Gas-like Behavior Research Articles

Surface tension and evaporation behavior of liquid fuel droplets at transcritical conditions: Towards bridging the gap between molecular dynamics and continuum simulations

The phase transition from subcritical to supercritical conditions, referred to as transcritical behavior, significantly impacts the evaporation and fuel–air mixing in high-pressure liquid-fuel propulsion systems. Transcritical behavior is characterized as a transition from classical two-phase evaporation to a single-phase gas-like diffusion regime as surface tension and latent heat of vaporization reduce. However, the interfacial behavior represented by the surface tension coefficient and evaporation rate during this transition which are crucial inputs for Computational Fluid Dynamics (CFD) simulations of practical transcritical fuel spray is still missing. This study aims at developing new evaporation rate and surface tension models for transcritical n-dodecane droplets using molecular dynamics (MD) simulations irrespective of the droplet size. As MD simulations are primarily limited to the nanoscale, the new models can bridge the gap between MD and continuum simulations and enable the direct application of these findings to microscopic droplets. A new characteristic timescale, i.e., “undroplet time,” is defined which marks the transition from classical two-phase evaporation to single-phase gas-like diffusion behavior. The undroplet time indicates the onset of droplet core disintegration and penetration of nitrogen molecules into the droplet, which occurs after the vanishment of the surface tension. By normalizing the time with respect to the undroplet time, the rate of surface tension decay, evaporation rate, and the rate of droplet mass depletion become independent of the droplet size. Calculation of pairwise correlation coefficients for the entire MD results shows that both surface tension coefficient and evaporation rate are strongly correlated with the background temperature, while pressure and droplet size play a less significant role past the critical point. Therefore, new models for surface tension coefficient and evaporation rate spanning from sub- to supercritical conditions are developed as a function of background pressure and temperature, which can be used in continuum simulations. The identified phase change behavior based on the undroplet time shows a good agreement with the phase change regime maps obtained using microscale experiments and nanoscale MD predictions.

Atomic dynamics in fluids: Normal mode analysis revisited.

Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise from these unstable structures is still elusive. In this paper, we explore the instantaneous eigenmodes of dynamical matrices of various Lennard-Jones argon liquid and gas systems at high temperatures and show that the normal modes can be interpreted as an interpolation of T→∞ (gas) and T=0 (solid) mode descriptions. We find that normal modes become increasingly collisional and translational, recovering atomistic gaslike behavior rather than vibrational with increase in temperature, suggesting that normal modes in liquids may be described by both solidlike and gaslike modes.

Constrained optimization of s-CO2 compression train system through 1D approach under a new gas-like behavior constraint

The rising importance of carbon capture, utilization and storage industry due to global warming has increased the development of new technologies. Centrifugal compressor train is an essential system required for the storage of s-CO2 into oil wells, a process known as Enhanced Oil Recovery (EOR), in which the pressurized fluid works above the critical point and close to widom-line. The optimization methodology proposed in the present work is based on One-dimensional (1D) centrifugal compressor model, capable to predict the fluid flow aspects that are not modeled through traditional and direct thermodynamic analysis. Furthermore, a Gas-like Behavior Margin (GBM) is presented to avoid high variations of thermodynamic properties on the compression process close to widom-line, avoiding transcritical phenomena that can cause damage to the equipment. A centrifugal compressor train has been proposed and an optimal configuration with four stages reduced the total power consumed by 14.09% when compared to the baseline configuration, ensuring that the fluid flow constraints as the Acceleration Margin to Condensation and Mach number at the throat is met. Moreover, a 3D CFD model for the fourth stage was built which demonstrated good agreement with 1D model and experimental data, proving the validity and clear advantages of using the methodology developed herein, as a fast and low-cost way to obtain a preliminary geometry for centrifugal compressors of train systems.

Technegas, A Universal Technique for Lung Imaging in Nuclear Medicine: Technology, Physicochemical Properties, and Clinical Applications.

Technegas was developed in Australia as an imaging radioaerosol in the late 1980s and is now commercialized by Cyclomedica, Pty Ltd. for diagnosing pulmonary embolism (PE). Technegas is produced by heating technetium-99m in a carbon crucible for a few seconds at high temperatures (2750 °C) to generate technetium-carbon nanoparticles with a gas-like behaviour. The submicron particulates formed allow easy diffusion to the lung periphery when inhaled. Technegas has been used for diagnosis in over 4.4 m patients across 60 countries and now offers exciting opportunities in areas outside of PE, including asthma and chronic obstructive pulmonary disease (COPD). The Technegas generation process and the physicochemical attributes of the aerosol have been studied over the past 30 years in parallel with the advancement in different analytical methodologies. Thus, it is now well established that the Technegas aerosol has a radioactivity aerodynamic diameter of <500 nm and is composed of agglomerated nanoparticles. With a plethora of literature studying different aspects of Technegas, this review focuses on a historical evaluation of the different methodologies' findings over the years that provides insight into a scientific consensus of this technology. Also, we briefly discuss recent clinical innovations using Technegas and a brief history of Technegas patents.

