2011-2013 Ph.D. Graduates

 

Rahman Abdulmohsin

Rahman Abdulmohsin, Fall 2013

 Thesis title : Gas Dynamics and Heat Transfer in a Packed Pebble-Bed Reactor for the 4th Generation Nuclear Eenergy

Abstract

Proper analyses of axial dispersion and mixing of the coolant gas flow and heat transport phenomena in the dynamic core of nuclear pebble-bed reactors pose extreme challenges to the safe design and efficient operation of these packed pebble-bed reactors.

The main objectives of the present work are advancing the knowledge of the coolant gas dispersion and extent of mixing and the convective heat transfer coefficients in the studied packed pebble-beds. The study also provides the needed benchmark data for modeling and simulation validation. Hence, a separate effect pilot-plant scale and cold-flow experimental setup was designed, developed and used to carry out for the first time such experimental investigations.  Advanced gaseous tracer technique was developed and utilized to measure in a cold-flow randomly packed pebble-bed unit the residence time distribution (RTD) of gas. A novel, sophisticated fast-response and non-invasive heat transfer probe of spherical type was developed and utilized to measure in a cold-flow packed pebble-bed unit the solid-gas convective heat transfer coefficients. The non-ideal flow of the gas phase in pebble bed was described using one-dimensional axial dispersion model (ADM), tanks-in-series (T-I-S) model and central moments analyses (CMA) method. Some of the findings of this study are:

¿ The flow pattern of the gas phase does not much deviate from the idealized plug-flow condition which depends on the gas flow rate and bed structure of the pebble-bed.

¿ The non-uniformity of gas flow in the studied packed pebble bed can be described adequately by the axial dispersion model (ADM) at different Reynolds numbers covers laminar and turbulent flow conditions. This has been further confirmed by the results of tanks in series (T-I-S) model and the central moment analyses (CMA).

¿ The obtained results indicate that pebbles size and hence the bed structure strongly affects axial dispersion and mixing of the flowing coolant gas while the effect of bed height is negligible in packed pebble-bed. At high range of gas velocities, the change in heat transfer coefficients with respect to the gas velocity reduces as compared to these at low and medium range of gas velocities.

¿ The increase of coolant gas flow velocity causes an increase in the heat transfer coefficient and the effect of gas flow rate varies from laminar to turbulent flow regimes at all radial positions of the studied packed pebble-bed reactor.

¿ The results show that the local heat transfer coefficient increases from the bed center to the wall due to the change in the bed structure and hence in the flow pattern of the coolant gas.

¿ The results and findings clearly indicate that one value as overall heat transfer coefficient cannot represent the local heat transfer coefficients within the bed and hence correlations to predict radial and axial profiles of heat transfer coefficient are needed.

 

Min Li

Min Li, Summer 2013

 Thesis title : Modeling and Analysis of Dynamic Behavior of Adsorption Stratified Column Beds Packed with Partially Fractal Porous Adsorbent Particles

Abstract

In this work novel porous adsorbent media having partially fractal structures are constructed whose intraparticle pore surface areas are significantly increased with increases in the total number of the recursion of the fractal. The adsorbent media possessing partially fractal structures have high adsorptive capacities and reduced intraparticle mass transfer resistances when compared to conventional porous adsorbent media, because they have large intraparticle pore surface areas which are easily accessible due to the desirable pore-size distributions and high pore connectivities that characterize the intraparticle pore structure of partially fractal adsorbent media. Furthermore, novel packed-bed structures forming stratified packed-beds are constructed having the radii of the packed porous adsorbent particles decreasing in size along the axis of bulk flow by packing the largest in size particles at the inlet section of the column and the smallest in size particles at the section of the column where the bulk fluid exits. This structure of a stratified packed-bed decreases both the external and intraparticle mass transfer resistances and increases the dynamic separation performance of the adsorbent column. When porous partially fractal adsorbent particles are packed in a stratified packed bed, this, in effect, creates a partially fractal adsorbent column that can provide highly effective and efficient chromatographic separations and high dynamic capacities when higher throughputs are required, and, thus, increases in the velocity of the flowing fluid stream in the column are implemented. Also in this work, the rigorous modeling and analysis of the dynamic behavior and performance of conventional and stratified column bed systems packed with either conventional or partially fractal porous adsorbent particles under various design and operating conditions, were studied. The results show that conventional columns packed with porous adsorbent particles having partially fractal structures provide larger breakthrough times and dynamic utilization of the adsorptive capacity of the column when compared with the separation performance obtained from conventional columns employing conventional adsorbent particles whose pore structure does not have a partially fractal architecture. Furthermore, the results show that stratified column beds exhibit superior species separation performance than conventional columns when they are packed with either conventional or partially fractal porous adsorbent media. The results show that stratified column beds packed with porous adsorbent media having partially fractal structures in general provide very effective separations and dynamic adsorptive capacities, and their highly desirable separation performance is increased further when the spatial ligand density distribution in the adsorbent particles is non-uniform and is described by certain functional forms. Furthermore the stratified columns packed with partially fractal porous adsorbent media provide to the designer and user of chromatographic column systems, especially in preparative and process chromatography, more degrees of freedom with respect to the number of variables and parameters that could be controlled in the design, construction, and operation of efficient chromatographic adsorption separation systems. The models, porous media, and systems constructed in this work can also be extended into other relevant fields including chemical catalysis, biocatalysis, and membrane separations which employ porous media. 

