Publications

1999
Cipelletti, L. ; Weitz, D. A. Ultralow-angle dynamic light scattering with a charge coupled device camera based multispeckle, multitau correlator. Review of Scientific Instruments 1999, 70, 3214-3221.Abstract
We use a charge coupled device (CCD) camera and a multi-tau software correlator to measure dynamic light scattering (DLS) at many angles simultaneously, from 0.07 degrees to 5.1 degrees. Real-time autocorrelation functions are calculated by averaging both over time and over CCD pixels, each corresponding to a different coherence area. In order to cover the wide spectrum of decay times associated with the large range of accessible angles, we adopt the multitau scheme, where the correlator channel spacing is quasilogarithmic rather than linear. A detailed analysis is presented of the effects of dark noise, stray light, and finite pixel area, and methods to correct the data for these effects are developed, making a CCD camera a viable alternative for a DLS detector. We test the apparatus on a dilute suspension of colloidal particles. Very good agreement is found between the particle radius derived from the CCD data, and that obtained with a conventional DLS setup. (C) 1999 American Institute of Physics. [S0034-6748(99)05008-X].
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Times Cited: 100
Aranda-Espinoza, H. ; Chen, Y. ; Dan, N. ; Lubensky, T. C. ; Nelson, P. ; Ramos, L. ; Weitz, D. A. Electrostatic repulsion of positively charged vesicles and negatively charged objects. Science 1999, 285, 394-397.Abstract
A positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area and another zone that repels additional negative objects, Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, this mechanism helps to explain recent experiments on surfactant vesicles.
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Times Cited: 42
Ansari, R. R. ; Hovenac, E. A. ; Sankaran, S. ; Koudelka, J. M. ; Weitz, D. A. ; Cipelletti, L. ; Segre, P. N. ; ElGenk, M. S. Physics of colloids in space experiment. In Space Technology and Applications International Forum - 1999, Pts One and Two; 1999; Vol. 458, pp. 108-113.Abstract

The Physics of Colloids in Space (PCS) experiment was proposed by investigators Weitz and Pusey. It is scheduled to be conducted on the International Space Station (ISS) in the year 2000. The broader objective is to study physics of colloidal particles dispersed in a fluid. This includes nucleation and growth of colloidal crystals and behavior of binary colloidal crystal alloys. The structure and properties of colloidal particles with attractive interactions (depletion interactions) induced by the addition of a non-adsorbing polymer, behavior of large-scale fractal aggregates, and gels will also be studied. A multi-purpose light scattering apparatus will be employed in these studies. This apparatus is being designed and built by the NASA Lewis Research Center and is capable of performing dynamic light scattering (DLS), static light scattering (SLS), and Bragg scattering experiments. The flight experiment hardware will be located on the EXPRESS rack mounted in the ISS US Laboratory Module. It is anticipated that the long-term benefit of this research will be to fabricate novel materials that may have applications in opto-electronic display technology. Materials could be fabricated that could act as light switches and could control the direction or color of the displayed light.

1998
Poulin, P. ; Weitz, D. A. Inverted and multiple nematic emulsions. Physical Review E 1998, 57, 626-637.Abstract
We investigate experimentally the structures that form when small colloidal particles are suspended in a nematic solvent. These structures are anisotropic, and their formation is driven by interactions arising from the orientational elasticity of the nematic solvent. By using inverted and multiple nematic emulsions composed of water droplets dispersed in a thermotropic liquid crystal, we identify the nature of these interactions, and demonstrate that they can be controlled by the anchoring of the liquid crystal molecules at the surfaces of the droplets. When the anchoring is normal, the droplets form linear chains, suggesting a long-range dipole-dipole attraction between the particles. By contrast, the interactions are repulsive at short range, and prevent contact of the droplets, thereby stabilizing them against coalescence. When the anchoring is planar, the droplets generate distortions that have a quadrupolar character. The resultant elastic interactions lead to more compact, but still anisotropic, clusters.
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Times Cited: 303
Krall, A. H. ; Weitz, D. A. Internal dynamics and elasticity of fractal colloidal gels. Physical Review Letters 1998, 80, 778-781.Abstract
The dynamic structure factor of fractal colloidal gels is shown to exhibit a stretched exponential decay to a finite plateau with an exponent of about 0.7. The value of the plateau depends on both initial particle volume fraction phi(0) and scattering wave vector. We show that this behavior results from the contribution of internal elastic modes of many length scales, and present a model which accounts for the data. From the observed plateau we determine that the very small elastic modulus scales as G similar to phi(0)(3.9), in agreement with predictions, and with direct mechanical measurements.
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Times Cited: 167
Kaplan, P. D. ; Weitz, D. A. A quasi-elastic scattering microscope for cellular dynamics. Biophysical Journal 1998, 74, A227-A227.