Dynamically structured bubbling in vibrated gas-fluidized granular materials.

The dynamics of granular materials are critical to many natural and industrial processes; granular motion is often strikingly similar to flow in conventional liquids. Food, pharmaceutical, and clean energy processes utilize bubbling fluidized beds, systems in which gas is flowed upward through granular particles, suspending the particles in a liquid-like state through which gas voids or bubbles rise. Here, we demonstrate that vibrating these systems at a resonant frequency can transform the normally chaotic motion of these bubbles into a dynamically structured configuration, creating reproducible, controlled motion of particles and gas. The resonant frequency is independent of particle properties and system size, and a simple harmonic oscillator model captures this frequency. Discrete particle simulations show that bubble structuring forms because of rapid, local transitions between solid-like and fluid-like behavior in the grains induced by vibration. Existing continuum models for gas-solid flows struggle to capture these fluid-solid transitions and thus cannot predict the bubble structuring. We propose a constitutive relationship for solids stress that predicts fluid-solid transitions and hence captures the experimental structured bubbling patterns. Similar structuring has been observed by oscillating gas flow in bubbling fluidized beds. We show that vibrating bubbling fluidized beds can produce a more ordered structure, particularly as system size is increased. The scalable structure and continuum model proposed here provide the potential to address major issues with scale-up and optimal operation, which currently limit the use of bubbling fluidized beds in existing and emerging technologies.

Structural Markers of the Frenkel Line in the Proximity of Widom Lines.

We have performed a neutron scattering experiment on supercritical fluid nitrogen at 160 K (1.27 TC) over a wide pressure range (7.8 MPa/0.260 g/mL-125 MPa/0.805 g/mL). This has enabled us to study the process by which nitrogen changes from a fluid that exhibits gaslike behavior to one that exhibits rigid liquidlike behavior at a temperature close to, but above, the critical temperature by crossing the Widom lines followed by the Frenkel line on pressure (density) increase. We find that the Frenkel line transition is indicated by a transition to a regime of rigid liquidlike behavior in which the coordination number remains constant within error, in agreement with our previous work at 300 K. The Frenkel line transition takes place at approximately the same density at 160 and 300 K. The data do not conclusively show an additional transition at the location of the known Widom lines. We find that behavior remains gaslike until the Frenkel line is crossed and our data support the hypothesis that Widom line transitions are density increase-driven.

Pseudo-boiling and concentration gradient in an enclosed supercritical binary mixture

Recent publications have shown the effectiveness of the analogy between fluids at sub- and supercritical pressures and confirmed there is a distinguishable crossover between liquid-like and gas-like behavior in supercritical fluids. This work is devoted to extending such an analogy to miscible zeotropic binary mixtures, whose well-known feature in liquid-gas coexistence under a subcritical pressure is the concentration difference between liquid and gas. Due to the availability of thermophysical properties, C2H6−CO2 binary mixture is taken as a reference fluid. Through numerical simulations for the steady states of an enclosed binary mixture at supercritical pressures subjected to temperature differences, this paper reveals the coexistence of liquid-like and gas-like states is featured by a concentration gradient naturally established by the critically enhanced Soret effect. It suggests that the concentration gradient is enhanced as the critical pressure is approached or as the temperature difference is enlarged. Besides, the low heat-conducting nature of the gas-like state compared to the liquid-like one gives rise to a pressure drop. This work gives new insights into the similarities of binary mixtures under sub- and supercritical pressures. For the convenience of latter studies, an open-source code is provided to calculate the physical properties of C2H6−CO2 binary mixture.

Gas or Liquid? The Supercritical Behavior of Pure Fluids.

By definition, the distinction between a gas and a liquid ceases to exist beyond the critical point for pure fluids. Nevertheless, there remains a strong desire to attribute gas-like or liquid-like behavior to fluids corresponding to different parts of the supercritical region, especially as this becomes important for understanding and designing the properties of supercritical fluids. Here, we use a combination of fluctuation solution theory and accurate equation of state data to elucidate an easily accessible dividing line and corresponding transition regime between liquid-like and gas-like behavior in the supercritical region of all pure fluids. Liquid-like behavior in the supercritical region is characterized by a negative skewness in the particle number distribution for an equivalent open system, indicating that particle deletion is favored for liquids, whereas gas-like behavior is characterized by a positive skewness, indicating that particle insertion is favored for gases. Identical behavior is observed either side of the liquid-vapor line. The possible consequences for the behavior of fluids at the critical point are also discussed.

