Publications

2000
Page, J. H. ; Cowan, M. L. ; Weitz, D. A. Diffusing acoustic wave spectroscopy of fluidized suspensions. Physica B 2000, 279, 130-133.Abstract
The velocity fluctuations and local strain rate in a fluidized suspension of particles are investigated using the new ultrasonic technique of diffusing acoustic wave spectroscopy. DAWS probes the relative motion of the particles at very short length scales down to the inter-particle separation, and allows the spatial correlations of the velocity fluctuations to be probed by varying the transport mean free path of the diffusing ultrasonic waves. Our results demonstrate the power of this ultrasonic technique to probe the dynamics of sedimenting particles at larger length scales and Reynolds numbers than can be achieved by light scattering methods. (C) 2000 Elsevier Science B.V. All rights reserved.
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Times Cited: 11 5th International Conference on the Electrical Transport and Optical Properties of Inhomogeneous Media (ETOPIM5) Jun 21-25, 1999 Hong kong baptist univ, hong kong, hong kong Lee Hysan Fdn; Croucher Fdn; KC Wong Educ Fdn; Schlumberger Doll Res; ARO FE; AFOSR AOARD; ONRASIA
Mason, T. G. ; Gisler, T. ; Kroy, K. ; Frey, E. ; Weitz, D. A. Rheology of F-actin solutions determined from thermally driven tracer motion. Journal of Rheology 2000, 44, 917-928.Abstract
We report measurements of the frequency-dependent complex shear modulus of semidilute F-actin solutions based on optical observations of the thermally excited motion of monodisperse tracer microspheres. Because the tracer spheres cause incident laser light to be strongly scattered, we determine their average motion using diffusing wave spectroscopy. From the measured mean square displacement, we extract the retardation spectrum of the actin solution using st regularized fit based on a discretized model involving a linear superposition of harmonically bound Brownian particles. At an actin concentration of C = 1.2 mg/ml and for microspheres of radius a = 0.8 mu m, we find that the complex modulus exhibits a dominant low frequency plateau modulus and a high frequency rise with the loss modulus dominating above a crossover frequency. Over a limited range of frequencies well above the crossover frequency, the magnitude of the high frequency storage modulus G'(omega) is consistent with the power law scaling omega(3/4). The observed gradual crossover appears to be at odds with previous theoretical predictions, but it corresponds to a simple structure of the retardation spectrum. (C) 2000 The Society of Rheology. [S0148-6055(00)00904-4].
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Times Cited: 50 71st Annual Meeting of the Society-of-Rheology Oct, 1999 Madison, wisconsin Soc Rheol
Lin, K. H. ; Crocker, J. C. ; Prasad, V. ; Schofield, A. ; Weitz, D. A. ; Lubensky, T. C. ; Yodh, A. G. Entropically driven colloidal crystallization on patterned surfaces. Physical Review Letters 2000, 85, 1770-1773.Abstract
We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fee crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.
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Times Cited: 189
Hiddessen, A. L. ; Rodgers, S. D. ; Weitz, D. A. ; Hammer, D. A. Assembly of binary colloidal structures via specific biological adhesion. Langmuir 2000, 16, 9744-9753.Abstract
We present a novel approach to the fabrication of binary colloidal materials where specific biomolecular cross-linking drives the self-assembly of bidisperse colloidal suspensions. In particular, we have employed low-affinity immune system carbohydrate-selectin interactions to mediate the heterotypic assembly of binary colloidal structures. Using small (0.94 mum) and larger (5.5 mum) diameter particles coated with complementary chemistry, we show that a progressive series of structures, such as colloidal micelles (a large particle coated with smaller particles), colloidal clusters, rings, and elongated chains, can be made by decreasing the number fraction, N-A/N-B, Of Small (A) to large (B) particles (200 greater than or equal to N-A/N-B greater than or equal to 2) at low total volume fraction (phir = 10(-4)-10(-3)). The assembly is due to specific molecular interactions, as control experiments in which the molecules are blocked or eliminated do not lead to the assembly of these structures. The size of the structures can be modulated by time or total volume fraction. Currently, our methods employ high molecular surface densities, such that the structures result from kinetically trapped, diffusion-limited assembly. Ultimately, with the ability to control the strength of the interaction (using different chemistries and molecular surface densities) as well as the lengths of the molecular tethering arms, particle number densities, and physical properties of the colloidal components, this colloidal assembly driven by specific interactions should yield new materials with many potential technological applications including optical filters, sensors, and separation media.
