Digital Signal Processing of TIRM Data Trajectories
Edward S. Atman, Paul Odiachi and Dennis C. Prieve
Department of Chemical Engineering

TIRM data sequences may be corrupted by variations in incident intensity as well as background illumination. These effects may be studied by applying spectral analysis to the TIRM signal and applying low pass filtering techniques to remove noise from the data.

Manipulating Adsorbed Polymer Layers Via Polymer-Surfactant Complexation
Alan D. Braem, Dennis C. Prieve, Robert D. Tilton
Department of Chemical Engineering

The prevalence of multicomponent complex fluids in nature and industry challenges us to understand interfacial phenomena in mixtures, particularly those containing polymers and surfactants. Dynamics are especially important in these systems, since the adsorbed layers are often kinetically trapped in non-equilibrium states. Here we present a study coadsorption of an amphiphilic ethylene oxide-propylene oxide-ethylene oxide triblock copolymer, Pluronic F108, and the anionic surfactant sodium dodecyl sulfate (SDS) to a negatively charged silica surface. The polymer and surfactant bind strongly in solution, yielding a polyelectrolyte-like complex with varying charge, depending on the relative concentrations of the components and the ionic strength. Using optical reflectometry we find that these complexes show several unique adsorption characteristics. We observed a surfactant shuttling effect, whereby the normally non-adsorbing surfactants are carried to the surface by the polymer. We also observe that the overall adsorbed amount can be tuned by altering the relative surfactant/polymer concentration or the ionic strength of the system. We propose colloidal solution depletion measurements and electrokinetic measurements to probe the structure and composition of the adsorbed layer under these varying conditions.

Predicting Van der Waals forces at Rough Interfaces with Diffuse Polymer Coatings
Raymond R. Dagastine, Dennis C. Prieve and Lee R. White
Department of Chemical Engineering

Many theoretical predictions of van der Waals forces use model systems and smooth interfaces while actual colloidal systems have rough or coated interfaces. This work treats rough interfaces as a series of homogenous layers between two smooth interfaces. These homogenous layers have material properties that are intermediate between the smooth materials on each side of the interface. This approach is expanded to diffuse polymer coatings as well to provide theoretical predictions that model actual interfaces.

The van der Waals interaction for multiple homogenous layers at interfaces are calculated using Lifshitz theory to model rough PS surfaces in an aqueous solution. Surface topography information was obtained from atomic force microscopy (AFM) images of the PS surfaces. The rough interface is sectioned off into discrete layers and average material properties for each layer are calculated based on the volume fraction of PS as a function of height from the smooth surface. A Clausius-Mosotti mixing rule was used to calculate average dielectric properties for each layer for use in the Lifshitz calculation. The theoretical results are compared to experimental TIRM results for a PS sphere and plate in water.

This method was expanded to a diffuse polymer coating of Pluronic(r) F-108 absorbed on rough PS in an aqueous solution, where the discrete layers were averaged based on volume fractions of all three components. The theoretical predictions were also compared to experimental TIRM measurements of a PS sphere and plate with a Pluronic(r) F-108 absorbed polymer coating.

Atom Transfer Radical Polymerization of t-Butyl Acrylate and Preparation of Amphiphilic and Multi-block Copolymers
Kelly A. Davis, Krzysztof Matyjaszewski*
Department of Chemistry

t-Butyl acrylate was polymerized using atom transfer radical polymerization (ATRP) techniques. An activated alkyl halide, methyl 2-bromopropionate, and a CuBr/N,N,N~,N~,N~-pentamethyldiethylenetriamine catalyst system were used to obtain a controlled polymerization. The addition of 5 mol % added CuBr2, relative to [Cu(I)], and small amounts of solvent (20-25%) resulted in linear plots of ln ([M]0/[M]) vs. time, indicating negligible termination during the reaction. Molecular weights increased linearly with conversion and agreed well with the theoretically predicted values. Polydispersities were low, however, increasing the polarity of the solvent lowered the rate of polymerization and the final polydispersity, presumably through an increased concentration of deactivator in solution. These polymers have been hydrolyzed to afford poly(acrylic acid). Amphiphilic block copolymers were prepared by using poly(t-butyl acrylate) as a macroinitiator for further ATRP reactions, followed by a deprotection step to remove the t-butyl group. Poly(t-butyl acrylate) macroinitiators were also used successfully to prepare ABC block copolymers of various chain architectures.

Thermocapillary Flow and Aggregation of Bubbles on a Solid Surface
Hiroki Kasumi, Yuri Solomentsev, Paul J. Sides
Department of Chemical Engineering

Oxygen bubbles electrolytically evolved on vertically oriented tin oxide electrode have been reported to migrate on the surface toward each other by Sides and Tobias. The attraction was independent of orientation with respect to gravity. A model based on thermocapillary flow around a bubble is developed to explain this phenomenon.

