The freezing of model atmospheric aerosols – specifically, model cirrus cloud particles – was investigated through laboratory studies of supercooled water aerosols. Water droplets with radii of 1 – 2.7 µm were exposed to well-defined temperature profiles ranging from 240 – 230 K in a cryogenic flow tube apparatus, and observed using infrared extinction spectroscopy. A computational characterization procedure, based on theories of light scattering, was used to determine the size and phase composition of aerosols from extinction spectra. The procedure showed large ice fractions at uncharacteristically warm temperatures, which was attributed to the formation of ordered, “ice-like” clusters of molecules in supercooled water. Temperature-dependent complex indices of refraction were determined from the supercooled water extinction spectra, and showed changes reflecting this ordered formation. Taking the “ice-like” character of clusters into account, the homogeneous nucleation point for micrometre-sized water aerosols was determined to be 236.2 K. A microphysical model was developed to determine temperature-dependent, volume- and surface-based homogeneous nucleation rates from experimental freezing data. The model results indicated that surface nucleation was the dominant process for our range of experimental conditions. This was supported by separate studies of smaller, 0.63 and 0.75 µm radius aerosols, with larger surface-to-volume ratios. An optical microscopy apparatus was placed in the cryogenic flow tube to allow real-time imaging of particles in freezing experiments. The imaging studies demonstrated the utility of the microscopy apparatus for the observation and classification of ice crystal habits. Ray tracing and image processing algorithms were used to analyze particle geometry and size. The latter was used to validate the size retrievals from the aerosol characterization procedure. Additional studies probed the changes in the optical properties of crystalline ammonium sulfate, (NH₄)₂SO₄, due to the paraelectric-to-ferroelectric transition at 223 K. Temperature- dependent refractive indices were determined from crystalline (NH₄)₂SO₄ extinction spectra. Only small changes in these values were observed down to 223 K, below which significant changes were observed, due to the changes in lattice structure accompanying the ferroelectric transition.

DNA condensates have attracted the attention of biophysicists, biochemists and polymer physicists for more than thirty years. In the biological community, the quest to understand DNA toroid formation has been motivated by its relevance to gene packing in certain viruses and by the potential use of DNA toroids in artificial gene delivery (e.g. gene therapy). In the physical sciences, DNA toroids are appreciated as a superb model system for studying particle formation by the collapse of a semiflexible, polyelectrolyte polymer. The thesis includes an analysis of the kinetic and thermodynamic factors governing DNA condensate morphology in solution, and discusses implications for future applications of DNA condensation in vitro as a model system for testing theories of polyelectrolyte collapse. In addition, DNA condensation by folded bovine protamine, a naturally occurring multivalent oligopeptide responsible for packing genomic DNA in bovine sperm cells, has been studied as well. The analysis of morphology, size, DNA strand packing density, and the stability of structural integrity of DNA condensates obtained with folded bovine protamines suggests that we have reconstituted native sperm cell chromatin. The results of this study were used to model the local structure of bovine sperm cell chromatin.

There is strong evidence that variations in seawater chemistry occurred during the Phanerozoic Eon. Of particular importance are the changes in the Mg2+/Ca2+ ratio because they have been attributed to the oscillations between “calcite” and “aragonite seas” over time. In addition to the Mg2+/Ca2+ ratio variations, there were also major changes in pCO2 levels and alkalinity that could also affect the calcium carbonate (CaCO3) polymorph that precipitates from seawater. Experiments were conducted in seawater where the initial alkalinity and pCO2 levels were varied and then slow degassing of CO2 resulted in a gradual increase of saturation state with respect to CaCO3 and eventually nucleation. The pH was continually monitored throughout the experiments and it was used in combination with the initial alkalinity to calculate the pCO2 and saturation state of aragonite (sigmarag) at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively. It was observed that the initial alkalinity greatly affected the nucleation pCO2 value and the CaCO3 polymorph that was precipitated. In seawater with Mg2+/Ca2+=1.2 and ~10 mM alkalinity and a pCO2 below 2,500 μatm, calcite that was overgrown with aragonite was the dominate polymorph nucleated, while pure aragonite precipitated when the pCO2 was above 2,500 μatm. Seawater with Mg2+/Ca2+=1.2 and a wide range of initial alkalinities (5-50 mM) produced variable results. Seawater with Mg2+/Ca2+=1.7 produced only aragonite at lower alkalinities, but calcite was nucleated when the alkalinity and pCO2 values were exceptionally high, typically above 11 mM. These results point to the need to also consider the effects of the carbonic acid system in the “critical” Mg2+/Ca2+ region of about 1 to 2 for “calcite seas” and “aragonite seas” at various times throughout the Phanerozoic Eon.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 76-81). Also available in electronic version. Access restricted to campus users.