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Times Cited: 68
Gittings, M. R. ; Cipelletti, L. ; Trappe, V. ; Weitz, D. A. ; In, M. ; Marques, C. Structure of guar in solutions of H2O and D2O: An ultra-small-angle light-scattering study. Journal of Physical Chemistry B 2000, 104, 4381-4386.Abstract
We examine the structure of guar as a function of concentration in both H2O and D2O using several different scattering techniques. The range of scattering vectors spans 5 decades (143 cm(-1) < q < 10.3 x 10(6) cm(-1)), providing insight into the supramolecular and local, organization of the chains. This allows us to directly characterize the large-scale aggregate structure of the guar, which can be on the order of 100 mu m. The aggregates are most likely loosely interconnected with single chains and other aggregates, and the structure and organization are critical in determining solution viscoelastic properties. The solubility is poorer in D2O, as evidenced by larger aggregates, higher scattering intensities, a slightly higher fractal dimension, and a sublinear concentration dependence of the intensity. Aggregates were found in dilute neutral guar solutions as well as in cationic guar solutions (in H2O), whether screened or unscreened. The presence of aggregates at all concentrations for neutral and charged guar indicates the difficulty in determining a molecular weight of the guar molecule.
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Times Cited: 32
Crocker, J. C. ; Valentine, M. T. ; Weeks, E. R. ; Gisler, T. ; Kaplan, P. D. ; Yodh, A. G. ; Weitz, D. A. Two-point microrheology of inhomogeneous soft materials. Physical Review Letters 2000, 85, 888-891.Abstract
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 thr polymer's mesoscopic inhomogeneity. Measurements in an F-actin solution suggest conventional microrheology measurements may not reflect the true bulk behavior.
2000_physrevlet_crocker.pdf
Times Cited: 305
Cowan, M. L. ; Page, J. H. ; Weitz, D. A. Velocity fluctuations in fluidized suspensions probed by ultrasonic correlation spectroscopy. Physical Review Letters 2000, 85, 453-456.Abstract
Velocity fluctuations in a fluidized suspension of particles are investigated using two new ultrasonic correlation spectroscopies: diffusing acoustic wave spectroscopy and dynamic sound scattering. These techniques probe both the local strain rate and rms velocity of the particles, providing important information about the spatial extent of velocity correlations. Our results demonstrate the power of these techniques to probe particle dynamics of fluidized suspensions, and suggest that the velocity correlations are essentially independent of Reynolds numbers for Re-p < 1.
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Times Cited: 46
Cipelletti, L. ; Manley, S. ; Ball, R. C. ; Weitz, D. A. Universal aging features in the restructuring of fractal colloidal gels. Physical Review Letters 2000, 84, 2275-2278.Abstract
We use multispeckle dynamic light scattering to measure the dynamic structure factor, f(q, tau), of gels formed by aggregation of colloids. Although the gel is an elastic solid, f(q, tau) nearly completely decays on long time scales, with an unusual form, f(q, tau) similar to exp{-(tau/tau(f))(mu)}, with mu approximate to 1.5 and with tau(f) proportional to q(-1) A model for restructuring of the gel with aging correctly accounts for this behavior. Aging leads to a dramatic increase in tau(f) however, all data can be scaled on a single master curve, with tau(f) asymptotically growing linearly with age. This behavior is strikingly similar to that predicted for aging in disordered glassy systems, offering convincing proof of the universality of these concepts.
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Times Cited: 236
1999
Zapotocky, M. ; Ramos, L. ; Poulin, P. ; Lubensky, T. C. ; Weitz, D. A. Particle-stabilized defect gel in cholesteric liquid crystals. Science 1999, 283, 209-212.Abstract
Dispersions of colloidal particles in cholesteric liquid crystals form an unusual solid by stabilizing a network of linear defects under tension in the ideal layered structure of the cholesteric. The large length scales of the cholesteric liquid crystals allowed direct observation of the network structure, and its properties were correlated with rheological measurements of elasticity. This system serves as a model for a class of solids formed when particles are mixed with layered materials such as thermotropic and lyotropic smectic liquid crystals and block copolymers.
