Oral Presentation Abstracts

Talks 1 through 50

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1

Elucidating Metal Sorption Mechanisms at the Mineral/Water Interface: From the Macroscopic to the Molecular Scale

Donald L. Sparks, University of Delaware, Department of Plant and Soil Sciences, Newark, DE 19717-1303

A fundamental understanding of the kinetics and mechanisms of metal sorption in natural systems is important in assessing the speciation, mobility, and bioavailability of metals in soil and water environments. To definitively describe the reaction mechanisms, it is preferable to employ both macroscopic and molecular scale approaches. In this paper, recent advances will be discussed in elucidating the dynamics and mechanisms of metal adsorption and precipitate transformations on soil minerals and soils and in directly ascertaining metal speciation in contaminated soils. It will be shown that with metals such as Ni and Zn, metal hydroxide precipitates form on an array of mineral surfaces that are common in the natural environment and in laboratory- and field-contaminated soils. The precipitates often occur on time scales of minutes, at metal surface loadings far below a theoretical monolayer coverage, and in a pH range well below that at which metal hydroxide precipitates would be expected to form according to the thermodynamic solubility product. Dissolution studies indicate that the formation of these precipitates greatly enhances the sequestration of the metal. Using in-situ XAFS and DRS, coupled with HRTGA, the enhanced stability was ascribed to the stepwise transformation of an initial metal-hydroxide precipitate to a precursor metal phyllosilicate phase. The inclusion of both adsorbed and precipitated phases must be included in sorption models to accurately predict the fate of metals in the environment.


2

Influence of Cd2+, Cu2+, and Pb2+ on the Proton Affinity Distributions at the Oxide/Solution Interface

B. Prelot, F. Thomas, F. Villieras, Laboratoire Environnement et Minéralurgie, INPL et CNRS UMR 7569, BP40, 54501 Vandoeuvre lès Nancy Cedex, France.

The effect of surface heterogeneity on proton adsorption was investigated experimentally by conducting high-resolution titrations with up to 300 data points. The titration curves are considered as proton adsorption isotherms. Following the so-called Derivative Isotherm Summation (DIS) procedure, various energetic domains can be identified on the derivative titration curves. A local pK or Point of Zero Charge then characterizes each energetic domain. The proton affinity distributions were calculated from the derivative titration curves on silica, anatase and goethite, in the absence or in the presence of metallic cations (Pb2+, Cd2+ and Cu2+) in solution. Silica, anatase and goethite are characterized by three or four energy domains, which reflect different coordinances of the surface hydroxyls on the crystal faces, defects, impurities, or local dissolution. Metallic ions modify differently the proton affinity distributions. In the presence of oxide surface, the hydrolysis of the three metals is shifted towards lower pHs. For instance the proton affinity patterns of goethite are strongly modified by each of the three metals. Cadmium and lead are adsorbed on anatase at specific sites and do not alter the affinity distribution, whereas copper is hydrolysed as an adsorbed species.


 

3

Monitoring Ion Adsorption and Desorption on Aqueous Oxide Surfaces using Optical Waveguide Lightmode Spectroscopy

Jan Sefcik, Marek Kroslak and Massimo Morbidelli, Swiss Federal Institute of Technology (ETH), Department of Chemical Engineering, Universitatstr. 6, CH-8092, Switzerland

Dynamics and equilibria of ions interacting with oxide surfaces in aqueous solutions are important for many environmental, geological, chemical and materials applications. Silica is a common component in environmental and synthetic materials. Various organic and inorganic cations play important role in catalyzing growth and dissolution of silica and structurally directing crystallization of specific silica polymorphs, e.g. pure silica zeolites. For example, tetrapropylammonium cations are necessary to direct nucleation and crystal growth of silicalite while being incorporated into the crystal structure itself. On the other hand, sodium cations apparently play a catalytic role in the process of silicate condesation in building the covalent framework. We use Optical Waveguide Lightmode Spectroscopy (OWLS) to study kinetics of adsorption and desorption of various cations on silica/water interface. Time resolved measurements of an effective refractive index at the aqueous silica surface were performed by OWLS for a range of process conditions (ion charge and size, solution pH and ionic strength, temperature) in a flow through reactor. From this data we determine relative adsorption and desorption rates of individual ions and corresponding activation energies and we discuss implications for surface chemistry of silica in aqueous environments.


4

Zn Sorption to Hydrous Metal Oxides: An Investigation with XAS

Paras Trivedi and Lisa Axe, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, NJ 07102.

The mobility and the bioavailability of toxic metals such as Zn in soils and sediments are largely controlled by their sorption to hydrous oxides of Fe (HFO) and Mn (HMO). Previous macroscopic studies revealed this sorption as a two-step process: rapid physical sorption to the external surface followed by slow intraparticle diffusion. To better understand the surface complexation mechanism, X-ray absorption spectroscopic (XAS) studies were conducted on Zn sorbed to HFO and HMO as a function of pH, loading, and time. In these sorption samples, the first shell, fitted using zinc oxide as reference, consisted of 5-6 oxygen atoms at an average radial distance of 2.00 to 2.04 Å. These fits are consistent with the primary solvation shell of aqueous zinc nitrate suggesting that zinc ions sorb physically to these amorphous oxides. Additionally, this sorption mechanism is independent of pH and adsorbate loadings. The XAS measurements of long-term (diffusion) samples also revealed that the local structure of these sorbed Zn ions was invariable as a function of time. Because as much as 90% of the adsorption sites are located along the micropore walls of these oxides, the adsorption mechanism appears to be the same for internal and external sites.


5

Residence Time Effects of Arsenate Adsorption/Desorption Mechanism at the Aluminum Oxide-Water Interface

Yuji Arai and Donald L. Sparks, University of Delaware, 152 Townsend Hall, Newark, DE 19717-1303

Soils and sediments are nearly always at disequilibrium with respect to ion transformations, therefore the rate of contaminant (metal/metalloid) bioavailability can be reduced or increased over time. In this study, we investigated the residence time effects (1day-11months) on As(V) adsorption/desorption mechanisms at the aluminum oxide-water interface using a combination of batch adsorption (pH 4 and 8, 5g L-1,[As(V)]o=1mM, I=0.1M NaNO3) and desorption experiments and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS). The As(V) adsorption kinetics at both pHs show an initial fast reaction (<4h) followed by slow continuous adsorption over 24h. The As(V) adsorption increases with decreasing pH, resulting 87% of the total adsorption after 24h at pH 4 and whereas only 40% of the total absorption occurred at pH 8. The EXAFS data indicate that As(V) predominantly formed inner-sphere bidentate binuclear complexes between 1day and 1month at constant loading levels (@ 0.18mM g-1 at pH 4 and @ 0.10mM g-1 at pH 8), as evidenced by a As(V)-Al bond distance of @ 3.11 Å. The As(V) adsorption mechanisms were further investigated at aging times up to 11months. The stability of aged samples was also investigated using the batch desorption experiments, replenishing with buffered background electrolyte every 24h for 25days.


6

Transmission 57Fe-Mössbauer Spectroscopy Applied to Men+-Substituted Amorphous ferric (hydr)oxides.

