Molecular Dynamics Simulations Of Nucleation Research Articles

A novel constant pressure control method for nucleation boiling of CO2/lubricant in molecular dynamics simulation

Conventional molecular dynamics simulations of nucleation boiling typically employ the fixed volume method, resulting in a non-constant pressure during the boiling process. In this paper, we propose an additional pressure control method, placing a cover plate at the top of the liquid phase to achieve liquid phase pressure control during boiling. The novel method yields a pressure control error within ±5 %. CO2 and CO2/PEC4 lubricating oil mixture are used to investigate the differences in heat transfer characteristics between non-constant and constant pressure nucleation boiling. Furthermore, the effect of PEC4 lubricating oil on CO2 nucleation boiling under a fixed pressure condition is explored. The results show that both the bubble nucleation and growth time of CO2 and the CO2/PEC4 mixture are prolonged under constant pressure. This causes a compression in CO2 molecular spacing, thereby reducing CO2 diffusion and mass transfer capabilities. Heat transfer from the solid to the liquid, and within the liquid itself, is diminished, while the distribution of PEC4 molecules within the liquid phase becomes more even. The migration rate of PEC4 molecules in the liquid phase slows down. Under the constant pressure model, PEC4 increases the mass transfer resistance of CO2. As the lubricant mass fraction rises, The nucleation and growth rate of CO2 bubbles decline. In comparison to CO2, PEC4 molecules exhibit a stronger interaction energy with the Cu wall surface. During boiling, some PEC4 molecules accumulate on the Cu surface, forming an oil film that impedes the direct contact between CO2 molecules and the Cu wall.

Enabling molecular dynamics simulations of helium bubble formation in tritium-containing austenitic stainless steels: An Fe-Ni-Cr-H-He potential

Formation of helium bubbles impacts mechanical properties of materials used in nuclear applications. An Fe-Ni-Cr-H-He quinary potential capable of direct molecular dynamics simulations of nucleation and growth of helium bubbles from randomly-born helium interstitial atoms has been developed. This is accomplished by incorporating helium into an existing Fe-Ni-Cr-H quaternary potential while addressing three challenging paradoxes characterizing helium in austenitic stainless steels: (a) interstitial He atoms form tightly bound dimers and larger clusters in the lattice but He atoms are only bound by weak van de Waals forces in the pure gas phase, (b) He atoms diffuse readily in host metals yet significantly distort the lattice causing large volume expansions, and (c) He atoms prefer tetrahedral interstitial sites as opposed to the larger octahedral sites despite large repulsive interactions with metal atoms within the lattice. We demonstrate that our potential reproduces density functional theory results on important properties relevant to helium bubble nucleation and growth. In addition to molecular statics validation of static properties, molecular dynamics simulation tests establish that our potential leads to the nucleation of helium bubbles from an initial random distribution of He interstitial atoms while at the same time capturing the equation of state in the pure He phase.

Routes to cubic ice through heterogeneous nucleation

The freezing of water into ice is one of the most important processes in the physical sciences. However, it is still not understood at the molecular level. In particular, the crystallization of cubic ice ([Formula: see text])-rather than the traditional hexagonal polytype ([Formula: see text])-has become an increasingly debated topic. Although evidence for [Formula: see text] is thought to date back almost 400 y, it is only in the last year that pure [Formula: see text] has been made in the laboratory, and these processes involved high-pressure ice phases. Since this demonstrates that pure [Formula: see text] can form, the question naturally arises if [Formula: see text] can be made from liquid water. With this in mind, we have performed a high-throughput computational screening study involving molecular dynamics simulations of nucleation on over 1,100 model substrates. From these simulations, we find that 1) many different substrates can promote the formation of pristine [Formula: see text]; 2) [Formula: see text] can be selectively nucleated for even the mildest supercooling; 3) the water contact layer's resemblance to a face of ice is the key factor determining the polytype selectivity and nucleation temperature, independent of which polytype is promoted; and 4) substrate lattice match to ice is not indicative of the polytype obtained. Through this study, we have deepened understanding of the interplay of heterogeneous nucleation and ice I polytypism and suggest routes to [Formula: see text] More broadly, the substrate design methodology presented here combined with the insight gained can be used to understand and control polymorphism and stacking disorder in materials in general.

Molecular Dynamics Simulation of Nucleation from Undercooled Melt of Nickel–Aluminum Alloy and Discussion on Polymorphism in Nucleation

Molecular Dynamics Simulation of Nucleation from Undercooled Melt of Nickel–Aluminum Alloy and Discussion on Polymorphism in Nucleation

Temperature dependence of the solid-liquid interface free energy of Ni and Al from molecular dynamics simulation of nucleation.