Failure in structural materials occurs initially by localization of deformation, and subsequently through a process of nucleation, growth and coalescence of voids. Predicting material failure requires a careful investigation of the different stages of damage evolution at the multiple scales. The main objective of this thesis is to explore the evolution of damage and to correlate this with the deformation of the material at the continuum and microstructural levels. This is accomplished through macroscopic measurements of strain evolution using digital image correlation and microscale measurements of strain and damage using optical and scanning electron microscopy. Three materials with different microstructure were examined. In oxygen-free, high-conductivity copper, a high-purity material without appreciable second phase particles, strain levels in the order of three were observed in the material without any trace of damage. Failure was observed to be triggered by plastic instability in the form of shear bands and the emergence of a prismatic cavity that grows in a self-similar fashion by an alternating slip mechanism. In Al 6061-T6, a material with a dispersion of second phase particles at a volume fraction of about 0.01, nucleation of damage does not appear until plastic strain levels of 0.5 to 1.0. Once damage in the form of particle fracture or decohesion at the interface initiates, subsequent failure follows by the void nucleation, growth and coalescence; but, dominated by the fluctuations in the distribution of second phase particles, final separation occurs in a highly localized layer of material on the order of the grain size, corresponding to a small increase in the overall strain. In nodular cast iron, a material with an initial porosity of about 0.10, growth of voids was observed initially, but this was terminated by a transition of the deformation into a localized region. Phenomenological models based on strain-to-failure and micromechanical models based on a mechanistic description of the microscale deformation are evaluated in light of the above examination of failure in these three classes of materials. / text

An optical microscope coupled to a flow cell was used to investigate ice nucleation on five atmospherically relevant mineral dusts at temperatures ranging from 233 to 247 K. Kaolinite and muscovite particles were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite particles, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K, but poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between these data and previously published work. This can be explained by several factors including mineral source, particle size, observation time and surface area available for nucleation. Heterogeneous nucleation rates (Jhet) were calculated from the onset data. Jhet values ranged from 60 to 1100 cm-²s-¹ for the five minerals studied. These values were then used to calculate contact angles (θ) for each mineral according to classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º; for quartz and calcite the contact angles were much higher, ranging from 25 to 27º. The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and above 20º at higher temperatures. The reported Jhet and θ values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. The roles of H₂SO₄ and (NH4)₂SO₄ coatings on the ice nucleating properties of kaolinite were also investigated. Onset data was collected for H₂SO₄ coated and (NH4)₂SO₄ coated kaolinite particles at temperatures ranging from 233 to 247 K. In contrast to uncoated kaolinite particles, which were effective ice nuclei, H₂SO₄ coated particles were found to be poor ice nuclei, requiring relative humidities close to water saturation before nucleating ice at all temperatures studied. (NH4)₂SO₄ coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K.

The number and size distributions of particles between 10 and 400 nm were measured in Toronto and rural Egbert during May 2007 to May 2008 to compare nucleation and growth at paired urban and rural locations. Particle formation and growth were observed in Egbert more frequently than in Toronto, and simultaneous events occurred on 34 out of 368 days. In contrast, formation and growth rates were both higher in Toronto. Further, a linear regression analysis suggested that compounds contributing to nucleation and growth processes were different in Toronto and Egbert. Vehicular emissions seemed to suppress particle formation in downtown Toronto. Nucleation also appeared to be suppressed by long-range transported pollutants originating from industrial regions in southwestern Ontario and northern Ohio in the United States. A Nucleation Indicator (NI) was developed by combining relevant parameters, and it was found to provide a reasonable measure of the probability of nucleation events occurring.

