Donguk Suh

Nanoparticle growth analysis by molecular dynamics: Spherical Seed

Three-dimensional condensation on a spherical nanoscale seed was simulated by classical molecular dynamics. In order to observe the effects of the dimension of seeds and thermodynamic conditions on the condensation characteristics, initial seed size and system supersaturation ratio were the factors that were examined. At supersaturation ratios above the critical value, two stages of nucleation were found to exist within the system, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the growth and homogeneous nucleation characteristics were each decomposed and analyzed separately. The Yasuoka-Matsumoto method was used to calculate the nucleation and growth rate. The homogeneous nucleation characteristics coincided with the classical nucleation theory. The condensation characteristics, however, showed a discrepancy with the modified classical nucleation theory for completely wetted heterogeneous nucleation, where no supersaturation ratio influence could be observed. The seed size was found to have a reciprocal effect on the growth rate, but showed to be insignificant on the homogeneous nucleation characteristics for this system. The critical nucleus size from kinetic analysis showed a greater difference compared to the first nucleation theorem, classical nucleation theory, or free energy analysis. All in all, the classical nucleation theory showed relatively good agreement compared to previous homogeneous nucleation studies by molecular dynamics, but a modification was found to be necessary when applying to heterogeneous growth of nanoparticles.

Kinetic Analysis on Nanoparticle Condensation by Molecular Dynamics

Condensation on a cubic seed particle was simulated by classical molecular dynamics (MD). Seed size and supersaturation ratio of the system were the factors that were examined in order to observe the effects of the dimension of seeds and thermodynamic conditions. Two stages of nucleation were observed in the phenomenon, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the nucleation rate and growth rate were each calculated by the Yasuoka–Matsumoto (YM) method. As the seed size increased, the growth rate decreased, but there was no clear seed influence on the homogeneous nucleation characteristics. Besides, the classical nucleation theory (CNT), cluster formation free energy and kinetic analysis were conducted. The free energy in the exponential term of the classical nucleation theory and that obtained from the cluster formation free energy showed different characteristics.

Condensation on nanorods by molecular dynamics

Many recent experimental studies have been conducted on constructing nanorods and nanowires to use in a wide range of applications. In this study, molecular dynamics is used to directly examine the condensation rate of nanorods and the results are compared with other basic configurations such as cubes or spheres. According to previous studies conducted by Suh and Yasuoka [J. Phys. Chem. B 115, 10631 (2011); 116, 14637 (2012)], a simple change in the configuration of the seed produces a shape effect, where the curvature of the solid seed surface directly affects the growth generating an orderly difference depending on the curvature. Nanoscale cuboids or nanorods were studied to find an aspect ratio effect when condensation occurs on the surface. Various aspect ratios were examined for different nanorod sizes over a wide range of supersaturation ratios. The results show that the growth rate of the nanorod is independent of the supersaturation ratio, which was also observed for the sphere and cube. The growth rate for the rod fell between those of the cube and the sphere, and this is due to an increase in the surface area of the nanorod compared to the cube and curvature effect in comparison with the sphere. A clear size dependence of the seed was observed, which is also similar to the cube and sphere. Furthermore, no aspect ratio influence was seen for the growth rate. This does not mean that the actual amount of condensation is the same for longer seeds, but rather from the definition of the growth rate, the amount of accumulation per unit area is the same for all seed lengths.

Molecular dynamics simulation of heterogeneous nucleation on nanotubes

Vapour-to-liquid nucleation of argon on silicon nanotubes is studied by means of classical molecular dynamics. The aim of this simulation study is primarily to address the question of whether condensation is faster on the inner surface or on the outer surface of the nanotube. A constant particle number, volume and temperature ensemble was used for the molecular dynamics simulation with the system having different supersaturation ratios, tube lengths, and pore sizes. For a larger pore, the growth rate of droplets was higher on the inner surface, whereas for a smaller pore, a crossover occurred depending on the supersaturation ratio. Pore plugging was strongly affected by the tube diameter, where initial clogging was critical in expediting the filling process inside the tube. Furthermore, in order to examine how the pore existence affects the surrounding vapour, lids on both ends of the tube were placed. In terms of growth, the open-ended tube was typically the slowest, whereas the fully filled cylinder generally gave rise to the highest growth rate.

Heterogeneous nucleation of bubbles by molecular dynamics

A homogeneous liquid system and a heterogeneous system with an impurity inserted inside it were used for investigation of bubble nucleation by molecular dynamics simulation. A constant particle number, volume, and temperature ensemble was used. The systems with the impurities showed an overall increase in bubble formation, which is consistent with previous studies. The shape of the impurities was changed to see if there was any direct influence on the bubble nucleation rate. With the limited number of systems investigated, the occurrence of a shape effect was inconclusive. As observed in previous heterogeneous nucleation studies with walls, the bubble initially forms remotely from the impurity and remains at some distance from the seed.