 

Mohammed Al-Mesfer, Spring 2013

 Thesis title : Effects of Dense Heat Exchanging Internals on the Hydrodynamics of Bubble Column Reactors Using Non-Invasive Measurement Techniques

Abstract

Bubble columns as multiphase reactors are employed in a wide range of applications in chemical and biochemical industries. Given their efficiency and capital cost reduction, bubble/slurry bubble column reactors are the best choice for the Fischer-Tropsch (FT) synthesis, offering clean alternative fuels and chemicals. FT synthesis is an exothermic process that requires many heat exchanging tubes in order to remove heat efficiently and maintain the desired temperature and isothermal operating condition. The impact of the heat exchanging tubes (internals) on the hydrodynamics is not fully understood. Reliably designing and scaling up bubble column reactors requires accurate hydrodynamics information, as well as heat and mass transfer parameters.

The main objective of this work is to advance the understanding of the effect of internals (25% covered cross-sectional area to meet FT needs) on hydrodynamics in bubble columns. Single-source γ-ray Computed Tomography (CT) and Radioactive Particle Tracking (RPT) were used to compute the time-averaged gas holdup, liquid velocity field and turbulent parameter profiles.

The main findings obtained in this study can be summarized as follows:

¿ The presence of internals at a given superficial gas velocity causes:

   ¿ An increase in gas holdup and the axial centerline liquid velocity

   ¿ A sharp decrease in turbulent parameters

¿ The increase in superficial gas velocity in the presence of internals causes:

   ¿ An increase in gas holdup, axial centerline liquid velocity and turbulent parameters

 

Sreekanta Aradhya

Shreekanta Aradyha, Spring 2013

 Thesis title : Scaleup and Hydrodynamics Study   of Gas-Solid Spouted Beds

Abstract

A thorough understanding of the complex flow structure of gas-solid spouted bed is crucial for design, scale-up and performance. Advanced gas-solid optical probes were developed and used to evaluate different hydrodynamic parameters of spouted beds. These optical probes measure solids concentration, velocity and their time series fluctuations. Since solids concentration needs to be converted to solids holdup through calibration, for meaningful interpretation of results, a novel calibration method was proposed (which is inexpensive and reliable compared to the current reported methods) and validated in the present study. The reported dimensionless groups approach of spouted bed scale-up was assessed and was found that the two different spouted beds were not hydrodynamically similar. Hence, a new scale-up methodology based on maintaining similar or close radial profiles of gas holdup was proposed, assessed and validated. CFD was used after it was validated as an enabling tool to facilitate the implementation of the newly developed scale-up methodology by identifying the new conditions for maintaining radial profiles of gas holdup while scaling up. It can also be implemented to quantify the effect of various variables on their hydrodynamic parameters. Gamma Ray Densitometry (GRD), a non-invasive radioisotope based technique, was developed and demonstrated to montior on-line the conditions for the scale-up, flow regime and spouted beds operation. The solids holdup in spout region increases with axial height due to movement of solids from the annulus region. However, solids velocity in the spout region decreases with axial height. In the annulus region the solids move downward as a loose packed bed and the solids velocity and holdup do not change with axial height. Using factorial design of experiments it was found that solids density, static bed height, particle diameter, superficial gas velocity and gas inlet diameter had significant effect on the identification of spout diameter. Flow regimes in spouted bed were determined with the help of optical probes, pressure transducers and GRD. It was found that the range of stable spouting regime is higher in 0.152 m beds and the range of stable spouting decreases in the 0.076 m beds. The newly developed non- invasive radioisotope technique (GRD) was able to successfully identify different flow regimes and their transition velocities besides scale-up conditions and operation. 