Particle stability and structure on the peritoneal surface in pressurized intra-peritoneal aerosol chemotherapy (PIPAC) analysed by electron microscopy: First evidence of a new physical concept for PIPAC.

Pressurized intra-peritoneal aerosol chemotherapy (PIPAC) has been introduced to the clinical setting as a novel approach for the treatment of peritoneal metastasis. The local interaction of chemoaerosol droplets with the peritoneal surface as well as their distribution pattern is considered the main advantage over conventional liquid intraperitoneal chemotherapy. The aim of the present study was to investigate the behavior of these aerosol particles during PIPAC application via electron microscopy. Solutions of doxycycline, liposomal doxorubicin and macrophage cells were aerosolized using an established ex-vivo model. PIPAC was performed on peritoneum samples via microcatheter (MC) at a pressure of 12 mmHg C02 at 27°C. Following PIPAC the surface structure of applied particles was measured via electron microscopy. The aerosol particle contact of doxycyclin created a nanofilm of ~200 nm height on the peritoneal surface, and this height was revealed to be independent of the size of the initial particle hitting. These nanofilm blocks of ‘cylinders’ are of different diameters depending on the initial aerosol particle hitting that spot. Diameters of these ‘cylinders’ are far wider than the original diameter of the initial aerosol particle. However, coated particles such as liposomal doxorubicin and macrophages remained intact following contact with the peritoneal surface. Based on this and other data, the concept that aerosol particles exhibit a gas-like behavior in the abdomen creating a therapeutic capnoperitoneum should be revised. Fluid aerosol particles collide with the peritoneum creating a nanofilm. The interaction of pressurized intraperitoneal aerosol on the peritoneum is therefore closer to the distribution of a liquid film than to that of a gas. Further studies are required to further analyze the interaction of this nanofilm on the peritoneum.

Study of distance dependence of hydrophobic force between two graphene-like walls and a signature of pressure induced structure formation in the confined water.

We examine the separation distance dependence of the hydrophobic force by systematically varying the distance (d) between two walls. The hydrophobic force exhibits a distance mediated crossover from a liquid-like to a gas-like behavior at around d ∼ 12 Å for 1 atm pressure. The distance dependence can be fitted to a bi-exponential form, with the longer distance part displaying a correlation length of 20 Å. In addition, the crossover is found to be accompanied by a divergent-like growth of the local relative number fluctuation of the water molecules confined between the two surfaces. Furthermore, at a fixed separation (d = 20 Å), we observe a pressure induced structural modification of confined water at high pressure. The confined water is found to form an ordered structure at high pressure (10 000 atm) and room temperature, in agreement with the experimental study [G. Algara-Siller et al. Nature 519(7544), 443 (2015)].

Molecular interactions of CO2 with the CuBTC metal organic framework: An FTIR study based on two-dimensional correlation spectroscopy

The sorption of Carbon dioxide in the prototype metal-organic framework CuBTC was examined in detail by FTIR spectroscopy. The spectra were collected in-situ under isothermal equilibrium conditions at two different temperatures (308 and 273 K). The results were analyzed by difference spectroscopy (DS) and 2D correlation spectroscopy (2D-COS). In particular, 2D-COS was employed to improve the resolution of the spectral profiles and to verify the occurrence of multiple molecular species. The spectroscopic monitoring of the sorption process provided precise quantitative data as well as relevant molecular-level information. Distinct interaction sites were identified in the framework structure, in agreement with first-principles simulations. A minor fraction of probe molecules characterized by a gas-like behavior was identified by analysis of the ν2 frequency range.

Solid-like features in dense vapors near the fluid critical point.

