ORCID Profile
0000-0001-7333-4923
Current Organisation
University of Helsinki
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Publisher: Copernicus GmbH
Date: 06-2023
Abstract: Abstract. Kinetics of collision-sticking processes between vapor molecules and clusters of low-volatility compounds govern the initial steps of atmospheric new particle formation. Conventional non-interacting hard-sphere models underestimate the collision rate by neglecting long-range attractive forces, and the commonly adopted assumption that every collision leads to the formation of a stable cluster (unit mass accommodation coefficient) is questionable for small clusters, especially at elevated temperatures. Here, we present a generally applicable analytical interacting hard-sphere model for evaluating collision rates between molecules and clusters, accounting for long-range attractive forces. In the model, the collision cross section is calculated based on an effective molecule–cluster potential, derived using Hamaker's approach. Applied to collisions of sulfuric acid or dimethylamine with neutral bisulfate–dimethylammonium clusters composed of 1–32 dimers, our new model predicts collision rates 2–3 times higher than the non-interacting model for small clusters, while decaying asymptotically to the non-interacting limit as cluster size increases, in excellent agreement with a collision-rate-theory atomistic molecular dynamics simulation approach. Additionally, we calculated sticking rates and mass accommodation coefficients (MACs) using atomistic molecular dynamics collision simulations. For sulfuric acid, a MAC ≈1 is observed for collisions with all cluster sizes at temperatures between 200 and 400 K. For dimethylamine, we find that MACs decrease with increasing temperature and decreasing cluster size. At low temperatures, the MAC ≈1 assumption is generally valid, but at elevated temperatures MACs can drop below 0.2 for small clusters.
Publisher: AIP Publishing
Date: 15-05-2023
DOI: 10.1063/5.0148013
Abstract: Molecular dynamics (MD) simulations of gas-phase chemical reactions are typically carried out on a small number of molecules near thermal equilibrium by means of various thermostatting algorithms. Correct equipartitioning of kinetic energy among translations, rotations, and vibrations of the simulated reactants is critical for many processes occurring in the gas phase. As thermalizing collisions are infrequent in gas-phase simulations, the thermostat has to efficiently reach equipartitioning in the system during equilibration and maintain it throughout the actual simulation. Furthermore, in non-equilibrium simulations where heat is released locally, the action of the thermostat should not lead to unphysical changes in the overall dynamics of the system. Here, we explore issues related to both obtaining and maintaining thermal equilibrium in MD simulations of an exemplary ion–molecule dimerization reaction. We first compare the efficiency of global (Nosé–Hoover and Canonical S ling through Velocity Rescaling) and local (Langevin) thermostats for equilibrating a system of flexible compounds and find that of these three only the Langevin thermostat achieves equipartition in a reasonable simulation time. We then study the effect of the unphysical removal of latent heat released during simulations involving multiple dimerization events. As the Langevin thermostat does not produce the correct dynamics in the free molecular regime, we only consider the commonly used Nosé–Hoover thermostat, which is shown to effectively cool down the reactants, leading to an overestimation of the dimerization rate. Our findings underscore the importance of thermostatting for the proper thermal initialization of gas-phase systems and the consequences of global thermostatting in non-equilibrium simulations.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0NR02043E
Abstract: In this study, we have investigated the influence of the tip on the three-dimensional scanning force microscopy (3D-SFM) images of calcite–water interfaces by experiments and simulations.