 

 Kan Huang

Kan Huang, Spring 2013

 Thesis title : Nanoscale Metal Oxide and Supported Metal Catalysts for Li-Air Battery

Abstract

The thesis work focuses on research and development of durable nanoscale catalysts and supports for rechargeable Li-air batteries that use aqueous catholytes. Transition metal oxides, TiO2 and Nb2O5 in particular, were prepared from a sol-gel process in the form of nanocoatings (5~50 nm) on carbon nanotubes (CNTs) and studied as catalyst supports. Carbon doping in the oxides and post annealing significantly increased their electronic conductivity. Pt catalyst on the support with TiO2 (Pt/c-TiO2/CNTs) showed a much better oxygen reduction reaction (ORR) activity than a commercial Pt on carbon black (Pt/C). Negligible loss (< 3%) in ORR activity was found in Pt/c-TiO2/CNTs as compared to more than 50% loss in Pt/C, demonstrating a significantly improved durability in the developed catalysts. However, Pt/c-Nb2O5/CNTs was found to be worse in ORR activity and durability, suggesting that c-Nb2O5/CNTs may not be a good support.

CNTs have fibrous shape and would provide a unique porous structure as electrode. Their buckypapers were made and used to support catalysts of Pt and IrO2 in the cathodes of Li-air batteries with sulfuric acid catholyte. At low Pt loading (5 wt.%) without IrO2 on the buckypaper cathode, the Li-air cell achieved a discharging capacity of 306 mAh/g and a specific energy of 1067 Wh/kg at 0.2 mA/cm2. A significant charge overpotential reduction (~ 0.3 V) was achieved when IrO2 was also used to form a bifunctional catalyst with Pt on the buckypapers. The round trip efficiency was increased from 72% to 81% with the bifunctional cathode, demonstrating a higher energy conversion efficiency. 

 

Moses Kagumba

Moses Kagumba, Spring 2013

 Thesis title : Heat transfer and bubble dynamics   in bubble and slurry bubble columns with internals for Fischer-Tropsch synthesis of clean alternative fuels and chemicals

Abstract

Synthesis gas, a mixture of CO and H2 obtained from coal, natural gas and biomass are increasingly becoming reliable sources of clean synthetic fuels and chemicals and via Fischer-Tropsch (F-T) synthesis process. Slurry bubble column reactor is the reactor of choice for the commercialization of the F-T synthesis. Even though the slurry bubble column reactors and contactors are simple in structures, their design, scale-up, operation, and performance prediction are still challenging and not well understood due to complex interaction of phases. All the studies of heat transfer have been performed without simultaneously investigating the bubble dynamics adjacent to the heat transfer surfaces, particularly in slurry with dense internals. This dissertation focuses on enhancing the understanding of the role of local and overall gas holdup, bubble passage frequency, bubble sizes and bubble velocity on the heat transfer characteristics by means of a hybrid measurement technique comprising an advanced four-point optical probe and a fast response heat transfer probe used simultaneously, in the presence and absence of dense internals. It also seeks to advance a mechanistic approach for estimating the needed parameters for predicting the heat transfer rate in two phase and three phase systems. The results obtained suggest that the smaller diameter internals gives higher heat transfer coefficient, higher local and overall gas holdup, bubble passage frequency and specific interfacial area but smaller bubble sizes and lower axial bubble velocities. The presence of dense internals enhances the heat transfer coefficient in both the large and smaller columns, while increased column diameter increases the heat transfer coefficient, axial bubble velocity, local and overall gas holdup, bubble chord lengths and specific interfacial area. Addition of solids (glass beads) leads to increased bubble chord lengths and increase in axial bubble velocity, but a decrease in local and overall gas holdup, a decrease in bubble passage frequency and decrease in the heat transfer coefficient. Further, a mechanistic assessment of the dependence of the heat transfer coefficient on the bubble dynamics shows that the contact time needed in the heat transfer coefficient estimation is indeed a function of the bubble passage frequency and local gas holdup. Hence the variation of the heat transfer coefficient with contact time is via bubble passage frequency and local gas phase holdup, which are related with sizes and velocity.

