Molecular Dynamics Simulations of Homogeneous Nucleation

Nucleation is the first step in a phase transition where small nuclei of the new phase start appearing in the metastable old phase, such as the appearance of small liquid clusters in a supersaturated vapor. Nucleation is important in various industrial and natural processes, including atmospheric new particle formation: between 20 % to 80 % of atmospheric particle concentration is due to nucleation. These atmospheric aerosol particles have a significant effect both on climate and human health. Different simulation methods are often applied when studying things that are difficult or even impossible to measure, or when trying to distinguish between the merits of various theoretical approaches. Such simulation methods include, among others, molecular dynamics and Monte Carlo simulations. In this work molecular dynamics simulations of the homogeneous nucleation of Lennard-Jones argon have been performed. Homogeneous means that the nucleation does not occur on a pre-existing surface. The simulations include runs where the starting configuration is a supersaturated vapor and the nucleation event is observed during the simulation (direct simulations), as well as simulations of a cluster in equilibrium with a surrounding vapor (indirect simulations). The latter type are a necessity when the conditions prevent the occurrence of a nucleation event in a reasonable timeframe in the direct simulations. The effect of various temperature control schemes on the nucleation rate (the rate of appearance of clusters that are equally able to grow to macroscopic sizes and to evaporate) was studied and found to be relatively small. The method to extract the nucleation rate was also found to be of minor importance. The cluster sizes from direct and indirect simulations were used in conjunction with the nucleation theorem to calculate formation free energies for the clusters in the indirect simulations. The results agreed with density functional theory, but were higher than values from Monte Carlo simulations. The formation energies were also used to calculate surface tension for the clusters. The sizes of the clusters in the direct and indirect simulations were compared, showing that the direct simulation clusters have more atoms between the liquid-like core of the cluster and the surrounding vapor. Finally, the performance of various nucleation theories in predicting simulated nucleation rates was investigated, and the results among other things highlighted once again the inadequacy of the classical nucleation theory that is commonly employed in nucleation studies.Nukleaatio on olomuodonmuutoksen ensimmäinen askel, jossa metastabiiliin vanhaan faasiin ilmaantuu pieniä uuden faasin ytimiä, kuten esimerkiksi pienten nestemäisten hiukkasryppäiden ilmaantuessa ylikylläiseen höyryyn. Nukleaatio on eräs tärkeimmistä ilmakehän aerosolien muodostumismekanismeista: 20 % - 80 % ilmakehän hiukkaspitoisuudesta on peräisin nukleaatiosta. Ilmakehän aerosolit puolestaan vaikuttavat maapallon säteilytasapainoon ja sitä kautta myös ilmastoon, ja koostumuksesta riippuen niillä voi olla kielteisiä terveysvaikutuksia. Kokeellisen ja teoreettisen nukleaatiotutkimuksen lisäksi ja niitä täydentämään käytetään usein erilaisia simulaatiomenetelmiä. Tässä työssä on molekyylidynamiikkaa käyttäen simuloitu homogeenista nukleaatiota, eli nukleaatiota joka tapahtuu suoraan höyrystä tiivistymällä eikä tiivistymällä valmiille pinnalle. Molekyylidynamiikkasimulaatioiden perusajatuksena on laskea tutkittavan systeemin aikakehitys ratkaisemalla numeerisesti hiukkasten liikeyhtälöt. Hiukkaset vuorovaikuttavat ennalta määrätyn potentiaalin mukaisesti, ja tässä työssä potentiaali on ollut yleisesti käytetty Lennard-Jones -potentiaali argonin parametreilla. Simulaatioihin sisältyy sekä suoria nukleaatiosimulaatioita, joissa lähtötilanteena on ylikylläinen höyry ja nukleaatiotapahtuma havaitaan simulaation kuluessa, että simulaatioita, joissa hiukkasrypäs on tasapainossa ympäröivän höyryn kanssa. Jälkimmäiseen menetelmään joudutaan turvautumaan höyryn kyllästyssuhteen ollessa niin alhainen, että suorissa simulaatioissa nukleaatiota ei tapahdu mielekkäässä ajassa. Työssä tutkittiin pienten hiukkasryppäiden ominaisuuksia ja sitä kautta nukleaatioteorioihin sisältyvien oletusten pätevyyttä (mm. hiukkasryppäiden pintajännityksen kokoriippuvuutta) ja eri teorioiden menestystä simulaatiotulosten ennustamisessa. Yleisesti käytetty klassinen nukleaatioteoria menestyi vertailluista teorioista huonoiten. Lisäksi tutkittiin eräiden simulaatioteknisten seikkojen, kuten lämpötilan säätelyyn käytetyn menetelmän, vaikutusta saatuihin tuloksiin

Mass Accommodation of Water: Bridging the Gap Between Molecular Dynamics Simulations and Kinetic Condensation Models

The condensational growth of submicrometer aerosol particles to climate relevant sizes is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. We have simulated the mass accommodation and evaporation processes of water using molecular dynamics, and the results are compared to the condensation equations derived from the kinetic gas theory to shed light on the compatibility of the two. Molecular dynamics simulations were performed for a planar TIP4P-Ew water surface at four temperatures in the range 268–300 K as well as two droplets, with radii of 1.92 and 4.14 nm at T = 273.15 K. The evaporation flux from molecular dynamics was found to be in good qualitative agreement with that predicted by the simple kinetic condensation equations. Water droplet growth was also modeled with the kinetic multilayer model KM-GAP of Shiraiwa et al. [ Atmos. Chem. Phys. 2012, 12, 2777]. It was found that, due to the fast transport across the interface, the growth of a pure water droplet is controlled by gas phase diffusion. These facts indicate that the simple kinetic treatment is sufficient in describing pure water condensation and evaporation. The droplet size was found to have minimal effect on the value of the mass accommodation coefficient. The mass accommodation coefficient was found to be unity (within 0.004) for all studied surfaces, which is in agreement with previous simulation work. Additionally, the simulated evaporation fluxes imply that the evaporation coefficient is also unity. Comparing the evaporation rates of the mass accommodation and evaporation simulations indicated that the high collision flux, corresponding to high supersaturation, present in typical molecular dynamics mass accommodation simulations can under certain conditions lead to an increase in the evaporation rate. Consequently, in such situations the mass accommodation coefficient can be overestimated, but in the present cases the corrected values were still close to unity with the lowest value at ≈0.99

