Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2006. / Includes bibliographical references (p. [159]-163). / The coupling between polymer models and experiments has improved our understanding of polymer behavior both in terms of rheology and dynamics of single molecules. Developing these polymer models is challenging because of the wide range of time and length scales. Mechanical models of polymers have been used to understand average heological properties as well as the deviation a single polymer molecule has from the average response. This leads to more physically significant constitutive relations, which can be coupled with fluid mechanic simulations to predict and understand the theological response of polymer solutions and melts. These models have also been used in conjunction with single molecule polymer experiments. While these have provided insight into the dynamics of polymers in rheological flows, they have also helped to design single molecule manipulation experiments. Promising research in this area includes DNA separation and stretching devices. A typical atomic bond has a length of 10-10m and vibration time scale of 10-14s. A typical experiment in a microfluidic device has lengths of order 10-5m and times of order 102s. It is not possible to capture these larger length and time scales of interest while capturing exactly the behavior at the smaller length and time scales. / (cont.) This necessitates a process of coarse-graining which sacrifices the details at the small scale which are not necessary while retaining the important features that do affect the response at the larger scales. This thesis focuses on the coarse-graining of polymers into a series of beads connected by springs. The function which gives the retractive force in the spring as a function of the extension is called the spring force law. In many new microfluidic applications the previously used spring force laws produce significant errors in the model. We have systematically analyzed the coarse-graining and development of the spring force law to understand why these force laws fail. In particular, we analyzed the force-extension behavior which quantifies how much the polymer extends under application of an external force. We identified the key dimensionless group that governs the response and found that it is important to understand the different constraints under which the polymer is placed. This understanding led to the development of new spring force laws which are accurate coarse-grained models by construction. We also examined the response in other situations such as weak and strong flows. / (cont.) This further illustrated the disadvantages of the previous force laws which were eliminated by using the new force laws. This thesis will have practical impact because the new spring force laws can easily be implemented in current polymer models. This will improve the accuracy of the models and place the models on firmer theoretical footing. Because the spring force law has been developed independently of other coarse-grained interactions, this thesis will also help in determining the best parameters for other interactions because they will not need to compensate for an error in the spring force law. These new spring force laws will help form the framework of coarse-grained models which can help understand a wide range of situations in which the behavior at a small scale affects the large time and length scale behavior. / by Patrick Theodore Underhill. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 136-145). / Multifunctional polymeric microparticles have shown the great potentials in a variety of fields. While the advance in particle synthesis allows for fine tuning of their physical properties and chemical functionality, particle manipulation is still appealing, but challenging issue in colloidal science. In order to expand the utility of microparticles, many particle manipulation techniques have been developed to arrange large-scale of particles at precise locations. However, current approaches cannot simultaneously fulfill desired capabilities of arrangement: scalability, precision, specificity, and versatility. This thesis explores the ability to synthesize particles with a controllability of characteristics, and development of a new microfluidic platform, porous microwell arrays, to create structured large-scale microparticle arrays using a scaling theory, which is a function of particles' characteristics. Lastly, we demonstrate the potential of generated particle arrays in various bioengineering application and material sciences. First, we synthesize anisotropic, cell-adhesive microparticles using stop flow lithography (SFL) and carbodimide coupling. Synthesized microparticles are functionalized with collagen or poly-L-lysine using streptavidin-biotin interaction, resulting in cell-adhesiveness. After functionalization, target cells are spread on the particles and spatially patterned only on the functionalized region. Thus, cells are not exposed to potentially harmful components of particle synthesis processes, photoinitiators and ultraviolet light, ensuring no