CiteULike: weeks's Weeks

Experimental and numerical studies of an eastward jet over topography

Yudong Tian — 2008-06-18T01:27:04-00:00

Journal of Fluid Mechanics, Vol. 438, No. -1. (2001), pp. 129-157.

Motivated by the phenomena of blocked and zonal flows in Earth's atmosphere, we conducted laboratory experiments and numerical simulations to study the dynamics of an eastward jet flowing over wavenumber-two topography. The laboratory experiments studied the dynamical behaviour of the flow in a barotropic rotating annulus as a function of the experimental Rossby and Ekman numbers. Two distinct flow patterns, resembling blocked and zonal flows in the atmosphere, were observed to persist for long time intervals. Earlier model studies had suggested that the atmosphere's normally upstream- propagating Rossby waves can resonantly lock to the underlying topography, and that this topographic resonance separates zonal from blocked flows. In the annulus, the zonal flows did indeed have super-resonant mean zonal velocities, while the blocked flows appear subresonant. Low-frequency variability, periodic or irregular, was present in the measured time series of azimuthal velocity in the blocked regime, with dominant periodicities in the range of 6–25 annulus rotations. Oscillations have also been detected in zonal states, with smaller amplitude and similar frequency. In addition, over a large region of parameter space the two flow states exhibited spontaneous, intermittent transitions from the one to the other. We numerically simulated the laboratory flow geometry in a quasi-geostrophic barotropic model over a similar range of parameters. Both flow regimes, blocked and zonal, were reproduced in the simulations, with similar spatial and temporal characteristics, including the low-frequency oscillations associated with the blocked flow. The blocked and zonal flow patterns are present over wide ranges of forcing, topographic height, and bottom friction. For a significant portion of parameter space, both model flows are stable. Depending on the initial state, either the blocked or the zonal flow is obtained and persists indefinitely, showing the existence of multiple equilibria.

Anomalous diffusion resulting from strongly asymmetric random walks

Eric Weeks — 2008-06-15T22:33:09-00:00

Physical Review E, Vol. 57, No. 5. (1 May 1998), 4915.

We present a model of one-dimensional asymmetric random walks. Random walkers alternate between flights (steps of constant velocity) and sticking (pauses). The sticking time probability distribution function (PDF) decays as P(t)∼t-ν. Previous work considered the case of a flight PDF decaying as P(t)∼t-μ [Weeks et al., Physica D 97, 291 (1996)]; leftward and rightward flights occurred with differing probabilities and velocities. In addition to these asymmetries, the present strongly asymmetric model uses distinct flight PDFs for leftward and rightward flights: PL(t)∼t-μ and PR(t)∼t-η, with μ≠η. We calculate the dependence of the variance exponent γ (σ2∼tγ) on the PDF exponents ν, μ, and η. We find that γ is determined by the two smaller of the three PDF exponents, and in some cases by only the smallest. A PDF with decay exponent less than 3 has a divergent second moment, and thus is a Lévy distribution. When the smallest decay exponent is between 3/2 and 3, the motion is superdiffusive (1<γ<2). When the smallest exponent is between 1 and 3/2, the motion can be subdiffusive (γ<1); this is in contrast with the case with μ=η.

Transitions Between Blocked and Zonal Flows in a Rotating Annulus with Topography

Eric Weeks — 2008-06-15T22:28:54-00:00

Science, Vol. 278, No. 5343. (28 November 1997), pp. 1598-1601.

The mid-latitude atmosphere is dominated by westerly, nearly zonal flow. Occasionally, this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer. Experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the Coriolis force. In the presence of two symmetric ridges at the bottom of the annulus, the resulting flows were nearly zonal at high forcing or blocked at low forcing. Intermittent switching between blocked and zonal patterns occurs because of the jet's interaction with the topography. These results shed further light on previous atmospheric observations and numerical simulations.

Electric field line diagrams don't work

Alan Wolf — 2008-06-15T22:23:52-00:00

American Journal of Physics, Vol. 64, No. 6. (1996), pp. 714-724.

