Computational investigation of structure, dynamics and nucleation kinetics of a family of modified Stillinger-Weber model fluids in bulk and free-standing thin films

In recent decades, computer simulations have found increasingly widespread use as powerful tools of studying phase transitions in wide variety of systems. In the particular and very important case of aqueous systems, the commonly used force-fields tend to offer quite different predictions with respect to a wide range of thermodynamic and kinetic properties, including the ease of ice nucleation, the propensity to freeze at a vapor-liquid interface, and the existence of a liquid-liquid phase transition. It is thus of fundamental and practical interest to understand how different features of a given water model affect its thermodynamic and kinetic properties. In this work, we use the forward-flux sampling technique to study the crystallization kinetics of a family of modified Stillinger-Weber (SW) potentials with energy ($\epsilon$) and length ($\sigma$) scales taken from the monoatomic water (mW) model, but with different tetrahedrality parameters ($\lambda$). By increasing $\lambda$ from 21 to 24, we observe the nucleation rate increases by 48 orders of magnitude at a supercooling of ${\zeta}=T/T_m=0.845$. Using classical nucleation theory, we are able to demonstrate that this change can largely be accounted for by the increase in $|\Delta\mu|$, the thermodynamic driving force. We also perform rate calculations in freestanding thin films of the supercooled liquid, and observe a crossover from a surface-enhanced crystallization at $\lambda = 21$ to a bulk-dominated crystallization for $\lambda\ge22$.Comment: 10 pages, 9 figures, five table

Static and dynamic correlation lengths in supercooled polymers

A key point to understand the glass transition is the relationship between structural and dynamic behavior experienced by a glass former when it approaches Tg. In this work, the relaxation in a simple bead-spring polymer system in the supercooled regime near its glass transition temperature was investigated with molecular dynamic simulations. We develop a new manner to look at the dynamic length scales in a supercooled polymeric system, focusing on correlated motion of particles in an isoconfigurational ensemble (that is, associated with the structure), as measured by Pearson’s correlation coefficient. We found that while the usual dynamic four-point correlation length deviates from the structural (mosaic or point-to-set) length scale at low temperatures, Pearson’s length behaves similarly to the static length in the whole temperature range. The results lead to a consensus of similar scaling of structural and dynamical length scales, reinforcing the idea of the theories of Adam-Gibbs and random first order transition.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Gianetti, Melisa Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Soulé, Ezequiel Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Nucleation and crystallization kinetics of tetrahedral liquids

El objetivo principal de esta tesis consistió en el estudio de la cinética de nucleación y crecimiento de agua y líquidos similares a esta en cuanto a su estructura tetraédrica, mediante simulaciones computacionales y experimentos. Para los estudios computacionales se utilizó un método de muestreo avanzado denominado Forward- flux sampling (FFS), que permite el estudio de eventos raros como la nucleación. Para este estudio se utilizó el modelo de agua (mW), basado en el modelo de Stillinger y Weber (SW) y reparametrizado por Molinero y Moore (J. Chem. Phys. B (2009) 113, 4008). Este modelo cuenta con un término de interacción de tres cuerpos que depende de un parámetro de tetraedricidad (λ). En este trabajo se analizó la dependencia de la cinética de nucleación con λ en sistemas bulk, películas delgadas con interfase líquido-vapor y películas delgadas en contacto con superficies de variada hidro-filicidad. En sistemas bulk se observó que la cinética de nucleación está principalmente determinada por la dependencia de la diferencia del potencial químico entre las fases líquida y sólida. En interfases líquido-vapor se observó que el efecto de la interfase en la cinética de nucleación depende de λ, siendo la velocidad de nucleación mayor para películas delgadas que para sistemas bulk para λ = 21, observándose lo opuesto para λ ≥ 22. La presencia de una superficie acelera la cinética de nucleación, siendo esta influencia mucho mayor que el efecto de la estructuración del líquido, dado por λ. Además, se estudió la cinética de crecimiento de hielo sobre mica por Microscopía de Fuerza Atómica (AFM). Estos experimentos, permitieron estimar la velocidad de crecimiento entre 2,5 y 5,0 K de sobreenfriamiento mediante el análisis de curvas de fuerza. Los resultados obtenidos indican que la velocidad de crecimiento de hielo en este intervalo de temperaturas y HR de alrededor de 120% es de 26±12 nm•min^1. La importancia de estos resultados radica en que el presente trabajo es el primero en el que la técnica de AFM es utilizada para estudiar la cinética de crecimiento de hielo sobre superficies.The main idea of this thesis was the study of nucleation and crystallization kinetics of water and liquids with tetrahedral structure through simulations and experiments. The advanced sampling technique known as Forward-flux sampling (FFS), was employed to study nucleation. FFS is indicated to study rare events like nucleation. The monoatomic model of water mW, based on the Stillinger-Weber potential and reparametrized by Molinero and Moore (J. Chem. Phys. B (2009) 113, 4008) was used. The three-body interaction term of this potential contains a tetrahedral parameter (λ). In this work the dependence of the nucleation kinetics with λ was investigated. Bulk, thin films with liquid-vapor interface and thin films in contact with surfaces of varied hydrophilicity were analyzed. Nucleation kinetics in bulk is principally described by the chemical potential difference between the liquid and solid phases. For thin films, the nucleation rate has a strong dependence with λ. A crossover between the surface-dominated freezing at λ = 21, and bulk-dominated freezing at λ ≥ 22 was observed. Surfaces enhance the nucleation rate, and this enhancement is higher than the tetrahedrality effect. Force curves taken with an AFM were used to estimate ice crystallization rates over mica at 2.5 and 5.0 K of supercooling. For this supercooling and RH of 120 %, the ice growth rate is 26±12 nm•min^1. This is, to our knowledge, the first ice crystallization rate determined with AFM for this system.Fil: Gianetti, Melisa Mariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

