Gas-liquid Mixture Research Articles

Gasified acid solution in pre-transition state for well stimulation

In this paper, a new class of fluids is considered, which are single-phase gas-liquid mixtures in the pre-transition state. The mentioned single-phase mixture is a liquid with precritical gas bubbles of nanometer size. The stability of nanobubbles can be increased by introducing surfactants into the gas-liquid mixture. The rheology of a gasified acid solution in a pre-transition has been studied. Based on the conducted research, the method of treating the wellbore zone with a gasified acid solution in a pre-transition state is proposed in order to increase the efficiency of acid treatment. The introduction of a cationic surfactant (0.1%) promotes hydrophobization of the pore walls and slows down the reaction between rock and acid. Improved filtration of single-phase gasified acid solution in pre-transition state increases liquid flow rate at constant pressure drop and reduces the consumption of gaseous agent. The effective viscosity of the gasified acid solution in the pre-transition state is 10 times lower than that of the non-carbonated acid solution. The resulting gasified acid solution flows uniformly into zones of varying permeability, increasing the treatment coverage through both the formation thickness and its depth. Compared to existing acid treatment technologies, such as the use of foam acid and viscoelastic surfactant-based systems, the gasified acid solution in the pre-transition state has a penetration capacity of 40-70% and a coverage factor of 100% higher. In order to reveal the effect of gasified acid solution in pre-transition state on the porous medium, the COMSOL Multiphysics simulation was used.

Experimental Research on the Ignition Characteristics and Inhibition Strategy for Venting Emissions Mixture of Failure LiFePO4 Battery

When the concentration of a gas is below its lower flammable limit and the content of a liquid is below its minimum explosible concentration, their combined fuel mixture can be ignitable. The flammability characteristics and inhibition strategies for battery emission mixtures deserve further in-depth research attention. This article presents experimental research on the ignition characteristics and inhibition strategy for a venting emission mixture of a failure LiFePO4 battery. By identifying the components of venting emissions, ignition experiments for gases, electrolyte mist, their combination fuels, and mixtures with additives are performed to determine the flammable parameters, including ignition sensitivity and severity. The hybrid combination of non-flammable venting gases and electrolyte mist has the potential to induce ignition. However, there still exists a non-ignition region, where the gas concentration ratio (mg) is below 0.15 and the liquid concentration ratio (ml) is below 0.1. A safety design principle can be proposed: increasing ignition temperature, prolonging ignition time, and reducing maximum pressure. Adhering to this principle, a non-flammable electrolyte consisting of 1 mol LiPF6 in EC:DEC = 1:1 vol%, with FEC at 10% and VC at 1%, can be considered as an optimization strategy. In comparison to the original gas–liquid mixtures, the region where no ignition occurs becomes wider when both the mg is below 0.45 and the ml is below 0.3. The new two-phase mixture has an ignition temperature of 835 °C, which is, respectively, 50% higher than that of the original mixture. Overall, this experimental research demonstrates an innovative methodology for assessing the battery venting emission mixture safety while proposing a design principle for modifying non-flammable electrolyte functional materials. Consequently, these findings can contribute to formulating more suitable preventive and protective measures for commercial electric vehicles and battery energy storage systems’ thermal safety designs.

Numerical simulation of fluid flow characteristics and ultrasonic cavitation performance in novel-designed stirred tank sonoreactor

A novel-designed stirred tank sonoreactor was developed to well solve large-scale dispersion of nanoparticles in liquid phase system. The fluid flow characteristics and ultrasonic cavitation performance in the sonoreactor were numerically simulated by CFD method. By comparing the pressure, cavitation bubble and velocity distribution under different situations, it is found that larger ultrasonic amplitude, relatively smaller ultrasonic frequency, higher saturated vapor pressure and lower viscosity of liquid medium are beneficial to ultrasonic cavitation. Besides, the acoustic flow action is strengthened with the increase of ultrasonic frequency and decrease of gas-liquid mixture's average density. For the designed sonoreactor, it is critical that high-frequency transducers should be determined near the bottom and upper region of the kettle, and large-amplitude transducers can be confirmed in the middle position. The research findings will provide theoretical and technical supports for developing state-of-the-art sonoreactors and optimizing ultrasonic process parameters in the industrialized preparation of nanocomposites.

