Browsing by Author "Peter Kilpatrick, Committee Member"

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Characterization and Engineering of the Process of Directed Particle Self-assembly in Thin Films and Sessile Droplets
(2007-09-11) Kuncicky, Daniel M.; Stefan Franzen, Committee Member; Orlin Velev, Committee Chair; Peter Kilpatrick, Committee Member; John van Zanten, Committee Member
Directed self-assembly of colloidal particles confined between a solid and liquid interface has been studied as a versatile tool for organizing 2D and 3D crystalline arrays on solid substrates. The overarching goal of this study was to engineer the process of assembly to achieve simple and cost effective solutions for application needs were self-assembled particle structures work better than microfabricated ones. Two technologically relevant geometrical motifs of assembly were studied in detail. A thin film assembly technique based on controlled withdrawal of a meniscus was employed for modifying solid substrates with arrays of organic and inorganic colloidal particles. The assembled particles were used as a template for sterically directing the meso- and micro structure of metallic films for control over their electro-optical functionality. A sessile droplet templating technique was developed for fabricating arrays of discrete colloidal crystal patches of controlled shape and size. The process of assembly was studied in detail for each motif to optimize the deposition conditions and formulate protocols for controlling the size, composition, internal particle symmetry and overall shape. The methods and the results developed are relevant to different disciplines, including self-assembly, surface chemistry, Atomic Force Microscopy methodology, biological research and spectroscopy. Convective assembly and latex templating of gold nanoparticles was used to fabricate highly efficient nanostructured substrates for surface enhancing Raman scattering (SERS)-based sensors. The structure-dependent performance of these SERS substrates was systematically characterized with cyanide in a flow milli-fluidic chamber to simulate on-line continuous water monitoring. A matrix of experiments was designed to isolate the SERS contributions arising from meso- and microscale porosity, long range ordering of the micropores, and the thickness of the nanoparticle layer. The SERS results were compared to the substrate structure observed by scanning electron microscopy and optical microscopy to correlate substrate structure to SERS performance. A single-step method for rapidly assembling tobacco mosaic virus (TMV) into nanocoatings and macroscopically ordered fibers was developed. Uniform films with long-range alignment or arrays of virus bundles were formed through a combination of shear and dewetting. Discrete, contiguous arrays of the TMV fibers were coated over centimeter length scales using only microliters of TMV suspension. The density and branching of the wire structure were controlled by varying the substrate wettability and meniscus withdrawal speed. The ability to precisely control the wire structure of the bio-scaffold allowed for the fabrication of architectures with advanced chemical and physical functionality. As an example, a procedure was developed where the TMV fibers were conjugated to Au particles followed by Ag enhancement for metal deposition. The procedure developed was used to convert the virus fibers into anisotropically conductive arrays of long wires. A systematic study of a sessile droplet templating process for fabrication of colloidal crystals in small micropatches was undertaken. The methodology was based on drying of a particle suspension on a substrate of controlled contact angle. The process of assembly was correlated to the dynamics of the receding contact line. The kinetics of drying were examined by measurement of droplet profiles and it was found that the rate matched well with diffusion-limited dynamics. The effects of major parameters controlling the process: contact angle, particle concentration, and electrolyte were investigated in detail. A variety of micropatch shapes were observed and categorized within the parameter space. Based on the understanding developed from this cycle of experiments we fabricated arrays of gold SERS substrates in the form of flat, uniformly-shaped micropatches with diameters ranging from microns to millimetres. We also demonstrated that the assemblies can serve as a new class of microidenters that can be used directly for biomechanical characterization and hydraulic permeability studies of whole cells and tissue.