Je-Hun Jang (J.-H. Jang), Brian A. Dempsey, and William D. Burgos, Civil and Environmental Engineering, 212 Sackett, Pennsylvania State University, University Park, PA 16802

Gary L. Catchen, Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802

Using Mössbauer spectroscopy, we have observed the effects of various cations on transformation of ferrihydrite to more stable ferric oxides. FeCl3 solutions (final 50 mM) were titrated to pH 7 using NaOH, aged for one day, and dried at 40ºC or at 110ºC. The precipitates were re-suspended in DI water and aged at 65ºC. Samples dried at 40ºC for 1 day exhibited Mössbauer parameters that are identical to published values for ferrihydrite. Samples aged in water at 65ºC and samples dried at 110ºC and then kept dry resulted in different transformation trends, and the solid phases have not yet been identified. FeCl3 (final 50 mM) and MeCl2 solutions (final 25 mM) were combined and titrated to pH 7 using NaOH. These samples were not dried and were aged at 65ºC up to 5 days. Mössbauer parameters indicate the presence of ferrihydrite. In addition a magnetic hyperfine field typical for hematite was observed in the presence of Zn(II) or Mn(II), after five and three days respectively. Other metals induced a structural distortion in the sequence Co2+>Cu2+~Ni2+>Al3+, based on values for quadrupole splitting.


7

Entropically Driven Self-Assembly and Interaction in Suspension

Arjun G. Yodh, University of Pennsylvania, 209 S. 33rd Street, Philadelphia, PA 19104

The addition of small macromolecules (particles) to a suspension of large particles causes an entropic attraction to arise between the large particles. The origin of this attractive force is an increase in volume (i.e. entropy) available to the smaller particles that arises when the large particles are moved close together [1]. We review this effect and experimentally explore its manifestations near walls and wall-structures [2], and in semi-dilute solutions of polymeric DNA [3]. Light force microscopies to measure colloidal particle interactions are introduced and experimentally demonstrated in some of these systems [3]. Finally, entropically driven nucleation and assembly on grating surface templates is demonstrated [4].

References

  1. S. Asakura and F.J. Oosawa, Chem. Phys. 22, 1255 (1954); Vrij, A., Pure and Applied Chem. 48, 471 (1976).
  2. P.D. Kaplan, J.L. Rouke, A.G. Yodh, and D.J. Pine, Phys. Rev. Lett. 72, 582-85 (1994); A.D. Dinsmore, A.G. Yodh, and D.J. Pine, Nature 383, 239-242 (1996); A.D. Dinsmore and A.G. Yodh, Langmuir 1999, 314-31`6 (1999); A.D. Dinsmore, D.T. Wong, P. Nelson, and A.G. Yodh, Physical Review Letters 80 409-412 (1998).
  3. Ritu Verma, J.C. Crocker, T.C. Lubensky and A.G. Yodh, Physical Review Letters 81, 4004-4007(1998); J.C. Crocker, J.A. Mateo, A.D. Dinsmore and A.G. Yodh, Physical Review Letters 82, 4352-55 (1999).
  4. Lin, K-H, Crocker, J.C., Prasad, V., Schofield, A., Weitz, D.A., Lubensky, T.C., and Yodh, A.G., Physical Review Letters 85, 1770-1773 (2000).


 

8

Enhanced Aggregation of Colloidal Platelets With Weakly or Non –Adsorbing Polysaccharides.

Jerome M. Labille, Isabelle Bihannic, Bruno S. Lartiges, Fabien Thomas, Laurent Michot,

CNRS-INPL / LEM Pole de l’Eau – 15 Avenue du Charmois – BP 40 – F-54501 Vandoeuvre Cedex

Shear-aggregation of montmorillonite particles was investigated in the presence of a coagulating salt and polysaccharide macromolecules using laser diffraction. Negatively charged montmorillonite colloids were first destabilized with either CaCl2 or NaCl to yield stable aggregates about 15 µm in size. Polysaccharide macromolecules (Dextran (neutral polymer) or Succinoglycan (slightly anionic polymer)) of various molecular weights (110 000 to 2 106)were then added to the aggregated clay suspension. Upon addition of polysaccharide, further aggregation of montmorillonite aggregates occurred. The steady-state aggregate size increased with polysaccharide concentration and molecular weight in the range investigated. The growth rate of aggregates was about 0.01 µm/s indicating a very weak interaction between the polymer and the coagulated montmorillonite. Changing the coagulating electrolyte did not modify strongly the aggregation mechanism, as both growth kinetics and equilibrium aggregate size were of the same order of magnitude for clay aggregates of similar initial size. This suggests that the enhanced aggregation occurring upon addition of polysaccharide is more likely to be due to a depletion mechanism rather than polymer adsorption and bridging. The same aggregation enhancement was also observed with positively charged double layered hydroxides coagulated with SO42- anions in the presence of Dextran, thus supporting a depletion effect.


9

The Effect of Size and Surface Potential Heterogeneity on Depletion Interaction in Charged Systems

John Walz, Martin Piech, Yale University, 9 Hillhouse Ave., New Haven, CT 06520

The effect of heterogeneity in the size and/or surface potential of nonadsorbing polyelectrolytes on the depletion interaction between a charged plate and a colloidal particle and between two charged particles has been investigated both theoretically and experimentally. A force balance approach was used to calculate the interparticle pair potential produced by an equilibrium solution of nonadsorbing polyelectrolyte material. The experimental technique of atomic force microscopy was then employed to measure the depletion force in a variety of systems with a controlled degree of heterogeneity in the size and/or the surface potential of the nonadsorbing polyelectrolyte. These experimental results were compared to the theoretical predictions.


10

Correlation Effects in Polymer-Colloid Suspensions

K.S.Schweizer, Dept. of Materials Science, University of Illinois, 1304 W. Green Street, Urbana, IL 61801, Dr. Matthias Fuchs, Dept. of Physics and Astronomy, JCMB King’s Buildings, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom

Athermal mixtures of flexible polymers (radius of gyration, Rg) and spherical polymers (radius, R) undergo entropy driven fluid-fluid phase separation. Classic theories idealize the polymer as a phantom coil which interacts as a hard sphere with particles ; an effective 1-component description is often adopted where small polymers induce a pair decomposable depletion attraction between large colloids. Such approaches generally predict miscibility is reduced as size asymmetry, Rg/R, increases. Recent experiments with nanoparticles (surfactant micelles, proteins) and 50 nm silica colloids undergo depletion-driven demixing but miscibility is found to improve as Rg/R increases. We have developed a novel microscopic 2-component liquid state theory of athermal polymer-particle mixtures (Europhysics Letters 51, 621(2000)) which treats all the distinguishing macromolecular features including connectivity correlations and polymer-polymer repulsions. Many body depletion and length scale dependent clustering are also included. These physical effects stabilize the mixed phase resulting in miscibility enhancement as Rg/R increases, in accord with experiments. The theory also predicts homogeneous phase thermodynamic properties and structural correlations, examples of which will presented and contrasted to recent light scattering measurements.