Publisher: Springer International Publishing
Date: 2015
Publisher: Springer Science and Business Media LLC
Date: 24-11-2017
DOI: 10.1038/S41598-017-16460-9
Abstract: Mechanical properties of nanoscale objects can be measured with an atomic force microscope (AFM) tip. However, the continuum models typically used to relate the force measured at a certain indentation depth to quantities such as the elastic modulus, may not be valid at such small scales, where the details of atomistic processes need to be taken into account. On the other hand, molecular dynamics (MD) simulations of nanoindentation, which can offer understanding at an atomistic level, are often performed on systems much smaller than the ones studied experimentally. Here, we present large scale MD simulations of the nanoindentation of single crystal and penta-twinned gold nanorod s les on a silicon substrate, with a spherical diamond AFM tip apex. Both the s le and tip sizes and geometries match commercially available products, potentially linking simulation and experiment. Different deformation mechanisms, involving the creation, migration and annihilation of dislocations are observed depending on the nanorod crystallographic structure and orientation. Using the Oliver-Pharr method, the Young’s moduli of the (100) terminated and (110) terminated single crystal nanorods, and the penta-twinned nanorod, have been determined to be 103 ± 2, 140 ± 4 and 108 ± 2 GPa, respectively, which is in good agreement with bending experiments performed on nanowires.
Publisher: IOP Publishing
Date: 23-01-2017
Publisher: American Chemical Society (ACS)
Date: 18-12-2013
DOI: 10.1021/CT3008342
Abstract: High resolution atomic force microscopy (AFM) in liquids offers atomic scale insight into the structure at water/solid interfaces and is perhaps the only tool capable of resolving the nature of formed hydration layers. However, convolution between the imaging signal and the tip/surface interactions and hydration layers means that interpretation is far from straightforward. Modeling the complex imaging mechanism of atomic force microscopy in liquids requires calculation of the free energy profile as a function of the distance between AFM tip and surface. Its derivative is the best approximation for the force acting on the AFM tip, including entropic contributions from interactions with water molecules in hydration layers over the surface and around the tip apex. In order to establish a reliable approach for these simulations, we compare two methods of calculating free energy profiles from atomistic molecular dynamics simulations, umbrella s ling and free energy perturbation, on two model surfaces, calcium fluoride and calcium carbonate. Our results demonstrate that both methods effectively provide equivalent free energy profiles but offer different possibilities in terms of efficiency, constraints, and analysis of the free energy components.
Publisher: Copernicus GmbH
Date: 11-06-2019
DOI: 10.5194/ACP-2019-400
Abstract: Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical non-interacting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We have studied the statistics of collisions of sulfuric acid molecules in vacuum by atomistic molecular dynamics simulations. We have found that the effective collision cross section of the H2SO4 molecule, as described by an OPLS-All Atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor 2.2, compared to kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, fitted to the attractive part of the atomistic intermolecular potential of mean force.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-7758
Abstract: & & Small particles of silver iodide (AgI) are known to have excellent ice nucleating capabilities and have been used in rain seeding applications. It is widely believed that the silver terminated (0001) surface of & #946 -AgI acts as a template for the basal plane of hexagonal ice. However, the (0001) surface of ionic crystals with the wurtzite structure is polar and will therefore exhibit reconstructions and defects. Here, we use atomistic molecular dynamics simulations to study how the presence of defects on AgI(0001) affects the rates and mechanism of heterogeneous ice nucleation at moderate supercooling at -10 & #186 C. We first consider AgI(0001) surfaces exhibiting vacancies, step edges, terraces, and pits, and compare them to simulations of the corresponding ideal surface. We find that, while point defects have no significant effect on ice nucleation rates, step edges, terraces, and pits reduce both the nucleation and growth rates by up to an order of magnitude, which can be understood from the atomistic details extracted from the simulations. The reduction of the ice nucleation rate correlates well with the fraction of the surface area around the defects where perturbations of the hydration layer hinder the formation of a critical ice nucleus.& Finally, we consider more realistic AgI(0001) surfaces with 5x5 surface reconstructions that cancel the surface dipole, and report on their ice nucleating abilities.& &
Publisher: IOP Publishing
Date: 28-11-2014
Publisher: American Chemical Society (ACS)
Date: 12-04-2021
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-14945
Abstract: Collisions between ions and dipolar molecules can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models or simple parametrizations have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners.& & & & & & & & To determine the accuracy and applicability of these theoretical approaches, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectory simulations, s ling a relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters.& & & & & & & & We find excellent agreement between the collision cross sections and rate coefficients obtained from the molecular dynamics trajectory simulations and the central field model. Therefore, we conclude that the effective interactions between the collision partners are highly isotropic, and the central field model is able to capture the relevant physicochemical properties of the system.& & & & & & & & Comparing the molecular dynamics trajectory simulations with the often-used parametrization by Su and Chesnavich (1982), we find that the latter can predict the collision rate coefficient quite well for systems with a molecular dipole, but the agreement worsens for systems with a dipolar cluster.& & & & & & & & Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.& & & & & & & & We are currently using the combination of potential of mean force calculation and central field model, as well as the atomistic trajectory simulations, to understand the relatively large increase in rate coefficient observed in chemical ionization mass spectrometers when sulfuric acid is charged with acetate, as compared to nitrate (Fomete, 2022).& Su, T. and Chesnavich, W. J.: J. Chem. Phys. 76, 5183& #8211 , 1982.Fomete. S. et al.: J. Phys. Chem. A 126, 44, 8240& #8211 , 2022.