 

Chen Wang

Chen Wang, Spring 2013

 Thesis title : Behavior and Effects of Nanoparticles in Complex Fluid Systems

Abstract

The development of nanoscale technologies has greatly expanded the scientific and engineering horizons and also has the potential to turn molecular discoveries into worldwide benefits on multiple fronts.  One key ingredient in this development is nanometer-sized solid particles, which have unique properties unavailable in either isolated atoms or bulk materials, significant surface area per unit volume, and great capability to be further functionalized by surface coated organic and inorganic molecules.  Despite recent extensive efforts, the focus of nanoparticle studies has been on their synthesis and various possible derivatives.  In this dissertation, molecular based modeling and simulation studies have been performed to characterize and understand the behavior, properties, and effects of nanoparticles in some important material systems.  The results show that small nanoparticles that are highly faceted have significant shape effects, which are not captured by classical continuum-based approaches.  When dissolved in confined base oil as a nanolubricant additive, the surfactant-coated nanoparticles are found to disturb the layering tendency of the lubricant molecules and result in smoother force and dimension transitions when the normal load and surface separation are changed.  Under the condition where nanoparticles are located in the proximity of a sliding surface, they reduce the number of molecules adjacent to the surface, thereby lowering the surface-fluid interaction and the effective shear stress.  When dextran chains are used to coat nanoparticle surface, they can be well solvated by water molecules to retain a high level of helical structures.  Such dextran-coated nanoparticles are not only soluble in aqueous solutions but also endowed with pore structures from interactive dextran chains that may be suitable for bioengineering applications.

 

Faraj Zaid

Faraj Zaid, Spring 2013

 Thesis title : Gas-Solid Fluidized Bed Reactors: Scale-Up, Flow Regimes Identification and Hydrodynamics

Abstract

This research studied the scale-up, flow regimes identification and hydrodynamics of fluidized beds using 6-inch and 18- inch diameter columns and different particles. One of the objectives was to advance the scale-up of gas-solid fluidized bed reactors by developing a new mechanistic methodology for hydrodynamic similarity based on matching the radial or diameter profile of gas phase holdup, since gas dynamics dictate the hydrodynamics of these reactors. This has been successfully achieved. However, the literature reported scale-up methodology based on matching selected dimensionless groups was examined and it was found that it was not easy to match the dimensionless groups and hence, there was some deviation in the hydrodynamics of the studied two different fluidized beds.

A new technique based on gamma ray densitometry (GRD) was successfully developed and utilized to on-line monitor the implementation of scale-up, to identify the flow regime, and to measure the radial or diameter profiles of gas and solids holdups. CFD has been demonstrated as a valuable tool to enable the implementation of the newly developed scale-up methodology based on finding the conditions that provide similar or closer radial profile or cross sectional distribution of the gas holdup.

As gas velocity increases, solids holdup in the center region of the column decreases in the fully developed region of both 6 inch and 18 inch diameter columns. Solids holdup increased with the increase in the particles size and density. Upflowing particles velocity increased with the gas velocity and became steeper at high superficial gas velocity at all axial heights where the center line velocity became higher than that in the wall region. Smaller particles size and lower density gave larger upflowing particles velocity. Minimum fluidization velocity and transition velocity from bubbly to churn turbulent flow regimes were found to be lower in 18 inch diameter column compared to those obtained in 6 inch diameter column. Also the absolute fluctuation of upflowing particles velocity multiplied by solids holdups  as one of the terms for solids mass flux estimation was found to be larger in 18-inch diameter column than that in 6-inch diameter column using same particles size and density. 