11

Entropical Colloidal Interaction in Rod-like Molecular Solution

Keng-hui Lin, Dept. of Physics and Astronomy, U. of Pennsylvania, Philadelphia, PA 19104, Ana Carolina Zeri, Dept. of Chemistry and Biochemistry, U. of California at San Diego, La Jolla, CA 92093, John C. Crocker, Applied Physics Dept, California Institute of Technology, Pasadena, CA 91125, Arjun G. Yodh, Dept. of Physics and Astronomy, U. of Pennsylvania, PA 19104

We report direct measurements of the functional form of the depletion interaction between two colloidal spheres in a rod-like molecule suspension the line-scanned optical tweezer. The rod like molecules are bacteriaphage fd with length (L) 880 nm and diameter(D) 6.5 nm and TMV with L 300 nm and D 200 nm. We probed different ratios of sphere radius R and rod length L and compared with theoretical models of Yaman*. The experimental data agrees with the model with slight discrepancy due to the flexibility of rod molecules. At high salt concentration, we also observed the steric repulsion due rod molecule stuck on the spheres. We gratefully acknowledge support from the NSF (DMR-9623441) and MRSEC (DMR-9632598).

*K. Yaman, C. Jeppesen, C.M. Marques, Europhys. Lett. 42, 221 (1998)


12

Phase Behavior and Thermodynamic Properties of Colloid-Polymer Mixtures

S. Ramakrishnan and C. F. Zukoski, Department of Chemical Engineering, University of Illinois at Urbana-Champaign, 114 Roger Adams Lab, 600 S. Mathews Avenue, Urbana IL 61801.

The presence of non adsorbing polymers in a suspension of colloidal particles gives rise to attractive interactions between the particles. Based on the range and strength of these interactions, the suspensions can either gel or phase separate into a colloidal gas/colloidal liquid or colloidal liquid/colloidal solid. These observed phase changes are often compared to model calculations requiring a detailed knowledge of the pair interaction potential or the thermodynamic properties of the suspension. Very few studies in literature have focussed on measuring an independent measure of the strength of the particle interaction in addition to phase behavior and how these interactions change with polymer molecular weight and as the polymer concentration is increased into the semidilute limit. Here we systematically characterize the depletion interactions between hard sphere colloidal silica particles in the presence of non-adsorbing polystyrene in toluene - a good solvent for polystyrene. The Phase diagram of the particles in presence of the polymer is first measured. The range of interaction is controlled by varying polymer molecular weight yielding R/Rg from larger than unity to of order unity. Once the phase diagram is determined, the interactions are characterized by measuring the osmotic compressibility curves of the suspensions right up to the solubility point by turbidity. This gives us an independent measure of the interactions with which we can predict the phase behavior. After characterizing the polymer solution thermodynamic properties, the structure factors are compared with predictions of the Aasukara Oosawa model and Polymer Reference Interaction Site Model (PRISM). Here we systematically explore the ability of these models to describe the strength of interactions between the particles as a function of R/Rg and polymer concentration.


13

Action of Surface-Active Automotive Lubricant Additives Studied with a SFA

H. Ohtani1, Y. Zhu2, M. L. Greenfield1, M. Ruths3, and S. Granick2, 1Chemistry Department, Ford Research Laboratory, USA, 2Department of Materials Science and Engineering, University of Illinois, USA3, Max-Planck-Institut für Polymerforschung, Mainz, Germany

The precise control of friction has been one of the most important issues in designing automatic transmissions with better fuel efficiency. In order to understand the action of friction modifying additives (FMs) in the boundary lubrication regime, the nanorheological properties of fluids in confined spaces have been studied in situ. As a first step, simple model lubricants consisting of tetradecane and small amounts of FMs (polar alkanes) were examined. A modified surface forces apparatus (SFA) developed by Granick's group at University of Illinois was used, which was equipped with a device for oscillatory shearing and thus was capable of obtaining elastic and viscous responses of test fluids under a wide range of shear conditions. The model FMs were found to form monolayers on sliding surfaces and altered the shear response of the remaining fluid between the monolayer films: stick-slip was eliminated and the limiting shear stress at high shear rates was reduced. A similar response was obtained with a complex fully-formulated ATF, despite the many other competing additives present. Thus our model experimental approach appears to be a valid way to scrutinize the molecular actions of lubricant additives. Future directions for nanorheological study of automotive lubrication will also be discussed.


 

14

Desorption Kinetics of Oligomeric Lubricants from Surfaces

Andrew J. Gellman and Kris. R. Paserba, Dept. of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

The evaporation of lubricants from the surfaces of hard magnetic data storage media is one of the important sources of loss over the course of time. Those lubricants are oligomers of perfluorinated ethers known as perfluoropolyethers (PFPEs). The dynamics of oligomer desorption from surfaces has been studied by measuring the desorption kinetics of a set of straight chain alkanes ( H(CH2)nH, with n = 5 to 60) from the surface of single crystalline graphite. Desorption is observed to be a first-order process and the pre-exponent of the desorption rate constant has a value of v = 1019.6 ± 0.5 sec-1 and is independent of the oligomer chain length. More interestingly we find that the barrier to desorption has a non-linear dependence on chain length and takes the form D Edes = A + Bna , with the exponent having a value of a = 0.50 ± 0.01. A model has been derived to describe this oligomer desorption process which can be applied to the desorption of PFPEs from magnetic storage media.


15

In-situ Raman Spectroscopy of Lubricate Sliding Contacts

Peter C. Stair and Cheng U. Amanda Cheong, Northwestern University, Department of Chemistry, Evanston, IL 60208-3113

Ultraviolet Raman spectroscopy has been used to study the chemical reactions of a Krytox lubricant and Fomblin lubricants with two different molecular weights in a ball-on-disk sliding contact under operation conditions. Lubricant reaction to produce aromatic, amorphous carbon was observed for Krytox and the lower molecular weight Fomblin lubricants in a steel-on-sapphire sliding contact. No reaction was observed for a sapphire-on-sapphire contact or in the absence of sliding. UV Raman spectroscopy was also used to measure the lubricant temperature and pressure in a sliding contact using silicone grease. The increase in lubricant temperature was ca. linear in the sliding speed. The measured pressure was consistent with Hertzian contact theory. Possible mechanisms for the amorphous carbon formation and the potential UV Raman spectroscopy for in-situ studies of lubricant temperature, pressure, and chemical reaction will be discussed.


16

Humidity Uptake on Lubricated Carbon Overcoats

Nisha Shukla, Seagate Technology, 2403 Sidney Street, Pittsburgh, PA 15203, Andrew J. Gellman, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, Xiaoding Ma, Jing Gui, Seagate Technology, Recording Media Operations, Fremont, CA 94538

A quartz crystal microbalance has been used to measure contaminant adsorption on magnetic data storage media under controlled conditions. This apparatus has been developed to make measurements of contaminant adsorption at the level of 0.1 ng/cm2 with a time resolution on the order of seconds. Initially we have measured humidity uptake on amorphous carbon overcoats coated with lubricants. We have been able to estimate the amount of water adsorbed on lubricated carbon overcoats at room temperature and at moderate humidity levels (~ 25% RH ). Adsorption and desorption is fast indicating that equilibrium with ambient humidity is reached on timescales of minutes, much faster than the timescales for fluctuations in ambient humidity. We have also studied water adsorption on different types of lubricants deposited at different thicknesses. Interestingly, the amount of water adsorbed on lubricated and unlubricated carbon overcoats is similar suggesting that water adsorption is primarily dependent on the properties of the carbon. This observation appears to contradict the suggestion that a lubricant layer with high contact angle and low surface energy should prevent water adsorption from humid environments. Our results suggest that water adsorption occurs in the pores of the carbon film and is independent of the lubricant on the magnetic media.


