Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D36: Colloids II |
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Sponsoring Units: DFD Chair: Penger Tong, Univ. of Sci. and Tech Room: LACC 510 |
Monday, March 21, 2005 2:30PM - 2:42PM |
D36.00001: AC electrokinetics of microparticles in suspensions K.W. Yu Under the application of electric fields, the structure of electrorheological (ER) solids can be changed from the body- centered tetragonal lattice (ground state) to other lattices. For a particle in the lattice, we have derived its dipole moment by taking into account both the local-field effect arising from all the other particles and the multipolar interaction between two touching particles, through the Ewald dipole formulation and multiple image method. To simplify the study, the dipole moments have been expressed in the dielectric dispersion spectral representation (DDSR) exactly. To this end, it has been found that the electrorotation (EOR) spectrum of ER solids can be affected significantly by the structure transformation. Our results have been understood in the spectral representation theory. Thus, it is possible to monitor the structure of ER solids by detecting its EOR spectrum. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D36.00002: New Method to Calculate Electrical Forces Acting on a Sphere in an Electrorheological Fluid Kwangmoo Kim, David Stroud, Xiangting Li, David Bergman We describe a method to calculate the electrical force acting on a sphere in a suspension of dielectric spheres in a host with a different dielectric constant, under the assumption that a spatially uniform electric field is applied. The method uses a spectral representation for the total electrostatic energy of the composite. The force is expressed as a certain gradient of this energy, which can be expressed in a closed analytic form rather than evaluated as a numerical derivative. The method is applicable even when both the spheres and the host have frequency-dependent dielectric functions and nonzero conductivities, provided the system is in the quasistatic regime. In principle, it includes all multipolar contributions to the force, and it can be used to calculate multi-body as well as pairwise forces. We also present several numerical examples, including host fluids with finite conductivities. The force between spheres approaches the dipole-dipole limit, as expected, at large separations, but departs drastically from that limit when the spheres are nearly in contact. The force may also change sign as a function of frequency when the host is a slightly conducting fluid. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D36.00003: Charged Colloids Near an Oil-Water Interface: Collossal Crystals and Colloidosomes W.B. Russel, M.E. Leunissen, A. van Blaaderen, A.D. Hollingsworth, M.T. Sullivan, P.M. Chaikin In a cyclohexyl bromide (CHB)-decalin mixture sterically stabilized PMMA spheres have a positive charge. Together with a Debye screening length of several microns this results in long-ranged repulsive interactions. Water forms a stable interface with CHB and, with its high dielectric constant, absorbs ions from the oil phase. This reduces the screening even further and leads to `Coulombic' crystals with extreme lattice constants (tens of microns). The oil-water interface often is charged, repelling the similarly charged particles in the oil phase, leaving a large zone depleted ($\sim $ 50 microns). However, independent of the sign or magnitude of interfacial charges, at short range the attraction between the charged particle and its image, in the higher dielectric constant water, dominates. Thus, the charged hydrophobic particles get permanently bound to the interface, forming a 2D `surface crystal'. The crystal density can be controlled with an electric field that drives the particles to the surface. Using the specific properties of this particle-oil-water system we can make particle-coated droplets (`colloidosomes'), geometric particle packings enveloped by a water sheath and particle transporting channels. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D36.00004: Attraction between like-charged colloidal particles at a water-air interface Wei Chen, Tai Kai Ng, Penger Tong, Susheng Tan , Warren Ford Like-charged colloidal particles at aqueous interfaces have been found to experience attractive interactions but the origin of the attraction is not well understood. Here we report a detailed experimental study of attractive interactions between micron-sized charged polystyrene latex spheres at a water-air interface. Atomic force microscope images show 100nm-sized patchy regions of ionized polystyrene-rich domains on the colloidal surface, indicating that the surface charge distribution of the particles is not uniform. The surface heterogeneity will introduce in-plane diploes for the interfacial particles, causing attractive interactions when the inter-particle distance becomes small. The experiment reveals that the interfacial particles can form stable bounded clusters with different configurations. A theoretical model is developed to explain the experimental results using a combined interaction potential of in-plane dipole attraction and out-of-plane dipole repulsion as well as screened-Coulomb repulsion at short distances. Work supported by the Research Grants Council of Hong Kong SAR under Grant No. HKUST603003. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D36.00005: From Random Walk to Single File Diffusion Binhua Lin, Mati Meron, Bianxiao Cui, Stuart A. Rice, Haim Diamant We report an experimental study of diffusion in a quasi-one-
dimensional weakly interacting colloid suspension. The time
range studied encompasses regions that are both shorter and
longer than the time between collisions in the system. The mean
squared displacement as a function of time is described well
with an empirical expression for the entire time range measured.
