acase@tulane.edu

The hydrodynamic and colloidal interactions between a particle and stationary surface play an important role in a number of practical applications involving particle deposition and transportation In this work, the interaction between polystyrene latex particles and a BK-7 glass slide was investigated using the optical technique of total internal reflection microscopy (TIRM). The interacting surfaces were characterized using scanning electron microscopy and atomic force microscopy. SEM micrographs of polystyrene latex particles show the presence of asperities on the surface. This work describes the effect of these asperities on the hydrodynamic and colloidal interactions between a particle and flat plate In the first section, the sedimentation of slightly Brownian particles toward a solid glass plate was studied using TIRM. The average sedimentation rate was found to agree closely with theory only if the measured separation distances were adjusted by a constant offset which is believed to reflect the asperities on the particle surface. For the 15 $\mu$m diameter particles, the average offset was 19 nm. In addition, the Brownian movements observed during sedimentation were found to be normally distributed, in agreement with theory. The roughness limited the distance of closest approach, but did not affect the particle-surface hydrodynamics In the second section a set of analytical equations were developed to calculate interaction energies between a rough particle and a smooth plate. The particle roughness was modeled as hemispherical asperities. The van der Waals interaction energy was calculated using a pairwise summation technique, accounting for retardation effects, while the double-layer repulsion was calculated using the linear superposition approximation. It is found that at large separation distances, the roughness resulted in a shallower secondary potential well Finally, the TIRM technique was used to validate the predictions of the surface roughness model. The measured wells were consistently shallower than predictions made using the traditional equations for smooth surfaces. However, including asperities with an average height of 26 nm produced excellent agreement with the measured potential energy profiles. This asperity size was consistent with the values determined independently from the sedimentation experiments / acase@tulane.edu

Salt water intrusion often troubles ecosystems and municipalities. Several fields of engineering and science have quantified and described these flows under varying conditions with both physical and numerical modeling. However, no investigation has been performed into the effects of complex longitudinal channel geometry on a propagating salt water intrusion front. This study was undertaken in order to examine this situation and develop a relationship between longitudinal channel geometry and important parameters of flow. A predominantly transparent open channel flume was built that could modularly form several curved channel geometries. This flume was attached to a reservoir fitted with a release gate. The reservoir was filled with salt water and the flume with fresh water, both at equal heads. The gate was released and the intruding wedge was tracked via video recording. The data was then correlated using dimensionless parameters. The apparatus was verified using a straight channel setup and comparing it to data gathered by previous researchers. Additionally, a full experimental uncertainty analysis was performed to show the accuracy with which the data was taken Fifty-six experiments were performed. For each channel geometry four experiments were performed in duplicate under varying conditions (10-ppt, 25-ppt, 60-ppt, and 25-ppt with rough channel bottom). There were 7 experimental setups including large (22.5-inch centerline) radius 180-degree bend, small radius (12.5-inch centerline) radius 180-degree bend, 90-degree large and small bend, S-curve large and small bend, and straight channel. The results in the straight channel experiments not only verified the setup versus previous researchers, but also served as controls for the curved channel geometry experiments. Comparing the results of the straight channel experiments to each of the curved channel geometries showed that there were no measurable differences within the experimental uncertainty / acase@tulane.edu

acase@tulane.edu

This dissertation examines the impact of government policies on the international reserves of Ecuador. Specifically, it studies the appropriateness and effectiveness of domestic credit, central bank and private banks credit, ceilings on interest rates, advanced deposits on imports, tariffs, export duties, and export subsidies as instruments of control for the balance of payments The first part of the study describes the Ecuadorean economy, the Ecuadorean financial institutions and financial instruments, and the instruments of control available to the monetary authorities The second part of the dissertation compares and analyzes the effects on the level of international reserves of credit controls through central bank funds, ceilings on private banks credit, and advanced deposits on imports. The theoretical framework used for the analysis was the monetary approach to the balance of payments. It was found that the most effective policy instrument for control of foreign reserves is central bank credit whether it is used as open market operations, advanced deposits on imports, or changes in reserve requirements. Ceilings on the amount on credit private banks can grant to the private sector are appropriate instruments but are, in the case of Ecuador, 50% less effective than central bank credit. Finally, interest rate ceilings appear not to be appropriate instruments of control since rates of return on assets seem to have negligible effects on the demand for money The last issue studied here addresses to the effects on the balance of payments of policy instruments that affect the real sector of the economy. It has been argued that a policy that affects the real sector has an impact on the balance of payments only because it affects the monetary sector. In the absence of money such a policy has no permanent effect on the balance of trade. In this study, it has been found that, in Ecuador, policies such as import tariffs, export duties, and other policies that affect the relative prices of imports to exports have, indeed, no permanent (if any) effect on the balance of trade. However, the study cannot show that these policies have the desired effect on the balance of payments either. Thus, it can be concluded that policy instruments that affect the real sector are not very effective (and they may not even be appropriate) for balance of payments controls While this study provides insight into the effects of policy instruments for Ecuador it cannot be used to discern the controversy surrounding the essence of the balance of payments mechanism. Nevertheless, the theoretical framework and the empirical techniques are general enough so that they could be used for other countries in determining the effectiveness of balance of payments policies / acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

The vast majority of studies concerning the corrosion of implant alloys have tested materials in the polished, unstressed condition. However, most implants are either statically or dynamically loaded. Also, a current trend in orthopaedics is towards the use of implants with grit blasted or porous-coated surfaces. This dissertation research was undertaken to study the effect of surface condition and stress state on the corrosion behavior of implant alloys Commonly used implant alloys: polished, grit blasted, and porous-coated Co-Cr-Mo and Ti-6Al-4V ELI and polished 316L stainless steel, were tested electrochemically using linear polarization, AC impedance, and potentiodynamic polarization techniques. These techniques allowed a variety of measures, of both scientific and clinical interest, to be used to evaluate corrosion behavior. The AC impedance technique was able to detect fractures of the sintered bonds of the porous layer. The alloys were tested under a variety of loading conditions: statically stressed into the plastic region, dynamically stressed to levels typical in pattern and amplitude to those occurring in total hip prostheses during level walking, and dynamically stressed to overload levels Both surface and stress state affected the electrochemical behavior. Compared to the zero stress controls, all non-zero stress states caused stress enhanced ion release (SEIR) with grit blasted Co-Cr-Mo, and all but the dynamic overload stress state caused SEIR with porous-coated Ti-6Al-4V ELI. Statically stressing all Co-Cr-Mo surfaces, and polished 316L also caused SEIR Dynamically stressing 316L to typical levels also caused SEIR, as did dynamic overload stress states with all Co-Cr-Mo surfaces, polished Ti-6Al-4V ELI, and 316L. Dynamic overload stress was more damaging to the passive layers of all Co-Cr-Mo surfaces, grit blasted Ti-6Al-4V ELI, and 316L than was static stress, even though the magnitude of the static loading was much higher. SEIR was found at levels below the yield stress with all of these materials Inferring the in vivo situation from unstressed, polished material can lead to gross underestimates of the amount of material released, by as much as a factor of 38 / acase@tulane.edu

acase@tulane.edu