physiological changes. Second, we synthesize multi-striped, upconverting nanocrystal (UCN)-laden microparticles using SFL. Distinct upconversion emission colors are combined with the ability to spatial pattern them, providing superior encoding capacities. We can fine-tune upconversion emission by controlling the dopant composition in nanocrystal, and synthesize microparticles in a highly reproducible manner by SFL, allowing for the development of predictable decoding system. Two types of particles are synthesized with this appealing encoding strategy for two distinct applications: thermally stable particles for anti-counterfeiting application; and porous hydrogels for multiplexed microRNA detection. Third, we develop a microfluidic platform, porous microwell arrays, to manipulate microparticles while fulfilling all four desired capabilities (i.e. scalability, precision, specificity, and versatility). Microwells are fabricated on top of porous membrane by a vacuum-assisted molding method. Particles are guided and assembled into wells by hydrodynamic force associated with fluid flow through pores in microwell. Iteration of assembly and washing steps ensures high-throughput, large-scale particle arrangement with high yields on filling and capturing. Scaling theory allows for the rational design of platform to specifically position microparticles depending on their physical characteristics (i.e. size, shape, and modulus), enabling to generate complex patterns. We utilize this platform in three practical applications: high-throughput, large-scale single-cell arrays; microenvironment fabrication for neutrophil chemotaxis; and UCN-laden covert 2D tags for anti-counterfeiting. Lastly, we modified the porous microwell platform to a closed system, microfluidic channels, to park and isolate particles in monodisperse droplets surrounded by fluorinated oil. Rational modification retains the platform's desired capabilities, resulting in a single particle in a droplet with high yields on both parking and isolation. Particle-in-droplet arrays enables the observation of reaction in confined volume over the time. Such arrays can be utilized to accumulate the desired product from enzymatic reaction, amplifying the signal and improving the sensitivity of bioassays. We demonstrate the highly sensitive, multiplexed miRNA detections with these particle-in-droplet arrays. / by Jae Jung Kim. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February 2017. / Cataloged from PDF version of thesis. "November 2017." Handwritten on title page "February 2018." / Includes bibliographical references (pages 107-116). / Single-layer graphene membranes and other 2D membranes can realize very high gas permeation fluxes due to their atomic or unit cell thickness. Established modeling approaches for membrane transport consider transport through a finite and continuum thickness, and therefore they do not apply to the emerging field of 2D membranes, motivating the development of new theoretical treatments. In this thesis, I first developed an analytical theory for the transport of gases through single- layer graphene membranes, from the perspective of using pores in the graphene layer as a means for separation. I considered two pathways for the transport. The first being direct gas phase impingement on the pore, for which the large-pore separation factors are dictated by Knudsen selectivity, inversely proportional to the molecular weight; selectivity exceeding Knudsen is possible with smaller pores that reach a size commensurate with the size of the molecule, enabling separation by molecular sieving. The second pathway involves adsorption and transport on the graphene surface, similar to mechanisms in heterogeneous catalysis, which becomes more relevant for larger, strongly-adsorbing molecules. These models and pathways are applied for an estimate of a N2/H2 separation and as an explanation for results observed in the molecular dynamics literature. I applied our understanding of nanopore mechanisms and developed analysis of gas transport through graphene with approximately one selective nanopore etched into it, using experimental data from Bunch et al at Boston University for transport of He, H2 Ne, Ar, and CO2 through a small area graphene membrane with a single or few pores. The transport was measured by collaborators via monitoring the deflection of a graphene flake sealing a pressurized, 5[gamma]m diameter microcavity on the surface of a Si/SiO2 wafer. For this experimental system, I report on a mathematical formalism that allows one to detect and analyze stochastic changes in the gas phase fluxes from graphene membranes, extracting activation energies of pore rearrangements, 1.0 eV, and even identifying contributions from multiple, isolated pores.One opportunity that I identified is the use of a molecularly sized nanopore to 'direct write' the flux using a translatable platform. I performed an exploratory investigation of this concept of using a "nanonozzle," a nanometer scale pore that can deliver a flow of material locally, to grow nanoscale features. The model application was the growth of a graphene nanoribbon on a surface. I explored a variety of analytical mathematical models to understand the parameters and limitations of such a system. I developed a simple simulation of the nanoribbon growth and compared the results to the models for a range of parameters, considering the reasons for differences between the simulated and calculated results. This analysis provides considerations for the experimental design of such a system. Overall, the theories in this thesis and the analysis in they enable should aid the development of 2D membranes for separations applications and a novel direct write method for nanoscale patterning. / by Lee William Drahushuk. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 171-175). / It is essential to control crystallization in many areas of science and technology, such as the production of pharmaceuticals, pigments, concrete, semiconductors, as well as the formation of biominerals. In most practical circumstances, crystallization starts with heterogeneous nucleation at a foreign surface. Despite its widespread occurrence, mechanistic understanding of the role of a surface in heterogeneous nucleation is limited, especially in a solution environment. My thesis aims at elucidating the roles of surface chemistry and nanostructure on nucleation to enable rational design of surfaces for controlling crystallization from solution. To this end, I systematically investigated the role of surface chemistry, morphology, in particular porous structures of various polymeric materials on heterogeneous nucleation using small organic molecules as model compounds. I have demonstrated quantitatively the significance of surface chemistry to nucleation kinetics using a variety of polymer surfaces. By tuning the surface composition of the polymers, aspirin nucleation was promoted by up to an order of magnitude compared to the bulk. Further mechanistic investigations revealed that, macroscopically, it is through interfacial free energies that the surfaces influence the surface nucleation activity. Equipped with nucleation induction time statistics as a powerful tool, I found that nanoscopic pores of 50-100 nm accelerated nucleation by up to two orders of magnitude compared with surfaces without pores. Moreover, I demonstrated for the first time that the shape of surface nanopores is essential in determining the nucleation behavior, using lithographic methods for nanopatterning the polymer films. A molecular mechanism was further proposed based on additional mechanistic investigations. Furthermore, the nanoconfinement effect on nucleation was studied using polymeric microgels with tunable nanostructures and chemistry, whose mesh sizes range from 0.7-2 nm. We presented the first experimental evidence for the existence of an optimum confinement size at which the rate of nucleation was dramatically enhanced by up to four orders of magnitude. The degree of nucleation enhancement depends on the extent of polymer-solute interactions, whose role was elucidated from the perspective of adsorptive partitioning and nucleation-templating effect. In addition, the microgel nanostructure was also shown to play an important role in determining the crystal polymorphism of pharmaceutical compounds. / by Ying Diao. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1943. / Includes bibliographical references (leaves 86-87). / by Arthur W. Plummer. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2001. / Includes bibliographical references. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / With the growing desire for sustainable technologies, reactions in benign solvents, such as hydrothermal water and supercritical fluids, have become the focus of many investigations. Hydrothermal water has been used as a medium for chemical reactions, where the enhanced dissociation constant of water led to both acid- and base-catalysis without added reagents. Supercritical water oxidation (SCWO) has been proposed as an alternative technology for the treatment of aqueous-organic waste streams. At typical SCWO conditions (T = 550-650°C and P = 250-300 bar), mixed organic waste streams can be completely mineralized (>99.99%) in residence times of less than one minute. In an SCWO process, the waste stream is preheated to temperatures of >400°C in the absence of oxygen prior to the reactor. In the preheater, hydrolysis reactions may occur, significantly changing the composition of the reactor feed. To properly model these processes, a fundamental understanding of the reaction pathways and associated rates is essential. The rates of methylene chloride and methyl tert-butyl ether (MTBE) hydrolysis in sub- and supercritical water have been measured experimentally at 250 bar over a range of temperatures from 100 to 600°C. The rate constants for both compounds showed a local maximum below the critical temperature of water (374°C) followed by a local minimum just above the critical temperature. This behavior was qualitatively attributed to the changes in the solvent properties of water, shifting from a polar solvent in the subcritical region to a nonpolar solvent in the