Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G21: Colloids IV: Polymeric and Colloid-Polymer Systems |
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Sponsoring Units: DFD Chair: Cynthia Olson Reichhardt, Los Alamos National Laboratory Room: Baltimore Convention Center 318 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G21.00001: Self Assembly of Temperature-Sensitive Capsules using PNIPAm Microspheres D. B. Lawrence, T. Cai, Z. B. Hu, M. Marquez, A. D. Dinsmore We present a method for self assembly of novel semi-permeable capsules (colloidosomes) exhibiting temperature-dependent size and surface properties. These hollow micro-capsules are composed of micron-sized PNIPAm-co-acrylic acid particles that exhibit a reversible size transition at 32$^{o}$C. The PNIPAm particles self-assemble at the spherical interface between 2-Octanol and water droplets. Electrostatic cross-linking with the di-block copolymer Poly(butadiene-b-N-methyl 4-vinyl pyridinium iodide) locks the particles in place, forming a rigid, elastic membrane that remains after the 2-Octanol/Water interface is removed. Such self-assembled capsules retain the temperature sensitivity of the PNIPAm particles from which they are constructed, and exhibit reversible size transitions. Controlled variation of the permeability and potential applications in controlled release will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G21.00002: Femtonewton Measurements of Polymer-Mediated Colloidal Interaction Sven Behrens, Helmut Auweter The stability of colloidal dispersions hinges on the repulsive interaction between the particle surfaces. This interaction can be modified by adsorbed polymer and further tuned by the response of the adsorbate layer to changes in the surrounding solution.Total internal reflection microscopy has been used in this study to investigate the effect of charged and neutral polymer adsorbates on the interaction of a colloidal particle with a flat substrate. The results were correlated with light scattering studies of the employed polymers in solution and of polymer-coated particles; they reveal a subtle interplay of steric, electrostatic, and van der Waals forces. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G21.00003: Phase Behavior of Charged Colloid-Polymer Mixtures: A Simulation Study Ben Lu, Alan R. Denton We present a Monte Carlo simulation study of mixtures of charged colloids and nonadsorbing polymers. The multicomponent mixture of colloidal macroions, microions (counterions and salt ions), polymers, and solvent is modeled as an effective two-component mixture of pseudomacroions and neutral polymers. The pseudomacroions are assumed to interact via effective electrostatic interactions\footnote{A. R. Denton, {\it Phys. Rev.} E {\bf 62}, 3855 (2000).} (screened-Coulomb pair potential and one-body volume energy) and the polymers are treated as effective spheres that have hard interactions with the colloids but are mutually ideal. Previous studies have identified the volume energy as the driving force for phase separation of deionized colloidal suspensions\footnote{R. van Roij, M. Dijkstra, and J.-P. Hansen, {\it Phys. Rev.} E {\bf 59}, 2010 (1999); P. B. Warren, {\it J. Chem. Phys.} {\bf 112}, 4683 (2000).} and polymer depletion-induced attraction as the origin of colloid-polymer demixing. Here we apply the Gibbs ensemble Monte Carlo method to the effective binary mixture to examine the combined influences of electrostatic and depletion-induced interactions on phase behavior. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G21.00004: Demixing of Charged Colloid-Polymer Mixtures: Variational Theory$^1$ Shrikant Shenoy, Alan R. Denton We investigate thermodynamic phase behavior of mixtures of charged colloids and neutral nonadsorbing polymers using a variational method for the free energy. The mixture of macroions, microions, and solvent is first mapped onto an effective one-component system of pseudomacroions that interact via effective electrostatic interactions. The polymers are modeled as effective spheres that have hard interactions with the colloids but are mutually ideal. The charged colloid-polymer mixture is then mapped onto an Asakura-Oosawa model with effective colloid and polymer diameters$^2$. The free energy is approximated by combining thermodynamic perturbation theory for the colloids with free-volume theory for the polymers and minimizing with respect to the effective colloid diameter. Phase diagrams are computed by a coexistence analysis that ensures equality of pressures and of chemical potentials of all species in the two phases. The resultant phase behavior depends sensitively on colloid charge, polymer-to-colloid size ratio, and composition. Electrostatic repulsion between colloids is found to stabilize the mixture against polymer depletion-induced demixing, consistent with previous predictions$^3$ and experimental observations. \\[1ex] $^1$~Support from the National Science Foundation (grant DMR-0204020) is gratefully acknowledged. \\[0.5ex] $^2$~A. R. Denton and M. Schmidt, {\it J. Chem. Phys.