 

Satya Gowthami Achanta, Fall 2012

 Thesis title : Meso-Scale Fluidic Devices with Chemical Sensors for Biological Applications

Abstract

Molecular oxygen and humidity are some of the major environmental quantities being measured for various industrial and commercial applications. This dissertation focuses on the design, fabrication and characterization of optofluidic biosensor systems for oxygen and humidity quantification using color charge-coupled device (CCD) camera as a photodetector and LED panel as an excitation source. Meso-scale fluidic devices with integrated oxygen and humidity sensors for potential applications to hydrotropic and oxytropic studies of small plant roots have been investigated in this study. Meso-scale sensor platform was fabricated using porphyrine complex as the sensitive dye embedded within Ethyl Cellulose (EC) polymer matrix. Green LED light displayed from the light panel helped in exciting the oxygen complex by emitting varied fluorescence emission corresponding to oxygen. This method of optical oxygen imaging helps in wide area distribution over the sensor platform. The root tip response to environmental stimuli by directed growth plays a major role in plant development. With these tropic responses of roots, plants can help themselves during environmental risks such as drought conditions. Different fluidic devices were fabricated with embedded humidity sensors within the device to study the effect of tropic responses. Hydrotropic behavior of corn roots was analyzed along with humidity gradient quantification using color charge-coupled device (CCD) camera for both imaging of the plant root and profiling of humidity distribution. Successfully created and analyzed the humidity gradient which resulted in root orientation because of hydrotropic response indicating the effectiveness of this device for further biological applications.

 

Alexandria Niemoeller, Fall 2012

Thesis title: Free-Radical Maleation of Poly(Butylene Adipate-Co-Terephthalate) in SUpercritical Carbon Dioxide and Its Effect on the Percolation and Deformation Mechanism of Layered Silicate Nanocomposites

Abstract

The utilization of compostable polymers such as poly(butylene adipate-co-terephthalate) (PBAT)  in single-use packaging applications can mitigate the water,  solvents, and energy required for processing plastic waste.  PBAT exhibits exceptional elongational properties, but its moderate water vapor permeability and low tensile modulus limit the applications in which PBAT can be employed.  In this work, PBAT nanocomposites were produced at various loadings of organically-modified montmorillonite. The secondary structure of nanoparticles formed within the PBAT matrix was evident in the terminal region of the linear viscoelastic response under oscillatory shear, and a percolation threshold was determined to occur at 3.88% clay by weight.  Transmission electron microscopy (TEM), x-ray diffraction (XRD), and the water vapor transmission rate (WVTR) were also utilized to elucidate the clay nanoparticle structure and the effect of increased clay loading on thermal and mechanical and properties was also studied.

To improve the level of clay exfoliation, a graft copolymer was developed as a compatiblizer.  The maleation of PBAT was demonstrated via a solvent-free free-radical initiated grafting process in a supercritical carbon dioxide medium.  The grafted anhydride moieties were modified with benzylamine to aid in FTIR and 1H NMR spectroscopy, and it was calculated that PBAT-g-MA was produced at a graft level ranging between 1.16 and 1.63%.  The graft copolymer was then incorporated in PBAT nanocomposites to examine the compatibilization effects on the development of a nanoparticle network.  The addition of 5% PBAT-g-MA resulted in improved clay dispersion and exfoliation evidenced through TEM, XRD, permeation analysis, and tensile testing. Differential scanning calorimetry revealed that the presence of PBAT-g-MA nucleated early onset crystallization but the organized folding of the PBAT chains was hindered by the percolated clay structure resulting in a lower overall percent crystallinity.  Thermogravimetric analysis revealed that the compatibilized samples demonstrated onset degradation at lower temperatures and an increased char formation at 600°C.

 

Matthew J. Factor

Matthew J. Factor, Spring 2012

 Thesis title : Organoclay Dispersion in Linear Low-density Polyethylene and Maleated Linear Low-density Polyethylene via Supercritical Carbon Dioxide Processing

Abstract

Research into polymer-clay nanocomposites (PCN's) has been ongoing for decades as a result of the property enhancements offered by clay. To fully exploit these property enhancements, organically modified clays (organoclays) are utilized to promote clay delamination by reducing the disparity between the hydrophilicity of the clay and the hydrophobicity of the highly used polyolefin polymer. Since the organic modification of organoclays can degrade at temperatures typical to many polymers during melt-mix processing, this work utilizes the low-temperature processing fluid supercritical carbon dioxide (scCO2) to disperse an organoclay into the highly used polymer LLDPE and ascertains the associated processing conditions for achieving this goal. Investigations into the LLDPE resin size, scCO2 processing time, scCO2 capability and the processing component compatibility were undertaken to better understand the important parameters to achieving organoclay dispersion, in terms of infusion and intercalation/exfoliation behavior. A LLDPE pellet resin showed improved dispersion and obtainable information over that of a granule resin, securing the choice of resin for subsequent experiments. Experiments undertaken with pellet resin exhibited that a 1-hr processing time was insufficient for organoclay infusion into LLDPE, however when infusion occurs, intercalation/exfoliation can be affected by scCO2. Increasing the compatibility of LLDPE with clay and the processing fluid revealed that the increased compatibility had altered the effect of scCO2. Further analysis with the 93A-infused samples was conducted in order to gain a better understanding of the effect of scCO2 processing, such as the quantity and size of clay particles dispersed and changes to the polymer incurred by processing.

