Nucleation of liquid bridges and bubbles in nanoscale capillaries (original) (raw)
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The birth of a bubble: A molecular simulation study
The Journal of Chemical Physics, 2005
We study the nucleation of a bubble in a metastable Lennard-Jones ͑LJ͒ fluid, confined to a spherical pore with wetting walls, by a combination of grand canonical, canonical ensemble, and gauge cell Monte Carlo simulation methods complemented by the Voronoi-Delaunay tessellation analysis of statistical geometry of intermolecular cavities. We construct the isotherm of confined fluid in the form of a continuous van der Waals' loop, in which the unstable backward trajectory between the spinodals corresponds to bubble states. We show that as the degree of metastability increases and the fluid becomes progressively stretched, the decrease of fluid density is associated with the evolution of a population of interstitial intermolecular cavities. At the spinodal, the fluid becomes mechanically unstable: Interstitial cavities partly coalesce into a larger cavity located due to the system symmetry around the pore center. This cavity represents a bubble embryo, which grows at the expense of interstitial cavities. The nucleation barrier is calculated by direct thermodynamic integration along the isotherm. We compare our simulation results to the predictions of the classical nucleation theory and experiments on capillary condensation-evaporation of nitrogen in pores of hybrid organic-inorganic mesoporous molecular sieve HMM-3.
The Journal of Chemical Physics, 2006
Computer simulations are employed to obtain subcritical isotherms of small finite sized systems inside the coexistence region. For all temperatures considered, ranging from the triple point up to the critical point, the isotherms gradually developed a sequence of sharp discontinuities as the system size increased from ϳ8 to ϳ21 molecular diameters. For the smallest system sizes, and more so close to the critical point, the isotherms appeared smooth, resembling the continuous van der Waals loop obtained from extrapolation of an analytic equation of state outside the coexistence region. As the system size was increased, isotherms in the chemical potential-density plane developed first two, then four, and finally six discontinuities. Visual inspection of selected snapshots revealed that the observed discontinuities are related to structural transitions between droplets ͑on the vapor side͒ and bubbles ͑on the liquid side͒ of spherical, cylindrical, and tetragonal shapes. A capillary drop model was developed to qualitatively rationalize these observations. Analytic results were obtained and found to be in full agreement with the computer simulation results. The analysis shows that the shape of the subcritical isotherms is dictated by a single characteristic volume ͑or length scale͒, which depends on the surface tension, compressibility, and coexistence densities. For small reduced system volumes, the model predicts that a homogeneous fluid is stable across the whole coexistence region, thus explaining the continuous van der Waals isotherms observed in the simulations. When the liquid and vapor free energies are described by means of an accurate mean-field equation of state and surface tensions from simulation are employed, the capillary model is found to describe the simulated isotherms accurately, especially for large systems ͑i.e., larger than about 15 molecular diameters͒ at low temperature ͑lower than about 0.85 times the critical temperature͒. This implies that the Laplace pressure differences can be predicted for drops as small as five molecular diameters, and as few as about 500 molecules. The theoretical study also shows that the extrema or apparent spinodal points of the finite size loops are more closely related to ͑finite system size͒ bubble and dew points than to classical spinodals. Our results are of relevance to phase transitions in nanopores and show that first order corrections to nucleation energies in finite closed systems are power laws of the inverse volume.
The Journal of Chemical Physics, 2010
The free energy of forming a droplet and a bubble with a given particle number n and volume v within the pure-component Lennard-Jones supercooled vapor and superheated liquid, respectively, are further explored using density-functional theory. Similar to what was found previously ͓M. J. Uline and D. S. Corti, Phys. Rev. Lett. 99, 076102 ͑2007͒; M. J. Uline and D. S. Corti, J. Chem. Phys. 129, 234507 ͑2008͔͒, the limits of stability again appear within both free energy surfaces evaluated at two other metastability conditions, one closer to the binodal and one closer to the spinodal. Furthermore, an ad hoc bond connectivity criterion is also applied in an attempt, however approximately, to eliminate certain configurational redundancies that arise from the chosen droplet and bubble definitions. What results are free energy surfaces describing the formation of equilibrium embryos that should be an improved representation of the fluctuations that are relevant to those nonequilibrium embryos seen in an actual nucleation event. Finally, we discuss in some detail the use of the ͑n , v͒ reaction coordinate within the framework of an equilibrium-based theory and its relation to other descriptions of nucleation.