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Times Cited: 150
Zhang, Z. Q. ; Jones, I. P. ; Schriemer, H. P. ; Page, J. H. ; Weitz, D. A. ; Sheng, P. Wave transport in random media: The ballistic to diffusive transition. Physical Review E 1999, 60, 4843-4850.Abstract
The character of wave transport through a strongly scattering medium, excited by a pulsed plane-wave source, is investigated as a function of sample thickness over the range from about one to 13 mean free paths. To examine the behavior theoretically, we perform a first-principles calculation of both the frequency correlation function of the transmitted field and the time-domain profile of the transmitted intensity. These quantities are investigated experimentally using an ultrasonic technique, which allows us to separate the ballistic and scattered components of the total transmitted field, and hence to measure the scattered component unambiguously in thin samples. For sample thicknesses greater than about four mean free paths, we find good agreement between our theory, the diffusion approximation, and our experimental: data for both the frequency correlation function and the intensity time profile. In thinner samples, there are systematic differences between theory and experiment. To characterize the transition from ballistic to diffusive behavior in thin samples, we focus on the arrival time of the peak in the scattered component of the transmitted intensity; unexpectedly we find that the scattered peak arrival time exhibits an abrupt crossover between ballistic and diffusive behavior when the ratio of sample thickness to mean free path, L/l, is approximately equal to 3. Excellent agreement is obtained between our theory and experiment for this crossover behavior over the entire range of sample thicknesses investigated. [S1063-651X(99)12009-9].
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Times Cited: 42 B
Schriemer, H. P. ; Cowan, M. L. ; Page, J. H. ; Sheng, P. ; Liu, Z. Y. ; Weitz, D. A. Energy velocity of diffusing waves in strongly scattering media - Reply. Physical Review Letters 1999, 82, 2001-2001.
Times Cited: 4
Ramos, L. ; Lubensky, T. C. ; Dan, N. ; Nelson, P. ; Weitz, D. A. Surfactant-mediated two-dimensional crystallization of colloidal crystals. Science 1999, 286, 2325-2328.Abstract
Colloidal particles can form unexpected two-dimensional ordered colloidal crystals when they interact with surfactants of the opposite charge. Coulomb interactions Lead to self-limited adsorption of the particles on the surface of vesicles formed by the surfactants. The adsorbed particles form ordered but fluid rafts on the vesicle surfaces, and these ultimately form robust two-dimensional crystals. This use of attractive Coulomb interaction between colloidal particles and surfactant structures offers a potential new route to self-assembly of ordered colloidal structures.
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Times Cited: 79
Poulin, P. ; Bibette, J. ; Weitz, D. A. From colloidal aggregation to spinodal decomposition in sticky emulsions. European Physical Journal B 1999, 7 277-281.Abstract
Aggregation mechanisms of emulsions at high initial volume fractions (phi(0) >. 0.01) is studied using light scattering. We use emulsion droplets which can be made unstable towards aggregation by a temperature quench. For deep quenches and 0.1 > phi(0) > 0.01, the aggregation mechanism is identified as diffusion-limited cluster aggregation (DLCA). An ordering of the clusters, which is reflected by a peak in the scattering intensity, is shown to result from the intercluster separation, exhibiting different scaling than that observed at lower volume fractions. This manifests an increasing similarity to spinodal decomposition observed as phi(0) is increased. For bo > 0.1 and shallow quenches, different mechanisms, closer to spinodal decomposition, are observed. These results allow the subtle boundaries between DLCA and spinodal decomposition to be explored.
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Times Cited: 37
Pautot, S. ; Discher, B. M. ; Weitz, D. A. Controlled synthesis of large unilamellar vesicles with a reverse emulsion technique. Biophysical Journal 1999, 76, A435-A435.
Times Cited: 0 2
Page, J. H. ; Jones, I. P. ; Schriemer, H. P. ; Cowan, M. L. ; Sheng, P. ; Weitz, D. A. Diffusive transport of acoustic waves in strongly scattering media. Physica B 1999, 263, 37-39.Abstract
The diffusive transport of multiply scattered ultrasonic waves is investigated experimentally and theoretically in a simple system consisting of glass beads in water. New experimental results are presented using a novel method for measuring the frequency correlation function of the transmitted acoustic field. The wave diffusion coefficient D is found to vary strongly with frequency when the wavelength is comparable to the size of the scatterers, reflecting a substantial slowing down of wave propagation when the scattering is strongest. The results are interpreted using a model based on a spectral function approach that gives good agreement with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.
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Times Cited: 7 9th International Conference on Phonon Scattering in Condensed Matter (PHONONS 98) Jul 26-31, 1998 Lancaster, england Engn & Phys Sci Res Council; European Commiss; Int Union Pure & Appl Phys; US Army European Res Off; Lancaster Univ Dept Phys; Inst Phys; European Phys Soc
Meller, A. ; Gisler, T. ; Weitz, D. A. ; Stavans, J. Viscoelasticity of depletion-induced gels in emulsion-polymer systems. Langmuir 1999, 15, 1918-1922.Abstract
Using rheometry and light scattering, we have studied the viscoelasticity of gels formed by the kinetically arrested phase separation in an emulsion-polymer mixture; to prevent the gels From collapsing under their own weight, we have used an isopycnic solvent. At constant osmotic pressure (set by the polymer concentration) and droplet volume fractions phi well, above the gelation transition, we find the elastic modulus to increase roughly linearly with phi, indicating an entropic elasticity based on the cluster packing.