 

17

Molecular Modeling of Alkylsilane Monolayers

Mark J. Stevens, Sandia National Laboratories, P.O. Box 5800 MS 1111, Albuquerque, NM 87185-1111

Self-assembled monolayers of alkylsilanes have been of great interest the last 20 years. Recently, work has focused on using these SAMs as lubricant in micromachines which involve tiny gaps demanding new types of lubricants. One of the best candidates are alkyltrichlorosilanes. These systems are thought to get added stability by cross-polymerizing. I will show that cross-polymerization is impossible at full coverage due to steric effects and that it would yield monolayer densities that are too large. The effect of steric interactions on the monolayer coverage will be discussed particularly comparing alkyltrichlorosilanes and alkylmonochlorosilanes. Results of molecular dynamics simulations of the monolayers will be presented. The adhesive and friction forces between two SAMs as a function of separation for varying akyl tail length has been calculated and will be compared with experiment.


 

18

On Molecular and Continuum Boundary Conditions for a Binary Fluid

Colin Denniston and Mark O. Robbins, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218

We examine the boundary conditions at a solid surface bounding a two-component fluid using molecular dynamics simulations. In particular, we examine how concentration gradients and surface heterogeneity affect the slip at the boundary. We demonstrate several cases where certain average velocities demonstrate "no-slip" behavior while the individual species velocities do not, in contrast to what has been previously reported by other researchers.


19

How Do Self-Assembled Monolayers Grow?

Daniel K. Schwartz, Dept. of Chemical Engineering, University of Colorado, Boulder, CO 80309

Coating solid surfaces with well-organized single layers of amphiphilic organic molecules has become a popular method for surface modification allowing one to alter the physical or chemical properties of metal, oxide, or even polymer surfaces. These "self-assembled monolayers" (SAMs) have applications in areas including corrosion and bio-fouling inhibition, chemical and bio-sensors, photon harvesting, and molecular electronics. Although the formation of the monolayers takes place in solution instead of vacuum, we have found that the physical processes involved are analogous to those intrinsic to more traditional forms of epitaxial growth (e.g. molecular beam epitaxy), namely molecular adsorption, surface transport (2D diffusion), cluster nucleation and growth, etc. In situ atomic force microscopy observations, combined with spectroscopic data, allow us to put together a quantitative picture of monolayer growth and dissolution processes that draws heavily on 2D cluster growth models originally developed to explain vapor-phase epitaxial growth.


20

Slow Dynamics in Electrostatically-Adsorbed Layers

Nanthiya Hansupalak and Maria M. Santore, Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, PA 18015

This work examines the dynamic features of polyelectrolyte adsorption, focusing on the situation where both the polymer and surface adjust their charge density in response to the local pH. A model system for such a scenario is the adsorption of the cationic molecule, poly(dimethylaminoethyl methacrylate) DMAEMA on chemically-etched silica from aqueous solutions of controlled pH and ionic strength. This talk reports the adsorption kinetics on different regions of the apparent adsorption isotherm, demonstrating conditions where transport-limited kinetics dominate. Adsorption onto a bare surface is contrasted with the dynamic exchange between the solution and the interface. The latter is shown to be extremely slow, even for chains of modest length. The exchange dynamics are also much slower than previously observed for a model nonionic system, polyethylene oxide on silica, suggesting that the stronger segment-surface interaction of polyelectrolyte systems is responsible for slow dynamics.


21

Intrinsic Adsorption Kinetics of Triblock Copolymer Surfactants on Self-Assembled Monolayers Using Surface Plasmon Resonance Spectroscopy

Pietro Brandani and Pieter Stroeve, University of California, Davis, One Shields Ave, Bainer Hall, Davis, CA 95616

Pluronics are a series of poly(ethylene-oxide)-poly(propylene-oxide) (PEO-PPO-PEO) tri-block copolymers. They are surface active and find use in traditional industrial detergent applications as well as in biomedical applications. We have employed surface plasmon resonance (SPR) spectroscopy to study the intrinsic kinetics of adsorption of a series of Pluronics (P103,P104,P105,F108) with fixed PPO content and increasing content of PEO on a model hydrophobic surface. This technique in conjunction with the use of a custom-made laminar flow cell enabled us to resolve short time kinetics (sample time ~ 0.1s) and achieve hydrodynamic conditions where the kinetics were no longer influenced by mass transfer. We found the kinetics and equilibria of adsorption to be non-Langmuirian. At low contents of PEO in the copolymer, the adsorption isotherm exhibited a maximum at about 0.5 bulk CMC.


22

Structure and Packing of Interfacial Micelles: Experimental Results vs. Energy Optimization Calculations

M.B. Hay and S. Manne, Dept. of Physics, University of Arizona, Tucson AZ 85721

Recent results from atomic force microscopy (AFM) have shown that ionic surfactants at oppositely charged substrates self-assemble in the form of close-packed spheres, parallel cylinders or coherent bilayers, depending on the headgroup geometry and the substrate potential. Qualitatively, observed results are consistent with treating the substrate as a multivalent "planar counterion" that decreases the optimal micelle curvature by bringing headgroups closer together. Here we report results of global energy optimization calculations aimed at a more quantitative understanding of intermicellar spacing and morphology as a function of substrate potential. Micelles are treated as charged oil drops interacting with each other, and with the substrate, via DLVO interactions. Optimal spacings of spherical micelles are calculated by balancing intermicellar repulsions (calculated as a Madelung sum) with the micelle-surface attractive interaction. Interfacial sphere-to-rod transitions and comparisons to experiment are discussed.


23

Surface Mediated Solvation: Solvent Permittivity at Solid Liquid Interfaces

R. A. Walker, O. Esenturk, Department of Chemistry and Biochemistry, X. Zhang, Chemical Physics Program, University of Maryland, College Park, MD 20742

Chemists have long known that surfaces perturb the local structure of an adjacent solvent phase. What is less clear is how this altered solvent structure affects interfacial solvation. Here, the term solvation refers to the noncovalent solvent-solute interactions that control solute conformation, orientation and reactivity. We have used second order nonlinear optical spectroscopy to probe the interfacial dielectric environment created at different solid-liquid interfaces. Surface induced changes in solvent polarity lead to shifts in the electronic transition energies of adsorbed chromophores, allowing us to infer how interfacial permittivity differs from its bulk solution limit. By controlling substrate composition and solvent identity, we explore how both strongly and weakly interacting systems conspire to create discreet regions of varying dielectric character near surfaces. In addition, we have observed markedly reduced solvent polarity near aqueous-hydrophobic boundaries – a result that hints at significantly reduced solvent-solute interaction and possible of dry-layer formation.