Publisher: Informa UK Limited
Date: 20-02-2009
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D0CP05653G
Abstract: Molecular dynamics simulations reveal the structural and energetic properties of carbon dioxide clusters nucleating in the gas phase at extreme undercooling.
Publisher: American Chemical Society (ACS)
Date: 26-10-2022
Publisher: Springer Science and Business Media LLC
Date: 09-2013
DOI: 10.1007/S40544-013-0021-3
Abstract: Even though the slipperiness of ice is important both technologically and environmentally and often experienced in everyday life, the nanoscale processes determining ice friction are still unclear. We study the friction of a smooth ice-ice interface using atomistic simulations, and especially consider the effects of temperature, load, and sliding velocity. At this scale, frictional behavior is seen to be determined by the lubricating effect of a liquid premelt layer between the sliding ice sheets. In general, increasing temperature or load leads to a thicker lubricating layer and lower friction, while increasing the sliding velocity increases friction due to viscous shear.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-18959
Abstract: & & & span& Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical noninteracting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. & /span& We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS) all-atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid.& span& & As a consequence, the actual collision coefficient is enhanced by a factor of 2.2 at 300& #8201 K compared with kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard-sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, based on the attractive part of the atomistic intermolecular potential of mean force.& /span& & &
Publisher: Copernicus GmbH
Date: 15-02-2022
Abstract: Abstract. Ice clouds can form at low and moderate supercooling through heterogeneous ice nucleation on atmospheric particles. Typically, the nucleation requires active sites with special chemical and physical properties, including surface topology and roughness. This paper investigates microscopic mechanisms of how combinations of confinement by the surface topology and lattice match induced by the surface properties can lead to enhanced ice nucleation. We perform molecular dynamics simulations using both atomistic and coarse-grained water models, at very low supercooling, to extensively study heterogeneous ice nucleation in slit-like and concave wedge structures of silver-terminated silver iodide (0001) surfaces. We find that ice nucleation is greatly enhanced by slit-like structures when the gap width is a near-integer multiple of the thickness of an ice bilayer. For wedge systems we also do not find a simple linear dependence between ice nucleation activity and the opening angle. Instead we observe strong enhancement in concave wedge systems with angles that match the orientations of ice lattice planes, highlighting the importance of structural matching for ice nucleation in confined geometries. While in the slit systems ice cannot grow out of the slit, some wedge systems show that ice readily grows out of the wedge. In addition, some wedge systems stabilize ice structures when heating the system above the thermodynamics melting point. In the context of atmospheric ice nucleating particles, our results strongly support the experimental evidence for the importance of surface features such as cracks or pits functioning as active sites for ice nucleation at low supercooling.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-12521
Abstract: Nucleation of clusters from the gas phase is a widely encountered phenomenon, e.g. regional air quality and global climate are both directly impacted by particle formation from atmospheric trace gases [1]. Still, the underlying out-of-equilibrium dynamics of this process are not well understood. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, as in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial.To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics (MD) simulations of a supersaturated Lennard-Jones (LJ) vapor in the presence of thermalizing carrier gas. In addition, a previous study of homogeneous nucleation of carbon dioxide in argon carrier gas [2] was revisited for temperature analysis of the growing CO2 & clusters. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and the classical nonisothermal theory by Feder et al. [3], which also predicts the existence of cool subcritical clusters, and has been quite controversial.For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are indeed elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes. This resolves the apparent contradiction between elevated cluster temperatures and minor nonisothermal corrections to the nucleation rate, both often reported in literature, and is in excellent agreement with the theory of Feder et al. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.& References[1] M. Kulmala et al., Direct observations of atmospheric aerosol nucleation. Science 339, 943& #8211 (2013).[2] R. Halonen et al., Homogeneous nucleation of carbon dioxide in supersonic nozzles II: Molecular dynamics simulations and properties of nucleating clusters. Phys. Chem. Chem. Phys. 23, 4517& #8211 (2021).[3] J. Feder, K. C. Russell, J. Lothe, G. M. Pound, Homogeneous nucleation and growth of droplets in vapours. Adv. Phys. 15, 111& #8211 (1966).