Electric fields produced by coplanar point charges have often been represented by field line diagrams that depict two-dimensional slices of the three-dimensional field. Serious problems with these “conventional” field line diagrams (CFLDs) have been overlooked. Two of these problems, “equatorial clumping” and “false monopole moment,” occur because a two-dimensional slice lacks information vital to the accurate representation of an inherently three-dimensional field. Equatorial clumping causes most CFLDs to exhibit unphysical behavior such as irregular spacing between field lines terminating on negative charges. CFLDs can also mistakenly indicate that a neutral charge distribution has a significant monopole moment. Such phenomena make the visual estimation of local field strengths impossible and render CFLDs of little utility for representing three-dimensional fields. While these “projection” problems can be avoided by using two-dimensional field line diagrams to represent two-dimensional (1/r) electric fields, or by using three-dimensional field line diagrams to represent three-dimensional fields, other forms of distortion generally remain. ©1996 American Association of Physics Teachers.

Anomalous diffusion in asymmetric random walks with a quasi-geostrophic flow example

Eric Weeks — 2008-06-14T20:27:43-00:00

Physica D: Nonlinear Phenomena, Vol. 97, No. 1-3. (1 October 1996), pp. 291-310.

We present a model of one-dimensional symmetric and asymmetric random walks. The model is applied to an experiment studying fluid transport in a rapidly rotating annulus. In the model, random walkers alternate between flights (steps of constant velocity) and sticking (pauses between flights). Flight time and sticking time probability distribution functions (PDFs) have power law decays: P(t) ~ t-[mu] and t-[nu] for flights and sticking, respectively. We calculate the dependence of the variance exponent [gamma] ([sigma]2 ~ t[gamma]) on the PDF exponents [mu] and [nu]. For a broad distribution of flight times ([mu] < 3), the motion is superdiffusive (1 < [gamma] < 2), and the PDF has a divergent second moment, i.e., it is a Lévy distribution. For a broad distribution of sticking times ([nu] < 3), either superdiffusion or subdiffusion ([gamma] < 1) can occur, with qualitative differences between symmetric and asymmetric walks. For narrow PDFs ([mu] > 3, [nu] > 3), normal diffusion ([gamma] = 1) is recovered. Predictions of the model are related to experimental observations of transport in a rotating annulus. The Eulerian velocity field is chaotic, yet it is still possible to distinguish between well-defined sticking events (particles trapped in vortices) and flights (particles making long excursions in a jet). The distribution of flight lengths is well described by a power law with a divergent second moment (Lévy distribution). The observed transport is strongly asymmetric and is well described by the proposed model.

Chaotic advection in a two-dimensional flow: Lévy flights and anomalous diffusion

TH Solomon — 2008-06-14T20:25:52-00:00

Physica D: Nonlinear Phenomena, Vol. 76, No. 1-3. (1 September 1994), pp. 70-84.

Long-term particle tracking is used to study chaotic transport experimentally in laminar, chaotic, and turbulent flows in an annular tank that rotates sufficiently rapidly to insure two-dimensionality of the flow. For the laminar and chaotic velocity fields, the flow consists of v flow regimes, tracer particles stick for long times to remnants of invariant surfaces around the vortices, then make long excursions ("flights") in the jet regions. The probability distributions for the flight time durations exhibit power-law rather than exponential decays, indicating that the parrticle trajectories are described mathematically as Lévy flights (i.e. the trajectories have infinite mean square displacement per flight). Sticking time probability distributions are also characterized by power laws, as found in previous numerical studies. The mixing of an ensemble of tracer particles is superdiffusive: the variance of the displacement grows with time as t[lambda] with 1<[lambda]<2. The dependence of the diffusion exponent [lambda] and the scaling of the probability distributions are investigated for periodic and chaotic flow regimes, and the results are found to be consistent with theoretical predictions relating Lévy flights and anomalous diffusion. For a turbulent flow, the Lévy flight description no longer applies, and mixing no longer appears superdiffusive.

Structure and dynamics of biphasic colloidal mixtures

Ali Mohraz — 2008-06-13T21:12:16-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 6. (2008)

We investigate the structure and dynamics of biphasic colloidal mixtures composed of coexisting attractive and repulsive microspheres by confocal microscopy. Attractive gels formed in the presence of repulsive microspheres are more spatially homogeneous and, on average, are both more locally tenuous and have fewer large voids than their unary counterparts. The repulsive microspheres within these mixtures display heterogeneous dynamics, with some species exhibiting freely diffusive Brownian motion while others are trapped within the gel network during aggregation.

Shear-induced particle migration in binary colloidal suspensions

Denis Semwogerere — 2008-04-26T14:00:44-00:00

Physics of Fluids, Vol. 20, No. 4. (2008)

Evolving artificial neural networks to control chaotic systems

Eric Weeks — 2006-09-19T13:50:18-00:00

Physical Review E, Vol. 56, No. 2. (1997), 1531.