A temperature gradient perpendicular to the plate is established due to reaction irreversibility and ohmic heating. It creates a surface tension gradient at the bubble-liquid interface because surface tension is a function of temperature. The surface tension gradient leads to a shear stress gradient that pumps adjacent fluid away from the plate.
This flow of the fluid is called thermocapillary flow. Bubbles within proximity of a few radii mutually entrain each other until they aggregate.

From the governing equation of the thermocapillary flow, it is predicted that there are relationships between the rate of bubble aggregation and the temperature gradient, the bubble size, and the liquid viscosity. Bubble aggregation experiments under carefully controlled conditions without electrochemistry were performed. We found that scaling the experimental results according to the theory of thermocapillary flow collapsed all of the data onto a master curve, which is strong evidence that our bubble aggregation theory is correct. The governing equation is solved and it shows reasonable agreement with the experimental results when hindrance effect from the plate is taken into account. The hindrance effect is obtained from an independent measurement as well as calculation.

Monolayer Structure of Nonionic Surfactants at the Solid-Vapor Interface
B. Luokkala, S. Garoff, and R. Suter
Department of Physics

We investigate the structure of dry monolayers of the alkyl polyethyleneoxide surfactants, CmEn, using x-ray reflectivity. The monolayers are adsorbed to a hydrophilic silica surface from bulk surfactant solutions below the critical micelle concentration, and the sample is withdrawn from solution through the three phase contact line. For constant tail group chain length (m=12), we observe a nonmonotonic progression in both the thickness and the area per molecule as a function of the size of the head group (n=1-8). The thickness of the monolayer is maximum and the area per molecule minimum when n=4, corresponding to nearly equal head group and tail group chain lengths. The results suggest a crossover in the dominant interaction which determines the structure of the monolayer.

Gene Therapy: Colloidal Gold Vectors for DNA Formulation
Millicent M. Ow, Todd M. Przybycien
Department of Chemical Engineering

Gene therapy is used to treat genetic diseases ranging from cystic fibrosis to cancer. Its versatility is inherent: gene therapy is the process of introducing to any body tissue a normal segment of DNA in order to functionally substitute for a corresponding damaged or missing one. The body can use therapeutic DNA to permanently produce the protein drug it codes right at a diseased tissue site.

For gene therapy to be effective, therapeutic DNA must be safely transported to and processed inside the nuclei of cells in a mutant tissue. As previously studied gene delivery systems have not successfully implemented a non-toxic route to sufficient drug production, a colloidal gold based vehicle is suggested and examined.

13 nm diameter gold colloid was synthesized by citrate reduction and sized by transmission electron microscopy and dynamic light scattering. The suspension has optically been judged stable over months. An alkanethiol self-assembled monolayer is being added to the colloid's surface to provide cellular and nuclear targeting routes and to mediate electrostatic DNA adsorption. Preliminary Fourier transform infrared spectroscopic studies on the modified colloid confirm monolayer existence. Other current work includes the identification of cellular and nuclear targeting residues and the beginning of DNA adsorption modeling studies.

The Effect of Transient and Steady State Operating Conditions on the Morphology of Immiscible Polymer Blends in Complex Flows
Brian E. Priore, Lynn M. Walker
Department of Chemical Engineering

Continued growth of the use of immiscible polymer blends in commercial applications hinges on the ability to adequately control/predict blend morphology. In this work, we are probing in-situ blend morphology in mixed flow fields, an extension of the considerable amount of research on done in simple flow fields (e.g., pure shear or elongation at a constant rate). A specially designed slit flow cell provides a non-homogenous, shear-dominated flow field with a transient elongation pulse of varying magnitude and duration. The morphological findings for steady flow approaching a contraction have been examined. Specifically, the effects of viscosity ratio, droplet size, and contraction geometry (i.e., elongational intensity and duration) are quantified. The sensitivity of the blends to these mixed flows has been compared to predictions developed in simple flow fields. The morphological response to changes in processing flow rate have been investigated also. Long lasting, structural transients are observed under step-up conditions but not for start-up flows. These transients are attributed to morphological changes caused by flow induced coalescence at the lower processing rate. The effect of concentration, viscosity ratio, and rest time on the coalescence and break-up rate in mixed flows has been examined; a key result being that there is a critical viscosity ratio in which the coalescence time becomes much too long for significant structural changes to occur in the blend.