supercritical region. One of the primary objectives of this thesis was to develop a better understanding of the molecular-level effects of the solvent on the reaction rates and mechanistic pathways. The effects of water as a solvent on the hydrolysis reaction of CH2Cl2 were modeled as a dielectric continuum using Kirkwood theory. A correction factor, obtained from ab initio calculations, was applied to adjust the activation energy in order to account for differences in the free energy of solvation of the reactant and the transition state. Application of the Kirkwood correction to the empirical rate expression fit to data from 100-250°C yielded a model that quantitatively agreed with the experimentally measured reaction rate over the entire temperature range (from 100 to 500°C). To explain the extrema in the rate constant measured for MTBE hydrolysis, an acid-catalyzed mechanism was proposed. A new empirical rate expression was determined with a first-order dependence on the concentrations of H+ and MTBE. For the entire temperature range studied from 150 to 600°C, the empirical rate expression quantitatively modeled the experimentally measured decomposition rate within the uncertainty of the experiments. Further experiments were conducted with added HCl or NaOH that validated the acid-catalyzed hydrolysis pathway. The experimentally observed dependence on the concentration of H+ was slightly smaller than predicted by the proposed mechanism. Ab intio tools were employed to determine the relative contribution of a unimolecular decomposition pathway, which concluded that the pathway was not significant below 550°C. The unimolecular decomposition pathway set a lower limit on the overall reaction rate, which was observed experimentally under basic conditions where the acidcatalyzed pathway was effectively shut off. In addition to the kinetic measurements, two new experimental tools were developed to improve the capabilities of the supercritical fluids laboratory. Firstly, a new, large-bore tubular reactor system was built to address limitations in the current flow reactors in the supercritical fluids laboratory. The reactor was designed with the following advantages: 1) large diameter to minimize wall effects; 2) direct organic feed to eliminate hydrolysis during preheaters; 3) movable sampling probe; 4) sapphire windows to allow optical accessibility. The reactor was tested in preliminary runs up to 600°C and 250 bar. Secondly, a new reactor system was built to allow optical accessibility for in situ Raman spectroscopic measurement in supercritical fluids. The system was used in a study to probe local solvent effects in supercritical carbon dioxide. The effect of temperature, pressure, and density of CO2 on the vibrations of benzene and methylene chloride were investigated. As the density of CO2 increased, the vibrations shifted to lower frequency initially, and then leveled off at moderate densities. This leveling off may be due to local clustering of solvent molecules around solutes in these systems. / by Joshua David Taylor. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages [147]-156). / Knots seem to be found every time one encounters long, stringy objects. At the macroscopic scale, knots are seen every day in shoelaces, tangled hair, or woven clothing, yet they also present themselves at the microscopic scale in long polymer molecules. Knots can be found often in DNA packaged within the viral capsid, occasionally in proteins, and during the transcription and replication of genomic DNA. Biological knots are similarly thought to change the dynamics of viral ejection, protein digestion, and translocation of biomolecules through nanopores. Despite the prevalence of knots in important biological polymers, to date, the physics of knots is only partially understood. DNA has become a well-accepted model system for investigating the physics of single polymer molecules due to its tremendous biological significance and useful experimental properties. Recent advances in microscopy and nanofabrication have enabled the real-time manipulation and imaging of single DNA molecules, facilitating fundamental studies concerning the physics of individual polymers. Leveraging these experimental techniques, this thesis aims to explore the changes knots can impart on the static and dynamic properties of single DNA molecules. We first demonstrate a mechanism for the previously observed phenomenon of the compression and self-knotting of a single DNA molecule in the presence of an electric field. We then use this mechanism to study the process of stretching complex DNA knots in an extensional field. These knots dramatically alter the way DNA stretches in two ways: an initially arrested state and a subsequently slowed stretching phase. Our work consists of the first experimental support of these phenomena, originally predicted by simulation and theory. We then develop theoretical arguments, shown to agree with simulation results, for the physics that govern the distribution of sizes of knots that stochastically occur on DNA molecules, and more broadly, all semiflexible polymers. We