24

Numerical Simulations for the Development of Boundary Conditions for Moving Contact Line Problems

Shuang Xiao, Arijit Bose, Department of Chemical Engineering, University of Rhode Island, Kingston, RI 02881, Enrique Rame, National Center for Microgravity Research, Cleveland, OH, Stephen Garoff, Department of Physics, Carnegie Mellon University, Pittsburgh, PA

We are using numerical simulations as an investigative tool for understanding fluid physics in the vicinity of dynamic contact lines. Specifically, we are exploring the possibility of using these simulations to develop material boundary conditions for models of dynamic wetting processes. For inertialess (Reynolds number, Re = 0) flows at low Capillary numbers (Ca < 0.1), the slope of the interface in the intermediate region (where the meniscus shape is controlled by a balance between local viscous forces and surface tension) and the velocity field in that region serve as appropriate geometry-independent boundary conditions. Our recent work focuses on flows where inertia, and viscous forces in the outer region, cannot be ignored (Re ~ 100, Ca ~ 10). These regimes are of practical significance, but no analyses are currently available, making simulations a powerful tool for examining boundary conditions that may apply for these processes. Results from these recent simulations will be discussed.


25

Contact Angles in Sequential Wetting: Pentane on Water

Volker C. Weiss and B. Widom, Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301

In recent experiments, pentane deposited on water has been found to undergo two sequential wetting transitions as the temperature is increased [E. Bertrand et al., Phys. Rev. Lett. 85, 1282 (2000)]. At low temperatures (<25° C), pentane does not wet the water surface; rather, droplets of liquid pentane, which is in coexistence with pentane vapor, float on the water surface. At 25° C, there is a first-order wetting transition from discrete lenses to a wetting film of finite thickness (~ 100 Å). Increasing the temperature further leads to a growing film thickness, which ultimately diverges at 53 ° C, the critical wetting transition temperature. We calculate the contact angles of pentane lenses on the water surface (and, in principle, of those that sit on top of the finite wetting layer) in the temperature range 0-53 ° C and compute the magnitude of the discontinuity in the temperature-derivative of the contact angle at the first order transition. For these purposes, we use a modified Cahn theory, which incorporates the long-range interaction terms responsible for the occurrence of critical wetting and which accounts for a discrete layer of adsorbed pentane molecules at the water surface.


26

Prewetting and Wetting Transitions for Classical and Quantum Fluids from Computer Simulations

Wei Shi and J. Karl Johnson, University of Pittsburgh, Department of Chemical Engineering, 1249 Benedum Hall, Pittsburgh, PA 15261, USA

Fluids that interact very weakly with and adsorbent can exhibit prewetting and wetting transitions on the surface of the adsorbent, depending on a delicate balance between adsorption and surface tension forces. Computer simulations can be useful for studying prewetting and wetting phenomena. However, the precise transition points are very difficult to identify using conventional simulation techniques. We present multiple histogram reweighting calculations for simulations of both classical (argon) and quantum (hydrogen) fluids on weakly adsorbing substrates. Multiple histogram reweighting can be used to precisely locate the transition points for prewetting transitions by the equal area construction theorem. We demonstrate that the multiple histogram reweighting method can be applied to quantum fluids through a special implementation of the Feynman path integral formalism. The wetting transition temperature and the prewetting critical point are computed and compared with experiments where possible. Finite size effects are discussed.


27

Forced Spreading of Complex Fluids on Cylindrical Substrates

Mohan Srinivasarao1, Jung Ok Park1, and A. D. Rey2, 1School of Textile and Fiber Engg., Georgia Institute of Technology, Atlanta, GA 30332-0295, 2Department of Chemical Engineering, McGill University, Montreal, Canada

In many industrial applications it is necessary to coat a solid substrate with a fluid. This is usually accomplished by dragging the solid object through the fluid of interest at various velocities. At zero velocity the film thickness is zero and at infinite velocity it will be zero as well, since the fluid does not have enough time to form a coating. Thus a maximum wetting speed naturally enters the problem of forced wetting, as has been discussed in the literature. In this talk we are interested in addressing the issue of forced wetting of fluids where the fluids are complex fluids. In particular, we confine our attention to the special case of the solid substrate having a cylindrical geometry (fibers). We consider the case of a nematic fluid and a polymer solution whose concentration is above the overlap concentration being coated onto a polypropylene fiber. In both cases the initial thickness of the fluid film coated on the fiber is proportional to the capillary number, Ca (defined as the ratio of the viscous forces to those due to surface tension), to the first power. We present a model to account for the observed thickness dependence on the capillary number.


28

Micro- and Macroscopic Mechanisms of the Transition from Smooth Sliding to Stick-Slip

O. Braun, Institute of Physics, Ukrainian National Academy of Sciences, Kiev, Ukraine
J. Roder, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87544, USA

We show that the transition from smooth sliding to stick--slip motion in a single junction always takes place at an atomic-scale velocity of the substrates. The molecular dynamics technique for the simulation of a lubricant film, confined between two substrates moving relative to each other, and subject to external damping is developed to yield frictional behavior as a function of substrate separation and sliding velocity. The microscopic mechanism of stick-slip motion is either due to inertia effects in the solid lubricant, or to melting-freezing processes, if the lubricant-substrate interaction is much larger than the interaction within lubricant. The macroscopic mechanism of experimentally observed stick-slip is explained with an earthquake-like model.


29

Convergence of Molecular and Macroscopic Continuum Descriptions of Hydrophobic Hydration

Henry S. Ashbaugh, Princeton University, Department of Chemical Engineering, Olden St., Princeton, NJ 08544.

While molecular scale hydrophobic effects are beginning to be understood from a fundamental viewpoint, the gap between microscopic and macroscopic effects, which are the length scales relevant to self-assembly phenomena, is poorly understood. We present simulation studies of micro- and mesoscopic nonpolar solute hydration to gain insights to bridge these length scales. Hydrophobic interactions typically are assumed to be proportional to the solute surface area. Phenomenological correlations based on this presumption usually are fit to small nonpolar aqueous solubilities and extrapolated to macromolecular assemblies. We demonstrate, from explicit simulations of alkane hydration, that such correlations fail to simultaneously describe solubility and conformational equilibria. The breakdown of these correlations is traced to attractive interactions between water and the individual alkane groups. When only excluded volume interactions are taken into account a single value of the surface tension is determined which spans from methane to clusters the size of a micelle or globular protein.


30

Molecular Simulations of Peptides at Solid/Liquid Interfaces

Da Song and Daniel Forciniti, Chemical Engineering Depart., UMR, Rolla, MO 65409

Protein adsorption at solid/liquid interfaces has been extensively studied. However, the complexity of the phenomenon is such that it has precluded researchers to fully understand it at a molecular level. One approach that we have pursed to gain an insight into this phenomenon is to perform Monte Carlo simulations of systems consisting of a short peptide, a structured wall, and solvent (water) molecules. Each of the species is treated at an atomic level of detail. Our decision of using short peptides rather than entire proteins is aimed to capture the physics of the adsorption process while keeping the problem computationally treatable. The peptides we used are tailored to explore the effect that polar and non polar residues on a protein moiety have on its adsorption behavior. We demonstrate the effect that the hydrophobicity and charge of the surface have on the orientation of the adsorbed peptides. By monitoring the structure of water at the peptide and solid surfaces, we are able to quantify the entropic contribution to the free energy of adsorption.