Fluid Film Pulling in the Presence of Evaporation
Dan Qu, Stephen Garoff
Department of Physics

Volatile PDMS oils form films on SiO2 substrates when the substrates are pulled out of the bulk fluid. The films have constant length after steady state has been reached. The behavior of the evaporative film is fundamentally different from that of the non-evaporative film. The latter has a infinite length as the substrate is pulled out of the bulk fluid and the shape near the bulk meniscus is described by Bretherton's model. While the former has a finite length in steady state and has a different shape from that described by existing models. We have investigated the effect of evaporation rate, fluid viscosity and withdrawing velocity on the length and volume of the film. We found that the length and volume each has a power law dependence on the capillary number of the system. We also found that the film profile has a quadratic shape in steady state.

Hydrodynamics controlling relaxation process of contact angle
Yue Suo, Stephen Garoff
Department of Physics

In nature and in technological applications, contact lines are often accelerating or decelerating as fluid bodies spread on solid surfaces. We have examined the hydrodynamics near contact lines accelerating or decelerating toward their steady state value. We compare the liquid/vapor interface shape near the contact line and temporal relaxation of the contact angle with a simple model, which assumes the drag forces may be modeled as if the contact line were in steady state motion. At very early times we see deviations from this model indicating that the inertia has a measurable effect. At later times, our model describes the contact line behavior well, indicating that inertia is negligible during this time period.

The Effects of Nonionic Polymer on the Rheology of Dilute Rod-like Micellar Systems
My Hang T. Truong, Lynn M. Walker
Department of Chemical Engineering

In this work, rheological measurements and small-angle neutron scattering (SANS) are combined to characterize the structure of dilute aqueous systems of nonionic polymer and cetyltrimethylammonium tosilate (CTAT), a cationic surfactant which forms rod-like micelles. Rheological measurements show that polyethlyene oxide (PEO) and hydroxypropylcellulose (HPC) have dramatically different effects on CTAT rheology. At rest, SANS shows that both polymers have significant effects on micellar interactions and/or length, though the rod-like shape and micellar radius are preserved. Under shear, CTAT micelles align in the direction of flow, forming "shear-induced structures" (SIS). Consistent with rheological results, dilute concentrations of HPC tend to increase the critical shear rate required for SIS formation, while PEO has little or no effect. Despite their effects on micellar systems in the quiescent state and on the onset of SIS formation, the growth of the SIS is independent of the amount or type of polymer added. We hypothesize that the availability of the hydrophobic moieties of the polymer chain accounts for the different effects on CTAT rheology. Using SANS and rheology, we demonstrate that subtle changes in the physical chemistry of an added polymer can effectively control the rheology of rod-like micelles. Characterization of the structure of mixed systems will allow for better control of polymer-CTAT rheology as well as provide further insight into the SIS.

A Novel Total Internal Reflection Fluorescence Assay for Measuring Adsorbed Enzyme Activity
Eric F. Tsung, Robert D. Tilton
Department of Chemical Engineering

The catalytic activity of enzymes adsorbed at surfaces plays an important role in several technologies including the development of proteases for laundry detergents, the operation of catalytic enzyme reactors, the prevention of bio-fouling in packaging material, and the biocompatibility of artificial materials. Because of the difficulty in measuring this activity and other properties of adsorbed enzymes, relatively little is known on a molecular level about the structure-function relationships that govern the activity of these enzymes at surfaces. We are therefore developing laser based, optical probes that will enable us to explore these relationships.

To this end, we present here a new assay based on Total Internal Reflection Fluorescence (TIRF) to quantify the catalytic activity of adsorbed enzyme monolayers on macroscopically flat surfaces. More specifically, this assay gives us the ability to quantify the maximum catalytic activity and the binding constant of the enzyme, two values very closely related to the structure of the enzyme layer. The need for such an assay derives from a general shortage of assay methods that are sufficiently sensitive to measure reaction kinetics for just a single monolayer of enzymes. The assay is based on the enzymatic conversion of a soluble, non-fluorescent fluorogenic reagent to a soluble, highly fluorescent product. The reaction occurs at the solid-liquid interface where the enzymes are adsorbed. The fluorogenic substrates reach the surface by convective diffusion from solutions in steady laminar flow. The exponentially decaying evanescent wave produced by total internal reflected argon ion laser beam serves as a "spectroscopic ruler" to resolve the spatial concentration profile of fluorescent products in solution near the interface. By measuring the steady-state fluorescence signal as a function of the Peclet number that characterizes mass transfer conditions in the experiment, it is possible to determine the enzymatic reaction rate.