Design and Interfacial Activity of Copolymers with Controlled Monomer Sequence Distributions
(2004-01-13) Semler, James Joseph; Jan Genzer, Committee Chair; Carol Hall, Committee Member; Harold Ade, Committee Member; Peter Kilpatrick, Committee Member; Richard Spontak, Committee Member
We study the bulk and interfacial behavior of A-B copolymers. Emphasis is placed upon addressing the role of the monomer sequence distribution in A-B copolymers as it pertains to the copolymer's mobility in confined geometries and its ability to recognize chemical patterns on surfaces. Monte Carlo simulations are used to study the ability of block (A-b-B) and alternating (A-alt-B) copolymers to recognize chemical patterns on flat, impenetrable surfaces comprising two distinct chemical sites, C and D. The copolymer adsorption is driven by the repulsion between A and B segments along the copolymer chain and the attraction between B segments and D sites on the surface. The principle parameters that govern the ability of A-b-B and A-alt-B copolymers to recognize surface patterns are: the strength of the interaction between B segments and D surface sites, the A-B monomer sequence distribution, and the size and spatial distribution of adsorbing D sites. Our simulations reveal that both A-b-B and A-alt-B copolymers are capable of recognizing surface patterns and increasing the B-D attraction enhances the partitioning of A and B segments at the surface. Commensurability between the copolymer's monomer sequence distribution and the size and spatial distribution of the surface heterogeneities is also found to affect the ability of A-b-B and A-alt-B copolymers to recognize surface chemical patterns. When the adsorbing domain size exceeds the size of the copolymer's parallel component to the radius of gyration, A-b-B copolymers are found to transfer the surface pattern into the bulk with high fidelity. A-alt-B copolymers, however, are able to replicate the surface pattern into the bulk material when heterogeneous domain sizes are much smaller. We introduce a novel 'coloring' scheme to synthesize polystyrene-polybromostyrene (PS-co-PBrS) copolymers with statistically random (r-(PS-co-PBrS)) and random-blocky (b-(PS-co-PBrS)) monomer sequence distributions. Our results show that r-(PS-co-PBrS) and b-(PS-co-PBrS) copolymers with equivalent bromine content possess different intrinsic viscosities and radii of gyration. We attribute this behavior to the ability of b-(PS-co-PBrS) coils to form globular structures in toluene where PBrS forms a dense core and PS remains predominantly in a loose corona. This behavior is in contrast to that of r (PS co-PBrS) coils where both the PBrS and PS are homogeneously distributed. The interfacial behavior of the random and blocky copolymers is also found to differ. Specifically, thin films of r-(PS-co-PBrS) deposited on top of flat silica substrates covered with a semifluorinated self-assembled monolayer are found to dewet at a faster rate than b-(PS-co-PBrS) of comparable thickness at the same T−Tg, where Tg is the bulk glass transition temperature of the PS-co-PBrS copolymer. To our knowledge, this is the first experimental evidence that supports claims from computational studies arguing that the sequence distribution of random copolymers affects the chain's mobility on a surface. Molecular insights into the 'coloring' reaction are provided by Monte Carlo simulations of the experimental reaction scheme. The probability of chemically altering expanded homopolymer coils is found to be equal for all units along the length of the chain. In contrast, 'coloring' of collapsed homopolymer coils reveals that the probability of modification is widely distributed. These results further support our claim that copolymers with random and random-blocky monomer sequence distributions can be synthesized by 'coloring' expanded and collapsed homopolymer coils, respectively.
Free Meniscus Coating Using Compressed Carbon Dioxide
(2003-07-16) Novick, Brian Jeffery; Gregory Parsons, Committee Member; Ruben Carbonell, Committee Co-Chair; Jan Genzer, Committee Member; Joseph DeSimone, Committee Co-Chair; Orlin Velev, Committee Member; Peter Kilpatrick, Committee Member
This thesis investigates the use of compressed carbon dioxide as a replacement solvent for web based coating processes including the free meniscus based devices. We use theory, such as Tallmadge's Four Force Inertial Theory, to show why carbon dioxide based free meniscus coaters are advantageous over normal coating processes. We show theoretically that thinner films can be formed at faster rates, that important deposition forces can be controlled, that there is better penetration into porous materials, that there are less capillary forces, that films may have increased uniformity, and that there is better process control. This research also details how coatings can be applied by using a novel high pressure free meniscus coater (hFMC) to deposit thin films of important perfluorpolyether lubricants for microelectronics. The coater was designed as part of this thesis. We have investigated what substrates can be coated by showing that compressed gaseous carbon dioxide induces the wetting of low enery surfaces by low Mw coating precursors. We have shown that the hFMC device can be used to take advantage of the induced wetting. Biocompatible precursors have been coated onto porous PTFE and polymerized at high pressure. The coating process results in porous PTFE with significantly different properties than uncoated samples. We have also investigated what materials can be coated from carbon dioxide by studying the rheological effects of carbon dioxide on coating precursors. We find that changing the backbone structure, end groups, or side groups on the precursor affect the mixture viscosity. The results of this investigation open up new potential applications of this environmentally benign coating process.