In-beam spectroscopic study of 244Cf

The ground-state rotational band of the neutron-deficient californium (Z = 98) isotope 244Cf was identified for the first time and measured up to a tentative spin and parity of I = 20+. The observation of the rotational band indicates that the nucleus is deformed. The kinematic and dynamic moments of inertia were deduced from the measured gamma-ray transition energies. The behavior of the dynamic moment of inertia revealed an up-bend due to a possible alignment of coupled nucleons in high-j orbitals starting at a rotational frequency of about hw = 0.20 MeV. The results were compared with the systematic behavior of the even-even N = 146 isotones as well as with available theoretical calculations that have been performed for nuclei in the region

Octupole correlations in the structure of O2 bands in the N=88 nuclei150Sm Gd

Knowledge of the exact microscopic structure of the 01 + ground state and first excited 02 + state in 150Sm is required to understand the branching of double β decay to these states from 150Nd. The detailed spectroscopy of 150Sm and 152Gd has been studied using (α,xn) reactions and the γ -ray arrays AFRODITE and JUROGAM II. Consistently strong E1 transitions are observed between the excited Kπ = 02 + bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited Kπ = 02 + band and also in terms of the “tidal wave” model of Frauendorf.Web of Scienc

Implementation of state-of-the-art ternary new-particle formation scheme to the regional chemical transport model PMCAMx-UF in Europe

The particle formation scheme within PMCAMx-UF, a three-dimensional chemical transport model, was updated with particle formation rates for the ternary H2SO4-NH3-H2O pathway simulated by the Atmospheric Cluster Dynamics Code (ACDC) using quantum chemical input data. The model was applied over Europe for May 2008, during which the EUCAARI-LONGREX (European Aerosol Cloud Climate and Air Quality Interactions-Long-Range Experiment) campaign was carried out, providing aircraft vertical profiles of aerosol number concentrations. The updated model reproduces the observed number concentrations of particles larger than 4 nm within 1 order of magnitude throughout the atmospheric column. This agreement is encouraging considering the fact that no semi-empirical fitting was needed to obtain realistic particle formation rates. The cloud adjustment scheme for modifying the photolysis rate profiles within PMCAMx-UF was also updated with the TUV (Tropospheric Ultraviolet and Visible) radiative-transfer model. Results show that, although the effect of the new cloud adjustment scheme on total number concentrations is small, enhanced new-particle formation is predicted near cloudy regions. This is due to the enhanced radiation above and in the vicinity of the clouds, which in turn leads to higher production of sulfuric acid. The sensitivity of the results to including emissions from natural sources is also discussed.Peer reviewe

Lifetime measurements of excited states in ¹⁶³W and the implications for the anomalous B(E2) ratios in transitional nuclei

This letter reports lifetime measurements of excited states in the odd-N nucleus 163W using the recoil-distance Doppler shift method to probe the core polarising effect of the i13/2 neutron orbital on the underlying soft triaxial even-even core. The ratio B(E2:21/2⁺ → 17/2⁺)/B(E2:17/2⁺ → 13/2⁺) is consistent with the predictions of the collective rotational model. The deduced B(E2) values provide insights into the validity of collective model predictions for heavy transitional nuclei and a geometric origin for the anomalous B(E2) ratios observed in nearby even-even nuclei is proposed

Unityn Tilemap-ominaisuuden tutkiminen ja käyttö

Työssä tarkasteltiin Unity 2017.2f-versiossa tullutta Tilemap-ominaisuutta, sen käyttöä ja muokkaamista. Työn tavoitteena oli saada parempi kokonaiskuva Tilemap-ominaisuuden käytöstä ja muokkaamisesta käytännössä. Työllä ei ollut tilaajaa, vaan sen tarkoituksena oli kasvattaa omaa tietämystä Tilemap-ominaisuuden käytöstä. Työn teoriaosuudessa tarkasteltiin 2D-pelitasojen tekoon aikaisemmin käytettäviä menetelmiä, jonka jälkeen tutkittiin Unityn Tilemap-ominaisuuksia yksittäisinä osinaan. Teoriaosuus viimeisteltiin tutkimalla Unityn tekemiä ilmaiseksi saatavilla olevia automaatioratkaisuja. Lopuksi työssä tehtiin testiprojekti, jonka pelitasojen rakennuksessa käytettiin Unityn Tilemap-ominaisuutta, Unityn sille tekemiä valmiita automaatioratkaisuja sekä omaa kustomoitua sivellintä. Työssä huomattiin, että Tilemap-ominaisuutta käyttämällä 2D-pelintekijät voivat tehdä nopeasti yksinkertaisia ja modulaarisesti rakentuvia kenttiä. Tilemap-ominaisuuden jatkaminen omalla kustomoidulla siveltimellä sekä Unityn luomilla automaatioilla on kannattavaa, koska sillä voidaan merkittävästi vähentää kehitykseen käytettävää aikaa