Publisher: American Physical Society (APS)
Date: 09-10-2015
Publisher: Copernicus GmbH
Date: 09-2022
DOI: 10.5194/ACP-22-11155-2022
Abstract: Abstract. Ion–dipole collisions can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate the rate coefficients of ion–dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches under atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, s ling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles and charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physicochemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich (1982), which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and a central field model as a viable and elegant alternative to the brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-19008
Abstract: & & Understanding the formation of ice is of great importance to many fields of science. Sufficiently pure water droplets can remain in the supercooled liquid phase to nearly -40 & #186 C. Crystallization of ice in the atmosphere therefore typically occurs in the presence of ice nucleating particles (INPs), such as mineral dust or organic particles. These can trigger heterogeneous ice nucleation at clearly higher temperatures. Therefore, a better understanding of how the various types of aerosol particles present in the atmosphere affect ice nucleation (IN) in clouds would be an important advance in the field of atmospheric science.& & & & Experiments have shown in great detail what is the IN activity of different types of compounds, and recently also clarified the importance of small surface features such as surface defects, which function as active sites for ice nucleation. On most mineral dust particles, there may be only a few active sites for ice nucleation, typically around defects or pits (Holden et al., 2019). Simulations also showed enhanced ice nucleation efficiency in confined geometry such as wedges or pits (Bi, Cao and Li, 2017).& & & & We systematically study the effect of water confining defects with different surface geometries pyramidal pits, wedge-shaped cracks and slits with water confined between two parallel walls, using molecular dynamics simulations with both all-atom and monatomic water models, and show that that these defects enhance ice nucleation both at large supercooling and at very low supercooling.& & & & Results of simulations on pyramidal pits on Si (100) surfaces, realizable experimentally, show a clear (& #8710 T & 10 & #186 C) enhancement of ice nucleation compared to the very weakly IN active flat Si (100) or Si (111) surfaces. To show that water confinement can enhance IN also at very low supercooling, at temperatures above & #8722 & #186 C, we constructed wedge shaped structures with & #946 -AgI (0001) surface as one of the two side walls, and slit systems by positioning two & #946 -AgI (0001) slabs to mirror each other to cancel the dipole field from the polar surfaces. Depending on the wedge angle or the relation of the width of the gap between two slabs in the slit systems with the thickness of ice bilayers, ice nucleation can be clearly enhanced or hindered. We also clarify the different mechanisms behind IN enhancement at different geometries.& & & & Understanding the enhanced activity at surface features may enable characterization of ice nucleation active sites on some atmospheric particles, creation of IN active sites at otherwise poorly active materials such as silicon, and also enable enhancing very active IN materials such as AgI, to nucleate ice at nearly zero supercooling.& & & & This work was supported by the Academy of Finland Center of Excellence programme (grant no. 307331) and ARKTIKO project 285067 ICINA, by University of Helsinki, Faculty of Science ATMATH project, by the National Center for Meteorology (NCM), Abu Dhabi, UAE, under the UAE Research Program for Rain Enhancement Science, as well as ERC Grant 692891-DAMOCLES. Supercomputing resources were provided by CSC& #8211 IT Center for Science, Ltd, Finland.& &
Publisher: Elsevier BV
Date: 11-2015
Publisher: American Physical Society (APS)
Date: 13-03-2018
Publisher: American Chemical Society (ACS)
Date: 10-05-2021
Publisher: Royal Society of Chemistry (RSC)
Date: 2014
DOI: 10.1039/C4CP02375G
Abstract: We have used ab initio molecular dynamics (AIMD) and Kinetic Monte Carlo simulations to study the interaction of water with the NaCl surface.