The quasi-liquid layer of ice revisited : the role of temperature gradients and tip chemistry in AFM studies

In this work, we present new results of atomic force microscopy (AFM) force curves over pure ice at different temperatures, performed with two different environmental chambers and different kinds of AFM tips. Our results provide insight to resolve the controversy on the interpretation of experimental AFM curves on the ice\u2013air interface for determining the thickness of the quasi-liquid layer (QLL). The use of a Mini Environmental Chamber (mEC) that provides an accurate control of the temperature and humidity of the gases in contact with the sample allowed us for the first time to get force curves over the ice\u2013air interface without jump-in (jump of the tip onto the ice surface, widely observed in previous studies). These results suggest a QLL thickness below 1 nm within the explored temperature range ( 127 to 122\u25e6C). This upper bound is significantly lower than most of the previous AFM results, which suggests that previous authors overestimate the equilibrium QLL thickness, due to temperature gradients, or indentation of ice during the jump-in. Additionally, we proved that the hydrophobicity of AFM tips affects significantly the results of the experiments. Over-all, this work shows that, if one chooses the experimental conditions properly, the QLL thicknesses obtained by AFM lie over the lower bound of the highly disperse results re-ported in the literature. This allows estimating upper boundaries for the QLL thicknesses, which is relevant to validate QLL theories and to improve multi phase atmospheric chemistry models

Un "visual" sugli IDN

Available in italianIl piano di comunicazione del Registro .it per il biennio 2011/2012 prevede la prosecuzione delle attivit? del precedente piano, con azioni che abbiano come obiettivo la promozione dell\u27immagine, della visibilit?, della conoscenza del Registro e la diffusione della cultura di internet. Rispetto al precedente piano, si ? cercato di introdurre strumenti nuovi, in grado di veicolare una comunicazione rapida e di impatto, che fosse per? sempre coerente con l\u27immagine coordinata e con il ruolo istituzionale del Registro. E\u27 nata quindi l\u27idea di realizzare un "visual" di grafica animata su un argomento prettamente tecnico: l\u27introduzione dei caratteri speciali nei nomi a dominio, conosciuti con l\u27acronimo Idn (internationalized domain name)

Un web cartoon per il Registro

Available in italianLe avventure del Prof. Itt?-I virus" ? un web cartoon interamente progettato e realizzato all\u27interno dell\u27unit? Relazioni Esterne, Media e Comunicazione del Registro. Il cartone animato, creato per il portale Innovascuola.gov.it, ? stato poi pubblicato sul sito web del Registro e proiettato nel corso di fiere ed eventi. Il target ? rappresentato da bambini in et? scolare (6-10 anni) e ha per argomento la sicurezza informatica, in particolare i virus. Nella presente pubblicazione sono descritti gli obiettivi del progetto, le principali fasi di lavoro, alcune specifiche tecniche e le prospettive future, tra le quali la realizzazione di altri episodi sul tema della navigazione sicura

Development and Validation of a CFD Combustion Model for Natural Gas Engines Operating with Different Piston Bowls