31

Specific Interactions Between Polymer Pairs

Eva Blomberg1), Atte Kumpulainen1), Christelle David2) and Catherine Amiel2)

1) Dept of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden and Institute of Surface Chemistry, Stockholm, Sweden
2) Laboratoire de Recherche sur les Polymères, UMR C7581, CNRS, 2-8 rue H. Dunant, 94320 Thiais, France

The purpose with this study is to investigate the interactions between an adamantane end-capped poly(ethylene oxide) (PEO) and a polymer of b-cyclodextrin. The association of this system in solution has been studied by rheology, light scattering and fluorescence measurements. It was found that the adamantane-terminated PEO mixed with the b-cyclodextrin polymer give complexes where the inter-polymer links are formed by specific inclusion of the adamantane groups in the b-cyclodextrin cavities. This results in a higher viscosity of the solution and growth of intermolecular clusters. The surface force technique has been used to study the interactions between surfaces coated with a cationised b-cyclodextrin polymer across a water solution containing adamantane end-capped PEO polymers. It was observed that the adamantane/PEO polymer adsorbs onto the cyclodextrin surface and causes the surfaces to jump into contact due to an attractive force. This attractive force is interpreted as being due to a specific recognition between the hydrophobic adamantane groups on the PEO polymer and hydrophobic cavity in the b-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly compared to between b-cyclodextrin-coated surfaces, which is indicative of a specific interaction between the b-cyclodextrin polymer and the adamantane groups.


32

The Role Of Rings in the Phase Behavior of Equilibrium Polymers

James T. Kindt and William M. Gelbart, Department of Chemistry and Biochemistry, UCLA, Box 951569, Los Angeles, CA 90036-1569 , Richard P. Sear, Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom.

We use theory and simulation to investigate the influence of ring formation on equilibrium polymerization (that is, one-dimensional self-assembly) in the presence of either isotropic attractions or long-ranged isotropic repulsions. In the first case, we show that ring formation has a dramatic effect on the isotropic condensation of an equilibrium polymer in a poor solvent. In the second case, we show that self-assembly into rings can be particularly favored in the presence of long-ranged repulsions. The presence of long-ranged repulsions is proposed to account for the experimental observation of ring patterns formed by passivated metal nanoparticles suspended at the air-water interface.


33

Interactions of Hydrophobically Modified Polymers and Surfactant Lamellar Phases

Bing-Shiou Yang, Robert K. Prud’homme, William B. Russel, Princeton University, Princeton, NJ

We present an investigation of the anchoring of hydrophobically modified polymers in surfactant lamellar phases. The phase behavior of the system is determined by the balance between the loss of configurational entropy to confine the chain in the interlamellar spaces and the gain in hydrophobic energy to insert the hydrophobes in the surfactant membrane. The alteration of membrane properties has been measured using SANS and Neutron Spin Echo measurements. The lamellar phases can be sheared into multilamellar "onions" that are remarkably stable when hydrophobically modified polymers are added.


34

Why Do Polyelectrolytes Change the Bulk Self-Assembly of Similarly Charged Surfactants?

Stephanie Butler Velegol, Department of Chemical Engineering and Department of Civil and Environmental Engineering, Penn State University, University Park, PA 16802, Robert D. Tilton, Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

In systems where the surfactant and polyelectrolyte have opposite charges, it is the binding of the polyelectrolyte to the surfactant that is responsible for the change in bulk self-assembly. For this reason, in a system where the surfactant and polyelectrolyte have similar charges and do not bind in solution, it is generally often assumed that the polyelectrolyte will not affect the bulk self-assembly of the surfactant. However, we have found that the presence of a polyelectrolyte can significantly change the bulk self-assembly of a similarly charged surfactant. Cationic polylysine and oligolysine change the critical micelle concentration (cmc) of cationic cetyltrimethylammonium surfactants in low concentrations of background electrolyte. The cmc may either increase or decrease in the presence of the polyelectrolyte. We will discuss the effects of the polyelectrolyte concentration and molecular weight, as well as the type of counterions associated with either the surfactant or the polyelectrolyte. The results indicate that the polyelectrolyte acts like a co-ion and contributes its counterions for bulk self-assembly. Counterion exchange between the polyelectrolyte and the surfactant plays an important role.


35

Thermodynamics and Structure of Self-Assembling Branched Systems

A.G. Zilman, S.A. Safran, Dept. of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel

Branched self-assembling structures are encountered in a wide range of soft condensed matter systems, such as living polymers, wormlike micelles, physical gels, tubular microemulsions and dipolar liquids. We study the behavior of a generic assembly of self-assembling branched chains (living polymers) in terms of the `zero component` Heisenberg model, which has been used to describe solutions of polydisperse self-avoiding chains. The model is modified to include the possibility of branching. We calculate the phase diagram of a solution of branched self-assembling chains and show that it exhibits a first-order phase transition between unbranched chains, and a network, that terminates at a critical point. This transition is analogous to gelation transition. We calculate the structural properties of the system, such as density-density correlation function and the correlations between branching points and predict their behavior as a function of the relevant physical parameters.


36

Hydrogen Bonding and Self Assembly in Biopolymer Solutions

Yu Cheng, Robert K. Prud’homme, Princeton University, Princeton, NJ, Donald Rau, NIH, Bethesda MD

Weak hydrogen bonding interactions can be probed by the "Osmotic Stress" technique developed by Rau and Parsegian. By subjecting a solution to a known osmotic stress and measuring intermolecular distances by xray diffraction is is possible to produce molecular "force/distance" data. We demonstrate the role of hydrogen bonding in the assembly of guar galactomannans. At high osmotic pressure the chains assebmel into a two-dimensional sheet with inter-chain distances in the sheet of 4 A and intersheet spacings of about 16 A. The assembly into this sheet structure from a nematic crystal phase occurs at pressures that depend on the chemical derivatization of the polymer backbone. Increasing derivitization eventually eliminates the formation of the sheet-like structure and results in a the formation of only a compressible nematic phase. The effects of this substitution on enzymatic reactions and rheology of these polymers will be addressed.


37

Modulation of Guest Partitioning in Dendrimer-Surfactant Supramolecular Assemblies

Kerry K. Karukstis and Stacey C. Thonstad, Harvey Mudd College, Department of Chemistry, 301 E. Twelfth Street, Claremont, CA 91711

Electrostatic interactions are used to create a template-assisted supramolecular assembly consisting of a Starburst poly(amidoamine) dendrimer at the core and anionic surfactants positioned on the periphery. Three microenvironments within the dendrimer-surfactant complex are available for encapsulation of small guest molecules via non-covalent interactions - in the dendrimer inner core, at the termini of the dendritic branches, and within the surfactant tail region. Using a neutral fluorescent probe that exhibits an emission wavelength with an extreme sensitivity to the polarity of its surroundings, we have examined the dependence of the fluorophore distribution on dendrimer generation, surfactant structure, and surfactant:dendrimer concentration ratios. We delineate the microdomains within dendrimer-surfactant arrays through a deconvolution of the overall emission spectrum into a sum of overlapping Gaussian functions. Our spectroscopic investigations reveal that the electrostatic positioning of surfactants on the dendrimer surface can be used to modulate the location of guest molecules within the supramolecular complex.