Publisher: Copernicus GmbH
Date: 25-01-2023
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-16164
Abstract: & & Understanding the way in which ice forms is of great importance to many fields of science.& Pure water droplets in the atmosphere can remain in the liquid phase to nearly -40& #186 C. Crystallization of ice in the atmosphere therefore typically occurs in the presence of ice nucleating particles (INPs), such as mineral dust or organic particles, which trigger heterogeneous ice nucleation at clearly higher temperatures. The growth of ice is accompanied by a significant release of latent heat of fusion, which causes supercooled liquid droplets to freeze in two stages [Pruppacher and Klett, 1997].& & & & & & & & & We are studying these topics by utilizing the monatomic water model [Molinero and Moore, 2009] for unbiased molecular dynamics (MD) simulations, where different surfaces immersed in water are cooled below the melting point over tens of nanoseconds of simulation time and crystallization is followed.& & & & & & & & & With a combination of finite difference calculations and novel moving-thermostat molecular dynamics simulations we show that the release of latent heat from ice growth has a noticeable effect on both the ice growth rate and the initial structure of the forming ice. However, latent heat is& found not to be as critically important in controlling immersion nucleation as it is in vapor-to-liquid nucleation [Tanaka et al.2017].& & & & & & & & & This work was supported by the ERC Grant 692891-DAMOCLES, the Academy of Finland Flagship funding (grant no. 337549), and the University of Helsinki, Faculty of Science ATMATH project. Supercomputing resources were provided by CSC& #8211 IT Center for Science, Ltd., Finland.& & & & & & & & & REFERENCES& & & & & & & & & Pruppacher, H. R. and J. D. Klett (1997). Microphysics of Clouds and Precipitation. Vol. 18. Kluwer Academic.& & & & Molinero, V. and E. B. Moore (2009). J. Phys. Chem. B 113, 4008.& & & & Tanaka, K. K et al. (2017). Phys. Rev. E 96, 022804.& &
Publisher: American Chemical Society (ACS)
Date: 11-12-2019
Publisher: Proceedings of the National Academy of Sciences
Date: 05-07-2022
Abstract: Nucleation of clusters from the gas phase is a widely encountered phenomenon, yet rather little is understood about the underlying out-of-equilibrium dynamics of this process. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial. To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics simulations of a supersaturated vapor in the presence of thermalizing carrier gas. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and classical nonisothermal theory. For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes and resolve the apparent contradictions regarding cluster temperatures. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15132
Abstract: & & The condensation of carbon dioxide (CO2) is a topic of general interest in view of global decarbonization targets, e.g. in low-temperature CO2 capture technologies promoting the phase transition of CO2 gas is the crucial step. Homogeneous nucleation of a mixture of CO2 and argon gas in a supersonic nozzle has been studied at temperatures from 78 to 92 K, and CO2 partial pressures between 70 and 800 Pa. The consistency between the current data and measurements at higher temperature suggests the critical clusters remain liquid-like even at these low temperatures.& & & & Here we present large-scale atomistic molecular dynamics (MD) simulations of homogenous CO2 nucleation from the vapour phase at temperatures from 75 to 105 K. The MD approach is an unbiased method to study the nucleation process, including the phase and structure of even the smallest clusters. We used argon carrier gas as a heat bath for the CO2 molecules to avoid unphysical removal of latent heat.& & & & Simulations confirm that despite strong undercooling, nucleation proceeds through liquid-like clusters. Also, by applying standard steady-state cluster growth kinetics, we are able to calculate the cluster formation free energies from the MD simulations. The results suggest a curvature correction to the classical liquid drop model used in the classical nucleation theory. The correction depends only on the bulk liquid properties, and hence the simulation-based correction can be applied to predict the nucleation rates of real CO2.& & & & The simulation-based theory is able to capture the magnitude and the temperature-dependency of the nucleation rate rather well, whereas both standard CNT and its self-consistent version (SCNT) underestimate the rate by several orders of magnitude. Here we have corrected the theoretical values with the non-isothermal factor, which is about 0.01-0.1 for the studied system.& &