The phase diagram (pressure versus temperature) of the pure fluid is typically envisioned as being featureless apart from the presence of the liquid-vapor coexistence curve terminating at the critical point. However, a number of recent authors have proposed that this simple picture misses important features, such as the Widom line, the Fisher-Widom line, and the Frenkel line. In our paper, we discuss another way of augmenting the pure fluid phase diagram, lines of zero thermodynamic curvature R = 0 separating regimes of fluid solid-like behavior (R > 0) from gas-like or liquid-like behavior (R < 0). We systematically evaluate R for the 121 pure fluids in the NIST/REFPROP (version 9.1) fluid database near the saturated vapor line from the triple point to the critical point. Our specific goal was to identify regions of positive R abutting the saturated vapor line ("feature D"). We found the following: (i) 97/121 of the NIST/REFPROP fluids have feature D. (ii) The presence and character of feature D correlates with molecular complexity, taken to be the number of atoms Q per molecule. (iii) The solid-like properties of feature D might be attributable to a mesoscopic model based on correlations among coordinated spinning molecules, a model that might be testable with computer simulations. (iv) There are a number of correlations between thermodynamic quantities, including the acentric factor ω, but we found little explicit correlation between ω and the shape of a molecule. (v) Feature D seriously constrains the size of the asymptotic fluid critical point regime, possibly resolving a long-standing mystery about why these are so small. (vi) Feature D correlates roughly with regimes of anomalous sound propagation.

Membrane Mediated Cooperativity Facilitates Cadherin Clustering in Model Membranes

Cadherins are major cell-cell adhesion molecules in vertebrate tissue. They play a vital role in the development and maintenance of multicellular organisms. Cadherins show homophilic as well as heterophilic binding to members of their subfamily with a relatively low 3D binding affinity. In order to establish stable adhesion a combination of trans and cis interactions has been postulated to promote oligomerization and junction formation.In this project, we studied the dynamics of junction formation in a simplified model system consisting of a giant unilamellar vesicles adhering to a solid supported lipid bilayer via homophilic E cadherin - E cadherin bonds. We used reflection interference contrast microscopy to quantitatively extract vesicle membrane features (height and fluctuation amplitude) as well as to visualize the adhesion dynamics. Subtle differences in the initial membrane separation and fluctuation amplitude resulted in a variety of adhesion processes and steady states. We observed classical radial growth originating from one nucleation center leading to a dense array of bonds and strong adhesion. Other vesicles exhibited multiple nucleation centers or even gas-like behavior characterized by numerous bonds forming at the same time. The latter resulted in weak adhesion only and sometimes even unbinding.By developing a theoretical framework supported by Monte Carlo simulations, we are able to explain the extreme sensitivity of the experimentally observed growth processes. We suggest that the self-assembly of model adhesion junctions can be understood as an interplay between the effective 2D binding affinity, the characteristic nucleation time as well as the strength of membrane transmitted correlations. In vivo the observed sensitivity of the cadherin system can be exploited to regulate adhesion locally via biomembrane protrusions.

Dipolar bilayer with antiparallel polarization: A self-bound liquid

Dipolar bilayers with antiparallel polarization, i.e. opposite polarization in the two layers, exhibit liquid-like rather than gas-like behavior. In particular, even without external pressure a self-bound liquid puddle of constant density will form. We investigate the symmetric case of two identical layers, corresponding to a two-component Bose system with equal partial densities. The zero-temperature equation of state $E(\rho)/N$, where $\rho$ is the total density, has a minimum, with an equilibrium density that decreases with increasing distance between the layers. The attraction necessary for a self-bound liquid comes from the inter-layer dipole-dipole interaction that leads to a mediated intra-layer attraction. We investigate the regime of negative pressure towards the spinodal instability, where the bilayer is unstable against infinitesimal fluctuations of the total density, conformed by calculations of the speed of sound of total density fluctuations.

Complex singularities of the fluid velocity autocorrelation function

There are intensive debates regarding the nature of supercritical fluids: if their evolution from liquid-like to gas-like behavior is a continuous multistage process or there is a sharp well-defined crossover. Velocity auto-correlation function Z is the established detector of evolution of fluid particles dynamics. Usually, complex singularities of correlation functions give more information. For this reason, we investigate Z in complex plane of frequencies using numerical analytic continuation. We have found that naive picture with few isolated poles fails describing Z(ω) of one-component Lennard-Jones (LJ) fluid. Instead, we see the singularity manifold forming branch cuts extending approximately parallel to the real frequency axis. That suggests LJ velocity autocorrelation function is a multivalued function of complex frequency. The branch cuts are separated from the real axis by the well-defined “gap” whose width corresponds to an important time scale of a fluid characterizing crossover of system dynamics from kinetic to hydrodynamic regime. Our working hypothesis is that the branch cut origin is related to competition between one-particle dynamics and hydrodynamics. The observed analytic structure of Z is very stable under changes in the temperature; it survives at temperatures two orders of magnitude higher than the critical one.

The Widom line and dynamical crossover in supercritical water: Popular water models versus experiments.