 

Jason W. Picou

Jason W. Picou, Spring 2012

Thesis title: Glycerin Reformation in High Temperature and Pressure Water

Abstract

The noncatalytic reformation of glycerin in supercritical water was studied in a Haynes 282 tubular reactor. In order to determine which parameters were the most influential, a 2³ experimental matrix was conducted, with temperatures of 500 and 700°C, water/glycerin molar ratios of 3:1 and 13:1, and residence times of 30 and 90 seconds, all at a pressure of 24 MPa. It was found that temperature had the largest effect on the two gasification parameters deemed most important, gasification percentage and hydrogen yield. Based on this, the effect of temperature was further investigated by looking at 50°C intervals from 500 to 800°C. From this it was determined that a temperature of 700 to 750°C was most conducive to glycerin reformation. The results were compared to equilibrium, as calculated by Gibbs free energy minimization. It was found that at temperatures from 750°C to 800°C; most of the results were at equilibrium. Based on this, kinetic models were developed for experiments not in equilibrium. The first model is a pseudo first order model of the gasification, which compares favorably with other studies. The second kinetic model takes into account the carbon containing gaseous species. Three reactions are used to model the gaseous products: Complete gasification of the glycerin into carbon monoxide and hydrogen, water gas shift of the resulting carbon monoxide, and a reaction in which glycerin and hydrogen combine to produce methane. Other reaction pathways were tested, and they either did not fit the data as well, or were thermodynamically impossible. The reactions are also capable of predicting hydrogen production for most conditions.

 

Jared Scott Bouquet

Jared S. Bouquet, Spring 2012

 Thesis title : The Effect of Supercritical Water on Crude Glycerin Solution

Abstract

Crude glycerin solution, a principal byproduct of the conventional biodiesel process, was reacted in supercritical water using a specially designed Haynes® Alloy 282 reactor system for the production of hydrogen rich syngas. The effects of temperature, pressure, water-to-carbon molar ratio, reactor residence time, and glycerin-to-methanol weight ratio on the extent of carbon gasification and gas composition were explored. Based on the results, the extent and selectivity of reactions that concurrently occur when methanol and glycerin are in the presence of supercritical water were analyzed. A decomposition reaction pathway for glycerin conversion to syngas was hypothesized, and then utilizing the results of the crude glycerin solution experiments, as well as experiments using pure methanol and pure glycerin with supercritical water, the hypothesized decomposition pathway was evaluated. Also an empirical equation was formulated to predict the gas composition and carbon gasification of crude glycerin using the reaction conditions of temperature and reactor residence time. To gain a better understanding of how the functional groups of different hydrocarbon molecules react when exposed to supercritical water, experiments were also conducted to determine the effect of hydroxyl groups on hydrocarbons in supercritical water. These experiments used isopropanol, propylene glycol, and glycerin as monohydric, dihydric, and trihydric alcohol feeds. The effect that the number and the position of hydroxyl groups in the molecular structures had on carbon gasification and gas composition were determined at multiple temperatures, of hydroxyl groups were shown to have a great impact on the decomposition mechanism of hydrocarbons.