Investigations Into the Mechanisms Responsible for the Yield Stress of Protein Based Foams
(2005-04-22) Davis, Jack Parker; Chris Daubert, Committee Member; Peter Kilpatrick, Committee Member; E. Allen Foegeding, Committee Chair; John Cavanagh, Committee Member
Proteins function as natural surfactants in many industrial processes involving foam formation. Egg white proteins have traditionally served this role in the food industry, although substitution with other proteins, including those derived from bovine milk is becoming more prevalent. Above a critical gas phase, foams transition from viscous liquids to semi-solid materials that display a yield stress. Measurements of yield stress relate well to the empirical concept of foam robustness and more robust foams are generally desirable from a food science perspective, in order to withstand the rigors of processing, including pumping, heating, coating, etc. Accordingly, the general goal of this research was to investigate mechanisms responsible for foam yield stress on a fundamental level, in order to more efficiently utilize whey proteins (derived from bovine milk) as a foaming ingredient, with an emphasis on their capacity to regulate foam yield stress. In the first study, the yield stress of whey protein isolate (WPI) foams as affected by electrostatic forces was investigated by whipping 10% w/v protein solutions prepared over a range of pH levels and salt concentrations. Measurements of foam overrun, protein adsorption kinetics at the air/water interface, and dilatational rheological characterization, aided data interpretation. Interfacial measurements were also made with the primary whey proteins, beta-lactoglobulin and alpha-lactalbumin. Yield stress of WPI foams was dependent on pH, salt type and salt concentration. In the absence of salt, yield stress was highest at pH 5.0 and lowest at pH 3.0. The addition of NaCl and CaCl2 significantly increased yield stress at pH 7.0, with equivalent molar concentrations of CaCl2 as compared to NaCl increasing yield stress to greater extents. Salts minimally affected foam yield stress at pH 3.0 or 5.0. Comparisons with interfacial rheological data suggested the protein's capacity to contribute towards yield stress was related to its potential at forming strong, elastic interfaces throughout the structure. Dynamic surface tension data for beta-lactoglobulin and alpha-lactalbumin were similar to WPI, while the interfacial rheological data displayed several noticeable differences. In the second study, polymerized WPI (pWPI) was investigated for its potential as a functional foaming ingredient. Note that pWPI is a soluble complex of covalently bound whey protein formed via controlled heating. Foam yield stress displayed a parabolic response to increasing concentrations of pWPI to native WPI, peaking at 50%. Foam air phase volume steadily decreased with increasing pWPI content, whereas equilibrium surface tension steadily increased. Dynamic surface tension measurements revealed that native WPI adsorbed much more rapidly than pWPI, presumably due to the former's smaller size. Interfacial dilatational elasticity also displayed a parabolic trend with increasing pWPI content, peaking at 50%. This suggested that pWPI coadsorbs with native WPI, bolstering the dilatational elasticity of native WPI interfaces. However, too much pWPI caused a weakening of the network. A positive, curvilinear relationship between dilatational elasticity and yield stress was observed, consistent with earlier data, further suggesting a general link between these parameters. In the third study, beta-lactoglobulin, which is the primary whey protein, was hydrolyzed with three different proteases and subsequently evaluated for its foaming potential. Two heat treatments designed to inactive the enzymes, 75 degrees C/30 min and 90 degrees C/15 min, were also investigated for their effects on foam functionality. All unheated hydrolysates improved yield stress as compared to unhydrolyzed beta-lactoglobulin, with those of pepsin and Alcalase 2.4® being superior to trypsin. Heat inactivation negatively impacted foam yield stress, although heating at 75 degrees C/30 min better preserved this parameter than heating at 90 degrees C/15 min. The previously observed relationship between dilatational elasticity and yield stress was generally confirmed for these hydrolysates. Additionally, the three hydrolysates imparting the highest yield stress not only had high values of dilatational elasticity, they also had very low phase angles (essentially zero). This highly elastic interfacial state is presumed to improve foam yield stress indirectly by improving foam stability and directly by imparting resistance to interfacial deformation. In the final study, the foaming and interfacial properties of WPI and egg white protein (EWP) were directly compared. The highest dilatational elasticity and resistance to drainage were observed for standard EWP, followed by EWP with added 0.1% w/w sodium lauryl sulfate, and then WPI. Previously observed relationships between yield stress and interfacial rheological measurements did not hold across the protein types; however these interfacial measurements did effectively differentiate foaming behaviors within EWP-based ingredients and within WPI. Addition of 25% w/w sucrose to the solutions increased yield stress and drainage resistance of the EWP-based ingredients, but it decreased yield stress of WPI foams and minimally affected their drainage rates. These differing sugar effects were reflected in the interfacial measurements, as sucrose addition increased the dilatational elasticity and decreased the interfacial phase angle for both EWP-based ingredients, while sucrose addition imparted the exact opposite effects on WPI.