The temperature dependence of the solid-liquid interfacial free energy, γ, is investigated for Al and Ni at the undercooled temperature regime based on a recently developed persistent-embryo method. The atomistic description of the nucleus shape is obtained from molecular dynamics simulations. The computed γ shows a linear dependence on the temperature. The values of γ extrapolated to the melting temperature agree well with previous data obtained by the capillary fluctuation method. Using the temperature dependence of γ, we estimate the nucleation free energy barrier in a wide temperature range from the classical nucleation theory. The obtained data agree very well with the results from the brute-force molecular dynamics simulations.

Direct Molecular Dynamics Simulation of Nucleation during Supersonic Expansion of Gas to a Vacuum.

We develop a methodology for direct molecular-level simulation of adiabatic expansion of gas through a small orifice to a vacuum. The gas attains supersonic speeds, cools, and nucleates. The proposed approach combines equations of frictionless fluid dynamics with molecular dynamics simulation in an expanding periodic box. There are two key components of the proposed algorithm: (i) a time-reversible integrator tailored to an expanding system, and (ii) an iterative procedure employed to satisfy the condition of steady flow. For a conical nozzle (opening angle of 60°), the simulations with argon and water vapor predict cluster sizes in agreement with the experiment. Clusters of irregular shapes observed in the experiment [J. Lengyel et al. Phys. Rev. Lett. 2014, 112, 113401] are not reproduced. The role of friction, turbulence, and sonic boom originating at the sharp nozzle edge is discussed.

Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics Simulation.

Nucleation of salt crystals in confined media occurs in many processes of high importance, such as injection of CO2 in geological formations for its sequestration. In particular, salt precipitation in clays, a main component of sedimentary rock, is an important phenomenon. The crystals precipitate on the pores' surface, modify the pore space morphology, and reduce its flow and transport properties. Despite numerous efforts to understand the mechanisms of nucleation of salt crystals in confined media, the effect of the clay's chemistry on the growth, distribution, and properties of the crystals is not well understood. We report the results of extensive molecular dynamics simulation of nucleation and growth of NaCl crystals in a clay pore using molecular models of two types of clay minerals, Na-montmorillonite and kaolinite. Clear evidence is presented for the nucleation of the salt crystals that indicates that the molecular structure of clay minerals affects their spatial distribution, although the nucleation mechanism is the same in both types of clays.

Molecular Dynamics Simulation of Nucleation in CO2

Molecular Dynamics Simulation of Nucleation in CO₂

Submicrometer-scale molecular dynamics simulation of nucleation and solidification from undercooled melt: Linkage between empirical interpretation and atomistic nature

Nucleation and solidification from the undercooled iron melt are investigated from the atomistic point of view by large-scale molecular dynamics (MD) simulations up to 12 million atoms in systems of the submicrometer-scale. There exist some amount of atoms with icosahedral configuration in the undercooled iron melt and these atoms increase with decreasing temperature. It is expected that accumulation of atoms with icosahedral configuration in the initial β-relaxation regime of nucleation is the key to initiate the formation of bcc phase. On the other hand, mobility of atoms in the undercooled melt decreases drastically with decreasing temperature. These two competing factors in the atomistic scale are considered to derive a critical temperature at which nucleation rate becomes maximum, which agrees with a classical theory for homogeneous nucleation. Moreover, the Avrami exponents during solidification are approximately estimated to be close to 3 and 4 in two- and three-dimensional grain growths, respectively, which also agrees with empirical interpretation. Our novel approach utilizing the high parallel efficiency of the GPU supercomputer successfully links the empirical interpretation in metallurgy with the atomistic behavior of nucleation and solidification, which enlarges the application range of MD simulations for the study of structural materials.

Molecular dynamics simulations of nucleation from vapor to solid composed of Lennard-Jones molecules

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10(4)-10(5) Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S(0) ≃ 10-10(8)) and temperature (kT = 0.2-0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.

Cluster sizes in direct and indirect molecular dynamics simulations of nucleation

We performed molecular dynamics simulations of a Lennard-Jones fluid, and compared the sizes of critical clusters in direct simulations of a nucleation event in vapor phase with the sizes of clusters in stable equilibrium with the surrounding vapor. By applying different cluster criteria it is shown that both the critical clusters and the equilibrium clusters have dense cores of similar size but the critical clusters have more outlying cluster atoms surrounding this core. The cluster definition introduced by ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)], where each cluster atom must have at least five neighboring atoms within the distance of 1.5 times the Lennard-Jones length parameter, agrees well with the cluster size obtained from classical nucleation theory, and we find this agreement to be independent of temperature. The cluster size obtained from the observed nucleation rates by the first nucleation theorem is larger than the classical estimate and much smaller than the size given by the density profile of the equilibrium cluster.

Evaluating nucleation rates in direct simulations

We compare different methods for obtaining nucleation rates from molecular dynamics simulations of nucleation, using the condensation of Lennard-Jones argon as an example. All methods yield the same nucleation rate at the conditions where they can be applied correctly, with discrepancies smaller than a factor of 2. We critically examine the different approaches and highlight their respective strengths and possible limitations.