Nowadays, an accurate and precise description of the combustion phase is essential in spark-ignition (SI) engines to drastically reduce pollutant and greenhouse gas (GHG) emissions and increase thermal efficiency. To this end, computational fluid dynamics (CFD) can be used to study the different phenomena involved, such as the ignition of the charge, combustion development, and pollutant formation. In this work, a validation of a CFD methodology based on the flame area model (FAM) was carried out to model the combustion process in light-duty SI engines fueled with natural gas. A simplified spherical kernel approach was used to model the ignition phase, whereas turbulent flame propagation was described through two variables. A zero-dimensional evolution of the flame kernel radius was used in combination with the Herweg and Maly formulation to take the laminar-to-turbulent flame transition into account. To estimate the chemical composition of burnt gas, two different approaches were considered—one was based on tabulated kinetics, and the other was based on chemical equilibrium. Assessment of the combustion model was first performed by using different operating points of a light-duty SI engine fueled with natural gas and by using the original piston. The results were validated by using experimental data on the in-cylinder pressure, apparent heat release rate, and pollutant emissions. Afterward, two other different piston bowl geometries were investigated to study the main differences between one solution and the others. The results showed that no important improvements in terms of combustion efficiency were obtained by using the new piston bowl shapes, which was mainly due to the very low ((Formula presented.)) or null increase in turbulent kinetic energy during the compression stroke and due to the higher heat losses ((Formula presented.)) associated with the increased surface area of the new piston geometries

The Quasi-Liquid Layer of ice revisited: the role of temperature gradients and tip chemistry in AFM studies

In this work, we present new results of atomic force microscopy (AFM) force curves over pure ice at different temperatures, performed with two different environmental chambers and different kinds of AFM tips. Our results provide insight to resolve the controversy on the interpretation of experimental AFM curves on the ice–air interface for determining the thickness of the quasi-liquid layer (QLL). The use of a Mini Environmental Chamber (mEC) that provides an accurate control of the temperature and humidity of the gases in contact with the sample allowed us for the first time to get force curves over the ice–air interface without jump-in (jump of the tip onto the ice surface, widely observed in previous studies). These results suggest a QLL thickness below 1 nm within the explored temperature range (−7 to −2 ∘C). This upper bound is significantly lower than most of the previous AFM results, which suggests that previous authors overestimate the equilibrium QLL thickness, due to temperature gradients, or indentation of ice during the jump-in. Additionally, we proved that the hydrophobicity of AFM tips affects significantly the results of the experiments. Overall, this work shows that, if one chooses the experimental conditions properly, the QLL thicknesses obtained by AFM lie over the lower bound of the highly disperse results reported in the literature. This allows estimating upper boundaries for the QLL thicknesses, which is relevant to validate QLL theories and to improve multiphase atmospheric chemistry models.Fil: Gelman Constantin, Julián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; ArgentinaFil: Gianetti, Melisa Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Longinotti, María Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Corti, Horacio Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentin

Environmental chamber with controlled temperature and relative humidity for ice crystallization kinetic measurements by atomic force microscopy

The present work describes the development of an environmental chamber (EC), with temperature and humidity control, for measuring ice growth kinetics over a substrate with an atomic force microscope (AFM). The main component of the EC is an AFM fluid glass cell. The relative humidity (RH) inside the EC is set by the flow of a controlled ratio of dry and humid nitrogen gases. The sample temperature is fixed with an AFM commercial accessory, while the temperature of the nitrogen gas inside the EC is controlled by circulating cold nitrogen vapor through a copper cooler, specially designed for this purpose. With this setup, we could study the growth rate of ice crystallization over a mica substrate by measuring the force exerted between the tip and the sample when they approach each other as a function of time. This experimental development represents a significant improvement with respect to previous experimental determinations of ice growth rates, where RH and temperature of the air above the sample were determined far away from the ice crystallization regions, in opposition to the present work.Fil: Gianetti, Melisa Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Gelman Constantin, Julián. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Corti, Horacio Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; ArgentinaFil: Longinotti, María Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Modelling of a Free-Piston Generator Based on Spark-Ignition Engine, Linear Electric Machine and Active Rectifier

The Free-Piston Linear Generator (FPLG) technology is meant to serve as a link between the period of fossil fuels and the next green power generation. It consists of a Free-Piston Internal Combustion Engine (ICE) mechanically coupled with a Linear Electric Machine (LEM). No constraints on the piston movement and the inherently variable Compression Ratio (CR) result in a low vibrations, low friction, low maintenance costs, multifuel and high efficiency ICE, with a significant reduction of fuel consumption and greenhouse-gas emissions. In this paper the modelling of this new electricity generation system is investigated. The models of a gasoline single-piston Spark Ignition (SI) Free-Piston ICE and of the electric conversion system based on a three-phase LEM and a Space Vector Pulse Width Modulation (SVPWM) controlled active rectifier are developed in the CFD open-source OpenFOAM and Matlab Simulink environments, respectively