Nanoscale droplet vaporisation by molecular dynamics

Nanoscale droplet vaporisation was studied by molecular dynamics, which allows the calculation of properties for droplets statistically without considering the discontinuous interface between a liquid droplet and surrounding gas. An argon droplet was created and immersed inside its vapour. After equilibration, the periphery of the system was heated by a carrier gas to vaporise the droplet. Replications were conducted to check the variation in the phenomenon. Thermodynamic properties such as the density, pressure and temperature profiles were sampled for each interval. The evolution of the surface tension of the droplet undergoing vaporisation was investigated. Moreover, the vaporisation rate of nanodroplets was compared with the kinetic theory-based Hertz–Knudsen–Langmuir equation and two diffusion-based models, which are the D2 evaporation law and Kincaid and Longley model [Kincaid DC, Longley TS. A water droplet evaporation and temperature model. Trans ASAE. 1989; 32(2):457–463]. The kinetic model underestimates the vaporisation rate by one order of magnitude whereas the two diffusion-based models overestimate the rate by one order of magnitude.

Molecular dynamics simulation of heterogeneous nucleation on nanorods

Vapor-to-liquid growth on various solid nanoparticle configurations was studied by classical molecular dynamics. Earlier studies have shown a curvature effect to exist in the initial growth process, which increases the growth rate around one order of magnitude between a sphere and cube. In this study, two cuboids in different supersaturation ratios were used to see if the aspect ratio influences growth. Based on the current results, there is no clear evidence that the change in aspect ratio affects the growth of nanoparticles at all even though there is a large difference in the surface areas. No supersaturation ratio effect could be observed, which coincides with previous studies.

Heterogeneous cavitation and crystallisation with an impurity by molecular dynamics

Abstract A homogeneous metastable liquid system and a heterogeneous system with an immersed impurity were used to investigate the cavitation and crystallisation characteristics by molecular dynamics. An isochoric quench was conducted to the systems, which initially produced a cavity, where the entire system subsequently crystallised. The systems with the impurities showed an overall increase in the crystal nucleation rate and a modest but clear shape effect was observed. Similar to previous droplet condensation studies, a clear curvature effect was observed to influence the rate of the phenomenon. At the onset of quenching, crystallisation occurred on the faces of the seed, so a flat surface promoted local crystallisation, which propagated the formation of the cavity, and thereafter increased the density of the metastable liquid, so to advance overall crystallisation within the system.

Molecular Dynamics Simulation of Ice Crystal Growth Inhibition by Hexadecyl-trimethyl-ammonium Bromide

Recent experiments have found hexadecyl-trimethyl-ammonium bromide (CTAB) to have superior ice nucleation inhibition properties [ J. Phys. Chem. B 121, 6580]. The mechanism of how the inhibition takes place remains unclear. Therefore, molecular dynamics was used to simulate ice crystallization of a water/CTAB/ice system. The ice crystallization rate for a pure water system was compared for the basal [0001], first prism [101̅0], and secondary prism plane [112̅0], where the basal plane grew the slowest followed by the first prism plane. When CTAB was added to the ice-liquid water system, crystallization was clearly impeded. Even when ice starts growing away from the CTAB molecule, the hydrophilic head would at some point protrude and get caught in the water/ice interface. Once the head of the CTAB was encapsulated in the advancing interface, the hydrophobic body would wriggle around and disrupt the formation of hydrogen bond networks that are essential for ice growth. When the interface clears the length of the body of the CTAB molecule, ice crystallization resumes at its normal pace. In summary, the inhibition of ice growth is a combination of the hydrophilic head acting as an anchor and the dynamic motion of the hydrophobic tail hindering stable hydrogen bonding for ice growth.

Kinetic analysis of homogeneous droplet nucleation using large-scale molecular dynamics simulations

Studies on homogeneous nucleation have been conducted for decades, but a large gap between experiment and theory persists when evaluating the nucleation rate because the classical nucleation theory (CNT) with all its modifications still cannot fully incorporate the kinetics of homogeneous nucleation. Recent large-scale molecular dynamics (MD) simulations on homogeneous nucleation estimated a nucleation rate around the same order of magnitude as that obtained in experiments. This immensely improved agreement between experiment and theory is exciting because MD can provide detailed information on molecular trajectories. Therefore, a better understanding of the kinetics of homogeneous nucleation can now be obtained. In this study, large-scale MD simulations on homogeneous nucleation were performed. Through kinetic analysis of the simulation results, the nucleation rate, free energy barrier, and critical cluster size were found. Although the nucleation rates directly obtained from the simulations differed from those calculated from the CNT by 8-13 orders of magnitude, when the parameters calculated from the molecular trajectories were substituted into the classical theory, the discrepancy between the nucleation rates decreased to within an order of magnitude. This proves that the fundamental formulation of the theoretical equation is physically sound. We also calculated the cluster formation free energy and confirmed that the free energy barrier decreases with increasing supersaturation ratio. The estimated barrier height was twice that determined by theory, whereas the critical cluster size showed very good agreement between simulation and theory.