 

 

David A. Harney, Spring 2012

 Thesis title : Global Homotopy Continuation and Stability Analysis: Optimal Application to Chemical Engineering Design

 

 

Gautham Unni, Spring 2011

Thesis title: Effect of Hydrophobicity and Surface Roughness on Two-Phase Flow in Rectangular Microchannels

Abstract

Two-phase flows in microchannels have received significant attention recently, and have become the cornerstone of numerous microfluidic devices. Microscale devices used for bioengineering applications, oil recovery, and chemical and catalytic microreactor applications involve the transport of bubbles in confined fluidic networks in channels of micrometer length scale. These types of two-phase flows result in pressure variations, leading to an overall increase in pressure drop.  Among various flow parameters, pressure drop is extensively used in design of microfluidic devices. There are several parameters that affect the pressure drop across two-phase flow in microchannels. In the present study, the goal is to be able to predict the pressure drop of two-phase flow in rectangular microchannels as a function of hydrophobicity, surface roughness, and bubble size.  The SU-8 channels are fabricated using photolithography to ensure a perfectly smooth surface, which eliminates the effect of surface roughness. The fabricated channels are treated to alter the contact angle of water on SU-8, isolating the effects of hydrophobicity. Pressure drop data of air-water two- phase flow across the channels was collected, and compared to a previously published model, which predicts the pressure drop across a smooth hydrophilic rectangular microchannel with an air bubble flowing through it.  Deviations of the experimental pressure drop from the predicted values were observed as a function of hydrophobicity and bubble size, into the existing model. A method of fabricating rough SU-8 channels was proposed to isolate the effects of surface roughness. The model was validated using channels of varying aspect ratios.  It was found that the proposed model was independent of the aspect ratio.

 

Mahin Shahlari, Spring 2011

Thesis title: Biodegradable Polymer/Clay Nanocomposites Based on Poly (Butylene Adipate-Co-Terephthalate) and Polylactide

Abstract

In offering a scientifically succinct methodology which leads to property-enhanced biodegradable polymer products for broader application, as well as in promoting the use of bio-sourced renewable polymers such as Polylactide (PLA) in biodegradable polymers with desirable properties from petroleum based polymers such as PBAT, this study and its results both directly and indirectly contribute to the technological solutions that address growing environmental and energy problems caused by the increased use of and reliance upon nondegradable, nonrenewable, petroleum-based plastics.  This work studied blends of PBAT and PLA compounded with organoclay through direct melt mixing in a twin-screw extruder and a batch mixer, determined the most compatible organoclay for this blend, evaluated the role of the particles on the physical properties and morphologies of the blends, compared different mixing procedures, and evaluated the effect of clay loading on the properties.  Blended properly with PLA, PBAT showed significantly enhanced modulus, which was further increased by the addition of organoclay.  Cloisite 30B was the organoclay focused on for this blend and the incorporation of this organoclay was found to improve the thermomechanical performance of the composites.  In blends with equal weight contents of PBAT and PLA, organoclay platelets exhibited better dispersion when they were mixed with either of the two polymers first, than being premixed with the blend.  Migration of particles from one phase to the others was observed in both sequentially mixed blends.  In addition, the effect of an AC electric field, based on the rheological and thermal analysis, resulted in a significant increase on the dispersion of the clay platelets in the PLA matrix.

 

Zhan Gao, Spring 2011

Thesis title: Surface Modification of Micro-Fluidic Devices

Abstract

The surface modification, fabrication and characterization of microfluidic sensor systems for oxygen and glucose measurements with the use of polymeric materials were investigated in this dissertation. This dissertation is prepared in publication format.  The first manuscript focuses on the surface modification of SU-8 layer to enhance the wettability.  The improvement of wettability is critical since biofouling can be minimized during bioanalysis. A novel in situ photopatternable grafting technology was used to attach biocompatible polymer layers on SU-8 surface, changing its property from hydrophobicity to hydrophilicity. The results were confirmed by contact angle measurement and Fourier Transform Infrared (FTIR) spectra.  The second manuscript demonstrates the use of modified SU-8 for applications to microfluidic sensors for dissolved oxygen measurements based on quenching behavior of fluorophore.  A new chemical anchoring method was introduced to lithographically pattern sensing elements inside a complete SU-8 microfluidic channel.  A stable Stern-Volmer relationship between oxygen content and fluorescent intensity was observed for two months.  The third manuscript is about the fabrication and characteristic of fluorescent glucose microfluidic sensor by using an optically transparent dry film, PerMX 3050. It was possible to determine the glucose concentrations by the normalized initial reaction rates, derived from the fluorescent intensity change caused by enzymatic oxygen consumption. Novel polymer materials and surface modification technologies introduced in this dissertation have been successfully applied to optical microfluidic sensors and exhibit a large potential for many other biomolecules assay.