratios. The pool rotation diversely affects the structures of the basic flow and further varies the destabilization mechanisms. The pool rotation enhances the flow stability at Ta < 400 and reduces the flow stability at Ta > 400 with Rσ=−0.5 and −0.8, while it always stabilizes the basic flow at Rσ=−1.25 and −2. The evolution of flow pattern depends remarkably on the Taylor number and the capillary ratio. Multiple flow bifurcations occur at small capillary ratios or high Taylor numbers, which results from the coupling of various driven forces. The reverse transition process is observed at Rσ=−0.5 and −0.8 with Taylor numbers slightly larger than the turning point. Meanwhile, abundant oscillatory flow patterns are shown. The effects of pool rotation on their characteristics are discussed.

Heterogeneous CFs induced unipolar and bipolar resistive switching behaviors in InGaZnO thin films

This paper investigates resistive switching behaviors of amorphous InGaZnO (a-IGZO) thin films for non-volatile resistive random access memory applications. Reversible transition process between bipolar and unipolar modes in Ag/InGaZnO/Pt memory cell takes place through changing the polarities of sweeping voltages, whereas the reset process cannot be operated by applying a positive voltage. The dominant conduction mechanisms in both modes are Ohmic conduction for the low resistance state, but different for the high resistance state, that is, Poole-Frenkel emission in the unipolar mode and trap-controlled space charge limited conduction in the bipolar mode. Temperature dependence of low resistance state and transmission electron microscopy analysis further indicate that the dominant conductive filaments (CFs) are composed of Ag atoms for the bipolar mode and oxygen vacancies for the unipolar mode, respectively. These two kinds of conductive filaments play a coordination role in resistive switching transition between bipolar and unipolar modes in Ag/InGaZnO/Pt memory devices. Based on the conductive filament model, the physical mechanisms for resistive switching are discussed.

A Simple and Sensitive Method for an Important Physical Parameter: Reliable Measurement of Glass Transition Temperature by AIEgens

The glass transition temperature (Tg) is an important physical parameter and signifies the reversible transition process between the glassy state and the rubbery state in polymeric materials. Herein, a simple yet reliable method, denoted as ADEtect, utilizing aggregation-induced emission luminogens (AIEgens) for Tg measurement was developed. The fluorescent images of AIEgen-doped polymer films taken at different temperatures were monitored by a camera under the same settings. By using a MATLAB program, the grayscale of the selected area in each image was calculated. This value decreased steadily with increasing the temperature, and a significant change was observed at Tg. By plotting a graph of the second derivative of relative grayscale against temperature, the Tg of a polymer can be unambiguously determined from the lowest point. The present method shows performance comparable to that of conventional DSC measurement in terms of sensitivity and reliability. Besides, it can measure multiple samples in par...

Bioinspired hybrid material composed of helical polymer grafts and graphene oxide: Reversible transformation of particulate and extended structures of the grafts and application in chiral enrichment

Abstract Inspired by jellyfish, this article reports a novel type of GO-derived reversible transition hybrid material, in which optically active helical substituted polyacetylene was covalently grafted on GO sheets. The polymer chains underwent a reversible transition between forming particles and dispersing state upon changing the solvent. Transmission electron microscopy (TEM) confirmed the reversible transition process. Circular dichroism and UV–vis absorption spectroscopies demonstrated the fascinating optical activity of the GO hybrids, which originated in the helical polymer chains adopting predominantly one helicity. The enantio-differentiating enrichment experiments demonstrated the potential applications of the novel hybrids as reusable chiral materials during the reversible transition process, in which l -dopamine and l -phenylalanine were preferentially adsorbed when the polymer chains formed particles.

A bile acid-induced aggregation transition and rheological properties in its mixtures with alkyltrimethylammonium hydroxide

A bile acid, lithocholic acid (LCA)-induced aggregation transition and rheological properties in the mixtures of LCA and alkyltrimethylammonium hydroxide were investigated. In the salt-free system of tetradecyltrimethylammonium hydroxide (TTAOH) and LCA at equimolar ratio, unilamellar vesicles were observed initially and then transferred to a suspension of tubes within several hours at 25 °C, even at a very low total concentration. When an equimolar amount of NaBr was added into the solutions at a higher temperature, huge vesicles with a diameter of tens to hundreds of micrometres were formed initially and crystals quickly emerged inside the vesicles when cooling to room temperature spontaneously. In the longer chain system of cetyltrimethylammonium hydroxide (CTAOH)/LCA, vesicles were also observed after the dissolution of LCA in CTAOH micelle solution, but the vesicles were transferred to helical ribbons after several days at higher concentrations and a lower temperature, and gel was obtained after a longer time of aging. The huge vesicles were more stable than those of the TTAOH/LCA system when an equimolar amount of NaBr was added. The tube suspensions of the TTAOH/LCA system at a high concentration formed a weak gel with relatively low viscoelastic properties, indicating a weak interaction between the aggregates. The tube and helical ribbon suspensions can quickly transfer to viscoelastic vesicular solutions when heated at 30 °C, indicating a reversible transition process.

Spectral dynamic of pulsating turbulent pipe flow

The present work investigates the effects of a forced harmonic oscillations of fixed frequency on the characteristics of a turbulent pipe flow with a mean Reynolds number based on the cross sectional bulk velocity and pipe diameter of 5900. The setup refers to an extremely thin Stokes layer so that the vorticity associated to the oscillating motion is generated in a small near wall region. When the ratio of the amplitudes of the oscillating and bulk velocities is increased from 1 to 11, the turbulence is affected by the harmonic forcing so much that the near wall coherent structures are substantially modified, although not suppressed through a reverse transition process. This results in an overall space and time averaged resistance reduction which, for the largest amplitude case, adds up to 27% of the non-pulsating flow at the same bulk Reynolds number. The present contribution is focused on the analysis of the phase locked averaged statistics of the Reynolds stress tensor components and velocity spectra. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved large Eddy simulations with a spectrally accurate Navier–Stokes code.