An Effective Method for Reproducing the Flow of a Gas-Liquid Mixture Using a Flow Standard

The appearance on the market of measuring equipment of multiphase flowmeters used in the oil and gas industry to measure the amount of extracted hydrocarbons directly at wells without preliminary separation and performing the function of operational accounting raised the question of their metrological support in accordance with the requirements of regulatory documents of the sphere of state regulation in this field of measurements. His decision was to create a reference base, the basis of which was the State primary special standard for units of mass flow of gas-liquid mixtures GET 195-2011 and a verification scheme for measuring multiphase flow rates, regulated by GOST 8.637-2013. When transmitting units of volume and mass flow of a multiphase mixture with the required accuracy, the main problem is the difficulty of reproducing and maintaining stability of several flow parameters at once – mass flow and content of liquid components (water and oil simulator) and volume flow of the gas phase. It should also be borne in mind that test programs for the purpose of approving the type of multiphase flow measuring instruments, as well as verification and calibration methods, provide for changing the modes of reproduction of a gas-liquid mixture, covering the measurement ranges of multiphase flowmeters. Such standards used in practice represent a very complex measuring system consisting of storage tanks for components, supply pumping and compressor units, control equipment, mixers and separators, measuring instruments for the physical parameters of multiphase flow components and an automated control system. Given the high cost of multiphase standards, when creating them, effective methods and methods should be used to reproduce the flow of a multiphase mixture with specified parameters, as well as to make a fairly rapid transition to the next mode provided by the program or methodology, which will reduce the time and production costs for the procedure of transferring units of measurement from the standard to the working measuring instrument. This article describes a method for reproducing the flow rate of a gas-liquid mixture according to the original technological scheme introduced during the development of the 1st category standard. Constructive solutions are presented that increase the efficiency of the liquid component separation apparatus to increase the dosing accuracy, as well as expand the range of reproduction of the flow rate and parameters of the multiphase flow, ensuring the consistency of the composition of the liquid mixture at a given regime. An original rational method of compressing and maintaining the pressure of the gas phase is described, which allows you to quickly switch to the next mode, confirmed by the results of experimental studies of the standard.

Experimental investigation of a novel gas-liquid homogenizer methodology for ESP in high GVF flow applications

This paper presents a new homogenizer for Electric Submersible Pumps (ESP). Experiments were carried out using a two-stage radial-type pump with perforated vanes running at 3400 RPM. A test rig was designed to measure pressure and observe flow patterns. Results showed excellent performance for intake GVF ranging from 25 % to 89 %. Using the proposed homogenizer in the multistage ESP improves pump performance for gas-liquid mixtures. The study explored parameters to improve pump performance in high GVF conditions. The proposed homogenizer is more cost-effective and efficient by integrating into the existing pump stage and allowing customizable perforations. This design flexibility improves performance while avoiding the need for extra space or components. Moreover, numerical simulations with a helico-axial rotating multiphase pump and ANSYS-CFX software were conducted to prove the concept numerically. The focus was on gas-liquid two-phase flow and the effects of inlet GVFs. Modifications to the impeller blades, such as grooves, slits, or holes, were designed to enhance performance. The study found that CFD simulations accurately predict pump performance and that modified impellers improve performance in specific GVF ranges.

АНАЛИЗ ПРОБЛЕМ, ВОЗНИКАЮЩИЕ ПРИ ЭКСПЛУАТАЦИИ ПЕСЧАНЫХ СКВАЖИН

The article considers the causes of the formation of mechanical impurities in deep pump wells and the consequences that it entails for the listed reasons. The impact of solid floating particles in the liquid on the well equipment and the importance of improving the filtration technology of sand- containing liquid to prevent it has been noted. As a method used in the fight against mechanical impurities, the advantages and disadvantages of the filter design, which washes the filter element without lifting the pumping equipment, and the V-shaped wire slotted grilles made of durable stainless steel located in the filter compartment are considered. In practical methods of combating mechanical impurities, the main directions of technology for equipping the bottom areas of wells with filters and the main criteria for combating the negative effects of mechanical impurities are indicated. Based on the special importance of the problem of reducing solid floating particles in the production of a gas-liquid mixture, several methods of insulating sand from impurities used in oil production are considered to protect deep pumping equipment. It is established that solving the problem of increasing the reliability of pumping equipment in the presence of solid float particles in the reservoir fluid requires an integrated approach to the selection of technical means. Analysis of the operability of existing filters in the intake of deep pumps showed that they have low efficiency in the fight against solid floating particles, including the need to periodically raise the pumping equipment for cleaning due to the adhesion of solid floating particles to the surface

Experimental study on plunging breaking waves impacting a vertical cylinder with three different model scales