Here we present the development of the method and its application to a test system of ?-galactosidase adsorbed to methylated silica surfaces. Here, we found that though the activity was significantly reduced when adsorbed to our silica surface, the specific activity was not affected by other surface factors such as surface concentration of the enzyme or age of the enzyme on the surface. We will also present results of trypsin, a structural homologue of thrombin, an important enzyme involved in biocompatibility. With trypsin, we found that the activity on the surface is retained over time when autolysis was prevented with Calcium or when the enzyme had not encountered its substrate.

The Investigation of Protein-Protein Interaction Phenomena in Bioprocessing Environments using Self-Interaction Chromatography and Quartz Crystal Microbalance
Angela Wilcox, Todd M. Przybycien
Department of Chemical Engineering

Protein-protein interactions and protein aggregation phenomena are prevalent throughout the bioprocess industry. Aggregation, a primary mode of protein degradation, often leads to immunogenicity of the protein and a loss of bioactivity. Therefore, it is necessary to formulate proteins to minimize aggregation as even small amounts of contaminating aggregates in the protein drug can cause it to be unacceptable for delivery to patients.

In the present effort, we are using two techniques to characterize protein aggregation behavior. In the first technique, Self-Interaction Chromatography (SIC), the target protein is immobilized randomly on a chromatographic support, and the elution of the same protein in a solution of interest is effected. Retention will occur as the mobile phase protein samples the possible self-interactions with the stationary phase. We have used SIC to rapidly screen excipients or formulation additives for the physical stabilization of recombinant human growth hormone (rhGH). We also employed SIC to study protein-protein interaction phenomena of lysozyme and ?-chymotrypsinogen by varying pH and ionic strength. Next, we are using quartz crystal microbalance (QCM), a mass sensitive detection system, to study the interactions between bovine somatotropin (bSt), and to investigate the feasibility of QCM as a possible formulation development tool. In this system, the protein of interest is covalently attached to the gold electrode of a quartz crystal and the same protein in solution is then placed in contact with the protein-coated gold electrode surface and the change in frequency is monitored. The frequency change reflects the strength of the protein-protein interaction. We vary solution conditions by altering pH, ionic strength, and the addition of excipients to find a suitable non-aggregating environment for the protein.

Tuning the interfacial and bulk self-assembly of cationic surfactants via counterions and non-binding polyelectrolytes
Stephanie Butler Velegol, Robert D. Tilton
Department of Chemical Engineering

The ability of surfactants to self-assemble, both in solution and on surfaces, is exploited in many processes. These include suspension stabilization, detergency and wetting. Our goal is to understand and control this behavior, especially at surfaces. By combining quantitative data from optical reflectometry and atomic force microscopy, we have learned that surface-induced self-assembly aggregates of the cationic surfactant, cetyltrimethylammonium halide (CTAX), display very different packing on silica than in solution. Furthermore the density and shape of these surface aggregates can be tuned by changing the binding affinity of the counterion.

We are now investigating the effect of adding a cationic polyelectrolyte, poly-l-lysine. Although it does not bind to CTAX, we have found that they do interact both in solution and at the silica surface. In solution, the presence of the charged polyelectrolyte increases the bulk surfactant concentration and therefore decreases the effective critical micelle concentration. At the surface, there is a competition between the polymer and the surfactant: The presence of PL significantly decreases the adsorbed amount of CTABr, especially at concentrations below the critical micelle concentration. Future work will include investigating the effect of the polymer molecular weight and the presence of various counterions on this competitive adsorption.

The Effect of Changing Solvent Quality on Steric Interactions
Michael Bevan and Dennis C. Prieve

Initial rates of flocculation have been measured using small angle light scattering for aqueous dispersions of polystyrene particles sterically stabilized with physisorbed Pluronic triblock polymer. Solvent quality for the adsorbed polymer layers, and thus the degree of stability imparted through steric interactions, was controlled through temperature control of the dispersion. Work is currently being performed to correlate potential energy profiles of steric interactions obtained using Total Internal Reflection Microscopy with experimental flocculation rates and theoretical predictions.

Colloidal and Interfacial Phenomena in Polymer/Surfactant Mixtures
Alan D. Braem, Dennis C. Prieve and Robert D. Tilton

The bulk properties of a complex fluid such as stability, rheology, and wettability are to a large degree determined by colloidal forces. We know that the presence of polymer and surfactant additives can greatly affect the forces between colloidal particles. Most research in this area has focused on the effects of adding a single component. However, typical industrial complex fluids contain multiple surface active components. In this research we address this problem by studying mixtures of neutral polymer and anionic surfactant.

We are studying two mechanisms in particular: coadsorption behavior and its effect on steric and electrostatic forces, and polymer/surfactant binding in solution and its effect on the depletion force. Our current work on coadsorption indicates that addition of normally non-adsorbing surfactant to an adsorbing block copolymer system drastically changes both the steady-state adsorbed amount and adsorption kinetics. Furthermore, we have found that polymer/surfactant binding greatly increases the depletion interaction. It is clear that multi-component effects can play a large role in determining complex fluid behavior.