Publisher: Copernicus GmbH
Date: 25-01-2023
DOI: 10.5194/EGUSPHERE-2022-1449
Abstract: Abstract. Kinetics of collision-sticking processes between vapor molecules and clusters of low volatile compounds govern the initial steps of atmospheric new particle formation. Conventional non-interacting hard-sphere models underestimate the collision rate by neglecting long-range attractive forces, and the commonly adopted assumption that every collision leads to the formation of a stable cluster (unit mass accommodation coefficient) is questionable for small clusters, especially at elevated temperatures. Here, we present a generally applicable analytical interacting hard-sphere model for evaluating collision rates between molecules and clusters, accounting for long-range attractive forces. In the model, the collision cross section is calculated based on an effective molecule-cluster potential, derived using Hamaker's approach. Applied to collisions of sulfuric acid or dimethylamine with neutral bisulphate-dimethylammonium clusters composed of 1–32 dimers, our new model predicts collision rates 2–3 times higher than the non-interacting model for small clusters, while decaying asymptotically to the non-interacting limit as cluster size increases, in excellent agreement with a collision rate theory-atomistic molecular dynamics simulation approach. Additionally, we calculated sticking rates and mass accommodation coefficients (MAC) using atomistic molecular dynamics collision simulations. For sulfuric acid, unit MAC is observed for collisions with all cluster sizes at temperatures between 200 K and 400 K. For dimethylamine, we find that MACs decrease with increasing temperature and decreasing cluster size. At low temperatures, the unit MAC assumption is generally valid, but at elevated temperatures MACs can drop below 0.2 for small clusters.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0CP02279A
Abstract: Small angle X-ray scattering and pressure measurements yield quantitative homogeneous nucleation rates for CO 2 near 80 K.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-6193
Abstract: Kinetics of collision-sticking processes between vapor molecules and clusters of low volatile compounds govern the initial steps of atmospheric new particle formation. Conventional non-interacting hard-sphere models underestimate the collision rate by neglecting long-range attractive forces, and the commonly adopted assumption that every collision leads to the formation of a stable cluster (unit mass accommodation coefficient) is questionable for small clusters, especially at elevated temperatures. Here, we present a generally applicable analytical interacting hard-sphere model for evaluating collision rates between molecules and clusters, accounting for long-range attractive forces. In the model, the collision cross section is calculated based on an effective molecule-cluster potential, derived using Hamaker& #8217 s approach. Applied to collisions of sulfuric acid or dimethylamine with neutral bisulphate-dimethylammonium clusters composed of 1-32 dimers, our new model predicts collision rates 2-3 times higher than the non-interacting model for small clusters, while decaying asymptotically to the non-interacting limit as cluster size increases, in excellent agreement with a collision rate theory-atomistic molecular dynamics simulation approach. Additionally, we calculated sticking rates and mass accommodation coefficients (MAC) using atomistic molecular dynamics collision simulations. For sulfuric acid, unit MAC is observed for collisions with all cluster sizes at temperatures between 200 K and 400 K. For dimethylamine, we find that MACs decrease with increasing temperature and decreasing cluster size. At low temperatures, the unit MAC assumption is generally valid, but at elevated temperatures MACs can drop below 0.2 for small clusters.