Times Cited: 0 2
Hsu, P. ; Poulin, P. ; Weitz, D. A. Rotational diffusion of monodisperse liquid crystal droplets. Journal of Colloid and Interface Science 1998, 200, 182-184.Abstract
We have prepared monodisperse suspensions of nematic liquid crystal droplets in water. The droplets are stabilized by polyvinyl alcohol, a polymer that induces planar anchoring of the liquid crystal molecules at the surface of the droplets. The resultant particles exhibit strong optical anisotropy while having a spherical shape. As a consequence, the light scattering from the particles contains a strong depolarized component. We report a simple application of this feature by performing dynamic light scattering experiments to measure the rotational diffusion of colloidal spheres in a dilute suspension. (C) 1998 Academic Press.
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Times Cited: 14
Gisler, T. ; Weitz, D. A. Tracer microrheology in complex fluids. Current Opinion in Colloid & Interface Science 1998, 3 586-592.Abstract
Recent results suggest that the motion of colloidal particles can be interpreted in terms of the viscoelasticity of the surrounding medium. New experimental techniques to extend these probe measurements and new methods for data interpretation have been developed.
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Times Cited: 68
1997
Weitz, D. A. Fluid dynamics - Diffusion in a different direction. Nature 1997, 390, 233-&. [PDF]
Times Cited: 13
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. Physical Review Letters 1997, 79, 3166-3169.Abstract
Measurements of the diffusive transport of multiply scattered ultrasonic waves show that the energy velocity is very similar in magnitude and frequency dependence to the group velocity. Our data are accurately described using a theoretical model that accounts for the renormalization of scattering by the coupling between neighboring scatterers, quantitatively predicting the scattering delay that causes the strong frequency dependence of these velocities seen in our experiments. This gives a unified physical picture of the velocities of energy transport by both diffusive and ballistic waves. [S0031-9007(97)04300-7].
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Times Cited: 40
Poulin, P. ; Stark, H. ; Lubensky, T. C. ; Weitz, D. A. Novel colloidal interactions in anisotropic fluids. Science 1997, 275, 1770-1773.Abstract
Small water droplets dispersed in a nematic liquid crystal exhibit a novel class of colloidal interactions, arising from the orientational elastic energy of the anisotropic host fluid. These interactions include a short-range repulsion and a long-range dipolar attraction, and they lead to the formation of anisotropic chainlike structures by the colloidal particles, The repulsive interaction can lead to novel mechanisms for colloid stabilization.
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Times Cited: 610
Poulin, P. ; Cabuil, V. ; Weitz, D. A. Direct measurement of colloidal forces in an anisotropic solvent. Physical Review Letters 1997, 79, 4862-4865.Abstract
We present a new method to measure attractive interactions between colloidal particles, and determine the nature of the attraction between particles suspended in a nematic liquid crystal, We confine droplets filled with ferrofluid to a thin layer and apply a magnetic field to induce dipole moments that drive the droplets apart. When the field is removed, the attractive interactions pull the droplets back together. The force is determined from the velocity because the motion is viscously damped. We confirm the dipolar character of the interaction between droplets in a nematic solvent.
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Times Cited: 173
Page, J. H. ; Schriemer, H. P. ; Jones, I. P. ; Sheng, P. ; Weitz, D. A. Classical wave propagation in strongly scattering media. Physica A 1997, 241, 64-71.Abstract
The transport of classical waves in strongly scattering media is investigated using ultrasonic techniques, allowing us to measure both the ballistic and scattered components of the wave field. We fmd that the ballistic propagation is dramatically slowed down by scattering resonances, although the group velocity remains well-defined. The propagation of the scattered waves is also strongly affected by resonant scattering, and is shown to be well described by using the diffusion approximation. A model based on the generalized coherent potential approximation gives a quantitative explanation of the experimental data.
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Times Cited: 11 4th International Conference on Electrical Transport and Optical Properties of Inhomogeneous Media (ETOPIM4) Jul 23-30, 1996 St petersburg, russia
Mason, T. G. ; Gang, H. ; Weitz, D. A. Diffusing-wave-spectroscopy measurements of viscoelasticity of complex fluids. Journal of the Optical Society of America a-Optics Image Science and Vision 1997, 14, 139-149.Abstract
We present a new use of dynamic light scattering that permits the determination of the viscoelastic behavior of a complex fluid. By describing the motion of a scattering particle in a viscoelastic medium in terms of a generalized Langevin equation with a memory function, we relate the time evolution of its mean-square displacement to the frequency-dependent storage and loss moduli of the medium. The utility of this technique is illustrated through the application of diffusing-wave spectroscopy to probe the viscoelastic behavior of two complex fluids. The properties of a concentrated suspension of colloidal particles interacting as hard spheres are shown to be strongly influenced by the incipient colloidal glass transition, which leads to an extended range of frequencies over which they behave like an elastic solid. Similar elasticity is observed in a compressed emulsion, resulting in this case from the additional interfacial energy of the deformed droplets. In both cases diffusing-wave spectroscopy is used to measure the frequency dependence of the storage and loss moduli, and these results are compared with those from mechanical measurements. Besides providing a purely optical method for measuring mechanical properties, this technique provides new insight into the origin of the viscoelastic behavior. (C) 1997 Optical Society of America.