Effect of Solvent Quality on Permeability in Membrane-Supported and Bulk Polyacrylamide Gels
Kristen Buehler and John L. Anderson

Gel-membrane composites are formed by synthesizing a crosslinked polymer gel in the pores of a semi-rigid membrane. Experimental studies with composite membranes made with polyacrylamide-based gels have demonstrated good selectivity in the transport of proteins. The integrity and tightness of the composite membrane are determined by measuring its hydrodynamic permeability.

Bulk polyacrylamide gels are shown to exhibit macroscopic deswelling when exposed to a poor solvent. This can be translated into a decrease in the effective pore size of the gel. However, when membrane-supported polyacrylamide is exposed to the same solvent the effective pore size increases as is shown by an increase in the hydrodynamic permeability. In this study supported gels are exposed to methanol:water solutions up to pure methanol and the permeability results are found to be reversible. This indicates that the supported gels are able to maintain their integrity even under extreme osmotic stress.

This study was further expanded to include different alcohols of decreasing solvent quality (ethanol and propanol) There was no observable effect between the different alcohols on the bulk gel. However, the supported-gel exhibited a larger increase in permeability for alcohol of decreasing solvent quality at the same concentration. These differences become smaller at higher alcohol concentrations.

The Influence of Viscoelasticity on the Atomization of Polymer Solutions
Yenny Christanti and Lynn M. Walker

Many applications such as spray coating involve spraying of polymer solutions where both drop size and drop size distribution are important. When sprayed, polymer solutions often produce larger drops than Newtonian fluids of the same shear viscosity due to their higher extensional viscosities. Our goal is to understand the role of extensional viscosity and viscoelasticity in the atomization of polymer solutions. Such knowledge combined with intelligent chemical formulation can be used to control the drop size and the drop size distribution.

We are systematically studying the effect of extensional viscosity on atomization by spraying a series of Boger fluids (constant viscosity elastic fluids) with markedly different extensional viscosities but similar values of shear viscosity, surface tension, and density. Our preliminary results show that higher extensional viscosity corresponds to larger drops and longer break up region. The initial results also suggest that opposed-nozzle extensional viscometer is a suitable indicator of the atomization quality. Next we will attempt to investigate the qualitative effect of viscoelasticity on the extensional rate of a liquid jet.

Van Der Waals Forces at Rough Interfaces with Diffuse Polymer Coatings
Raymond R. Dagastine, Dennis C. Prieve, and Lee R. White

Van der Waals forces are always present in colloidal systems, and they are often the cause of dispersion destabilization. A theoretical understanding of van der Waals forces is necessary for predicting dispersion stability. Most theoretical predictions of van der Waals forces use model systems and smooth interfaces while the actual colloidal systems have rough or coated interfaces.

Van der Waals interactions can already be predicted for homogenous coatings at interfaces using Lifshitz theory. This form of the van der Waals interaction energy is employed to model rough surfaces through the use of a lateral smoothing technique based on surface topography information. This approach is also expanded to diffuse polymer coatings with decaying density profiles. Work is currently being done to incorporate both rough surfaces and diffuse polymer layers to model interfaces that resemble actual systems.

Self-Assembly of Colloidal Particles via Electrophoretic Deposition
Scott Guelcher and John L. Anderson

Charged colloidal particles deposited near an electrode have been observed to self-assemble to form ordered layers in the presence of an applied dc electric field. The field is applied in the direction normal to the electrode, and the deposited particles move and self-assemble in the plane parallel to the electrode. A hydrodynamic model has been proposed for the observed self-ordering based on convection in the electroosmotic flow about deposited particles. To test the electroosmotic flow model, we have studied experimentally the aggregation of two particles deposited near an electrode to form a doublet. The mean experimental doublet trajectories scale with the particle zeta potential and the applied electric field as predicted by the model. As a more quantitative test, we have compared the experimental trajectories to those calculated from the electroosmotic flow model. To calculate the doublet trajectories, we must know the mobility of the deposited particles, which is expected to be less than that of particles in the bulk fluid because of the presence of the electrode. We have independently measured the mobility of the deposited particles by performing lateral diffusion and Total Internal Reflection Microscopy (TIRM) experiments. Reasonable agreement was found between the experimental trajectories and the trajectories calculated from the electroosmotic flow model and the measured values of the particle mobility. However, although good agreement between experiment and theory was demonstrated for the mean trajectories, we have observed considerable spread in the individual trajectories (typically ten) used to calculate the mean. By solving the convective diffusion equation using a method of moments technique, we have shown that the observed spread can be explained by the Brownian motion of the deposited particles.