Publisher: American Physical Society (APS)
Date: 23-11-2016
Publisher: IOP Publishing
Date: 09-09-2016
DOI: 10.1088/0957-4484/27/41/415709
Abstract: Frequency modulation atomic force microscopy (FM-AFM) experiments were performed on the calcite (10[Formula: see text]4) surface in pure water, and a detailed analysis was made of the 2D images at a variety of frequency setpoints. We observed eight different contrast patterns that reproducibly appeared in different experiments and with different measurement parameters. We then performed systematic free energy calculations of the same system using atomistic molecular dynamics to obtain an effective force field for the tip-surface interaction. By using this force field in a virtual AFM simulation we found that each experimental contrast could be reproduced in our simulations by changing the setpoint, regardless of the experimental parameters. This approach offers a generic method for understanding the wide variety of contrast patterns seen on the calcite surface in water, and is generally applicable to AFM imaging in liquids.
Publisher: American Chemical Society (ACS)
Date: 27-05-2019
Publisher: AIP Publishing
Date: 16-04-2013
DOI: 10.1063/1.4800770
Abstract: The critical quantity in understanding imaging using an atomic force microscope is the force the s le exerts on the tip. We put forward a simple one-to-one force to water density relationship, explain exactly how it occurs, and in which circumstances it holds. We argue that two wide classes of atomic force microscope (AFM) tip should lead to at least qualitative agreement with our model and represent a significant fraction of AFM tips as currently prepared. This connection between the short-range force and the unperturbed equilibrium water density removes the need to perform simulations for each tip location, conservatively speeding up simulations by around three orders of magnitude compared to current methods that explicitly calculate the force on a tip model at each point in space.
Publisher: Copernicus GmbH
Date: 08-08-2022
DOI: 10.5194/ACP-22-10099-2022
Abstract: Abstract. Ice clouds can form at low and moderate supercooling through heterogeneous ice nucleation on atmospheric particles. Typically, the nucleation requires active sites with special chemical and physical properties, including surface topology and roughness. This paper investigates microscopic mechanisms of how combinations of confinement by the surface topology and lattice match induced by the surface properties can lead to enhanced ice nucleation. We perform molecular dynamics simulations using both atomistic and coarse-grained water models, at very low supercooling, to extensively study heterogeneous ice nucleation in slit-like and concave wedge structures of silver-terminated silver iodide (0001) surfaces. We find that ice nucleation is greatly enhanced by slit-like structures when the gap width is a near-integer multiple of the thickness of an ice bilayer. For wedge systems we also do not find a simple linear dependence between ice nucleation activity and the opening angle. Instead we observe strong enhancement in concave wedge systems with angles that match the orientations of ice lattice planes, highlighting the importance of structural matching for ice nucleation in confined geometries. While in the slit systems ice cannot grow out of the slit, some wedge systems show that ice readily grows out of the wedge. In addition, some wedge systems stabilize ice structures when heating the system above the thermodynamics melting point. In the context of atmospheric ice-nucleating particles, our results strongly support the experimental evidence for the importance of surface features such as cracks or pits functioning as active sites for ice nucleation at low supercooling.
Publisher: Copernicus GmbH
Date: 16-05-2022
DOI: 10.5194/EGUSPHERE-2022-93
Abstract: Abstract. Ion-dipole collisions can facilitate the formation of atmospheric aerosol particles, and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches at atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, s ling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physico-chemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich [J. Chem. Phys., 76, 5183–5185, 1982], which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.
Publisher: Copernicus GmbH
Date: 30-10-2019
DOI: 10.5194/ACP-19-13355-2019
Abstract: Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical noninteracting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS). OPLS all-atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor of 2.2 at 300 K compared with kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard-sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, based on the attractive part of the atomistic intermolecular potential of mean force.
Start Date: 2014
End Date: 2017
Funder: Australian Research Council
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