More Like this

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

   https://doi.org/10.1016/j.fuel.2023.130187  

   Jangale, Prajesh; Hosseini, Ehsan; Zakertabrizi, Mohammad; Jarrahbashi, Dorrin (November 2023, Fuel)

   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. 

   more » « less
2. Asymmetry of the Ferroelectric Phase Transition in BaTiO ₃

   https://doi.org/10.1002/adma.202516507  

   Hershkovitz, Asaf; Hemaprabha, Elangovan; Mandal, Rajesh; Kavle, Pravin; Tanus, Jamil; Barzilay, Maya; Lin, Ching‐Che; Spirito, David; Gorfman, Semën; Wang, Bo; et al (May 2026, Advanced Materials)

   Abstract Symmetry changes during phase transformations fundamentally determine the behavior of thermodynamic systems, governing phenomena as diverse as water evaporation, fermion condensation, and epidemic spreading. Phase transitions are conventionally divided into two classes, and this classification is typically assumed to remain invariant upon reversing the transition. Here, an asymmetric phase transformation is uncovered in the ferroelectric–paraelectric transition of single‐crystal BaTiO₃, a model system for first‐order transitions. Under slow temperature variation (≤0.1 °C min⁻¹⁾, thermodynamic, dielectric, and domain‐structure measurements reveal that the ferroelectric‐to‐paraelectric transition exhibits latent heat, phase coexistence, and a discontinuous order parameter, while these signatures are absent upon cooling. Complementary phase‐field simulations demonstrate similar behavior, attributing it to distinct elastic strain energy accumulation and release during heating and cooling. These findings reveal a first‐order character upon heating but a second‐order‐like behavior upon cooling, challenging the conventional paradigm of symmetric phase‐transition classification and suggesting new possibilities for ferroelectric‐based energy storage. 

   more » « less
3. Atmospheric heat transport is governed by meridional gradients in surface evaporation in modern-day earth-like climates

   https://doi.org/10.1073/pnas.2217202120  

   Fajber, Robert; Donohoe, Aaron; Ragen, Sarah; Armour, Kyle C.; Kushner, Paul J. (June 2023, Proceedings of the National Academy of Sciences)

   Evaporation adds moisture to the atmosphere, while condensation removes it. Condensation also adds thermal energy to the atmosphere, which must be removed from the atmosphere by radiative cooling. As a result of these two processes, there is a net flow of energy driven by surface evaporation adding energy and radiative cooling removing energy from the atmosphere. Here, we calculate the implied heat transport of this process to find the atmospheric heat transport in balance with the surface evaporation. In modern-day Earth-like climates, evaporation varies strongly between the equator and the poles, while the net radiative cooling in the atmosphere is nearly meridionally uniform, and as a consequence, the heat transport governed by evaporation is similar to the total poleward heat transport of the atmosphere. This analysis is free from cancellations between moist and dry static energy transports, which greatly simplifies the interpretation of atmospheric heat transport and its relationship to the diabatic heating and cooling that governs the atmospheric heat transport. We further demonstrate, using a hierarchy of models, that much of the response of atmospheric heat transport to perturbations, including increasing CO 2 concentrations, can be understood from the distribution of evaporation changes. These findings suggest that meridional gradients in surface evaporation govern atmospheric heat transport and its changes. 

   more » « less
4. Monte Carlo simulation method for calculating pore size distributions from high-pressure CO2 adsorption in Micro-Mesoporous Carbons

   https://doi.org/10.1016/j.carbon.202011.059  

   Dantas, Silvio; Cychosz, Katie; Thommes, Matthias; Neimark, Alexander V. (January 2021, Carbon)

   null (Ed.)

   An original methodology is suggested for evaluating the pore size distribution in carbons in the wide range of micro- and mesopores from 0.385 to 10 nm from a single isotherm of high-pressure adsorption of CO2 at 273 K. The proposed method is based on the reference theoretical isotherms calculated by Monte Carlo simulations in model pores of slit and cylindrical geometry. The relationship between the pore size and the pore filling pressure is established. Special attention is given to predicting of the capillary condensation transitions in mesopores by using the meso-canonical ensemble (gauge cell) Monte Carlo simulations. The proposed technique is demonstrated and verified against the conventional N2 and Ar low temperature adsorption methods drawing on the example of micro-mesoporous carbons of the CMK family. Advantages and limitations of CO2 adsorption characterization of nanoporous materials are discussed and further improvements are proposed. 

   more » « less
5. Argon Adsorption on MCM-48 Mesoporous Crystal Studied by In Situ Synchrotron Powder X-ray Diffraction

   https://doi.org/10.1021/acs.jpcc.6c00603  

   Muroyama, Norihiro; Yoshimura, Arisa; Kubota, Yoshiki; Miyasaka, Keiichi; Ravikovitch, Peter I; Neimark, Alexander V; Matsuda, Ryotaro; Higuchi, Masakazu; Kitagawa, Susumu; Takata, Masaki; et al (April 2026, The Journal of Physical Chemistry C)

   The structure of the silica mesoporous crystal MCM-48 and the argon adsorption process were studied by in situ synchrotron powder X-ray diffraction (XRD). The diffraction intensity data were analyzed using the maximum entropy method (MEM). The Ar adsorption process within the mesopores was directly monitored through changes in the electron density distribution in the sample. The diffraction patterns clearly reflected different stages of adsorption as the gas pressure increased: gradual formation of an adsorbed film on the pore walls at low pressures followed by stepwise pore filling due to capillary condensation. The real-space imaging of diffraction data presented in this paper is a robust and promising approach for improving our understanding of gas−solid interactions during adsorption and for characterizing the pore structures of mesoporous crystals. 

   more » « less