Bubble Migration Due to Thermocapillary Flow
Hiroki Kasumi and Paul J. Sides

Bubbles electrolytically evolved on anodically polarized tin oxide approach each other. The motion is independent of bubble size and cannot be explained by gravity. A model based on thermocapillary flow around a bubble is developed to explain this phenomenon. A temperature gradient perpendicular to the plate due to reaction irreversibility and ohmic heating creates a surface tension gradient at the bubble which leads to a shear stress that pumps adjacent fluid away from the plate. While one bubble creates this flow pattern and the other bubble is entrained into this flow, the other bubble also creates its own flow pattern and entrains the first bubble as well. Thus these two bubbles approach each other. In order to confirm this aggregation mechanism of bubbles, the motion of two almost-equal size air bubbles in silicone oil has been observed with a microscope and the effects of bubble size, temperature gradient and viscosity have been investigated. The results support this theory. Bubbles come together when the plate of the experimental cell is heated. The motion is scaled linearly with temperature gradient, bubble size and oil viscosity.

Solubilization of Nonpolar Compounds in Micelles and Polymer-Surfactant Complexes
Joon-Hyung Kim, Michael M. Domach and Robert D. Tilton

Solubilization of nonpolar compounds in micelles and polymer-surfactant complexes has important applications in detergent industry, enhanced oil recovery, food preparation, cosmetics and paint formulations, pharmaceutical compositions and environmental remediation, where water is typically the solvent. While solubilization in micelles without polymers has been extensively studied, molecular detail of solubilization by polymer-surfactant complexes is still not well understood.

We measured the polymer-surfactant interaction and solubilization properties of poly (ethylene glycol) (PEG) - sodium dodecyl sulfate (SDS) mixtures by surface tension and spectroscopic techniques such as UV/vis, fluorescence vibronic band ratios and excimer fluorescence techniques. Using fluorescence method we can determine the critical association concentration, polymer saturation binding concentration and free micelle forming concentration, and these results can be confirmed by surface tension measurement. From the spectroscopic techniques we also found that the aggregation number of PEG-SDS complexes is less than that of single component or unbound SDS micelles. Each PEG-SDS complex has less solubilization capacity (number of solubilizates per micelle) than a free SDS micelle, while the total solubilization capacity of the PEG-SDS complex system is actually more than that of a polymer-free SDS solution.

Clay Induced Depletion Attraction
Paul Odiachi and Dennis C. Prieve

Mineral flotation, wastewater treatment and enhanced oil recovery are examples of industrial processes that involve dealing with dispersed clays. These clays are known to impact system rheology and subsequently process efficiency. In addition to understanding these rheological effects, there is a need to understand what effects the clays might have on the colloidal forces.

Total internal reflection microscopy (TIRM) has been used to study the effect of a model clay colloid (laponite) on the interparticle forces acting between a single 6 mm polystyrene sphere and a glass slide. It was found that the clay induces a depletion attraction at concentrations of 100 ppm or greater. Increasing the clay concentration increases the magnitude of the attraction, while increasing the electrolyte content of the system reduces both the magnitude and the range of the attraction. The attraction observed is due to the exclusion of the clay platelets from the gap between the PS sphere and glass slide.

Using Micropatterned Substrata to Identify Cell-Cell Adhesive Interactions in Sheets of Cells
Margaret Peel and Paul A. DiMilla

Many physiological phenomena depend on the adhesive interactions between cells, including cell migration, the immune response, and tissue permeability. Adhesion for sheets and masses of cells depends on both cell-cell interactions (i.e., between cell-surface receptors on adjacent cells) and cell-substratum interactions (i.e., between cell-surface receptors and substratum-bound ligands of the extracellular matrix). Previous research has focused primarily on either identifying the molecular components responsible for these adhesive processes or measuring adhesive interactions between isolated, non-interacting cells and its supporting matrix. However, both cell-cell and cell-substratum interactions may contribute to the overall adhesive behavior for sheets of semi-confluent and confluent cells. To test the hypothesize that the overall adhesive behavior of sheets of cells can be regulated by controlling the extent and strength of cell-cell and cell-substratum interactions, we have developed a systematic experimental approach in which micropatterning is used to limit cell-cell interactions. Our approach involves selectively irradiating with broad band UV light regions of a glass slide derivatized with poly(N-isopropylacrylamide) which is resistant to cell adhesion. Thus, micropatterning using masks with features of different sizes and geometries allows control of the architecture of a cell culture. In this current study we demonstrate that the extent of cell-cell interactions can be varied by changing the size of adhesive islands while setting cell-substratum adhesive properties by culturing for a fixed length of time.

