Gasser, U. ; Weeks, E. R. ; Schofield, A. ; Pusey, P. N. ; Weitz, D. A. Real-space imaging of nucleation and growth in colloidal crystallization. Science 2001, 292, 258-262.Abstract
Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.
Times Cited: 428
Dinsmore, A. D. ; Weeks, E. R. ; Prasad, V. ; Levitt, A. C. ; Weitz, D. A. Three-dimensional confocal microscopy of colloids. Applied Optics 2001, 40, 4152-4159.Abstract
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. (C) 2001 Optical Society of America.
Times Cited: 133
Weeks, E. R. ; Gasser, U. ; Dinsmore, A. D. ; Weitz, D. A. ; Osa, O. S. A. Light scattering using colloidal microscopy. In Photon Correlation and Scattering, Technical Digest; 2000; Vol. 47, pp. 15-15.Abstract
Colloidal Microscopy is a new microscopy, invented by whoever shortened the title. Paul Chaikin, the world's greatest physicist, will explain it in his talk. This talk will cover any details that Paul does not cover. (C) 2000 Optical Society of America.
Times Cited: 0 Conference on Photon Correlation and Scattering Aug 21-23, 2000 Whistler, canada Opt Soc Amer; NASA
Weeks, E. R. ; Crocker, J. C. ; Levitt, A. C. ; Schofield, A. ; Weitz, D. A. Three-dimensional direct imaging of structural relaxation near the colloidal glass transition. Science 2000, 287, 627-631.Abstract
Confocal microscopy was used to directly observe three-dimensional dynamics of particles in colloidal supercooled fluids and colloidal glasses. The fastest particles moved cooperatively; connected clusters of these mobile particles could be identified: and the cluster size distribution, structure, and dynamics were investigated. The characteristic cluster size grew markedly in the supercooled fluid as the glass transition was approached, in agreement with computer simulations; at the glass transition, however, there was a sudden drop in their size. The clusters of fast-moving particles were largest near the alpha-relaxation time scale for supercooled colloidal fluids, but were also present, albeit with a markedly different nature, at shorter beta-relaxation time scales, in both supercooled fluid and glass colloidal phases.
Times Cited: 881
Valentine, M. T. ; Popp, A. K. ; Weitz, D. A. ; Kaplan, P. D. ; Osa, O. S. A. Simultaneous imaging and light scattering from biological tissues. In Biomedical Topical Meetings, Technical Digest; 2000; Vol. 38, pp. 314-316.Abstract
Using a microscope capable of simultaneous imaging and static light scattering, we measure the optical properties of different tissues. We correlate the scattering patterns to local structures and heterogeneities. (C) 1999 Optical Society of America.
Times Cited: 0 Biomedical Topical Meeting Apr 02-05, 2000 Miami beach, fl
Trappe, V. ; Weitz, D. A. Scaling of the viscoelasticity of weakly attractive particles. Physical Review Letters 2000, 85, 449-452.Abstract
The rheological data of weakly attractive colloidal particles are shown to exhibit a surprising scaling behavior as the particle volume fraction, phi, or the strength of the attractive interparticle interaction, U, are varied. There is a critical onset of a solid network as either phi or U increase above critical values. For all solidlike samples, both the frequency-dependent linear viscoelastic moduli, and the strain-rate dependent stress can be scaled onto universal master curves. A model of a solid network interspersed in a background fluid qualitatively accounts for this behavior.
Times Cited: 165
Umbanhowar, P. B. ; Prasad, V. ; Weitz, D. A. Monodisperse emulsion generation via drop break off in a coflowing stream. Langmuir 2000, 16, 347-351.Abstract
We describe an experimental technique for the production of highly monodisperse emulsions (with minimum achievable polydispersities <3%). The phase to be dispersed is introduced into a coflowing, surfactant-laden continuous phase via a tapered capillary. Drops detach from the capillary when the streamwise forces exceed the force due to interfacial tension. Drop size is a function of the capillary tip diameter, the velocity of the continuous phase, the extrusion rate, and the viscosities and interfacial tension of the two phases. Emulsions composed of a variety of fluids and with drop sizes ranging from 2 to 200 mu m have been produced using this technique.
Times Cited: 224
Popp, A. K. ; Valentine, M. T. ; Kaplan, P. D. ; Weitz, D. A. Light scattering microscope as a tool to investigate scattering heterogeneity in tissue. In Optical Biopsy Iii; Alfano, R. R., Ed.; 2000; Vol. 1, pp. 22-32.Abstract
Rayleigh light scattering has not yet been used for quantitative investigations of heterogeneous systems. Preconditions for such an experiment are a well defined scattering geometry and independent information about the local state of the sample. We have designed a new instrument that meets these criteria: a light- scattering microscope with simultaneous imaging. We demonstrate the ability to characterize local differences within one tissue type as well as global differences between tissue types. Real space images of the sample are taken by normal video microscopy techniques. The light scattering pattern is analyzed by the evaluation of wave- vector dependence (form factor) and scattering direction of the scattered intensity. Statistical analysis of scattering patterns show what is important for the characterization and classification of tissues and heterogeneous structures. Real space images provide context for scattering analysis. The light scattering microscope is a powerful tool for characterization of local structural order in inhomogeneous structures like tissues.
Times Cited: 0 Conference on Optical Biopsy III Jan 23-24, 2000 San jose, ca SPIE; Int Biomed Opt Soc
Pautot, S. ; Weitz, D. A. Light scattering experiments to probe membrane fluctuations of monodisperse unilamellar liposomes. Biophysical Journal 2000, 78, 178A-178A.
Times Cited: 1 2
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.
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].
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.
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.
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.
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.
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.
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.
Times Cited: 236
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.
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].
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