We develop a genetic algorithm that produces neural network feedback controllers for chaotic systems. The algorithm was tested on the logistic and HÃ©non maps; for which it stabilizes an unstable fixed point using small perturbations; even in the presence of significant noise. The network training method [D. E. Moriarty and R. Miikkulainen; Mach. Learn. 22 ; 11 (1996)] requires no previous knowledge about the system to be controlled; including the dimensionality of the system and the location of unstable fixed points. This is the first dimension-independent algorithm that produces neural network controllers using time-series data. A software implementation of this algorithm is available via the World Wide Web.

Particle Tracking Using IDL: website

Eric Weeks — 2007-09-24T16:32:57-00:00

Forced motion of a probe particle near the colloidal glass transition

P Habdas — 2007-09-20T04:40:18-00:00

Europhys. Lett., Vol. 67, No. 3. (2004), pp. 477-483.

We use confocal microscopy to study the motion of a magnetic bead in a dense colloidal suspension, near the colloidal glass transition volume fraction phg. For dense liquid-like samples near phg, below a threshold force the magnetic bead exhibits only localized caged motion. Above this force, the bead is pulled with a fluctuating velocity. The relationship between force and velocity becomes increasingly nonlinear as phg is approached. The threshold force and nonlinear drag force vary strongly with the volume fraction, while the velocity fluctuations do not change near the transition.

Foam drainage on the microscale II. Imaging flow through single Plateau borders

SA Koehler — 2007-09-20T04:35:11-00:00

Journal of Colloid and Interface Science, Vol. 276, No. 2. (15 August 2004), pp. 439-449.

The liquid in foam forms an interconnected network, which is composed of Plateau borders, nodes, and films. One of the dominant pathways for foam drainage is flow through Plateau borders, and we use confocal microscopy to obtain experimental results for the flow fields inside individual Plateau borders. For three types of surfactants detailed comparisons are made with a model based upon the influence of surface viscosity at free boundaries between the gas in the bubbles and the liquid in the Plateau borders. The model describes the flows well, and we find good agreement between the surface viscosity predicted by this model and representative values found in the literature. We also give a qualitative description of the flow in the nodes.

Video microscopy of colloidal suspensions and colloidal crystals

Piotr Habdas — 2007-09-20T04:33:58-00:00

Current Opinion in Colloid & Interface Science, Vol. 7, No. 3-4. (August 2002), pp. 196-203.

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.

Subdiffusion and the cage effect studied near the colloidal glass transition

Eric Weeks — 2007-09-20T04:31:56-00:00

Chemical Physics, Vol. 284, No. 1-2. (1 November 2002), pp. 361-367.

Real-Space Imaging of Nucleation and Growth in Colloidal Crystallization

U Gasser — 2007-09-20T04:28:54-00:00

Science, Vol. 292, No. 5515. (13 April 2001), pp. 258-262.

Crystallization of concentrated colloidal suspensions was studied in real space using laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of nucleation and growth of crystalline regions, providing the first experimental measure of properties of the nucleating crystallites. By following their evolution, critical nuclei were identified, nucleation rates were determined, and the average surface tension of the crystal-liquid interface was measured. The structure of the nuclei was the same as the bulk solid phase, random-hexagonal-close-packed (rhcp), and their average shape was rather aspherical, with rough rather than faceted surfaces.

Drainage of single Plateau borders: Direct observation of rigid and mobile interfaces

Stephan Koehler — 2007-09-20T04:26:50-00:00

Physical Review E, Vol. 66, No. 4. (October 2002), pp. 040601-040601.

Foam drainage varies with surfactant. We present direct measurements of the flow velocity profiles across single Plateau borders; which make up the interconnected channel-like network for liquid flow. For protein foams the interface is rigid; whereas small-surfactant foams show significant interfacial mobility. The results agree with a model that takes into account the shearing of the liquid-gas interface transverse to the flow direction. A significant consequence is that bubble size and liquid volume fraction in a foam affect the relative importance of surface rheology on the drainage behavior.