38

Dynamic Effects on Surface Force Measurements

Olga I. Vinogradova and Gleb E. Yakubov, Max Planck Institute for Polymer Research, Postfach 3148, 55021 Mainz, Germany and Institute of Physical Chemistry, Russian Academy of Sciences, 31 Leninsky Prospect, 117 915 Moscow, Russia

The advances in both theory and experiment during the last two decades have brought about an enormous increase in understanding of the nature of surface forces. However, some of the non-DLVO interactions still challenge the fundamental notions on liquid structure and despite considerable theoretical efforts their origin remains controversial. One significant experimental technique to measure the forces is the surface force apparatus (SFA), another is the atomic force microscope (AFM). That some problems can be a consequence of the incorrect interpretation of the experimental data becomes apparent when one recognizes that the majority of the SFA and AFM measurements are non-equilibrium. An extra attraction or repulsion (in addition to that predicted by DLVO theory) is inferred to be present when experiment deviates from theory, and the dynamic contribution is either ignored or incorrectly subtracted. We suggest a way to distinguish between static and dynamic contribution to the total force measured. Our conclusions are illustrated by showing how hydrophobic slippage, electroviscous effect, viscoelastic phenomena, and rate-dependent adhesion implicate the results of the surface force measurements. On the other hand, we demonstrate that the dynamic nature of the force measurement devices allows one to study the above phenomena, as well as many others, such as the line tension, nucleation rate, plastic deformation, etc.


39

Direct Force Measurement at Liquid-Liquid Interfaces Using AFM

Raymond R. Dagastine, Dennis C. Prieve, Lee R. White, Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

Understanding the forces controlling the behavior of deformable interfaces such as oil-water and air-water interfaces are crucial in formulating and processing modern colloidal systems in applications such as mineral separation and oil recovery. Atomic force microscopy (AFM) was used to measure surfaces forces at a negatively charged tetradecane-water interface with a negatively charged borosilicate glass colloidal probe in a variety of electrolyte concentrations with and without surfactant. The interpretation of the AFM data to determine the force acting on the colloidal probe as a function of separation distance is complicated by surface deformation. We employ the results of an analytic theory, developed for this work, which accounts for the surface deformation of the interface based on the effect of the disjoining pressure between a rigid spherical probe and the liquid interface. Through iteration, the force versus separation distance and disjoining pressure of these systems were determined. Comparisons to disjoining pressures for these systems calculated from independently measured electrostatic parameters are also presented.


40

Thin Film Drainage between Solid And Fluid Interfaces with Control of Surface Forces

Jason N Connor and Roger G Horn, Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia

Measurements have been made of the thickness and profile of an aqueous thin film between a flat mica plate and a mercury drop as the two surfaces are brought together at constant speed. Optical interferometry allows the film thickness to be measured with sub-nanometer resolution at video rates, and application of an electrical potential to the mercury allows the double-layer force between the mercury and mica surfaces to be varied at will. Hydrodynamic resistance to film thinning results in classical dimpling behavior, in which the curvature of the mercury drop is reversed when the film thickness reaches a few hundred nanometers. The time evolution of the dimple is followed for various types of surface force: strongly or weakly repulsive and weakly or strongly attractive. It is shown that the surface force has a major influence on the drainage behavior of the film, even at film thicknesses of several Debye-lengths.


41

The Effect of Surfactant on the Drainage of an Aqueous Film between an Oil Droplet Approaching a Hydrophobic Solid Surface

Michelle Gee, School of Chemistry, University of Melbourne, Australia

The effect of CTAB concentration above the CMC on the profile of aqueous films between an approaching squalene droplet and a hydrophobic silica surface is investigated using the technique imaging reflectometry. Drainage through the film periphery was observed to display viscous fingering effects and occurred via dendritic channels. This is explained in terms of a combination of viscosity and interfacial tension effects. To our knowledge, this is the first time behaviour such as this has been experimentally observed in a system involving the approach of a droplet to a solid surface.


42

Interactions Between PEG Brushes in Aqueous Medium: Soft Surface Modification for Implants and Biomaterials

Uri Raviv, Pierre Laurat, Joseph Frey, Rafael Tadmor and Jacob Klein, Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel

Poly(ethylene glycol) (PEG) is a water soluble polymer compatible with the immune system of the human body. Thus it may be a good candidate to modify surfaces of artificial implants, or to serve as a soft spacer for modifying surfaces with different biomolecules. To examine this, direct measurements of the normal and shear forces between two atomically smooth mica surfaces bearing functionalized PEG immersed in pure (conductivity) water have been carried out as a function of surface separation. PEG (Mw=3.4k) that has been functionalized on both ends, was introduced into water and adsorbed only at one end onto the mica surface. A monotonically-increasing force-distance law was indicated, beginning at surface separations of ~100nm. This repulsion is shorter ranged than in the conductivity water, due to the shorter Debye length in the presence of higher ion concentration. High repulsion forces starting at ~5nm are due to the extensive compression of the grafted polymer layers. The range of the high repulsion forces (3-5nm) and the AFM images indicate that the modified PEG molecules form a uniform monolayer on the surface. Shear motion was then applied between the surfaces, at separations from some tens of nanometers down to closest approach distance, and the lubricating properties of the layers were investigated. Results, which suggests that such PEG molecules can be readily used for surface modification of implants as well as precursor for active biomolecules, will be presented at the meeting.


43

Surface Tension Gradients due to Bulk Flow Coupling at an Air/Water Interface

A. H. Hirsa, and R. Miraghaie, Department of Mechanical Engineering, Aeronautical Engineering and Mechanics, Rensselaer Polytechnic Institute, Troy, NY 12180. J. M. Lopez, Department of Mathematics, Arizona State University, Tempe, AZ 85287

The coupling between a bulk flow and a surfactant-influenced air/water interface has been examined through experiments and computations. The flow bounded by stationary inner and outer cylinders is driven by the constant rotation of the floor and the free surface is initially covered by a uniformly distributed insoluble monolayer. When driven slowly, we have the deep-channel surface viscometer where the flow is essentially azimuthal. The only interfacial property that affects the flow in this regime is the surface shear viscosity. When operated at higher Reynolds numbers, secondary flow drives the surfactant film towards the inner cylinder until the surface tension gradients balance the shear stress on the bulk fluid. Due to the small capillary number, the surface tension gradients dominate the surface viscosities in the radial stress balance, and the surface shear viscosity is only important in the azimuthal direction. Vitamin K1 was chosen since it forms an insoluble monolayer on water with essentially zero surface shear viscosity in our range of concentration. The flow near the interface was measured using digital particle image velocimetry and found to be steady and axisymmetric at Re = 8500. The Navier-Stokes equations were computed, and the coupled flow is found to critically depend on the nonlinear equation of state.


44

Surface Tension Driven Break up of Dilute Polymer Solutions Using Forced Disturbances

Yenny Christanti and Lynn M. Walker, Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

Industrial processes such as ink jet printing and particle production have motivated the study of capillary break up of liquid jets. We are investigating the relationship between viscoelasticity and drop formation aimed at the production of monodisperse colloidal sized droplets. For Newtonian fluids, the final break up occurs through jet pinching which is influenced by the bulk properties of the fluid, namely viscosity, surface tension, and density. The addition of highly extensible polymer molecules changes the rheology of the bulk fluid influencing interfacial break up. In addition to viscous stress, elastic stress induced by the presence of macromolecules balances the interfacial stress. We quantify the capillary break up of well-characterized dilute polymer solutions due to forced disturbance. Viscoelasticity retards the formation of satellite drops. Capillary pressure causes fluid to drain from the filaments into the beads, but the relaxation time of the fluid dictates the draining rate.


