Water utilities can experience problems from bubble formation during conventional treatment, including impaired particle settling, filter air binding, and measurement as false turbidity in filter effluent. Coagulation processes can cause supersaturation and bubble formation by converting bicarbonate alkalinity to carbon dioxide by acidification. A model was developed to predict the extent of bubble formation during coagulation which proved accurate, using an apparatus designed to physically measure the actual volume of bubble formation. Alum acted similar to hydrochloric acid for initializing bubble formation, and higher initial alkalinity, lower final solution pH, and increased mixing rate tended to increase bubble formation. Lastly, the protocol outlined in Standard Methods for predicting the degree of supersaturation was examined, and when compared to this work, the Standard Methods approach produces an error up to 16% for conditions found in water treatment. Air entrainment and ozonation are the key causes of dissolved gas supersaturation and eventual bubble formation in water treatment plants. Total dissolved gas probes (TDGP) are now available to directly measure supersaturation and have many advantages compared to conventional techniques. Bubble formation during coagulation-flocculation hindered particle sedimentation, producing settled turbidities double that of solutions without dissolved gases. In a filtration study, run time to one half of initial flow was decreased by 54% when the source water was increased from 0.1 to 0.2 atm supersaturation. Indeed, even at 0.05 atm supersaturation, run length was only 21 hours in solutions without added particulate matter. A case study confirmed that bubble formation can interfere with coagulation and filtration processes at conventional treatment plants. / Master of Science

Yes / Mandelic acids are prototypic chiral molecules where the sensitivity of crystallized forms (enantiopure/racemic compound/polymorphs) to both conditions and substituents provides a new insight into the factors that may allow chiral separation by crystallization. The determination of a significant number of single crystal structures allows the analysis of 13 enantiopure and 30 racemic crystal structures of 21 (F/Cl/Br/CH3/CH3O) substituted mandelic acid derivatives. There are some common phenyl packing motifs between some groups of racemic and enantiopure structures, although they show very different hydrogen-bonding motifs. The computed crystal energy landscape of 3-chloromandelic acid, which has at least two enantiopure and three racemic crystal polymorphs, reveals that there are many more possible structures, some of which are predicted to be thermodynamically more favorable as well as slightly denser than the known forms. Simulations of mandelic acid dimers in isolation, water, and toluene do not differentiate between racemic and enantiopure dimers and also suggest that the phenyl ring interactions play a major role in the crystallization mechanism. The observed crystallization behavior of mandelic acids does not correspond to any simple “crystal engineering rules” as there is a range of thermodynamically feasible structures with no distinction between the enantiopure and racemic forms. Nucleation and crystallization appear to be determined by the kinetics of crystal growth with a statistical bias, but the diversity of the mandelic acid crystallization behavior demonstrates that the factors that influence the kinetics of crystal nucleation and growth are not yet adequately understood. / EPSRC, Max Planck Society for the Advancement of Sciences, UCL-MPS Impact Ph.D. Fellowship, EU COST Action

Yes / Understanding the effect of physical parameters (e.g., temperature) on crystallisation dynamics is of paramount importance for the synthesis of nanocrystals of well‐defined sizes and geometries. However, imaging nucleation and growth is an experimental challenge owing to the resolution required and the kinetics involved. Here, by using an aberration‐corrected transmission electron microscope, we report the fabrication of precious metal nanocrystals from nuclei and the identification of the dynamics of their nucleation at three different temperatures (20, 50, and 100 °C). A fast, and apparently linear, acceleration of the growth rate is observed against increasing temperature (78.8, 117.7, and 176.5 pm min−1, respectively). This work appears to be the first direct observation of the effect of temperature on the nucleation and growth of metal nanocrystals. / The Royal Society. Grant Number: UF150295 Leverhulme Trust. Grant Number: ECF-2013-414 The Academy of Medical Sciences. Grant Number: SBF003\1170

No description available.

Usant de certaines conditions, la croissance épitaxiale de GaN sur un large panel de substrats donne lieu à une assemblée de nanofils. Cette géométrie filaire peut permettre la croissance d'hétérostructures libres de tous défauts cristallins étendus, ce qui les rendent attractives pour créer des dispositifs de hautes performances. En premier lieu, mon travail de thèse a visé à clarifier le mécanisme de nucléation auto-organisé des nanofils de GaN sur substrat de silicium. Dans ce but, une étude approfondie de la couche tampon d'AlN, déposée préalablement à la nucléation des nanofils, a été réalisée, mettant en évidence une inattendue forte réactivité de l'Al avec le substrat. La nécessité de la polarité azote pour la croissance des nanofils de GaN a été mise en lumière, bien que des nanofils contenant dans leur cœur un domaine de polarité Ga ont également été observés. Dans ces nanofils, une paroi d'inversion de domaine est présente et a été démontrée être optiquement active, exhibant une photoluminescence à 3.45 eV. Ensuite des hétérostuctures filaires GaN/AlN ont été synthétisée pour des caractérisations structurales et optiques. Il a été montré que le mode de croissance de l'hétérostructure peut être changé en fonction du diamètre du nanofil. En dernier lieu, en prenant avantage de la géométrie cylindrique des nanofils, des mesures de diffusion de porteurs de charge ont été réalisées dans des nanofils de GaN et d'AlN. / Using specific conditions, GaN can be epitaxially grown on a large variety of substrates as a nanowire (NW) array. This geometry allows the subsequent growth of wire-like heterostructures likely free of extended defects, which makes them promising for increasing device controllability and performance. First, my PhD work has been devoted to the understanding of self-organized nucleation of GaN NWs on silicon substrates. For this purpose, a deep characterization of the growth mechanism of the AlN buffer deposited prior to NW nucleation has been done, emphasizing an unexpected large reactivity of Al with the substrate. The requirement of the N polarity to nucleate GaN NWs has been evidenced, although the possible existence of NWs hosting a Ga polar core has been observed as well. In these NWs, an inversion domain boundary is present and has been demonstrated to be optically active, having a photoluminescence signature at 3.45 eV. Next, GaN/AlN wire heterostructures have been grown for structural and optical characterization. It has been shown that by changing the wire diameter, different growth mode for the heterostructure could be reached.At last, thanks to the cylindrical geometry of NWs, the measurement of diffusion length for charge carriers in GaN and AlN NWs have been performed.