Conformations of Laulimalide in DMSO-<i>d</i><sub>6</sub>

P Thepchatri — 2007-09-20T04:25:35-00:00

J. Am. Chem. Soc., Vol. 127, No. 37. (21 September 2005), pp. 12838-12846.

Abstract: Laulimalide is one of the newest naturally occurring macrolides known to act as a microtubule stabilizing agent with properties similar to Taxol. It also stands as being one of the most flexible with 18 rotatable bonds. This large number of rotatable bonds allows for approximately 318 potential conformers. To examine the conformational energy surface of laulimalide, we have performed an NAMFIS deconvolution analysis for laulimalide in DMSO-d6. The latter has been supplemented with a post-NAMFIS energy analysis at the Becke3LYP/6-31G* level that examines the opposing effects of internal hydrogen bonding and syn-pentane interactions. In this way, we have identified 15 laulimalide conformations that can be classified into 5 different families: Supine, Convex, Cobra, Stretch, and Concave motifs.

A Genetic Toolbox for Creating Reversible Ca$^2+$-Sensitive Materials

S Topp — 2007-09-20T04:24:08-00:00

J. Am. Chem. Soc., Vol. 128, No. 43. (1 November 2006), pp. 13994-13995.

Abstract: A major goal of polymer science is to develop "smart" materials that sense specific chemical signals in complex environments and respond with predictable changes in their mechanical properties. Here, we describe a genetic toolbox of natural and engineered protein modules that can be rationally combined in manifold ways to create reversible self-assembling materials that vary in their composition, architecture, and mechanical properties. Using this toolbox, we produced several materials that reversibly self-assemble in the presence of Ca2+ and characterized these materials using particle-tracking microrheology. The properties of these materials could be predicted from the dilute solution behavior of their component modules, suggesting that this toolbox may be generally useful for creating new stimuli-sensitive materials.

Experimental studies of the flow of concentrated hard sphere suspensions into a constriction

L Isa — 2006-06-07T11:46:33-00:00

Journal of Physics: Conference Series, Vol. 40, No. 1. (2006), pp. 124-132.

Interesting flow properties are observed when a concentrated suspension of colloidal particles flows into a geometrical constriction. We present here a description of two different experimental techniques used to study the pressure driven flow of dense suspensions of micron-sized hard spheres into glass capillaries. The first one involves the analysis of the driving pressure during the flow, the other one is based on fast confocal microscopy. Technical details are given, together with a selection of preliminary results.

Confocal microscopy

Denis Semwogerere — 2007-09-20T04:18:38-00:00

(2005)

A confocal microscope creates sharp images of a specimen that would otherwise appear blurred when viewed with a conventional microscope. This is achieved by excluding most of the light from the specimen that is not from the microscope’s focal plane. The image has less haze and better contrast than that of a conventional microscope and represents a thin cross-section of the specimen. Thus, apart from allowing better observation of fine details it is possible to build three-dimensional (3D) reconstructions of a volume of the specimen by assembling a series of thin slices taken along the vertical axis.

Phase Behavior and 3D Structure of Strongly Attractive Microsphere-Nanoparticle Mixtures

JF Gilchrist — 2007-09-20T04:13:37-00:00

Langmuir, Vol. 21, No. 24. (22 November 2005), pp. 11040-11047.

Abstract: We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.

Interparticle Interactions and Direct Imaging of Colloidal Phases Assembled from Microsphere-Nanoparticle Mixtures

CJ Martinez — 2007-09-20T04:12:36-00:00

Langmuir, Vol. 21, No. 22. (25 October 2005), pp. 9978-9989.

Abstract: We investigate the interparticle interactions, phase behavior, and structure of microsphere-nanoparticle mixtures that possess high size and charge asymmetry.1 We employ a novel Monte Carlo simulation scheme2 to calculate the effective microsphere interactions in suspension, yielding new insight into the origin of the experimentally observed behavior.3 The initial settling velocity, final sediment density, and three-dimensional structure of colloidal phases assembled from these binary mixtures via gravitational settling of silica microspheres in water and index-matched solutions exhibit a strong compositional dependence. Confocal laser scanning microscopy is used to directly image and quantify their structural evolution during assembly. Below a lower critical nanoparticle volume fraction (nano < L,C), the intrinsic van der Waals attraction between microspheres leads to the formation of colloidal gels. These gels exhibit enhanced consolidation as nano approaches L,C. When nano exceeds L,C, an effective repulsion arises between microspheres due to the formation of a dynamic nanoparticle halo around the colloids. From this stable fluid phase, the microspheres settle into a crystalline array. Finally, above an upper critical nanoparticle volume fraction (nano > U,C), colloidal gels form whose structure becomes more open with increasing nanoparticle concentration due to the emergence of an effective microsphere attraction,3 whose magnitude exhibits a superlinear dependence on nano.