The number and size distributions of particles between 10 and 400 nm were measured in Toronto and rural Egbert during May 2007 to May 2008 to compare nucleation and growth at paired urban and rural locations. Particle formation and growth were observed in Egbert more frequently than in Toronto, and simultaneous events occurred on 34 out of 368 days. In contrast, formation and growth rates were both higher in Toronto. Further, a linear regression analysis suggested that compounds contributing to nucleation and growth processes were different in Toronto and Egbert. Vehicular emissions seemed to suppress particle formation in downtown Toronto. Nucleation also appeared to be suppressed by long-range transported pollutants originating from industrial regions in southwestern Ontario and northern Ohio in the United States. A Nucleation Indicator (NI) was developed by combining relevant parameters, and it was found to provide a reasonable measure of the probability of nucleation events occurring.

An optical microscope coupled to a flow cell was used to investigate ice nucleation on five atmospherically relevant mineral dusts at temperatures ranging from 233 to 247 K. Kaolinite and muscovite particles were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite particles, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K, but poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between these data and previously published work. This can be explained by several factors including mineral source, particle size, observation time and surface area available for nucleation. Heterogeneous nucleation rates (Jhet) were calculated from the onset data. Jhet values ranged from 60 to 1100 cm-²s-¹ for the five minerals studied. These values were then used to calculate contact angles (θ) for each mineral according to classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º; for quartz and calcite the contact angles were much higher, ranging from 25 to 27º. The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and above 20º at higher temperatures. The reported Jhet and θ values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. The roles of H₂SO₄ and (NH4)₂SO₄ coatings on the ice nucleating properties of kaolinite were also investigated. Onset data was collected for H₂SO₄ coated and (NH4)₂SO₄ coated kaolinite particles at temperatures ranging from 233 to 247 K. In contrast to uncoated kaolinite particles, which were effective ice nuclei, H₂SO₄ coated particles were found to be poor ice nuclei, requiring relative humidities close to water saturation before nucleating ice at all temperatures studied. (NH4)₂SO₄ coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K.

Structures grown via self-assembly are a unique field in nanotechnology. The morphology of self-assembled structures is affected by the balance between kinetics and thermodynamics during growth. Hence structures with tailored morphologies and properties can be created with adjustments in growth conditions. In this thesis we study crystal nucleation and equilibration, for both real and model systems. The growth of thin bismuth films is investigated on three atomically flat surfaces; Mica, Molybdenum di Sulphide, and highly oriented pyrolitic graphite (HOPG). Films are grown under UHV conditions, and characterised using scanning electron microscopy and atomic force microscopy. For coverages of only a few monolayers, bismuth particles are found to aggregate into flat, isolated islands. Islands have characteristic heights and morphologies for each substrate. By altering the deposition flux and coverage, the island density and morphology can be manipulated. On HOPG substrates, planar islands grown at low flux are replaced by 1D structures at high temperature. These anisotopic structures result from an anisotropy in bond strengths at the crystal-vapour interface. Depositing Bi on HOPG substrates at low flux or high temperature conditions produces nanorods aligned (roughly) perpendicular to step edges on the graphite. The aspect ratios (ARs) of these 1D structures are found to increase as the deposition flux is lowered, or the substrate temperature is increased. The Arrhenius dependence of the AR is determined from experiment. A Kinetic Monte Carlo (KMC) model for high AR step-edge aggregates was developed, determining the likely growth mechanism for the nanorods. A scaling regime devised from the KMC results predicts the dependence of nanorod ARs on flux and temperature, and allows an estimation of the energy binding Bi dimers to the sides of nanorods. Thin films can also be grown via the self-assembly of atomic clusters. After deposition coalescence of clusters has implications for the film morphology, and properties. We use KMC simulations to investigate the coalescence of pairs of 3D atomic clusters (15000 to 130000 atoms in size) via lattice based surface diffusion. For early coalescence stages, the radius of the neck region connecting the two clusters is found to develop with a different powerlaw to classical theory. For later coalescence stages, when the nucleation of new atomic layers on facets of the cluster is required for further coalescence the temperature, cluster size, and cluster orientation all influence the coalescence. Equilibration times for clusters coalescing at high temperature are found to be limited by the dissociation of atomic layers.