Homogeneous Bubble Nucleation Driven by Local Hot Spots: A Molecular Dynamics Study
The Journal of Physical …, 2008
We report a Molecular Dynamics study of homogenous bubble nucleation in a Lennard-Jones fluid. The rate of bubble nucleation is estimated using forward-flux sampling (FFS). We find that cavitation starts with compact bubbles rather than with ramified structures as had been suggested by Shen and Debenedetti ( J. Chem. Phys. 111, 3581 (1999)). Our estimate of the bubblenucleation rate is higher than predicted on the basis of Classical Nucleation Theory (CNT). Our simulations show that local temperature fluctuations correlate strongly with subsequent bubble formation -this mechanism is not taken into account in CNT.
Comparison of heterogeneous and homogeneous bubble nucleation using molecular simulations
Physical Review B, 2007
NPT and NP zz T molecular dynamics simulations of Lennard-Jones atoms were used to compare homogeneous and heterogeneous nucleation. In the heterogeneous cases, the attraction between the fluid and a smooth fcc ͑100͒ surface was varied. Multiple simulations were used to determine nucleation times from which nucleation rates were estimated using a transient nucleation model. Calculations demonstrated a clear enhancement in nucleation rates in the heterogeneous cases compared to the homogeneous case. To obtain homogeneous nucleation rates similar to the heterogeneous cases required temperatures about 10 K higher. It was also found that void formation was favored as the attraction between the liquid and solid was decreased. Varying the system size, thermostatting method, and barostat time constant affected quantitative results, but not the qualitative trends.
The Journal of Physical Chemistry B, 2005
We report a comprehensive Monte Carlo (MC) simulation study of the vapor-to-droplet transition in Lennard-Jones fluid confined to a spherical container with repulsive walls, which is a case study system to investigate homogeneous nucleation. The focus is made on the application of a modified version of the ghost field method (Vishnyakov, A.; Neimark, A. V. J. Chem. Phys. 2003, 119, 9755) to calculate the nucleation barrier. This method allows one to build up a continuous trajectory of equilibrium states stabilized by the ghost field potential, which connects a reference droplet with a reference vapor state. Two computation schemes are employed for free energy calculations, direct thermodynamic integration along the constructed trajectory and umbrella sampling. The nucleation barriers and the size dependence of the surface tension are reported for droplets containing from 260 to 2000 molecules. The MC simulation study is complemented by a review of the simulation methods applied to computing the nucleation barriers and a detailed analysis of the vapor-todroplet transition by means of the classical nucleation theory.
Molecular Dynamics Simulations of Bubble Formation in Nanochannels
Volume 11: Micro and Nano Systems, Parts A and B, 2007
Thermodynamics and kinetics of nano-scale bubble formation in liquid metals such as Li and Pb were studied by molecular dynamics (MD) simulations at pressures typical for magnetic and inertial fusion. Two different approaches to bubble formation were developed. In one method, radial densities, pressures, surface tensions, and work functions of the cavities in supercooled liquid lithium were calculated and compared with the surface tension experimental data. The critical radius of a stable cavity in liquid lithium was found for the first time. In the second method, the cavities were created in the highly stretched region of the liquid phase diagram; and then the stability boundary and the cavitation rates were calculated in liquid lead. The pressure dependences of cavitation frequencies were obtained over the temperature range 700-2700ºK in liquid Pb. The results of MD calculations for cavitation rate were compared with estimates of classical nucleation theory (CNT).
Molecular dynamics simulation of vapour-liquid nucleation of water with constant energy
EPJ Web of Conferences, 2015
The paper describes molecular dynamics study of nucleation of water in NVE ensemble. The numerical simulation was performed with the DL_POLY. The metastable steam consisting of 10976 water molecules with TIP4P/2005 potential was driven on the desired energy level by a simulation at constant temperature, and then the nucleation at constant energy was studied for several tens of nanoseconds, which was sufficient for clusters to evolve at hundred molecules size. The results were compared with the previously published results and the classical nucleation theory predictions.