then extend our theory to the case where the entire DNA molecule is confined and elongated within a channel. Here, the complex non-monotonic behavior of the sizes of knots agrees with our modified theory. We finally present the results of dynamical simulations where knots on polymers interact with flows or forces. We first examine the behavior of a knot along a polymer extended by extensional flow. The flow may cause a knot to be swept off a polymer molecule, and the motion of a knot is consistent with a model. Different families of knots display different rates of motion, and we explain this difference with a simple topological mechanism. We then turn to examine the case of knots jamming on a polymer molecule extended with high tensile forces. A simple energy barrier hopping argument qualitatively explains the observed slowdown in dynamics of knots. We use these results to reexamine the problem of DNA knots jamming during nanopore translocation, and our results establish the potential for using knots to slow and control the rate of translocation by a ratcheting mechanism. The impact of this thesis is threefold. First, we have demonstrated a novel experimental platform capable of interrogating DNA knots, likely the most efficient of its kind. Second, we have established a theoretical framework for the size and probability of knotting in single molecules capable of directing experiments where these properties need to be controlled. Finally, we have shown how knotted topologies can be manipulated by external flows or forces, which have applications involving preconditioning molecules to unknotted states or the jamming of knotted molecules in nanopores. / by C. Benjamin Renner. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. [139]-146). / Understanding the ways that matter deforms and flows, which is the focus of the branch of science known as rheology, is essential for the efficient processing and proper function of such practically and technologically important materials as plastics, paints, oil-drilling fluids, and consumer products. Rheology is also powerful from a scientific perspective because of the correlation between rheological properties and the structure and behavior of matter on microscopic and molecular scales. The developing sub-field of microrheology, which explicitly examines flow and deformation behavior on microscopic length scales, provides additional clarity to this connection between rheology and microstructure. Aging materials, whose rheological properties evolve over time, are one class of materials that are of significant scientific and practical interest for their rheological behavior. Also, the unique field-responsive rheological properties of magnetorheological (MR) suspensions, which can be tuned with an applied magnetic field, have been used to create active vibration damping systems in such diverse applications as seismic vibration control and prosthetics. A material that undergoes rheological aging and that has received much attention from soft matter researchers is the synthetic clay Laponite® . This material is attractive as a rheological modifier in industrial applications and consumer products because a rich array of rheological properties, including a yield stress, viscoelasticity, and a shear-thinning viscosity, can be achieved at very low concentrations in aqueous dispersions (~ 1 w%). Though this behavior has been investigated extensively using traditional 'bulk' rheology, a number of important questions remain regarding the nature of the dispersion microstructure. The techniques of microrheology, in which rheological properties are extracted from the motion of embedded microscopic probe particles, could help to elucidate the connection between microstructure and rheology in this material. Microrheological studies can be performed using passive techniques, in which probes are subject only to thermal motion, and active techniques, in which external forces are applied to probes. Because aqueous Laponite® dispersions exhibit a significant yield stress, they could be beneficial as novel matrix fluids for magnetorheological suspensions. MR fluids consist of a suspension of microscopic magnetizable particles in a non-magnetic matrix fluid. When an external magnetic field is applied, the particles attract each other and align in domain-spanning chains of particles, resulting in significant and reversible changes in rheological properties. Because of the typically large density difference between the matrix fluid and the suspended magnetic particles, however, sedimentation is often problematic in MR fluids. A yield stress matrix fluid such as an aqueous Laponite® dispersion could help address this issue. In this thesis, bulk rheology and microrheology experiments are combined in order to provide a thorough characterization of the rheological properties of aqueous Laponite® dispersions. Multiple Particle Tracking (MPT), a passive microrheology technique, is used to explore the gelation properties of dilute dispersions, while an active magnetic tweezer microrheology technique is used to examine the yield stress and shear-thinning