Lateral Structuring and Stability Phenomena Induced by Block Copolymers and Core-Shell Nanogel Particles at Immiscible Polymer/Polymer Interfaces
(2010-03-08) Gozen, Arif Omer; Jan Genzer, Committee Co-Chair; Richard J. Spontak, Committee Co-Chair; Orlin Velev, Committee Member; Peter Kilpatrick, Committee Member
We have investigated the parameters such as copolymer/nanoparticle concentration, architecture and molecular weight combined with film thickness, time and temperature in order to develop a molecular-level insight on how lateral interfacial structuring occurs at immiscible polymer/polymer interfaces. In order to develop a molecular-level understanding of how these Ã¢â‚¬ËœsmartÃ¢â‚¬â„¢ self-assembling materials and core-shell nanogel particles interact both intra- and inter-molecularly and form ordered structures in bulk, as well as at immiscible interfaces, we first focused on the response of core-shell polymer nanoparticles, designated CSNGs, composed of a cross-linked divinylbenzene core and poly(methyl methacrylate) (PMMA) arms as they segregate from PMMA homopolymer. We have demonstrated that these nanogel particles exhibit autophobic character when dispersed in high molecular weight homopolymer matrices and segregate to the interface with another fluid. We have further explored the migration of these new-generation nanogel particles (CSNG-Rs) segregating from PS homopolymer to PS/PMMA interfaces. Unlike the instability patterns observed with the CSNGs, which exhibit classical nucleation and growth mechanism with circular hole formation, we have observed an intriguing dewetting pattern and CSNG-Rs forming lateral aggregates and tentacle-like structures at the interface. In parallel with our core-shell particle studies, we have also explored the structuring of copolymer molecules that are far from equilibrium in bulk and complex laminate of polymer thin films. Our early triblock copolymer studies have proven that molecular asymmetry has a profound effect on order-disorder transition temperature. We focused primarily on the effect of the copolymer chemical composition (i.e., block sizes) on the dewetting behavior of PS/SM thin films on PMMA. We elucidate the interfacial segregation and concurrent micellization of diblock copolymers in a dynamically evolving environment with changing boundary conditions as spherical holes develop. These studies reveal that in-plane interfacial nanostructures produced by block copolymers may not always provide stabilization of the bilayer; this behavior has been attributed to the interplay between copolymer micellization and copolymer segregation at the immiscible polymer interface. Lastly, we have investigated the dewetting behavior of PS/PMMA assemblies containing compositionally varied mixtures of mirrored copolymers, such as PS50-b-PMMA10 / PS10-b-PMMA50 and PS50-b-PMMA20 / PS20-b-PMMA50. The dewetting rates of systems composed of copolymer mixtures lie between those of systems modified with the neat copolymers. This observation suggests that the dewetting behavior of the double layer with a copolymer mixture may be approximated satisfactorily by a linear rule of mixtures.