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Times Cited: 25
Gisler, T. ; Ball, R. ; Weitz, D. A. Strain hardening of fractal colloidal gels. Physical Review Letters 1999, 82, 1064-1067.Abstract
We report on experiments on the rheology of gels formed by diffusion-limited aggregation of neutrally buoyant colloidal particles. These gels form very weak solids, with the elastic modulus, G'(omega), larger than the loss modulus, G "(omega), and with both G'(omega) and G "(omega) exhibiting only a very weak frequency dependence. Upon small but finite strains gamma < 0.45 the elastic modulus increases roughly exponentially with gamma(2). We explain the observed strain hardening with the highly nonlinear elastic response of the rigid backbone of the gel to elongational deformation.
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Times Cited: 67
Gisler, T. ; Weitz, D. A. Scaling of the microrheology of semidilute F-actin solutions. Physical Review Letters 1999, 82, 1606-1609.Abstract
The viscoelasticity of actin networks is probed over an extended range of frequencies using microrheology techniques, where the thermal motion of small beads in the network is measured using diffusing-wave spectroscopy. Despite large sample-to-sample variations, the data exhibit an unexpected scaling behavior and can all be collapsed onto a single master curve, indicative of a surprising universality in the elastic properties. The scaled data provide a precise measure of the average behavior of the actin networks and indicate that at high frequencies omega, the shear modulus, increases as omega(3/4).[S0031-9007(99)08465-3].
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Times Cited: 97
Gisler, T. G. ; Weitz, D. A. Optical microrheology: From gels to cells. In Slow Dynamics in Complex Systems; Tokuyama, M. ; Oppenheim, I., Ed. 1999; Vol. 469, pp. 68-76.Abstract
We describe a method for measuring the rheological properties of complex materials by embedding small probe particles within the material and monitoring the thermal motion of the probe using either light scattering or particle tracking. If the probe particle is large compared to the characteristic length scales of the surrounding material, then its motion will reflect the average behavior of the medium. This can be interpreted in terms of the frequency dependent elastic modulus of the medium, providing a simple, non intrusive method for measuring rheological properties. Examples are shown of the measurement of the actin networks using this method, and an unexpected scaling behavior is observed.
Times Cited: 0 8th Tohwa-University International Symposium on Slow Dynamics in Complex Systems Nov 09-14, 1998 Fukuoka, japan
Giasson, S. ; Weitz, D. A. ; Israelachvili, J. N. Interactions between surfactant-coated surfaces in hydrocarbon liquids containing functionalized polymer dispersant. Colloid and Polymer Science 1999, 277, 403-413.Abstract
The forces and viscosity between calcium benzene sulfonate surfactant-coated mica surfaces in various hydrocarbon liquids containing a polyamine-functionalized hydrocarbon polymer (M-W approximate to 8000) have been measured using the surface forces apparatus technique. The polymer is found to adsorb to the substrate surfaces by displacing the surfactant layer, and to produce forces that are monotonically repulsive. The forces have a maximum range of 50-100 nm (>3R(H)), indicating that tails play a particularly important role in the interaction of this relatively low molecular weight polymer. The forces become steeply repulsive below about 10 nm (similar to 0.6R(H)), at which point a "hardwall" repulsion comes in that can sustain pressures greater than 100 atm. Thin-film viscosity measurements indicate that the far-field positions of the slipping planes Delta(H) depend on the shear rate, showing that significant shear thinning/thickening effects occur within the outermost tail regions of the adsorbed layers during shear. The position of the slipping plane, or hydrodynamic layer thickness Delta(H), varies from 0.6R(H) to 2R(H) away from each surface (mica and surfactant-coated mica surfaces). Beyond the hydrodynamic layer the far-field fluid viscosity is the same as that of the bulk polymer solution. At separations below D = 2 Delta(H) the viscosity increases as each polymer layer is compressed. The static forces exhibited various time- and history-dependent effects, which further indicate that a number of different relaxation/equilibration processes are operating simultaneously in this complex multicomponent system. The results reveal that the interactions of tails of functionally adsorbed polymers play a more important role than previously thought. This is especially true in this study where the adsorbed polymers are of low molecular weight and where the tails may represent the largest fraction of interacting segments.
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Times Cited: 10

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