In a previous study [Gallo et al., Nat. Commun. 5, 5806 (2014)], we have shown an important connection between thermodynamic and dynamical properties of water in the supercritical region. In particular, by analyzing the experimental viscosity and the diffusion coefficient obtained in simulations performed using the TIP4P/2005 model, we have found that the line of response function maxima in the one phase region, the Widom line, is connected to a crossover from a liquid-like to a gas-like behavior of the transport coefficients. This is in agreement with recent experiments concerning the dynamics of supercritical simple fluids. We here show how different popular water models (TIP4P/2005, TIP4P, SPC/E, TIP5P, and TIP3P) perform in reproducing thermodynamic and dynamic experimental properties in the supercritical region. In particular, the comparison with experiments shows that all the analyzed models are able to qualitatively predict the dynamical crossover from a liquid-like to a gas-like behavior upon crossing the Widom line. Some of the models perform better in reproducing the pressure-temperature slope of the Widom line of supercritical water once a rigid shift of the phase diagram is applied to bring the critical points to coincide with the experimental ones.

Web applet for predicting structure and thermodynamics of complex fluids

Based on a recently introduced analytical strategy [Hollingshead et al., J. Chem. Phys. 139, 161102 (2013)], we present a web applet that can quickly and semi-quantitatively estimate the equilibrium radial distribution function and related thermodynamic properties of a fluid from knowledge of its pair interaction. We describe the applet's features and present two (of many possible) examples of how it can be used to illustrate concepts of interest for introductory statistical mechanics courses: the transition from ideal gas-like behavior to correlated-liquid behavior with increasing density, and the tradeoff between dominant length scales with changing temperature in a system with ramp-shaped repulsions. The latter type of interaction qualitatively captures distinctive thermodynamic properties of liquid water, because its energetic bias toward locally open structures mimics that of water's hydrogen-bond network.

Widom line and dynamical crossovers as routes to understand supercritical water.

Supercritical water is fundamental in many fields of applications and a precise characterization of the supercritical state is of uttermost importance for this liquid. In a fluid, when moving from the critical point into the single-phase region, the thermodynamic response functions show maxima reminiscent of the critical divergence. Here we study the thermodynamic properties of water in the supercritical region by analysing both available experimental data and our computer simulation results. We find that the lines connecting the maxima of the response functions converge on approaching the critical point in a single line, the Widom line. We further show that the Widom line coincides with a crossover from a liquid-like to a gas-like behaviour clearly visible in the transport properties. These thermodynamic and dynamic features show that the supercritical state in water is far more complex than what was so far believed, indicating a new perspective in the characterization of the thermodynamics of this state.

Collective excitations in soft-sphere fluids

Despite that the thermodynamic distinction between a liquid and the corresponding gas ceases to exist at the critical point, it has been recently shown that reminiscence of gaslike and liquidlike behavior can be identified in the supercritical fluid region, encoded in the behavior of hypersonic waves dispersion. By using a combination of molecular dynamics simulations and calculations within the approach of generalized collective modes, we provide an accurate determination of the dispersion of longitudinal and transverse collective excitations in soft-sphere fluids. Specifically, we address the decreasing rigidity upon density reduction along an isothermal line, showing that the positive sound dispersion, an excess of sound velocity over the hydrodynamic limit typical for dense liquids, displays a nonmonotonic density dependence strictly correlated to that of thermal diffusivity and kinematic viscosity. This allows rationalizing recent observation parting the supercritical state based on the Widom line, i.e., the extension of the coexistence line. Remarkably, we show here that the extremals of transport properties such as thermal diffusivity and kinematic viscosity provide a robust definition for the boundary between liquidlike and gaslike regions, even in those systems without a liquid-gas binodal line. Finally, we discuss these findings in comparison with recent results for Lennard-Jones model fluid and with the notion of the "rigid-nonrigid" fluid separation lines.

Pair correlation functions of two- and three-dimensional hard-core fluids confined into narrow pores: Exact results from transfer-matrix method

The effect of confinement is studied on the local structure of two- and three-dimensional hard-core fluids. The hard disks are confined between two parallel lines, while the hard spheres are in a cylindrical hard pore. In both cases only nearest neighbour interactions are allowed between the particles. The vertical and longitudinal pair correlation functions are determined by means of the exact transfer-matrix method. The vertical pair correlation function indicates that the wall induced packing constraint gives rise to a zigzag (up-down sequence) shaped close packing structure in both two- and three-dimensional systems. The longitudinal pair correlation function shows that both systems transform continuously from a one-dimensional gas-like behaviour to a zigzag solid-like structure with increasing density.