No description available.

Many experiments demonstrate that the effects of hydrogen solutes decrease macroscopic fracture stresses and strains in ductile materials. Hydrogen-related failures have occurred in nearly all industries involving hydrogen-producing environments. The financial losses incurred from those failures reaches millions if not billions of dollars annually. With the ever-urgent needs for alternative energy sources, there is a strong push for a hydrogen economy from government and private sectors. Safe storage and transportation of hydrogen increases the momentum for studying hydrogen-related failures, especially in ductile materials. To quantify ductile material damage with the effects of hydrogen embrittlement, it is necessary to add hydrogen effects into the void nucleation, void growth, and void coalescence equations. In this research, hydrogen-enhanced void nucleation is our focus, with hydrogen-enhanced void growth and void coalescence t be studied in the future. Molecular Dynamic (MD) and Monte Carlo (MC) simulations with Embedded Atom Method (EAM) potentials were performed to study how hydrogen affects dislocation nucleation, dislocation structure formation and nanovoid nucleation at nickel grain boundaries. The results were inserted into the continuum void nucleation model by Horstemeyer and Gokhale, and the relationships between stress triaxiality-driven void nucleation, grain boundary hydrogen concentrations and local grain geometries were extracted. MD and MC simulations with EAM potentials were also performed to study how hydrogen interstitials affect the dislocation nucleation, dislocation structure formation and subsequent anovoid nucleation of single crystal nickel in different hydrogen-charging conditions. Evolutions of dislocation structures of nickel single crystal with different hydrogen concentrations were compared. The effects of nanovoid nucleation stress and strain at different hydrogen concentrations were quantified. The results were also inserted into the Horstemeyer and Gokhale model and the relationship between stress triaxiality-driven void nucleation and hydrogen concentration caused by stress gradient, which showed similar trends as the grain boundary studies. From nanoscale studies and existing experimental observations, a continuum void nucleation model with hydrogen effects was proposed and used in a continuum damage model based upon Bammann and coworkers. The damage model was implemented into user material code in FEA code ABAQUS. Finite element analyses were performed and the results were compared to the experimental data by Kwon and Asaro.

No description available.

No / There is considerable interest, both fundamental and technological, in understanding how additives and impurities influence crystal nucleation, and in the modulation of nucleation in a predictable way by using designer additives. An appropriate additive can promote, retard, or inhibit crystal nucleation and growth, assist in the selective crystallization of a particular enantiomer or polymorphic form, or enable crystals of a desired habit to be obtained.[1¿3] Applications involving additives include the control of the nucleation of proteins,[4] the inhibition of urinary-stone formation[5] and of ice formation in living tissues during cryoprotection,[6] their use as antifreeze agents in Antarctic fish,[7,8] the prevention of blockages in oil and gas pipelines as a result of wax precipitation[9] and gas-hydrate formation,[10] crystal-twin formation,[11] and as a possible basis for the antimalarial activity of some drugs.[12]We report herein the mode of action and explicit (apparently intuitive) rules for designing additive molecules for the modulation of crystal nucleation. The mode of action and the design features have been derived from molecular-dynamics simulations involving simple models.[13] These findings will help to rationalize how known nucleation inhibitors and modulators exert their effect and aid in the identification or design of new additives for the inhibition or promotion of nucleation in specific systems.

This thesis presents a compendium of work on superheated liquid releases. Superheated liquid releases are often subject to flashing. Nucleation has been identified as an important process in the early stage of flashing. The presence of strong nucleation and therefore flashing depends on the output of the balance of the promoting forces and dissipation forces inside the fluid released. A one dimensional model to classify the type of jet to be formed after the release has been developed based on the balance of these forces. The analysis is based on the assumption that the nucleation process can be modelled as a second order damped system. The model parameters are defined as a function of the pressure, temperature, fluid properties and geometric characteristic of the system. The results obtained have good agreement with the experimental results available for releases of different fluids, including both hydrocarbons and water. The calculation of the velocity discharge, void fraction and mass flow of a flashing jet generated after the release is made based on the thermodynamics jump formulation approach. Due to the nature of the nucleation process, the assumptions of adiabatic flow with non reversible work for the surface tension forces are made. Those considerations are found to be more realistic that the isentropic condition used until now by different authors. Numerical techniques are only applied after the flashing jet is formed, no droplets generation or vapour generation are included. Droplets are imposed as part of the boundary conditions of a gas jet. Droplets transport mechanics and momentum exchange with the gas current is made using Droplet Disperse Model (DDM) on the commercial code Fluent Ò. DDM determines the distribution of the disperse phase over the continuous phase using a Lagrangian Eulerian approach. The influence of velocity, the dimension of the nozzle and mass flow used in the CFD modelling were analysed. Nozzle dimensions have a large impact on the core region length of the velocity profile. The k −e turbulent model was used. As expected, the numerical results do approach experimental values in the far region, suggesting that the momentum of the two phase jet is conserved. The one dimensional model thus provides the necessary boundary conditions for the application of numerical methods to superheated liquid releases including flashing.