In the empirical expression the inverse mean squared
displacement is represented as the sum of the inverse mean
squared displacement for short time normal diffusion (random
walk) and the inverse mean squared displacement for asymptotic
single-file diffusion ($\frac{1}{ |
Monday, March 21, 2005 3:30PM - 3:42PM |
D36.00006: Coarsening driven transport of gas in steady-state foam Klebert Feitosa, Douglas J Durian An experiment is performed to investigate the transport of gas through coarsening in a column of aqueous foam. The foam is maintained in a steady state by a constant flux of gas at the bottom. The liquid-fraction and bubble-size profiles are measured vertically in the sample. The results show that the liquid fraction profile in the steady state foam departs from the equilibrium capillary profile for constant bubble size. We compare the data to predictions of the coarsening equation computed in steady state together with the no-drainage condition. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D36.00007: Turning colloidal hard spheres into binary crystals of oppositely charged particles A. van Blaaderen, C.G. Christova, M.E. Leunissen, C.P. Royall, A.-P. Hynninen, M. Dijkstra Sterically stabilized colloidal particles dispersed in apolar solvents are almost always slightly charged ($\sim $100's of charges). However, this does not interfere with crystallization at high volume fractions. At low volume fractions the consequences can be dramatic compared to hard-sphere behavior. We show that dispersions of \textit{oppositely }charged colloids, that readily form equilibrium phases, can be made by using a mixture of sterically stabilized PMMA spheres. In the solvent mixture used both the density and index of the spheres can be matched. Among the binary crystals already observed are large ($\sim $0.5 mm) CsCl-type crystals made up of 2 micron sized spheres of opposite charge. Our findings open up a new direction of investigation for colloidal model systems: particles with (spherically symmetric) long-range attractive interactions. It also provides new avenues for the creation of binary crystals as it enables different crystal structures and easier and faster binary crystal growth. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D36.00008: Microrheology of FCC Hard Sphere Crystals M.T. Sullivan, K. Zhao, A.D. Hollingsworth, W.B. Russel, P.M. Chaikin, A.J. Levine We present a measurement of the low-frequency elastic moduli of hard sphere colloidal crystals. Our system consists of concentrated suspensions ($\phi > 0.49$) of monodisperse PMMA-PHSA particles suspended in a density-matching solvent mixture of decalin and tetrachloroethylene. Single crystals are heterogeneously nucleated from a templated surface. In particular, a square surface pattern is used to grow large (1mmx1mm laterally, 20 microns deep), face centered cubic (FCC) single crystals from the template surface. Using confocal microscopy and video analysis techniques, we have measured the thermally driven self- and cross- correlated particle motion in the (100) plane. The measured viscous and elastic responses show an angular dependence that is related to the anisotropy of the hard sphere FCC crystal. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D36.00009: Binary colloidal crystals of oppositely charged particles M.E. Leunissen, C.G. Christova, R. van Roij, A. Imhof, A. van Blaaderen We show that sterically stabilized PMMA spheres in a density and index matching cyclohexyl bromide-decalin mixture can form a system of \textit{oppositely} charged colloids with long-range attractive interactions. The two particle species (+ and -, both $\sim $ 2 micron) had a different fluorescent label, so as to distinguish them with confocal microscopy. As determined by (micro-) electrophoresis the particles carried tens to hundreds of charges, while the charge-ratio between the two species was asymmetric ($\sim $ 1:4). We observed dense CsCl-type crystals, which are so far only predicted for same charge particles. Crystals nucleate in $\sim $ 40 hours from a highly structured, dense fluid and coexist with a fluid or very dilute gas phase of small clusters for at least weeks. The large crystals ($\sim $ 0.2 mm a side) display `terraces' and contain point defects, like substitutional particles and vacancies, where a sphere of one species is missing. We used a static electric field to determine the sign of the particle charge \textit{in situ} and to induce melting of the crystals. We found other structures for different size ratios. Moreover, we made binary crystals with particles that have different chemical compositions. This offers the possibility to further modify the structure, by selectively treating one of the species after crystallization. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D36.00010: Growth and Deformation of Colloidal Crystals and Glasses Peter Schall, Itai Cohen, David A. Weitz, Frans Spaepen Plastic deformation in atomic crystals is governed by dislocations – line defects in the crystalline lattice. Understanding how these dislocations propagate, multiply, and interact is central for understanding the plastic response of a crystalline material to an applied stress. Unlike for crystals, not much is known about the deformation mechanism of glasses, and the microscopic processes leading to macroscopic deformation are still highly speculative. We use colloidal crystals and glasses as models to study the behavior of their atomic counterparts under applied stress. We use confocal microscopy to determine the position of the individual particles and to study defect propagation on the particle scale. The colloidal crystals exhibit dislocations that show remarkable similarities to dislocations in atomic crystals. The slow time scale of the colloidal suspension allows us to directly observe the nucleation of dislocations as the crystal is deformed. We have built a laser diffraction microscope, which is inspired by a transmission electron microscope used to study dislocations in hard materials, to image the strain field of the dislocation defects. This technique enables us to study dislocation motion and dislocation interaction on a much larger length scale. In the amorphous suspension, we are able to follow the motion of the individual particles and identify local shear events that give rise to the macroscopic deformation. Recent results indicate a correlation between the location of shear events and regions of low local particle density or high free volume. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D36.00011: Nonequilibrium state diagram of weakly attractive colloids Jacinta Conrad, Hans Wyss, Suliana Manley, Kunimasa Miyazaki, Laura Kaufman, David Reichman, Veronique Trappe, Dave Weitz We study a colloidal suspension with weak long-range attractions at several volume fractions ranging from $\phi=0.15$ to $\phi=0.35$. The magnitude of the attraction is controlled by varying the concentration of a non-adsorbing polymer. At the lowest interaction energies, the dynamics measured by light scattering reflect single-particle Brownian motion, and the viscous modulus measured by rheology is dominant at experimental frequencies. At intermediate interaction energies, the dynamics are arrested, yet the viscous modulus still dominates. At high interaction energies, the dynamics are still arrested and the elastic modulus becomes dominant. Gelation in the regime of long-range attractions may result from spinodal decomposition into colloid-rich and colloid-poor regions; in this picture, solidification occurs when the volume fraction in the colloid-rich region crosses an attractive glass transition. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D36.00012: Direct Measurements of Forces inside a Three-Dimensional Emulsion Jing Zhou, A. D. Dinsmore Disordered solids (\emph{e.g.}, glass, gels and sand piles) differ tremendously in their microscopic detail, yet may exhibit similar response to applied forces. To test this idea in three dimensions, we measure positions, orientations and magnitudes of inter-particle forces inside dense emulsion piles under gravity and other external stresses. Confocal microscopy of monodisperse and polydisperse emulsions provides quantitative information in the interior of samples. CdSe-TOPO nano-particles and CMPV bio-particles are used to stabilize and label the interfaces of droplets. A method of image analysis was developed to find these flat contact regions and thus map forces in three dimensions. The contact area between neighboring droplets is quantitatively related to the force $(f)$ between the droplets. We use Princen’s model to calculate forces, but also experimentally investigate the area-force relationship. We will present force autocorrelation functions and structure factors that quantify the spatial distribution of forces. This work is supported by NSF (DMR-0305395). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D36.00013: Capillary interactions between anisotropic colloidal particles Jean-Christophe Loudet, Ahmed M. Alsayed, Jian Zhang, Arjun G. Yodh We report on the behavior of micron-sized prolate ellipsoids trapped at an oil-water interface. The particles experience strong, anisotropic, and long-ranged attractive capillary interactions which greatly exceed the thermal energy $k_{B}T$. Depending on surface chemistry, the particles aggregate into open structures or chains. Using video microscopy, we extract the pair interaction potential between ellipsoids and show it exhibits a power law behavior over the length scales probed. Our observations can be explained using recent calculations, if we describe the interfacial ellipsoids as capillary quadrupoles. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D36.00014: Correlation between the particle mobility and dielectric constant in a giant electrorheological fluid Jason Shulman, Feng Chen, Stephen Tsui, Yuyi Xue, C. W. Chu, W. J. Wen The giant electrorheological (ER) fluid, which consists of a colloid of urea coated Ba$_{0.8}$Rb$_{0.4}$TiO(C$_{2}$O$_{4})_{2}$ nanoparticles suspended in silicone oil, has been shown to possess a large negative dielectric constant ($\varepsilon )$ in the presence of even modest electric fields, although $\varepsilon $ is large and positive ($\varepsilon \sim $10$^{4})$ with a zero dc bias. The underlying mechanism that gives rise to this exotic behavior is still unknown. Our work focuses on elucidating the physics encompassing this phenomenon by investigating the temperature effect,$ i.e$. by changing the fluid viscosity, on the sign- change and frequency dependence of $\varepsilon $. From this, information of the nanoparticle mobility and possible particle/particle interactions is extracted. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D36.00015: Chains of colloidal platelets in a electric, light and flow fields Kun Zhao, Christopher Harrison, Matthew Sullivan, Thomas Mason, William Russel, Paul Chaikin We have used photolithography to produce square, plate-like, PMMA colloidal disks of dimension 4.5x4.5x0.7(1.4) microns. In an AC electric field the disks form ribbon-like chains, which at low field are separate and at higher field aggregate as in other electrorheological fluids. Here we report a study on the micromechanics of single chains. We use laser tweezers to hold the midpoint of a chain that is subjected to a transverse viscous flow. The resulting parabolic deformed shape can be used to quantitatively characterize the interactions between disks. Using a dipole-dipole interaction model, we accurately predict the chain deformation and the result allows for evaluation of the effective dielectric contrast between the particles and solvent. [Preview Abstract] |
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