No description available.

No description available.

No description available.

No description available.

The Langevin equation is a stochastic differential equation frequently used in molecular dynamics for simulating systems with a constant temperature. Recent developments have given rise to wide uses of Langevin dynamics at different levels of spatial resolution, which necessitate time step and friction parameter choices outside of the range for which many existing temporal discretization methods were originally developed. We first study the GJ--F, BAOAB and BBK numerical algorithms, originally developed for atomistic simulations, on a coarse-grained polymer melt, paying close attention to the large time step regime. The results of this study then inspire our search for new algorithms and lead to a general class of velocity Verlet-based time-stepping schemes designed to perform well for all parameter regions, by ensuring that they faithfully reproduce statistical quantities for the case of a free particle and harmonic oscillator. This family of methods depends on the choice of a single free parameter function and we explore some of the methods defined for certain choices of this parameter on realistic coarse-grained and atomistic molecular systems relevant in material and bio-molecular science. In addition, we provide an equivalent splitting formulation of this one-parameter family which allows for enhanced insight into the hidden time scaling induced by the choice of the free parameter in the Hamiltonian and stochastic time scales. / Mathematics

Plasticizers are a commonly used additive used in the polymer industry to make the plastic more pliable by reducing the glass transition temperature, Tg and Young's modulus, Y. As the plasticizer aids in polymer process-ability and making it suitable for applications from industrial cables to sensitive medical equipment, the mechanism of plasticization is not fully understood. There are three theories used to explain plasticization: lubricity theory, gel theory, and free volume theory. The latter is a fundamental concept of polymer science that is used to calculate many polymer properties, but they all do not give a clear picture on plasticization. With molecular dynamics (MD) simulation, a coarse-grained (CG) model - which consist of a simple bead-spring model that generalizes particles as a bead and connects them via a finite spring – is used to explore the impact of plasticizer size throughout the polymer system. The interaction characteristics of the plasticizer is explored by representing the plasticizer molecules as a single bead of varying size. This gives better control on the variability of the mixture and pinpoint the significant contributions to plasticization. A path to understanding the the mechanism of plasticization will give insight in glass formation, and can later be used to find an optimal plasticizer architecture to minimize the migration of the additive by tuning the compatibility. Current results show a decoupling between the Tg and Y of the polymer-additive system. The overall understanding of finite-size effects shows: as additive of increasing size is added, the polymer free volume increases which in-turn would decrease the Y, but Tg is shown to increase because the polymer and additive are not as mobile to reduce caging effect of monomeric units. / Thesis / Master of Applied Science (MASc)

Flotation is regarded as the best available separation method for the recovery of valuable minerals such as chalcopyrite (CuFeS2), sphalerite (ZnS), etc., from mined ores. Practically all metals humans use today are produced by flotation. The process relies on controlling the stability of the thin liquid films (TLFs) of water formed between minerals and air bubbles (wetting film), air bubbles (foam film), and mineral particles (colloid films). In flotation, a desired mineral is rendered hydrophobic by surfactant coating as a means to destabilize the TLFs, so that they can be attached to the hydrophobic air bubbles. A TLF ruptures when the disjoining pressure (or surface forces per unit area) of the film becomes negative, i.e., Π < 0. Thermodynamically, a wetting film can rupture when the contact angle (θ) of a mineral surface is larger than zero. It would, therefore, be reasonable to consider the roles of the surface forces to better understand the fundamental mechanisms involved in flotation. The surface forces considered in the present work included the electric double layer (EDL), van der Waals (vdW), and attractive hydrophobic (HP) forces. A flotation model has been developed by using the hydrophobic force as a kinetic parameter, which made it possible to track the fates of mineral particles of different of size, surface liberation, and contact angle to predict both recovery and grades for the first time. The model has been validated against the plant survey data obtained from an operating copper flotation plant. The simulation results obtained using the first principles model have been utilized to address the limitations of current flotation practices. One such limitation is the presence of slow-floating target minerals present in the cleaner-scavenger tails (CST) that are routinely recycled back to the rougher flotation bank as circulating loads (CLs) to allow longer retention times for the slow-floating particles for additional recovery. The simulation results show also that opening a flotation circuit by treating the CST streams separately in an advanced circuit can substantially improve the plant performance. One of the major limitations of flotation is that the coarse particles in a feed stream are difficult to recover due to the low hydrophobicity associated with poor surface liberation. A new flotation model developed in the present work suggests various ways to address the problem. One is to increase the hydrophobicity of the composite (poorly liberated) particles using the Super Collectors that can increase the contact angles to 150 -170o. Simulation results obtained using the model developed in the present work show significant financial benefits of using Super Collectors. Flotation is controlled by surface forces as noted above. As particle size becomes larger than 150 µm, however, the gravitational force comes into the picture and can override the surface forces. A new flotation cell has been developed to mitigate the effects of the extraneous force by decreasing the effective specific gravity (SG) by attaching air bubbles to facilitate levitation and by creating a pulsation to allow particles to move according to SGs independent of particle size, which should help increase the upper particle size limit of flotation. Surface forces in foam and oil-in-water emulsion films have been measured at different temperatures to determine the changes in thermodynamic properties of the thin liquid films (TLFs) of water confined between two bubbles and two oil drops. The results show that the films are destabilized by the attractive hydrophobic forces created during the course of building H-bonded structures in confined spaces, which entails decreases in enthalpy (H < 0) and entropy (TS < 0), the second term representing the thermodynamic cost of building the structures. / Doctor of Philosophy / Flotation is a kinetic process designed to separate valuable minerals from mined ores. This process depends on several hydrodynamic and surface chemistry parameters making it hard to model. A U.S. patent was awarded to Sulman and Picard in 1905 for using air bubbles to selectively collect hydrophobic particles from the aqueous phase, leaving hydrophilic particles behind. Since then, the separation process known as flotation has been used to produce practically all metals humans use. Many investigators developed flotation models using hydrodynamic parameters, e.g., particle size, bubble size, energy dissipation rate, etc., but without a reference to particle hydrophobicity. Therefore, the models were successful in predicting recoveries but not product grades. Derjaguin and Dukhin (1961) were the first to model flotation using surface forces but without due consideration of the role of hydrophobic force in flotation. Therefore, it also failed to predict product grades. In the current work, a new flotation model has been developed using the hydrophobic force as a kinetic parameter. This approach made it possible to predict both recoveries and grades for the first time. The model has been reduced to a simple form mimicking the Arrhenius equation so that it can be used to delineate the different conditions required for optimizing coarse and fine particle flotation. The model has been derived by considering the surface forces in the thin liquid films (TLFs) of water confined between bubbles, and bubbles and particles. It has been found that the hydrophobic force plays a decisive role in destabilizing a wetting film and inducing bubble-particle attachment. The surface forces measured in the present work show that the hydrophobic interactions in macroscopic scales are controlled by enthalpy rather than entropy, which is contrary to the nanoscale hydrophobic interactions. The model has been validated against a full-scale plant operation and demonstrated predictive capabilities. The simulation results have been analyzed to determine the limitations of the current flotation practices. It was found that coarse particle flotation is difficult either due to the presence of composite particles reducing the particle contact angle or due to their poor hydrodynamic properties. Utilizing the insights from the model, various methods of alleviating these limitations have been developed and presented in the current work. References Derjaguin, B.V., Dukhin, S.S., 1961. Theory of flotation of small and medium-size particles. Inst. Min. Metall. 241–267. Sulman, H.L., and Kirkpatrick-Picard (1905). U.S. Patent No. 793,808.