Scale effects, which can influence the reliability and effectiveness of experimental results, are often unavoidable but seldom explored, particularly in the context of plunging waves. To investigate the scale effects of plunging wave impacts on a vertical cylinder, experiments were conducted in a wave flume at model scales of 1:20, 1:30 and 1:40. The wavemaker movements were scaled based on the Froude-similarity law and the pressures on the front surface of the cylinder were analyzed. The results indicated that plunging waves were generated across these three scales, with fine agreement in overall wave elevations and a local waveform similarity of 80–90%. The analysis of pressures showed that the distribution of peak pressures on the cylinders remained consistent across different scales; however, scale effects were closely related to the intensity of wave impacts. Specifically, in cases of relatively weak impact, such as when the wavefront struck the cylinder before breaking or even after an intense wave breaking with intense gas-liquid mixtures, the pressure discrepancies were less than 20% among these different scales. This discrepancy can exceed 50% near the wave peaks during the vertical wave front or early jet formation stages with strong impacts.

Computational fluid dynamics modeling of gas bubble separation efficiency in downhole separators and vertical / inclined annulus: An experimental validation and analysis

This study presents a comprehensive investigation into gas bubble separation efficiency in a vertical downhole separator using computational fluid dynamics (CFD) modeling, which is validated with experimental data. The research aims to provide detailed insights into the separation mechanisms and the factors affecting them, such as liquid velocity, gas bubble sizes, and positions. Using an Euler-Lagrange multiphase model, the behavior of individual gas bubbles within the liquid-gas mixture was simulated. The results demonstrate that separation efficiency decreases as the liquid flow rate increases. This reduction in efficiency occurs because the increased liquid flow rate drags more bubbles within the stream, making it harder for them to separate. The separation process is primarily driven by the buoyancy force acting on the bubbles and the velocity profile of the liquid flow. Smaller bubbles, which tend to be near the casing wall, separate more efficiently, while larger bubbles are predominantly located between the second and third quarters of the annulus space. The "quarters" refer to radial divisions from the casing wall to the tubing wall, with the first quarter closest to the casing wall and the fourth quarter closest to the tubing wall. The study introduces a nonlinear regression model based on the numerical results to predict liquid slug separation efficiency. This model, validated against experimental data, accurately predicts separation efficiency and offers a robust methodology for estimating the efficiency of gravity-driven gas-liquid separators. This methodology is crucial for refining separator design and improving the operational efficiency of downhole separation systems, which are vital for enhancing the performance of pumping systems in oil and gas wells. The contributions of this study include a deeper understanding of the downhole separation process, the development of a simplified model for assessing bubble separation efficiency, and the potential application of the proposed methodology to different gas-liquid separator designs and inclination angles, thereby informing the design and operation of downhole separation systems.

Experimental Investigation and Numerical Validation of a Roots Pump’s Performance Operating with Gas-Liquid Mixtures

To facilitate the high operating pressure of a novel underwater power cycle, the potential of Roots pumps for pressurizing gas-liquid mixtures is experimentally investigated in this paper. The experimental facility is constructed, and the effects of inlet gas volume fractions and rotational speeds on the pump performance are discussed. The results show that the increased inlet gas volume fraction is beneficial to increasing the pump efficiency. This is associated with the increased pressure ratio and the gas-liquid mixture compressibility. In addition, the increases in rotational speed and liquid phase volume fraction negatively affect the pump’s efficiency. These phenomena are caused by the resulting high pressure difference and subsequently the back-flow from the pump outlet, thereby increasing the gap leakage and decreasing the Roots pump’s operating efficiency. The numerical model is further compared against experimental resultsk and the maximum difference is found to be less than 7.53%. This paper experimentally tests the potential of Roots pumps for pressurizing gas-liquid mixtures.

Highly stable nanobubbles in the reduction of apparent viscosity of liquids during UF process