Squishy Materials

Piotr Habdas — 2007-09-20T04:08:32-00:00

The Physics Teacher, Vol. 44, No. 5. (2006), pp. 276-279.

Most people do not realize that many substances they use in the kitchen and the bathroom are not simple liquids or solids. Everyone is familiar with three states of matter: solids, liquids, and gases. However, creams, shampoo, toothpaste, and ketchup all have properties of both liquids and solids. This paper describes demonstrations and laboratory exercises1 that show intriguing properties of squishy substances, defined as materials that are not unambiguously solid, liquid, or gas. Unlike some areas of physics, the concepts behind squishy materials are understandable even by beginning students. Squishy physics can be used to show physics questions arising from everyday life and to convey the excitement of current research.

Invariance of Structure in an Aging Colloidal Glass

Gianguido Cianci — 2007-09-20T04:07:03-00:00

AIP Conference Proceedings, Vol. 832, No. 1. (2006), pp. 21-25.

We study concentrated colloidal suspensions, a model system which has a glass transition. The non-equilibrium nature of the glassy state is most clearly highlighted by aging — the dependence of the system's properties on the time elapsed since vitrification. Fast laser scanning confocal microscopy allows us to image a colloidal glass and track the particles in three dimensions. We analyze the static structure in terms of tetrahedral packing. We find that while the aging of the suspension clearly affects its dynamics, none of the geometrical quantities associated with tetrahedra change with age.

Correlations of structure and dynamics in an aging colloidal glass

Gianguido Cianci — 2007-09-20T04:05:51-00:00

Solid State Communications, Vol. 139, No. 11-12. (September 2006), pp. 599-604.

We study concentrated colloidal suspensions, a model system which has a glass transition. Samples in the glassy state show aging, in that the motion of the colloidal particles slows as the sample ages from an initial state. We study the relationship between the static structure and the slowing dynamics, using confocal microscopy to follow the three-dimensional motion of the particles. The structure is quantified by considering tetrahedra formed by quadruplets of neighboring particles. We find that while the sample clearly slows down during aging, the static properties as measured by tetrahedral quantities do not vary. However, a weak correlation between tetrahedron shape and mobility is observed, suggesting that the structure facilitates the motion responsible for the sample aging.

Semagenesis and the parasitic angiosperm Striga asiatica

William Keyes — 2007-08-10T15:40:09-00:00

The Plant Journal, Vol. 51, No. 4. (August 2007), pp. 707-716.

Aging of tetrahedral structure in a Lennard-Jones glass

Gianguido Cianci — 2007-09-20T04:02:45-00:00

Vol. 19 (2007), pp. 51-56.

We study the aging of a glassy Lennard-Jones binary mixture with molecular dynamics simulations. We follow the evolution of the packing as a function of the system's age tw, the time passed since the system is quenched to below its glass transition temperature. We focus on simple properties of all tetrahedra formed by the majority of particles. We find that both the averages and the distributions of the edge length of tetrahedra and of their standard deviation monotonically evolve over time: they age. Specifically the aging process decreases the irregularity of tetrahedra while loosening them up. This is in stark contrast with previous experiments on slightly charged hard-sphere colloidal suspension where tetrahedral geometry was found to be a poor indicator of age. Furthermore, we confirm that tetrahedral packings sample microscopic structure in a non-trivial way.

Structure of dense colloidal liquids in tight spaces

Eric Weeks — 2007-09-20T04:00:14-00:00

Vol. 19 (2007), pp. 45-49.

We use three-dimensional confocal microscopy to study the structure of a dense colloidal liquid confined between two parallel glass plates. The colloidal sample is at a volume fraction of 50\% and is a binary mixture of 2~$μ$m and 3~$μ$m diameter particles to prevent crystallization. The plate separation ranges from 50 small particle diameters to 3 small particle diameters. While particles form layers immediately adjacent to the confining walls, we otherwise see little influence of the confinement on structure.

Observation of anomalous diffusion and Lévy flights in a two-dimensional rotating flow

TH Solomon — 2007-09-19T16:39:29-00:00

Physical Review Letters, Vol. 71, No. 24. (13 December 1993), 3975.