Evaluation of Coating and Cleaning Techniques for PCB-Contaminated Concrete
Gonzalo Pizarro and David Dzombak

Because of stringent U.S. regulations governing exposure to polychlorinated byphenyl (PCB) compounds in the workplace and the environment under the Toxic Substances Control Act (TSCA), management of concrete containing PCBs is growing concern for environmental managers. Recently USEPA released regulations requiring exposure control via encapsulation or barrier construction where complete cleanup is not achievable in a cost-effective manner. We are performing a project to identify effective cleaning and coating (encapsulation) procedures for PCB-contaminated concrete. Specific objectives are to assess the effectiveness of selected concrete cleaning procedures in removing PCBs from the surface and near-surface of concrete contaminated with PCB-bearing oils; to assess the rate of bleedback of PCBs and oils in cleaned concrete and the factors affecting the bleedback phenomenon; and to assess the effectiveness of selected coating systems in encapsulating PCBs in concrete. Issues to be addressed in the study of coating systems will include assessment of general properties of coatings appropriate for particular conditions (e.g., temperature ranges, oil types); strength of coating adhesion to oil-contaminated concrete with time after cleaning and application; and effectiveness of coatings as diffusion barriers to PCBs contained in oils present in the concrete.

Small Angle Light Scattering Analysis of Immiscible Polymer Blends in Complex Flows
Brian E. Priore and Lynn M. Walker

Given their economic advantage over specialty homopolymers, the interest in polymer blends for product formulation has grown in recent years. The properties of the finished product depend strongly on the morphology of the phases formed during the melt stage processing of the blend. To intelligently control the morphology, the relationship between blend rheology and structure must be understood. To date, polymer blend research has focused primarily on morphological development in simple flow fields. We have initiated a program to examine these issues under more realistic processing flows involving non-homogeneous shear profiles and regions of transient elongation. Small angle light scattering (SALS) was chosen as a means to probe the blend microstructure in these flows. At this time, SALS is able to qualitatively describe the effects of the flow field and material parameters on the blend's morphological development.

Coadsorption of Strongly Interacting Macromolecules and Surfactants
Michelle Lourette Sun and Robert D. Tilton

The control of interfacial tension and colloidal forces via the adsorption of macromolecules and surfactants to solid and fluid interfaces is central to dispersion formulations and coatings. Currently, most industrial formulations are based on trial and error methods. Understanding the adsorption of polymers and surfactants at the molecular level can facilitate rational development of formulation methods. Adsorption from mixtures of surface active molecules often bears little resemblance to adsorption from single component solutions of any of the individual solutes. In fact, although industrial formulations are almost always multicomponent, nearly all previous adsorption studies have focused on single component systems. To more closely mimic industrial formulations, I investigate solution environment effects on coadsorption from two component aqueous solutions of macromolecules and surfactants to hydrophobic surfaces. I observe how competitive adsorption and macromolecule/surfactant binding in solution influence adsorption; I tune the strength of these interactions by changing the pH and ionic strength.

Using scanning angle reflectometry, a recently developed in situ optical method for measuring thin film properties and adsorption, I compare single-component adsorption kinetics with mixture coadsorption kinetics for the protein lysozyme and the anionic surfactants sodium dodecyl sulfonate (SDSo) and sodium dodecyl sulfate (SDS). Below the Krafft temperature of SDSo, electrostatically-driven complexation between SDSo and lysozyme creates a highly surface active aggregate that adsorbs rapidly, leading to synergistic adsorption behavior on polymethylmethacrylate (PMMA) surfaces. Synergism is also indicated at the air-water interface. However, the use of SDS above its Krafft temperature causes interaction with lysozyme to be more complex. Therefore I am currently investigating coadsorption in three SDS concentration regimes that are distinguishable by their unique protein/surfactant binding behavior.

Thin Polymer Melt Films in Front of Moving Contact Lines
Kroum Stoev and Stephen Garoff

Thin films are often present near contact lines: forming spontaneously from the fluid body, formed as the contact line recedes across the surface, or preexisting on the surface. When the contact line is moving, the hydrodynamics in and near these films control the macroscopic spreading of the fluid body. By measuring the liquid/vapor interface shape in a microscopic region near the contact line, we examine the hydrodynamics in these films and near the moving contact line. For wetting surfaces, thick films are deposited on the surface. Models of the moving contact line successfully describe the film thickness and interface shape at low speeds but fail at higher speeds.