45

Yield Stress and Compressibility of Solids-Laden Foams

Seung Ihl Kam and William R. Rossen, Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX 78712-1061

The yield stress and compressibility of a mixture of solids and foam is crucial to handling wastes stored at the Hanford nuclear reservation, in fracturing operations in the petroleum industry, and in tunneling through soft sediments using foam drilling fluids. These properties were investigated theoretically using 2D periodic model. The yield stress of a foamy sand increases with gas fraction at a given solid fraction and increases with solid fraction at a given gas fraction. Remarkably, at fixed liquid fraction, yield stress is relatively insensitive to gas or solid fraction alone, as reported empirically in the petroleum industry. There exists a minimum gas fraction below which the yield stress disappears. Theoretical values of yield stress are comparable to those measured in tunneling studies. We compare these results to previous experimental and theoretical studies of foam with and without solids. For sufficiently small solids and bubbles, capillarity significantly alters the compressibility from that for the same volume of ideal gas and for a dispersion of bubbles of the same size without solids present. There is a second-order phase transition at the point where bubbles touch each other between the solid particles.


46

Surface Yield Properties of Bitumen Droplets in Aqueous Media

Kevin Moran*, Anthony Yeung*, Jan Czarnecki# and Jacob Masliyah*, *Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6, #Syncrude Canada Ltd, Edmonton Research Centre, 9421 – 17 Avenue, Edmonton, Alberta, Canada T6N 1H4

Simple, micromechanical techniques allow direct measurements of interfacial properties of emulsion drops. Under isotropic interfacial tensions, elongated emulsion drops will always recover to their original spherical shapes. However, more complex interfaces can exhibit yield properties that prevent an extended droplet from returning to a sphere. To directly study this phenomenon, a novel, micromechanical procedure is developed that allows for the elongation of individual emulsion drops using micropipettes. One of the micropipettes is shaped into a cantilever for axial force measurements; the loading required for a given drop deformation is calculated from the cantilever deflection. Of present interest is the behaviour of bitumen drops in an aqueous medium containing calcium ions and montmorillonite clays. (This environment is known to severely impede the coalescence of such drops, which in turn reduces the recovery of bitumen from oil sands in water-based extraction processes.) The plasticity and other surface properties of these bitumen drops are discussed. A simple, lumped-parameter model is developed to describe the recovery of bitumen droplets to final non-spherical shapes.


47

Monodisperse Foams: Effect of Shear on Lyotropic Lamellar Phases Containing Air Bubbles

A. Alkahwaji, R. K. Prud'homme, Department of Chemical Engineering, Princeton University, Princeton, New Jersey, 08544, P. Herve, CNRS/Rhodia, Complex Fluids Laboratory UMR166, Cranbury, New Jersey, 08512-7500

We have conducted a study on the effect of shear on air droplet dispersions in lyotropic lamellar phases. Depending on the lamellar phase composition and on the shear rate, a quasi-monodisperse bubble size distribution can be obtained. The polydispersity in air droplet size is in general very broad at low shear rates, decreases as the shear rate increases. The quasi-monodisperse bubble size distribution is obtained at the higher values of the shear rate. In addition to the polydispersity the actual size of the air droplets decreases with increasing shear rate and also with increasing lamellar phase concentration. This variation may be understood as a balance between an effective shear stress and a Laplace pressure. Our study opens the way to produce controlled quasi-monodisperse foams. Several systems have been investigated and particularly good results were obtained using electrostatically stabilized lamellar phases of potassium didecyl phosphate ester in water.


48

Modeling of Foam Flow in Porous Media

Qiang Xu & William R. Rossen. Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX78712, United States

Foams are injected into subterranean porous layers for oil production and environmental remediation. In these applications, foam bubbles are larger than pores; most remain trapped, while those that flow move along paths not unlike a periodically constricted tube. Foam effective viscosity in this case comprises a yield stress arising from pore constriction and the drag of films along the pore wall. Earlier research showed that soap films moving quasi-statically through even radially symmetric pores jump spontaneously to asymmetric shapes, and thereby increase the yield stress. Here we extend this work to include drag, in two dimensions. As velocity increases, the asymmetric jump disappears either abruptly, replaced by a symmetric jump, or gradually; as a result the effective yield stress drops substantially, though the drag contribution increases. One result is that bubbles moving at a finite velocity can have lower pressure drop than at nearly zero velocity, or pressure drop can decrease as velocity increases over a finite range of velocities. Experiments confirm the transition from asymmetric to symmetric movement with increasing velocity in a model pore.


49

Information of Static Structure Factor from Frequency-Domain Photon Migration Measurements of Multiply Scattered Light in Colloidal Suspensions of Varying Volume Fraction and Ionic Strength

Yingqing Huang, Zhigang Sun, and Eva M. Sevick-Muraca, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA.

Debye length and the zeta potential decide the electric static force among the particles. The electric repulsive force between particles increases the static structure of colloidal systems and hinders scattering. Using frequency domain photon migration (FDPM), the scattering coefficients of polystyrene lattices were measured at two wavelengths (687 nm and 828 nm). The ionic strengths were adjusted using sodium chloride to 100 mM and 0.1 mM equivalents, which corresponds to the debye lengths of 1 nm and 30 nm. The polydispersity and the average size of the polystyrene lattices were determined by TEM and dynamic light scattering, respectively. Experimental results illustrated that, when the debye length equals 1 nm, the isotropic scattering coefficients are significantly greater than those when the debye length is equal to 30 nm. At the debye length of 1 nm, the isotropic scattering coefficients, can be accurately predicted by Mie theory and the Percus-Yevick polydisperse model for hard-sphere interaction. Also, average structure factors at the debye length of 30 nm were less than those at the debye length of 1 nm, which indicates that the polystyrene lattices are more structured at a large debye length.


50

Developing Rapid Screening Methods and Structural Studies for Soluble Macromolecules

Amir Y. Mirarefi and Charles F. Zukoski, University of Illinois, Urbana-Champaign, Department of Chemical Engineering, 114 RAL, Box C-3, 600 S. Mathews Ave., Urbana, IL 61801

Crystallization of globular macromolecules is a notoriously difficult process. Often the conditions resulting in x-ray quality crystals involves a large number of trial experiments where buffer conditions are screened. This process is slow, tedious and provides little guarantee that crystallization conditions will be found. In addition, screening processes provide no insights into why certain conditions work with some globular macromolecules and not others. One goal of the research discussed in this paper is to develop screening methods that ensure a higher likelihood of success. Our approach is to locate likely crystallization conditions based on characterization of the strengths of protein interactions. A wide range of studies show that when the strength of attractions of globular proteins can be driven into a range where the second virial coefficients, B2, lie in a narrow range, likelihood of crystallization of globular macromolecules is greatly enhanced. In this presentation we develop a dynamic light scattering (DLS) method for rapidly investigating the effects of changing solution conditions on the strength of protein interactions. The volume fraction dependence of the diffusivity extracted from DLS measurements contains information similar to that contained in B2. However, DLS has the advantage over static light scattering used to determine B2 of requiring less sample and less time than static light scattering (SLS). Initial studies will be presented demonstrating the feasibility of this method and the links between measurements of strength of interaction and likelihood of crystallization.


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