Chaotic transport in a laminar fluid flow in a rotating annulus is studied experimentally by tracking large numbers of tracer particles for long times. Sticking and unsticking of particles to remnants of invariant surfaces (Cantori) around vortices results in superdiffusion: The variance of the displacement grows with time as t δ with γ=1.65±0.15. Sticking and flight time probability distribution functions exhibit power-law decays with exponents 1.6±0.3 and 2.3±0.2; respectively. The exponents are consistent with theoretical predictions relating Lévy flights and anomalous diffusion.

Rheological Microscopy: Local Mechanical Properties from Microrheology

DT Chen — 2007-09-19T16:38:22-00:00

Physical Review Letters, Vol. 90, No. 10. (14 March 2003), 108301.

We demonstrate how tracer microrheology methods can be extended to study submicron scale variations in the viscoelastic response of soft materials; in particular; a semidilute solution of λ-DNA. The polymer concentration is depleted near the surfaces of the tracer particles; within a distance comparable to the polymer correlation length. The rheology of this microscopic layer alters the tracers’ motion and can be precisely quantified using one- and two-point microrheology. Interestingly; we found this mechanically distinct layer to be twice as thick as the layer of depleted concentration; likely due to solvent drainage through the locally perturbed polymer structure.

Anomalous Diffusion Probes Microstructure Dynamics of Entangled F-Actin Networks

IY Wong — 2007-09-19T16:37:04-00:00

Physical Review Letters, Vol. 92, No. 17. (2004)

We study the thermal motion of colloidal tracer particles in entangled actin filament (F-actin) networks, where the particle radius is comparable to the mesh size of the F-actin network. In this regime, the ensemble-averaged mean-squared displacement of the particles is proportional to , where 0<<1 from 0.1<<100 s and depends only on the ratio of the probe radius to mesh size. By directly imaging hundreds of particles over 20 min, we determine this anomalous subdiffusion is due to the dynamics of infrequent and large jumps particles make between distinct pores in the network.

Two-Particle Microrheology of Quasi-2D Viscous Systems

V Prasad — 2007-09-19T16:35:40-00:00

Physical Review Letters, Vol. 97, No. 17. (2006), 176001.

We study the spatially correlated motions of colloidal particles in a quasi-2D system (human serum albumin protein molecules at an air-water interface) for different surface viscosities s. We observe a transition in the behavior of the correlated motion, from 2D interface dominated at high s to bulk fluid dependent at low s. The correlated motions can be scaled onto a master curve which captures the features of this transition. This master curve also characterizes the spatial dependence of the flow field of a viscous interface in response to a force. The scale factors used for the master curve allow for the calculation of the surface viscosity s that can be compared to one-particle measurements.

Three-Dimensional Imaging of Colloidal Glasses under Steady Shear

R Besseling — 2007-09-19T16:31:56-00:00

Physical Review Letters, Vol. 99, No. 2. (2007), pp. 028301-028301.

Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time alpha-1" align="middle"> and diffusion constant D, as functions of the local shear rate , show marked shear thinning with alpha-1" align="middle">D0.8 over more than two decades in . In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.

Development of particle migration in pressure-driven flow of a Brownian suspension

D Semwogerere — 2007-09-19T16:15:32-00:00

J. Fluid Mech., Vol. 581 (2007), pp. 437-451.

Particle migration in pressure-driven flow of a Brownian suspension

M Frank — 2007-09-19T16:15:32-00:00

J. Fluid Mech, Vol. 493 (2003), pp. 363-378.

Confocal microscopy of colloids

V Prasad — 2007-09-19T15:10:32-00:00

Journal of Physics: Condensed Matter, Vol. 19 (2007), pp. 113102-113102.

Colloids have increasingly been used to characterize or mimic many aspects of atomic and molecular systems. With confocal microscopy these colloidal particles can be tracked spatially in three dimensions with great precision over large time scales. This review discusses equilibrium phases such as crystals and liquids, and non-equilibrium phases such as glasses and gels. The phases that form depend strongly on the type of particle interaction that dominates. Hard-sphere-like colloids are the simplest, and interactions such as the attractive depletion force and electrostatic repulsion result in more nontrivial phases which can better model molecular materials. Furthermore, shearing or otherwise externally forcing these colloids while under microscopic observation helps connect the microscopic particle dynamics to the macroscopic flow behavior. Finally, directions of future research in this field are discussed.