Yes / The impact behavior of deflection-hardening High Performance Fiber Reinforced Cementitious Concretes (HPFRCs) was evaluated herein. During the preparation of HPFRCs, fiber type and amount, fly ash to Portland cement ratio and aggregate to binder ratio were taken into consideration. HPFRC beams were tested for impact resistance using free-fall drop-weight test. Acceleration, displacement and impact load vs. time graphs were constructed and their relationship to the proposed mixture parameters were evaluated. The paper also aims to present and verify a nonlinear finite element analysis, employing the incremental nonlinear dynamic analysis, concrete damage plasticity model and contact surface between the dropped hammer and test specimen available in ABAQUS. The proposed modelling provides extensive and accurate data on structural behavior, including acceleration, displacement profiles and residual displacement results. Experimental results which are further confirmed by numerical studies show that impact resistance of HPFRC mixtures can be significantly improved by a proper mixture proportioning. In the presence of high amounts of coarse aggregates, fly ash and increased volume of hybrid fibers, impact resistance of fiberless reference specimens can be modified in a way to exhibit relatively smaller displacement results after impact loading without risking the basic mechanical properties and deflection-hardening response with multiple cracking.

Yes / This paper investigates the bond performance of recycled coarse aggregate (RCA) concrete with un-corroded/corroded reinforcing steel bars, with the main parameters being RCA content, corrosion level, bar diameter and embedment length. For this purpose, 60 pull-out specimens containing different percentages of RCA (i.e. 0%, 25%, 50% and 100%) and steel bars of two diameters (12 and 20 mm) and different embedment lengths were tested. In order to establish various levels of corrosion, specimens were exposed to an electrochemical corrosion for 2, 5, 10 and 15 days. The bond strength between RAC concrete and un-corroded/corroded steel are compared to current codes and equations proposed by other researchers. Experimental results showed that larger corrosion rate of steel bars was observed with the increase of the replacement level of RCA due to its high porosity and water absorption. The use of RCA had a slight influence on bond strength for un-corroded specimens compared to that obtained from conventional concrete. Furthermore, the bond strength of RCA concrete was strongly affected by corrosion products; bond strength slightly enhanced for up to about 2% corrosion rate, and then significantly decreased as the corrosion time further increased, similar to that of conventional concrete. However, the rate of bond degradation between RCA concrete and corroded steel bars was much faster than that observed in corroded conventional concrete.

Possibilities for increasing the upper limit of floatable particle sizs in the froth flotation process have been examined since the early beginnings of mineral flotation. The economic implications of such an incresae are far ranging; from decreased grinding costs and increased recoveries to simplified flow-sheet design and increased throughput, all leading to increased revenue. Bubble-particle detachment has been studied to better understand the factors influencing the strength of attachment and the energies involved. Direct measurements of bubble particle detachment were performed using a hanging balance apparatus (KSV Sigma 70 tensiometer) and using a submerged hydrophobic plate in water. Three experiments were used; direct force measurement of bubble-particle detachment, detachment force and energy of a bubble from a submerged hydrophobic plate, and detachment force and energy of a cetyltrimethylammonium bromide coated silica sphere from a flat bubble. Octadecyltrichlorosilane was used as a hydrophobic coating in the first two experimental methods. These experiments were recorded with a CCD camera to identify the detachment processes involved. Energies for both methods were calculated and divided into the two main steps of the detachment process: Three-Phase-Contact pinning and three phase contact line sliding. The first step represents the energy barrier which must be overcome before detachment can begin. It is directly related to contact angle hysteresis. Detachment occurs during the second step, where the solid-vapor interface is replaced by solid-liquid and liquid-vapor. This step corresponds to the work of adhesion. The effects of surface tension, contact angle and hysteresis were well demonstrated with the three experimental methods. Good correlation was found between theoretical work of adhesion and measured energies. / Master of Science