The effect of polymers on the dynamics of turbulence in a drag reduced flow

An experimental investigation of a polymer drag reduced flow using state-of-the-art laser-Doppler anemometry in a refractive index-matched pipe flow facility is reported. The measured turbulent stresses deep in the viscous sublayer are analyzed using the tools of invariant theory. It is shown that with higher polymer concentration the anisotropy of the Reynolds stresses increases. This trend is consistent with the trends extracted from DNS data of non-Newtonian fluids yielding different amounts of drag reduction. The interaction between polymer and turbulence is studied by considering local stretching of the molecular structure of a polymer by small-scale turbulent motions in the region very close to the wall. The stretching process is assumed to re-structure turbulence at small scales by forcing these to satisfy local axisymmetry with invariance under rotation about the axis aligned with the main flow. It is shown analytically that kinematic constraints imposed by local axisymmetry farce turbulence near the wall to tend towards the one-component state and when turbulence reaches this limiting state it must be entirely suppressed across the viscous sublayer. Based on this consideration it is suggested that turbulent drag reduction by high polymers resembles the reverse transition process from turbulent to laminar. Theoretical considerations based on the elastic behavior of a polymer and spatial intermittency of turbulence at small scales enabled quantitative estimates to be made for the relaxation time of a polymer and its concentration that ensure maximum drag reduction in turbulent pipe flows, and it is shown that predictions based on these are in very good agreement with available experimental data.

Average Time between Bursts in a Favorable pressure Gradient.

Effects of a favorable pressure gradient on the average time between bursts were studied by two different methods : the modified U-level method, and a proposed detection scheme that applies the U-level method to ejections and sweeps. Experiments were carried out in a boundary layer along a flat wall of two-dimensional asymmetric contraction. The results of experiments show that the average time between bursts scaled with inner variables increases with a decrement of the pressure gradient parameter Δp, and the ratio of the Eulerian micro-time scale to the average time between bursts gives a constant value of 0.1 at Y+=15. The proposed method also indicates that bursts still remain throughout the reverse transition process from a turbulent boundary layer to a laminar one.

長方形断面の縮流筒のコーナー流れにおける逆遷移

This paper presents an experimental study of reverse transition in a turbulent boundary layer along a corner of contraction of rectangular section. The reverse transition was examined by consideration of the velocity distribution and the contours of axial mean velocity and turbulent intensity. It is found that a turbulent corner flow reverses to laminar in the presence of favourable pressure gradient and the well-known secondary flow of second kind generated in a corner region disappears during the reverse transition process.

Heat transfer from boundary layers undergoing an acceleration induced reverse transition

AbstractAn experimental and analytical investigation of heat transfer in an acceleration induced transitional flow regime is presented. A previously developed criterion for the onset of reverse transition of turbulent boundary layers is combined with a theory of forward transition to derive the effect of free‐stream turbulence intensity on the critical value of an acceleration parameter for two initial velocity profiles within the turbulent boundary layer. The results are consistent with experimental observations.Local heat transfer measurements were obtained along converging walls of rectangular flow channels having convergence angles of 45, 60, and 70 deg. Free‐stream turbulence (1.6 to 11%) was induced by grids upstream of the start of convergence, and the local Reynolds number were varied by variation in channel size and stagnation pressures (200 to 500 lb./sq.in. abs.) at a gas stagnation temperature of 1,500°R.The results indicate a two‐stage reverse transition process along the converging wall with a forward transition between the two stages of reverse transition. The local Nusselt numbers obtained over a wide range of an acceleration parameter are correlated with the local Reynolds number, the acceleration parameter at the start of convergence, and the local flow area contraction ratio. The corrlation is in the form of laminarization paths having negative slopes for (NNu)r/(NPr)0.4 vs. (NRe)r at common area ratios. Local values of free‐stream turbulence intensity are required for proper application of the reverse transition criterion in conjunction with the heat transfer correlation presented.

Ignition and extinction of liquid fuel drops—Numerical computations

The equations for the steady state combustion of liquid fuel drops in an oxidizing atmosphere were integrated numerically for various fuel-oxygen systems. The results give a continuous description of the drop behaviour from the low mass burning rate kinetic controlled region to the high mass burning rate diffusion controlled region. Ignition was defined as the transition from kinetic to diffusion control and extinction as the reverse transition process. Ignition and extinction conditions were obtained as a function of ambient oxygen concentration for various reaction rates, reaction orders and stoichiometric ratios.

On the criteria for reverse transition in a two-dimensional boundary layer flow

Experiments on reverse transition were conducted in two-dimensional accelerated incompressible turbulent boundary layers. Mean velocity profiles, longitudinal velocity fluctuations$\tilde{u}^{\prime}(=(\overline{u^{\prime 2}})^{\frac{1}{2}})$and the wall-shearing stress (TW) were measured. The mean velocity profiles show that the wall region adjusts itself to laminar conditions earlier than the outer region. During the reverse transition process, increases in the shape parameter (H) are accompanied by a decrease in the skin friction coefficient (Cf). Profiles of turbulent intensity (u’2) exhibit near similarity in the turbulence decay region. The breakdown of the law of the wall is characterized by the parameter\[ \Delta_p (=\nu[dP/dx]/\rho U^{*3}) = - 0.02, \]whereU* is the friction velocity. Downstream of this region the decay of$\tilde{u}^{\prime}$fluctuations occurred when the momentum thickness Reynolds number (R) decreased roughly below 400.