} {\bf 122}, 2449111 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G21.00005: Preparation of PHSA-PMMA stabilizer for model hard sphere systems Andrew Hollingsworth, William Russel, Carlos van Kats, Alfons van Blaaderen Sterically-stabilized colloidal particles are an excellent model hard-sphere system used by many groups. One of the original stabilizers used for such systems was developed and patented by ICI more than 30 years ago. It consists of a `comb-like' stabilizer of a poly(12-hydroxystearic acid) which is soluble in aliphatic hydrocarbons. These pendant PHSA chains are grafted to an insoluble poly(methyl methacrylate) backbone that strongly adsorbs to polymer particles and thus provides a means of anchoring stabilizer to particle surfaces. Unfortunately, the PHSA-g-PMMA stabilizer is not commercially available. Furthermore, the three-step procedure (Antl, et al. 1986) is generally regarded by non-chemists as technique intensive and time-consuming. We have systematically studied the PHSA-PMMA stabilizer synthesis with the goal of taking the mystery out of the protocol and making the entire synthesis reproducible. Several important details, not published in the literature, will be discussed, along with the analytical results from mass spectroscopy, proton NMR, acid titration and gel permeation chromatography, all of which were used to characterize the polymer and its precursors. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G21.00006: Self-assembly of Asphaltenes: Enthalpy, Entropy of Depletion and Dynamics at Crossover I-experimental Natalia Lisitza, Denise Freed, Pabitra Sen, Yi-Qiao Song The continuous rise of global demand for energy and the difficulty of significantly increasing production have driven the petroleum industry to develop much more difficult oil reservoirs, such as deep-water fields. Asphaltenes, naturally-occurring large aromatic molecules in crude oils, are known to be the ``cholesterol'' of petroleum because they self-associate to form solid aggregates and eventually clog the production pipes and the rocks. An extraordinary amount of work using many analytical techniques has been applied to elucidate the properties of asphaltenes. However, many fundamental issues, such as the molecular architecture and the aggregation mechanisms, are still in debate. Here we use NMR to detect asphaltene aggregation in toluene solutions and to obtain both the enthalpy and entropy of this process. We observe an abrupt drop of the asphaltene diffusion constant which is indicative of a molecular conformation (shape) change. This change is intimately related to or possibly a prerequisite of the aggregation. The entropy of aggregation was found to be positive due to the excluded volume effect for the solvent. This is reminiscent of the depletion entropy force, which is considered to be the driving force for the aggregation of much large artificial nanoparticles. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G21.00007: Self-assembly of Asphaltenes: Enthalpy, Entropy of Depletion and Dynamics at Crossover II - theoretical Denise Freed, Natalia Lisitza, Pabitra Sen, Yi-Qiao Song NMR spin-relaxation is directly sensitive to molecular dynamics and is therefore an excellent tool for studying the formation of nano-aggregates. When aggregates form, the molecular rotations are slowed down, and the spin-relaxation rate is enhanced, which reduces the NMR signal. This gives rises to a remarkable kink in the NMR signal intensity at the Critical Nano-Aggregate Concentration (CNAC). In this talk, we use Debye's two-state model for micelle formation to describe asphaltene aggregation. We use the temperature dependence of the CNAC to determinate the enthalpy and entropy of aggregation. The enthalpy is negative, as expected, and indicates that the aggregate formation is energetically favorable due to $\pi$-stacking interactions. On the other hand, we find that the entropy is positive, which is quite a surprise. We propose that this increase comes from the depletion entropy of the solvent. As asphaltene molecules aggregate, the free volume available for the solvent molecules increases, giving rise to a depletion effect. An estimate of the depletion entropy from the sizes of asphaltene and toluene (solvent) is very close to the measured entropy gain. We will also discuss the applicability of Debye's two-state model to aggregate formation in asphaltene solutions. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G21.00008: Molecular Diffusivities of Asphaltene Monomers by Fluorescent Correlation Spectroscopy Ballard Andrews, Rodrigo Guerra, Pabitra Sen, Oliver Mullins Many analytical techniques have been applied to elucidate the properties of asphaltenes due to their extraordinary importance in the petroleum industry. However, many fundamental issues such as the molecular size are still in debate. Using Fluorescence Correlation Spectroscopy (FCS) we measure the translational diffusion coefficient of asphaltene molecules in toluene at extremely low dilutions. To avoid focal volume artifacts common in FCS we use a scaling procedure to determine asphaltene diffusivities relative to known molecules such as porphyrins, perylene and quantum dots. We estimate a diffusion coefficient for the asphaltene molecules of 0.3, 10$^{-5}$ cm$^2$/sec at room temperature. This value agrees with recent estimates from NMR studied at 10-100 fold higher concentrations. The Stokes-Einstein equation implies that the unaggregated hydrodynamic diameter of asphaltene monomers is 15-25 {\AA}. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G21.00009: Novel emulsions stabilized by pH and temperature sensitive microgels To Ngai, Helmut Auweter, Sven Behrens Poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-MAA) microgel particles in aqueous solution exhibit a volume phase transition that can be induced by changes of either pH or temperature. In the swollen state, these microgels self-assemble at an octanol-water interface and can be used to stabilize surfactant-free oil-in-water emulsions. This stabilizing efficiency is retained even in the collapsed state, provided that the microgels are fully charged. At very low charge (low pH), on the other hand, the microgels migrate completely into the oil phase, and the emulsion breaks. In an intermediate regime of practical interest the emulsion stability can be tuned by small adjustments of pH or temperature. Because of this unprecedented stability control, we believe that such stimulus-responsive charged microgels have a great potential for applications in the field of cosmetic or pharmaceutical formulations. Conceptually they belong to a new class of emulsifiers combining properties of both classical surfactants and solid particles. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G21.00010: Internal Dynamics of Double Emulsion Creams and Polymer-Dispersed Complex Droplets Hu Gang The availability of monodisperse double emulsions allows for detailed study of their stability and rheological properties. Practical use of double emulsions concerns the structure and structural evolution of the complex droplets. The usually achievable size of the complex droplets ranges from a micrometer to tens of micrometers. To obtain a homogeneous system, a double emulsion has to be in the form of cream or gel to avoid macroscopic phase separation. However, immobilizing the double emulsion drops does not cease the full dynamics since the internal smaller droplets are also subject to thermal agitation. The ability to track the evolution of the internal encapsulated phase is essential to the understanding of the kinetic stability of a new formulation. Conventional light scattering encounters significant difficulties to probe the structure of concentrated emulsions while diffusing-wave spectroscopy (DWS) shows a unique advantage. We prepare monodisperse W/O/W double emulsions and apply DWS to study the structure of double emulsion creams and gels. We also test the sensitivity of DWS when the amount of encapsulated phase is varied by induced coalescence or osmotic gradient. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G21.00011: Quasi-two-dimensional equilibrium and non-equilibrium thermodynamics of thin liquid films stabilized by colloidal particles Jerzy Blawzdziewicz, Eligiusz Wajnryb Due to the presence of oscillatory structural forces, static and dynamic properties of thin liquid films stabilized by colloidal particles, micelles, or macromolecules differ from the properties of particle-free films. During drainage process, particle-stabilized films often form a stepwise structure with coexisting regions (phases) of uniform but different thickness. We describe film phase equilibria using a quasi-two dimensional thermodynamic formalism. The key quantity in our approach is the film tension. We show that the particle contribution to this quantity results from the anisotropy of the particle osmotic-pressure tensor in the film. The quasi-2d description is also developed for non-equilibrium film states. We show that the motion of particle-stabilized films is analogous to the dynamics of a two-dimensional compressible fluid - the film thickness plays the role of mass density per unit area and film tension the role of pressure. In the linear-response regime, the film dynamics is characterized by the shear and extension viscosity coefficients. There are also two independent kinetic coefficients relating the particle flux to the gradients of the normal osmotic pressure and particle chemical potential. For a film stabilized by a suspension of hard spheres we have calculated these coefficients using a multipolar-expansion methods combined with a flow reflection technique. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G21.00012: Dynamics, Rectification, and Fractionation for Colloids on Flashing Substrates Andras Libal, Charles Reichhardt, Boldizsar Janko, Cynthia Olson Reichhardt We show that a rich variety of dynamic phases can be realized for mono- and bidisperse mixtures of interacting colloids under the influence of a symmetric flashing periodic substrate. These type of substrates have been attracting growing interest due to recent experimental breakthroughs that allow the creation of dynamic periodic arrays using optical and holographic techniques. With the addition of dc or ac drives, these system show phase locking, jamming, and new types of ratchet effects. We studied these effects in detail. The ratchet effect can be used for charge separation of a bidisperse system. In this system, in some regimes we find that the addition of a non-rathceting species increases the velocity of the ratcheting particles. We show that these effects occur due to the collective interactions of colloids. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G21.00013: Formation of Extended Optical Traps by Shape-Phase Modulation - Measuring Inter-Colloidal Interactions in Tailored Potential Landscapes Yohai Roichman, David G. Grier We describe methods for projecting holographic optical traps whose potential energy wells are extended along specified curves, typically a straight line, and whose intensity profiles also can be tailored. This class of optical traps is useful for manipulating elongated nano-items, creating anisotropic potential energy landscapes, and in particular for investigating the interactions and dynamics of micro-particles in reduced dimensionality. This new class of extended optical traps is created by modulating the shape and the phase of a complex hologram, projected by a phase-only diffractive optical element. We demonstrate rapid characterization of extended traps' potential wells through digital video microscopy of trapped colloidal spheres, and use arrays of calibrated traps for efficient measurement and screening of colloidal interactions. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G21.00014: Holographically Assembled Photonic Quasicrystals Yael Roichman, David G. Grier Recently, the photonic band structure of a centimeter-scale three-dimensional icosahedral quasicrystal was measured in the microwave region and shown to feature prominent photonic band gaps at the effective Brillouin zone boundary. We demonstrate that similar two- and three-dimensional quasicrystalline photonic structures can be assembled from micrometer-scale colloidal particles through dynamic holographic optical trapping. The resulting colloidal heterostructures feature lengthscales appropriate for photonic bandgap applications at visible wavelengths, and can include specifically engineered defects such as vacancies, active centers and waveguides. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G21.00015: The Intermediate Scattering Function in Fluorescence Correlation Spectroscopy Rodrigo Guerra, Ballard Andrews, Pabitra Sen We formulate the autocorrelation function for Fluorescence Correlation Spectroscopy (FCS) $G_D(\tau)$ in reciprocal space in terms of the of the Intermediate Scattering Function ISF($\overrightarrow{k},t$) and the fourier transform of the Optical Response Function ORF($\overrightarrow{k}$). In this way we may extend the use of FCS to processes that have been studied using NMR, DLS, and neutron scattering. This formalism is useful for the complicated propagators involved in confined systems and in the study of diffusion in cells: where diffusion is either restricted or permeation through membrane is important. Calculations in k-space produce approximate expressions for the ORF using cumulant expansions that are accurate for small wavevectors. This provides descriptions for longer timescales better suited for studying time-dependent diffusion $ISF($\overrightarrow{k},t$)\rightarrow exp[-tD(t)k^2]$ and provides a natural separation of contributions from system dynamics and from optical artifacts and aberrations. We will show an explicit derivation of a semi-analytical fit function for free diffusion based on standard electromagnetic analysis of a confocal optical apparatus. This fit function is then used to analyze a representative data set and has no free fit parameters other than the diffusion constant. [Preview Abstract] |
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