behavior in more concentrated dispersions. MPT results show strong probe-size dependence of the gelation time and the viscoelastic moduli, implying that the microstructure is heterogeneous across different length scales. We also demonstrate the first use of magnetic tweezers to measure yield stresses at the microscopic scale, and show that yield stress values determined from bulk and micro-scale measurements are in quantitative agreement in more concentrated Laponite® dispersions. With a thorough understanding of the clay rheology, we study the magnetorheology of MR suspensions in a yield stress matrix fluid composed of an aqueous Laponite® dispersion. Sedimentation of magnetic particles is prevented essentially indefinitely, and for sufficient magnetic field strengths and particle concentrations the matrix fluid yield stress has negligible effect on the magnetorheology. Using particle-level simulations, we characterize the ability of the matrix fluid yield stress to arrest the growth of magnetized particle chains. The methods and results presented in this thesis will contribute to the fundamental understanding of the rheology and microstructure of aqueous Laponite® dispersions and provide researchers with new techniques for investigating complex fluids on microscopic length scales. Additionally, our characterization of the effects of a matrix fluid yield stress on magnetorheological properties will aid formulators of MR fluids in achieving gravitationally stable field-responsive suspensions, and provide a new method for manipulating the assembly of particle building blocks into functional structures. / by Jason P. Rich. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 92-105). / In this thesis work, we aimed to explore crystallization processes for small molecule API compounds based on engineered polymer surfaces that could be used in continuous manufacturing. First, we identified a library of polymers that can be used and selected PVA as the model polymer based on its solution and film properties. We also illustrated a rational approach for designing and fabricating PVA film surfaces for increasing heterogeneous nucleation rate of different compounds and enable polymorph selection. The design philosophy was to select prevalent angles between major faces of crystals according to a selection of compounds, and to create substrate surfaces with indentations that include these angles. Nucleation induction time trends showed that heterogeneous nucleation rates were accelerated by at least an order of magnitude in the presence of PVA due to the favorable interactions between the model compounds and the polymer. Nucleation rates were further increased for patterned substrates with matching geometries. Surface indentations with non-matching angles resulted in faster nucleation rates than flat films but slower than matching geometries because they only increased the effective area of the films and their roughness. X-ray diffraction was used to reveal faces that preferentially interacted with the PVA side chains and to deduce possible arrangement of solute molecules at the corners of the indentations. Combining X-ray data and morphology of the crystal product, we suggest that matching geometries on the substrate enhanced nucleation of compounds. In addition to enhancing nucleation rate, polymorph selection was possible in the presence of the polymer substrate to yield a higher percentage of thermodynamically stable gamma indomethacin. Offline Raman experiments and in-line morphology determination confirmed that polymorph control of the final crystal product via kinetic control of the nucleation process was viable. For the aspirin system, the 85 degree angle lead to the highest rate of nucleation; for the polymorphic indomethacin system, XRPD results showed that gamma form preferentially formed on the PVA films with 65 and 80 degree angles leading to the largest reduction in nucleation induction time. Kinetic Monte Carlo simulation showed that a crystallizer incorporating both nucleation and crystal growth in the absence of active mass transfer would have too small a throughput and too large a footprint to be useful. The main reasons were long average nucleation induction times and slow crystal growth in the absence of convection. A set of batch desupersaturation experiments showed that mass transfer limited growth dominate the crystal growth kinetics at low supersaturations when nucleation events were suppressed. An increase in the bulk fluid velocity increased the effective growth kinetics in the system when mass transfer kinetics dominated. Steady state modeling based on the first principle approach was performed using a combination of Navier Stokes Equations and diffusion-convection mass transport equations. The modeling result demonstrated that for mass transfer from a moving fluid to a stationary surface, a thin momentum and concentration boundary layer existed at the leading edge, which resulted in much higher local mass transfer rates. In the absence of momentum boundary layers, mass transfer could only occur via diffusion, which resulted in slow growth kinetics. The first principle model was used to derive dimensionless number correlations for the continuous crystallizer. / by Li Tan. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007. / Includes bibliographical references (p. 147-153). / Electrospinning or electrostatic fiber spinning employs electrostatic force to draw a fiber from a spinneret. This fiber solidifies and lies down on a collector in the form of a non-woven fiber mat. Electrospinning has attracted much attention recently due to the ease with which fine fibers about 10 nanometers to 10 microns in diameter can be produced from both natural and synthetic polymers. Despite the large volume of publications on this technology, few publications discuss the mechanics of electrospinning. Most publications deal with the exploratory works on what material can be electrospun and the potential applications of the electrospun fibers. This work examined the electrohydrodynamics of the electrospinning process and developed this technology for making functional materials. The first part of this dissertation deals with the electrohydrodynamics of the process. The effects of processing parameters and material properties on the size and structure of electrospun fibers were studied. The experimental findings validated the analytical scaling model developed by Fridrikh and co-workers to predict how the final radius or "the terminal jet radius" of the electrospun fiber depends on the processing parameters. / (cont.) The scaling formula is derived from the force balance between surface tension and surface charge repulsion. The scaling model provides a powerful tool for controlling the fiber diameter just by adjusting the surface tension, the flow rate, and the electric current on the jet. The next part of this dissertation describes the role of fluid elasticity in the formation of fibers from polymer solution by electrospinning. Obtaining a uniform electrospun fiber can become problematic when the polymer solution is too dilute. In this case, experience suggests that the lack of elasticity prevents the formation of uniform fibers; instead, droplets or necklace-like structures know as "beads-on-string" are formed. Model fluids were prepared by blending small amounts of high molecular weight polyethylene oxide (PEO) with concentrated aqueous solutions of low molecular weight polyethylene glycol (PEG). The formations of beads-on-string and uniform fiber morphologies were observed in a series of solutions having the same polymer concentration, surface tension, shear viscosity, and conductivity but with different degrees of extensional viscosity. A high degree of extensional stress was observed to arrest the breakup of the jet, which was due to the Rayleigh instability. / (cont.) In some cases, the extensional stress was able to suppress the Rayleigh instability altogether. The susceptibility of the jet to the Rayleigh instability was examined in two ways. First, a Deborah number was defined as the ratio of the fluid relaxation time to the instability growth time and was shown to correlate with the arrest of droplet breakup, giving rise to electrospinning rather than electrospraying. Second, the critical extensional stress on the jet was shown to be large enough in some cases to completely suppress the Rayleigh instability. The next part of this dissertation describes ways to produce functional electrospun fibers for potential applications. It presents a method to electrospin into fibers materials that would otherwise be difficult or impossible to process using conventional extrusion or electrospinning. This method involves electrospinning two materials into fibers with core-and-shell morphology. The "electrospinnable" shell fluid serves as a processing aid to electrospin the core fluid. The shell of the fiber can be removed during post processing, while the core of the fiber remains intact. Several types of core/shell fibers were produced for the first time to illustrate the versatility of this technique. / (cont.) Finally, the dissertation presents a process to make transparent, electrospun-fiber reinforced-composite that has the ability to change reversibly, its color and transparency in response to stimuli, such as irradiation. Matching the refractive indexes of the fiber and the matrix is important in order to reduce haze and poor visibility. Any large mismatch will contribute to haze and poor visibility. Electrospinning is chosen to produce the reinforcing fiber for the following reasons. Electrospinning produces very fine fibers (average diameter ranging from 100 nm to 500 nm) that can minimize the scattering of light in case there is a slight mismatch in refractive indexes. Electrospinning can produce a non-woven mat that has a large ratio of surface area to mass for better bonding to the matrix material. It also allows the dye to disperse in the spin solution without compromising the chemical stability of the dye during electrospinning. The result is a mechanically tough, highly transparent composite that has low haze, the ability to change color, and selective transmittance. / by Jian Hang Yu. / Ph.D.