New Electrokinetic Techniques for Material Manipulation on the Microscale
(2008-11-25) Chang, Suk Tai; Glenn Walker, Committee Member; Saad Khan, Committee Member; Peter Kilpatrick, Committee Member; Orlin D. Velev, Committee Chair
We report the results of series of investigations how electrically induced forces and interfacial phenomena could be used to manipulate particles and fluids on the microscale. Particle microseparations in a droplet floating on a dielectrophoretic liquid-chip system were investigated. Particle-localized electroosmotic flow was used for designing autonomously moving microdevices and locally distributed micropumps/mixers. Microfluidics was adapted for enhancing mechanical properties of materials with embedded microchannel networks. Ionic current rectification in charged aqueous gels was used for constructing new types of "soft matter" diodes. Detailed analytical and numerical modeling was performed for each system. The results of this work can apply to new fields of microfluidics, self-propelling microdevices, and aqueous gel-based electronic components. In the first part of this work we explored unusual phenomena of colloidal particle transport and separation inside microdroplets floating in fluorinated oil on electrically controlled chips. Microspheres suspended in a drying droplet on liquid-liquid chips were rapidly separated in the dropletâ€™s top region due to water evaporation. During the evaporation process, a surface tension gradient emerged as a result of a non-uniform temperature distribution within the droplet. This interfacial gradient generated a Marangoni flow inside the evaporating droplet. The suspended colloidal particles driven by the convective flow were collected at the top of the droplets by the hydrodynamic flux compensating for the evaporation. The flow pattern and temperature distribution within the evaporating droplet were simulated using finite element calculation. The internal flow pattern calculated by the simulation was consistent with the experiments using tracer particles. The levitated microdroplets were used as templates for colloidal assembly and containers for microbioassays based on particle agglutination inside droplets. An alternative mechanism of self-propulsion based on electroosmotic force and the extension of this propulsion force to innovative microfluidic pumps/mixers were developed in the second part of this study. Various types of miniature diodes floating in water acted as self-propelling particles when powered by an alternating (AC) electric field. Direct (DC) electric field induced across the diodes as a result of rectification of the external AC field led to particle-localized electroosmotic flow. The resulting reactive force pushed the diodes in the direction opposite to the electroosmotic flux. The microelements began to move parallel to the electric field in the direction of either the cathode or the anode, depending on their surface charge. In effect, the semiconductor microelements harvest electric energy from external AC field and convert it into mechanical propulsion on the microscale. The particle-localized propulsion force was used in diode-actuated electroosmotic motors and actuators. Diodes embedded in microfluidic channel walls could serve as locally distributed pumps or mixers powered by a global AC external field. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows The viscoelastic properties of fluids inside microchannels were used in the development of novel microfluidic materials in the form of flexible sheets that can be solidified on demand to yield preprogrammed shapes. These materials were based on microfluidic channel networks in polydimethylsiloxane (PDMS) filled with photocurable polymers. When the elastic sheets with embedded microchannel networks were shaped and exposed by UV light, the photoresist inside the channels was solidified and acted as endoskeleton within the PDMS layer, acquiring the pre-arranged shape. Bending and stretching moduli of the materials with solidified endoskeleton increased drastically and once the external force was removed, the memorized shapes were recovered. The permanent preservation of the shape of solidified microfluidic sheets could be used in making instant packages and supports on demand. Finally, unidirectional ionic current flow across a fixed junction between two aqueous agarose gel phases containing oppositely charged polyelectrolytes was discovered. The non-linear current response of the interface between the cationic and anionic gels originated directly from anisotropy in the mobile charges within the system. The current densities in the forward bias and current rectifying ratios in the gel diodes were higher or comparable to those using ionic carries and junctions built from conductive polymers. The promising feature of this new type of rectifying junction is that it is operates on the basis of water-borne ions. The devices are extremely simple, inexpensive and possess good long-term stability in DC or AC conduction mode.