Molecular dynamics simulation of nucleation of KBr clusters confined within armchair single-walled carbon nanotubes

We carry out molecular dynamics (MD) simulation studies on the phase transition and nucleation of (KBr) N clusters confined within armchair single-walled carbon nanotubes ((R,R)-SWNTs) with Born–Mayer–Huggins’ potential functions for the ion–ion interactions and simple Lennard-Jones potentials for the ion–carbon interactions. A novel crystallite of KBr cluster inside (R,R)-SWNT is observed during slow cooling or quenching runs. Some important thermodynamic and dynamic parameters are estimated. The effect of (R,R)-SWNT on nucleation and phase transition of KBr clusters is analyzed and discussed.

Kinetic Reconstruction of the Free-Energy Landscape

We present a new general method to trace back a lot of valuable information from direct simulations and experiments of activated processes. In particular, it allows the reconstruction of the free-energy landscape for an arbitrary reaction coordinate directly from the out-of-equilibrium dynamics of the process. We demonstrate the power of this concept by its application to a molecular dynamics simulation of nucleation of a Lennard-Jones vapor. The same method can be also applied to Brownian dynamics and stochastic simulations.

Molecular-dynamics simulations of nucleation and crystallization processes of lasercrystallized poly-Si

The nucleation and crystallization processes of excimer-laser annealed Si on aSiO2 substrate for complete melting conditions have been investigated by using molecular-dynamicssimulations. In the early stage of nucleation, the preferential growth of nuclei with a{111} face normal to the surface was originated from the{111} twin boundaries with a low surface energy. The partial rotation of the dimer leads to the growth of{111}-oriented nuclei along twins that have different stacking sequences. Therecombination of vacancies and dimers at the solidification front is directly related to{111} growth from the twin boundaries.

323 Ar流体の構造とダイナミックスに関する分子動力学的研究(T06-1 マイクロ・ナノ熱流体システム(1) モデル,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

323 Ar流体の構造とダイナミックスに関する分子動力学的研究(T06-1 マイクロ・ナノ熱流体システム(1) モデル,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Comparative study on methodology in molecular dynamics simulation of nucleation

Gas-liquid nucleation of 1000 Lennard-Jones atoms is simulated to evaluate temperature regulation methods and methods to obtain nucleation rate. The Berendsen and the Andersen thermostats are compared. The Berendsen thermostat is unable to control the temperature of clusters larger than the critical size. Independent of the thermostating method the velocities of individual atoms and the translational velocities of clusters up to at least six atoms are accurately described by the Maxwell velocity distribution. Simulations with the Andersen thermostat yield about two times higher nucleation rates than those with the Berendsen thermostat. Nucleation rate is extracted from the simulations by direct observation of times of nucleation onset and by the method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)]. Compared to the direct observation, the nucleation rates obtained from the method of Yasuoka and Matsumoto are higher by a factor of 3.

1429 過飽和水蒸気中の核生成に関する分子動力学的研究(S38-1 ナノフルイズ,S38 マイクロ・ナノフルイズ)

1429 過飽和水蒸気中の核生成に関する分子動力学的研究(S38-1 ナノフルイズ,S38 マイクロ・ナノフルイズ)

Molecular-dynamics simulations of nucleation and crystallization in supercooled liquid silicon: Temperature-gradient effects

We have investigated atomistic processes of nucleation and crystallization in supercooled liquid silicon (Si) based on molecular-dynamics (MD) simulations using the Tersoff potential. MD cells composed of up to 8192 Si atoms were heated to produce melted Si, and then melted Si was quenched under various supercooled conditions with or without a temperature gradient and the corresponding nucleation processes were visualized. The critical nucleation radius was determined as a function of the amount of supercooling $\ensuremath{\Delta}T$ and it was found to be inversely proportional to $\ensuremath{\Delta}T.$ It was also found, in the case of supercooling under a linear temperature gradient, that nucleation first occurred at the lower-temperature region and then crystallization proceeded toward the high-temperature region with the (111) surface mostly parallel to the temperature gradient.

Large scale molecular dynamics simulation of nucleation in supercooled NaCl

Nucleation of NaCl was reproduced by using molecular dynamics simulation. We carried out MD simulations in the 13 824 ion system under the free boundary condition. Critical nucleus size, nucleation time lag, and nucleation rate were directly estimated from simulation results without using nucleation theory. We also carried out MD simulations in the 13 824 and 125 000 ion systems under the periodic boundary condition to compare the results with those of the free boundary condition. A single crystal and a polycrystal were formed in our simulation. We investigated the difference of the nucleation process of a single crystal and a polycrystal. All simulations were calculated by using the special purpose computer, MDGRAPE-2, for molecular dynamics simulation.