The Effect of Nonionic Polymer on Rod-Like Micellar Systems
My Hang Truong and Lynn M. Walker

Although polymer-surfactant systems are found in a multitude of industrial applications, the effect of polymer on the rheology of micellar systems is not well understood. The objective of this work is to understand the effect of nonionic polymers on an aqueous cationic surfactant system of cetyl trimethylammonium tosilate (CTAT). CTAT can form rod-like micelles and can exhibit significant thickening under shear deformation. Since structure dictates rheology, we examine the effect of polymer on the structure of CTAT micelles using small angle neutron scattering (SANS) while simultaneously investigating its effect on CTAT rheology. Initial SANS studies showed that hydroxy propylcellulose (HPC) has little effect on CTAT micellar length and shape, although a dramatic effect on intermicellar interaction was detected. In rheological work, a larger stress was required to induce shear-thickening when HPC was added to a CTAT system. Thus, the effect of HPC on intermicellar interactions dominates the rheological behavior of CTAT-HPC systems. Polyethyleneoxide (PEO) was also examined and was found to have the opposite effect on CTAT rheology; addition of PEO induces shear-thickening at lower stress levels. Currently, we are examining these two polymers, HPC and PEO, as well as other polymers in terms of their hydrophobicity, intrinsic viscosity, and rigidity. From this we aim to isolate key material properties that affect CTAT rheology. The ultimate goal is to be able to predict CTAT-polymer rheology based solely on the characteristics of the polymer.

A Novel Multi-Technique Qpproach to Measure the Reaction Kinetics for Adsorbed Enzyme Monolayers
Eric Tsung and Robert D. Tilton

The catalytic activity of enzymes at surfaces plays an important role in several technologies including the development of proteases for laundry detergents, the operations of catalytic enzyme reactors, the prevention of bio-fouling in packaging material, and the biocompatibility of artificial materials. Because of the difficulty in measuring this activity and other properties of surface enzymes, relatively little is scientifically known on a molecular level about the structure-function relationships that govern the activity of these enzymes at surfaces. We are therefore developing optical probes based primarily on Total Internal Reflection Fluorescence and Optical Reflectometry that will enable us to explore these relationships.

The Nano-Coating of Magnetic Hard Disks Using Perfluoropolyether (PFPE) Lubricants
David M. Phillips, Stephen J. Vinay III, and Myung S. Jhon

Understanding the role of nano/micro-coating processes is essential in achieving head-disk interface reliability in current state-of-the-art disk drives. A fundamental microscopic study of the spreading and replenishment properties of PFPE lubricants with various molecular weights and end group functionalities over different carbon overcoats (hydrogenated and nitrogenated) is performed to find the optimal choice of disk lubricant.

We have experimentally investigated lubricant spreading characteristics using scanning micro-ellipsometry, and a mathematical model of the spreading profiles using the Monte Carlo (MC) method was developed. A finite difference method (FDM) scheme was used to study replenishment characteristics. In MC, lubricants having either reactive or non-reactive end groups are modeled as "simple reactive spheres" (SRS). As SRS, lubricants are assigned either a of spin ±1 (reactive) or 0 (non-reactive), where a -1 spin implies attractive interactions between end groups and the surface. In our initial simulation, we pinned end groups to the surface and introduced anti-ferromagnetic coupling to describe the spreading behavior of Zdol over amorphous carbon surfaces. We are currently modifying this pinning effect by introducing a damped anti-ferromagnetic coupling to describe spreading behavior over hydrogenated and nitrogenated surfaces. Using FDM and experimental diffusion coefficient data, we are able to predict the lubricant recovery time for a given dimension of displaced lubricant.

Effect of Changing Solvent Quality on Steric Interactions
Mike Bevan and Dennis Prieve

Coadsorption of Triblock Copolymer and Anionic Surfactant to Solid Surfaces
Alan Braem and Robert Tilton

The Ordering of Ferritin at an Interface
Stephanie Butler and Robert Tilton

Pyrene Solubilization in Surfactant Solutions
Joon-Hyung Kim, Robert D. Tilton and Michael M. Domach

Coadsorption of Strongly Interacting Proteins and Surfactants
Michelle Lourette and Robert D. Tilton

Evaluation of physical/chemical mechanisms controlling PCB release from river sediments
Enrique Ortiz and Richard G. Luthy

Organic phase resistance to dissolution of polycyclic aromatic hydrocarbon compounds
Enrique Ortiz, Mathias Kraatz and Richard G. Luthy

Effects of Cell-Cell Interactions on the Strength of Cell Adhesion for Sheets of Cells
Margaret M. Peel, T. Oguz Acarturk and Paul A. DiMilla

Probing Microscopic Structures of Surfactant Monolayers with Atomic Force Microscope
Dan Qu and Stephen Garoff