Spin Coating and Photolithography Using Liquid and Supercritical Carbon Dioxide
(2002-10-04) Hoggan, Erik Nebeker; Joseph DeSimone, Committee Co-Chair; Peter Kilpatrick, Committee Member; Gregory Parsons, Committee Member; Ruben Carbonell, Committee Co-Chair; Christine Grant, Committee Member
This thesis details work on the utilization of dense phase carbon dioxide (CO2) in semiconductor processing. In particular, work is presented on the formulation of CO2 soluble photoresists and the spin coating of those photoresists using only liquid CO2 as a solvent. As part of this spin coating work, a novel high-pressure CO2 spin coater was designed and constructed, and the theoretical equations governing its performance were derived. Also discussed in this thesis are 248 and 193 nm exposures of these CO2 spun films and subsequent development in supercritical CO2. Resist stripping was also performed in CO2. In short, this thesis details the first steps towards a complete replacement of all aqueous and organic solvents in the conventional photolithographic processes of spin coating, developing, and resist stripping. This change not only confers significant environmental advantages, but opens up many new avenues in resist chemistry and promises improvements in large scale film uniformity, elimination of feature collapse, elimination of extraneous processing steps, and improved control of the lithographic process.
Subcritical Water and Chemical Pretreatments of Cotton Stalk for the Production of Ethanol
(2006-09-10) Williams, Kelly Caldwell; Larry Stikeleather, Committee Member; Peter Kilpatrick, Committee Member; Ratna Sharma-Shivappa, Committee Chair
The objective of this study was to explore the potential of sodium hydroxide, sodium percarbonate and subcritical water as pretreatments for cotton stalk to aid ethanol production. Sodium hydroxide was tested at concentrations of 0, 4 and 8 % (w/v) and sodium percarbonate was tested at concentrations of 0, 1, 2, 4 and 8 % (w/v). Cotton Stalks were pretreated with the chemicals by autoclaving 10% solids slurry at 121 degrees C and 15psi for 30, 60 or 90 minutes. Both chemical pretreatments used a factorial experimental design where each time was run with each concentration. NaOH degraded more lignin with the maximum being 63.4% at 4% concentration and 30-minute treatment time. The maximum lignin degradation by Na-percarbonate was 42.0% at 8% concentration and 90-minute treatment time. Higher concentrations of both chemicals degraded more lignin. Based on the HPLC carbohydrate analysis, NaOH produced significantly higher xylan solubilization than Na-percarbonate. The maximum solubilizations for NaOH and Na-percarbonate, respectively, were 82.7% with 8% concentration and 90-minute treatment time and 59.0% with 2% concentration and 30-minute treatment time. Higher concentrations of NaOH produced higher xylan solubilizations but the solubilization values did not change significantly (p>0.05) for concentrations above 0% Na-percarbonate. Subcritical water was tested at temperatures of 230, 275 and 320 degrees C, holding times of 2, 6 and 10 minutes and ground particles sizes of 1⁄8, 3⁄16 and 1⁄4 inches using a response surface model experimental design. Whole and smashed cotton stalks were also pretreated with subcritical water for each time-temperature combination. Enzymatic hydrolysis was performed on the three ground sample combinations showing the highest lignin degradation and the three with the highest percent total sugars. Lignin analysis was done on all pretreated samples and sugars were analyzed using DNS assay for the subcritical water pretreatments and HPLC for the chemical pretreatments. The highest percent total sugar of 46.3% was found for 230 degrees C, 10 minutes and 3/16 inches and the highest percent lignin reduction of 36.7% was found for 320 degrees C, 2 minutes and 3⁄16 inch particle size. This suggests that lower temperatures produce more total sugars and higher temperatures produce higher lignin degradation. Response surface models were developed for percent total sugars and percent lignin reduction as a function of time, temperature and ground particle size with R-squared values of .6048 and .5112, respectively. The maximum percent total sugar for the whole cotton stalks was 44.8% for the 230 degrees C, 10 minutes combination and for the smashed stalks it was 22.6% for the 320 degrees C, 2 minutes combination. Except in one case, smashing the stalks did not increase the percent total sugars and did not increase the lignin reduction in any cases when compared to the whole stalks. Also, the ground stalks did not show any significant increase in lignin degradation when compared to whole and smashed but there was an increase in the percent total sugars for two of the whole sample sets and four of the smashed sample set. Pure lignin and cellulose samples were pretreated with subcritical water revealing that interaction between the components of lignocellulosic biomass affects the effectiveness of subcritical water pretreatment.