Direct visualization of ageing in colloidal glasses

Rachel Courtland — 2005-08-20T23:20:53-00:00

J. Phys.: Condens. Matter, Vol. 15 (2003), pp. S359-S365.

We use confocal microscopy to directly visualize the dynamics of ageing colloidal glasses. We prepare a colloidal suspension at high density, a simple model system that shares many properties with other glasses, and initiate experiments by stirring the sample. We follow the motion of several thousand colloidal particles after the stirring and observe that their motion significantly slows as the sample ages. The ageing is both spatially and temporally heterogeneous. Furthermore, while the characteristic relaxation timescale grows with the age of the sample, nontrivial particle motions continue to occur on all timescales.

Three-Dimensional Direct Imaging of Structural Relaxation Near the Colloidal Glass Transition

Eric Weeks — 2005-06-19T21:01:36-00:00

Science, Vol. 287, No. 5453. (28 January 2000), pp. 627-631.

Three-Dimensional Confocal Microscopy of Colloids

AD Dinsmore — 2007-06-11T21:35:48-00:00

Applied Optics, Vol. 40, No. 24. (2001), pp. 4152-4159.

Confocal microscopy is used in the study of colloidal gels, glasses, and binary fluids. We measure the three-dimensional positions of colloidal particles with a precision of approximately 50 nm (a small fraction of each particle s radius) and with a time resolution sufficient for tracking the thermal motions of several thousand particles at once. This information allows us to characterize the structure and the dynamics of these materials in qualitatively new ways, for example, by quantifying the topology of chains and clusters of particles as well as by measuring the spatial correlations between particles with high mobilities. We describe our experimental technique and describe measurements that complement the results of light scattering.

Properties of Cage Rearrangements Observed near the Colloidal Glass Transition

Eric Weeks — 2007-04-02T22:33:06-00:00

Physical Review Letters, Vol. 89, No. 9. (2002), pp. 095704-095704.

We use confocal microscopy to study particle motion in colloidal systems. Near the glass transition; motion is inhibited; as particles spend time trapped in transient “cages” formed by neighboring particles. We measure the cage sizes and lifetimes; which; respectively; shrink and grow as the glass transition approaches. Cage rearrangements are more prevalent in regions with lower concentrations and higher disorder. Neighboring rearranging particles typically move in parallel directions; although a nontrivial fraction moves in antiparallel directions; usually from particle pairs with initial separations corresponding to local maxima and minima of the pair correlation function g ( r ); respectively.

Contribution of Slow Clusters to the Bulk Elasticity Near the Colloidal Glass Transition

Jacinta Conrad — 2007-08-15T16:26:46-00:00

Physical Review Letters, Vol. 97, No. 26. (2006)

We use confocal microscopy to visualize individual particles near the colloidal glass transition. We identify the most slowly-relaxing particles and show that they form spatially correlated clusters that percolate across the sample. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales. In contrast, in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.

Short- and long-range correlated motion observed in colloidal glasses and liquids

Eric Weeks — 2007-08-29T17:41:41-00:00

Journal of Physics: Condensed Matter, Vol. 19, No. 20. (2007), pp. 205131-205131.

We use a confocal microscope to examine the motion of individual particles in a dense colloidal suspension. Close to the glass transition, particle motion is strongly spatially correlated. The correlations decay exponentially with particle separation, yielding a dynamic length scale of O(2-3s) (in terms of particle diameter s). This length scale grows modestly as the glass transition is approached. Further, the correlated motion exhibits a strong spatial dependence on the pair correlation function g(r). Motion within glassy samples is weakly correlated, but with a larger spatial scale for this correlation.

Two-Point Microrheology of Inhomogeneous Soft Materials

John Crocker — 2006-08-31T12:33:47-00:00

Physical Review Letters, Vol. 85, No. 4. (24 July 2000), pp. 888-891.

We demonstrate a novel method for measuring the microrheology of soft viscoelastic media; based on cross correlating the thermal motion of pairs of embedded tracer particles. The method does not depend on the exact nature of the coupling between the tracers and the medium; and yields accurate rheological data for highly inhomogeneous materials. We demonstrate the accuracy of this method with a guar solution; for which other microscopic methods fail due to the polymer's mesoscopic inhomogeneity. Measurements in an F-actin solution suggest conventional microrheology measurements may not reflect the true bulk behavior.

Colloidal Glass Transition Observed in Confinement

Carolyn Nugent — 2007-07-13T19:38:05-00:00

Physical Review Letters, Vol. 99, No. 2. (2007), pp. 025702-025702.

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.