Nano-bubbles (NBs) have attracted more attention in various chemical processes due to their different physicochemical properties from macro-bubbles. However, little attention has been paid to the variations in physicochemical properties of the liquids caused by NBs. In this study, stable NBs in the range of 120–360 nm (mean diameters of 221 nm) are generated at an air pressure of 0.3 MPa. Meanwhile, the surface tension of H2O and sodium dodecyl sulfate (SDS) solution decrease with increasing amounts of NBs, indicating that NBs altered the physicochemical properties of liquids. Consistent with the variations in surface tension, the apparent viscosity of pure H2O and 400 mg L−1 sodium alginate (SA) solutions decreased by 5.6 % and 6.6 % after bubbling for 180 min, respectively, which can be attributed to the long retention time of NBs in the mixture of NB-liquids. The correlations between the apparent viscosity of the gas–liquid mixture (μ) and the gas holdups (V) are then deduced as μ=μ0(1-αV), where μ0 is the apparent viscosity of liquid in the absence of NBs and α is the correction factor. The CFD results further confirm that the apparent viscosity of the gas–liquid mixture decreases with increasing gas holdups. During the UF process, NBs significantly increase the normalized flux of NB-H2O from 1.00 to 1.35 and from 0.72 to 1.02 for NB-10 mg L−1 SA solution, which from the other side confirms that NBs decrease the apparent viscosity of the gas–liquid mixture, thus improving the normalized flux of filtrates. This study provides valuable clues for the significance of NBs in chemical processes.

Spatiotemporal Evolution of Gas in Transmission Fluid under Acoustic Cavitation Conditions

The presence of gas in transmission fluid can disrupt the flow continuity, induce cavitation, and affect the transmission characteristics of the system. In this work, a gas void fraction model of gas–liquid two-phase flow in a transmission tube is established by taking ISO 4113 test oil, air, and vapor to accurately predict the occurrence, development, and end process of the cavitation zone as well as the transient change in gas void fraction. This model is based on the conservative homogeneous flow model, considering the temperature change caused by transmission fluid compression, and cavitation effects including air cavitation, vapor cavitation, and pseudo-cavitation. In this model, the pressure term is connected by the state equation of the gas–liquid mixture and can be applied to the closed hydrodynamic equations. The results show that in the pseudo-cavitation zone, the air void fraction decreases rapidly with pressure increasing, while in the transition zone from pseudo-cavitation to air cavitation, the air void fraction grows extremely faster and then increases slowly with decreasing pressure. However, in the vapor cavitation zone, the vapor void fraction rises slowly, grows rapidly, and then decreases, which is consistent with the explanation that rarefaction waves induce cavitation and compression waves reduce cavitation.

Experimental Investigation of Spherical Particles Settling in Annulus Filled with Rising-Bubble-Containing Newtonian Fluids

During the drilling of ultra-deep wells, gas kick often occurs, influenced by the complex void pressure profile. The accurate description of particle settling behavior in the gas–liquid mixture is of great significance to effectively deal with gas kicks and ensure drilling safety. In this study, the gas–liquid two-phase annulus flow with different gas volume fractions is created through the transparent annular pipe, constant pressure air pump, and gas flowmeter. High-speed photography is used to record and analyze the sedimentation of particles in gas–liquid mixtures. This study is based on 288 tests. The main parameters in this experiment include the particle Reynolds number, the gas fraction, and liquid viscosity. The effects of wall and gas fraction on the drag coefficient were analyzed. The correlation of particle terminal settling velocity was established. The results obtained show a correlation with average absolute errors (AAE) of 10.7%. This study reveals the settling characteristics of particles in the annular gas–liquid mixed flow, provides an accurate terminal settling velocity prediction explicit formula, and provides guidance for the calculation of bottom hole pressure under the condition of gas kick.

Improving the technology for Inflow Profile Evaluation in horizontal Wells by temperature changes Due to calorimetric Mixing

The paper is devoted to the problem of increasing the effectiveness of temperature logging quantitative interpretation for the inflow profile evaluation in horizontal production wells draining heterogeneous reservoirs with low permeability. Such wells are characterized by an extremely uneven distribution of inflow along the length of the wellbore. One of the ways of quantitative interpretation of temperature logging is based on the effect of calorimetric mixing. It’s widely used to quantify the share of local producing intervals in the total flow rate.The low accuracy of interpretation is associated with the lack of reliable information about the temperature of the fluid flowing into the wellbore. The authors propose an estimate of this parameter based on the similarity of the behavior of the temperature field vs time in the near-wellbore zone of a reservoir during periods of stable production and the periods of the well shut-in. This relationship is confirmed by the results of modeling the temperature field of the “well – reservoir” system, taking into account changes in a wide range of reservoir permeability and thermal properties of the reservoir, the geometry of hydraulic fractures in the reservoir, the flowing fluids, as well as the parameters of the well production targets. The logging technology recommended by the authors involves the registration of several temperature profiles along the length of the wellbore at the beginning of the well production with the maximum rate and drawdown and during the well shut-in. Their integrated analysis based on the behavior of the temperature field features in time identified on the basis of modeling makes it possible to evaluate with a high degree of certainty the dynamics of the temperature of the gas-liquid mixture coming from reservoirs to the wellbore during production periods. This provides the required accuracy in the quantitative assessment of the inflow profile from mixing anomalies.The proposed approaches to the interpretation of thermograms are applicable in the analysis of the results of non-stationary temperature logging results in horizontal and vertical wells during the production from heterogeneous reservoirs both through perforation and multi-stage hydraulic fracturing.

The study of solid particles effect on the gas bubble dispersion dynamics of complex gas-liquid mixtures at the intake screen of submersible pump

The study of gas phase dispersion at the intake of submersible pumping equipment is an actual issue, since changes in gas phase dispersion can lead to changes in natural separation. At the same time, the gas separation efficiency can be affected by the presence of solid phase in the well products, which would change the structure of gas-liquid mixture and operation mode of electric submersible pump. The results of the research are novel, which takes into account the influence of the properties of multi-component gas-liquid mixture on the character of operation of ESP well. The new dependences of gas bubble diameter at the submersible equipment intake on gas content for four model gas-liquid mixtures: «water – air», «water – surfactants – air», «water – air – solids» and «water – surfactants – air – solids» with the concentration of suspended particles in the flow of 1.0 g/l have been experimentally obtained in conditions of gauge pressure at the intake 0.04 - 0.05 MPa, gas content of mixture was increased up to 71%, liquid flow rate was changed in the range of values from 13 to 68 m3/day. High-speed video recording with fixing the structure of gas-liquid mixture in the near-intake space of the ESP was carried out during the tests. Visualization allows us to clarify the boundary of the flow regime transition from «bubble» to «slug/churn» compared to the Caetano method. There were developed new correlations of natural gas separation coefficient taking into account the influence of solid phase at different operating modes of ESP well. Keywords: gas-liquid mixture (GLM); gas bubble; natural gas separation; solid particles; surfactants.

Influence of connecting a new gas pipeline to the operating gas pipeline on the flow rate of production wells

In some cases, it is necessary to connect new line to the pipeline. From this point of view, in the presented work, serious changes occurring in the pipeline as a result of connecting a new gas pipeline to the operating gas pipeline are investigated. The changes occurring in the pipeline are transmitted to the formations, which causes a violation of the stable operation of the wells in the initial period. After a certain period of time, the dynamic processes formed in the system as a result of changes in the transport line become stationary. From this point of view, it is of great practical importance to consider the stationary state of the processes occurring in the system. Therefore, in the presented work, as a result of the addition of a new line to the pipeline, the change in the production of gas wells in stationary regime conditions is studied. The effect of these changes on the operating mode of the wells, the effect on the change in production of operational wells is studied. Taking into account the interaction of the layer-pipeline system, a mathematical model of the movement process of gas and gas-liquid mixture is established, using the equations of motion and continuity, the resulting system of related differential equations is solved. With the help of the obtained formulas, the pressure drop and production reduction for each well are calculated during the connection of the new line. Numerical calculations are performed using various practical values of system parameters and the obtained results are analyzed. Keywords: production; layer; pressure; pipeline; stationary state; gas-liquid mixture; motion and continuity equation.

Automated hyperpolarized 129Xe gas generator for nuclear magnetic resonance spectroscopy and imaging applications

We describe a method and an apparatus for producing hyperpolarized (HP) Xe gas of high concentration without Xe condensation. The HP Xe generator works under atmospheric pressure and employs a quasi-continuous method to provide a continuous supply of HP Xe gas by adopting technologies for supplying a highly pure gas, a gas control system and precise pressure control. The apparatus has a glass cell containing solid Rb metal and solid Xe in vacuum at a low temperature and is heated so that the Xe becomes a gas and Rb exists in a gas-liquid mixture. Then a magnetic field is applied with laser beam irradiation to produce polarized Xe gas of high concentration. The device was evaluated using a 2T magnetic resonance imaging (MRI) scanner. Long-term experimental operation demonstrated the continuous collection of 30 ml syringes of HP Xe gas with a sufficient polarization rate for fast one scan acquisition. A practical device for the automated manufacture of HP 129Xe gas was thus successfully developed.

Unified closure relationship for slug liquid holdup

AbstractSlug flow is a common flow pattern in pipes operating with gas–liquid mixtures. The slug flow's periodic behaviour affects the performance of multiphase transportation systems, which is generally predicted using mechanistic models. Mechanistic models are based on one‐dimensional mass and momentum conservation equations and require additional relationships to define a complete system of equations. These additional equations are usually called closure relationships. The slug liquid holdup () is one of the required closure relationships. In this paper a unified closure model for is proposed considering the energy balance and the interaction between inertial, gravitational, viscous, and surface tension forces. The model parameters' behaviour is described as a function of a new dimensionless number. The experimental data analysis shows that this number can quantitatively define two flow categories: ‘low viscosity flow’ and ‘medium/high viscosity flow’. Slug liquid holdup changes its behaviour in these regions due to the predominance of inertial–surface tension forces or gravitational–viscous forces, respectively. The proposed closure performs better than the existing mechanistic and empirical models.

Numerical Study on the Local Aggregation of Helium Bubbles in Liquid Lithium and Its Thermal Analysis

Abstract Liquid lithium is widely regarded as an optimal cooling medium for space nuclear reactors due to its exceptional heat transfer properties and low density. However, the helium bubbles generated by liquid lithium under space irradiation pose significant hazards to the safe and stable operation of nuclear reactions. This study employs the COMSOL finite element software to construct the level-set two-phase flow models and bubble stream model separately to investigate the local accumulation of helium bubbles and the overall flow of low-concentration gas–liquid mixtures. The main focus is on examining the different distributions of multiple helium bubbles randomly generated in local liquid lithium and the influence of boundary conditions on their accumulation morphology, as well as the impact of low-concentration bubble stream on their overall heat transfer performance. Agglomerated bubbles with radii between 5 μm and 150 μm are classified into three categories based on local concentrations: circular (≤20.37%), irregular elongated (up to 30.44%), and banded (up to 36.31%).The interconnected banded bubbles can be up to 8 times larger than irregularly elongated ones, and they have a positive effect on the distribution of physical quantities and wall temperature perturbations in the pipeline. The increase in inlet velocity triggers bubble impacts and fragmentation, further reducing thermal resistance and enhancing heat transfer performance. When the bubble diameter is less than 15 μm and the bubble concentration is within 1%, the influence of the mixed flow on overall heat transfer is not significant. This study provides insights for manipulating bubble structure and guiding localized and comprehensive thermal analyses.

The kinetic energy transfer analysis between the gas and the liquid in flow-focusing of the micro-jet

The present study uses computational fluid dynamics to analyse the kinetic energy transfer from the gas to the liquid phase, considering the significant influence of surface tension. The considered situation is the gas dynamic virtual nozzle, where the co-flowing gas focuses and accelerates the liquid jet. The experimentally validated half-space three-dimensional gas-liquid mixture model addresses the unsteady, incompressible, isothermal, Newtonian, low-turbulent two-phase flow. The continuity, momentum and the k-ω SST turbulence model are employed to resolve the fluid flow. The numerical solution is based on the finite volume method and volume of fluid approach with a geometric reconstruction scheme for tracking the gas-liquid interface. The total pressure of the gas, an indication of its energy, is tracked along streamlines and analysed spatially and temporarily. It is found that around 50 % of the focusing gas energy is transferred to the liquid jet before its breakup for the nozzle with Weber number 3.5, and gas and jet Reynolds number 1842 and 108, respectively. The linear regression between jet length and energy transfer efficiency is discovered. The presented methodology represents an essential tool for analysing and understanding the energy transfer process between the focusing gas and the liquid jet.

Near-critical dark opalescence in out-of-equilibrium SF6

The first-order phase transition between the liquid and gaseous phases ends at a critical point. Critical opalescence occurs at this singularity. Discovered in 1822, it is known to be driven by diverging fluctuations in the density. During the past two decades, boundaries between the gas-like and liquid-like regimes have been theoretically proposed and experimentally explored. Here, we show that fast cooling of near-critical sulfur hexafluoride (SF6), in presence of Earth’s gravity, favors dark opalescence, where visible photons are not merely scattered, but also absorbed. When the isochore fluid is quenched across the critical point, its optical transmittance drops by more than three orders of magnitude in the whole visible range, a feature which does not occur during slow cooling. We show that transmittance shows a dip at 2eV near the critical point, and the system can host excitons with binding energies ranging from 0.5 to 4 eV. The spinodal decomposition of the liquid-gas mixture, by inducing a periodical modulation of the fluid density, can provide a scenario to explain the emergence of this platform for coupling between light and matter. The possible formation of excitons and polaritons points to the irruption of quantum effects in a quintessentially classical context.