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Times Cited: 101
Mason, T. G. ; Lacasse, M. D. ; Grest, G. S. ; Levine, D. ; Bibette, J. ; Weitz, D. A. Osmotic pressure and viscoelastic shear moduli of concentrated emulsions. Physical Review E 1997, 56, 3150-3166.Abstract
We present an experimental study of the frequency omega dependence and volume fraction phi dependence of the complex shear modulus G*(omega,phi) of monodisperse emulsions which have been concentrated by an osmotic pressure Pi. At a given phi, the elastic storage modulus G'(omega)=Re[G*(omega)] exhibits a low-frequency plateau G'(p), dominating the dissipative loss modulus G''(omega)=Im[G*(omega)] which exhibits a minimum. Above a critical packing fraction phi(c), we find that both Pi(phi) and G'(p)(phi) increase quasilinearly, scaling as (phi-phi(c))(mu), where phi(c) approximate to phi(c)(rcp), the volume fraction of a random close packing of spheres, and mu is an exponent close to unity. To explain this result, we develop a model of disordered droplets which interact through an effective repulsive anharmonic potential, based on results obtained for a compressed droplet. A simulation based on this model yields a calculated static shear modulus G and osmotic pressure Pi that are in excellent agreement with the experimental values of G'(p) and Pi.
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Times Cited: 132 B
Krall, A. H. ; Huang, Z. ; Weitz, D. A. Dynamics of density fluctuations in colloidal gels. Physica A 1997, 235, 19-33.Abstract
We use dynamic light scattering to measure the dynamic structure factor of density fluctuations occurring in colloidal suspensions that have attained a quiescent state long after aggregation. We find a stretched-exponential decay to a finite plateau. Our interpretation of the arrested decay is that these systems are gels, i.e., systems possessing a finite elastic modulus G. We develop a theory for the internal elastic modes of a fractal cluster and use it to derive G and the arrested, stretched-exponential behavior of colloidal gel dynamics. Good agreement between experiment and theory is obtained.
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Times Cited: 16 Proceedings of the Workshop on Colloid Physics Nov 30-dec 02, 1995 Univ konstanz, konstanz, germany
1996
Weitz, D. A. Materials - Foams flow by stick and slip. Nature 1996, 381, 475-476. [PDF]
Times Cited: 1
Page, J. H. ; Sheng, P. ; Schriemer, H. P. ; Jones, I. ; Jing, X. D. ; Weitz, D. A. Group velocity in strongly scattering media. Science 1996, 271, 634-637.Abstract
Investigation of the ballistic propagation of acoustic waves through a resonantly scattering, Inhomogeneous medium indicates that although the ballistic signal remains coherent with the incident pulse, it is nevertheless strongly affected by scattering resonances. These resonances cause considerable frequency dispersion and substantially reduce the phase and group velocities. The experimental data are quantitatively described by a theoretical model that correctly accounts for the coupling between the resonant scatterers, leading to an effective renormalization of the scattering within the medium. This approach; resolves a long-standing problem in the definition of the group velocity in strongly scattering materials.
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Times Cited: 59
Mason, T. G. ; Gang, H. ; Weitz, D. A. Rheology of complex fluids measured by dynamic light scattering. Journal of Molecular Structure 1996, 383, 81-90.Abstract
We introduce a method for using dynamic light scattering to measure the frequency-dependent linear viscoelastic moduli of complex fluids. The technique exploits the fluctuation dissipation theorem, which relates the relaxation of thermal excitations of a probe particle to the viscoelastic properties of the surrounding medium. The relaxation of the thermal excitations of probe particles are determined by measuring the time evolution of the mean square displacement using dynamic light scattering. A Langevin equation with a time-dependent damping term is used to relate this mean square displacement to the dynamic shear modulus of the medium. This method probes the linear viscoelastic moduli over a much larger frequency range than traditional mechanical means, and in particular, easily extends their measurement to much higher frequencies.
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Times Cited: 41 Conference on the Horizons in Small Angle Scattering From Mesoscopic Systems Sep 27-30, 1995 Stromboli, italy
Mason, T. G. ; Bibette, J. ; Weitz, D. A. Yielding and flow of monodisperse emulsions. Journal of Colloid and Interface Science 1996, 179, 439-448.Abstract
We have measured the yield transition of monodisperse emulsions as the volume fraction, phi, and droplet radius, alpha, are varied. We study the crossover from the perturbative shear regime, which reflects the linear viscoelastic properties, to the steady shear regime, which reflects nonlinear, plastic flow. For small oscillatory strains of peak amplitude gamma, the peak stress, tau, is linearly proportional to gamma. As the strain is increased, the stress becomes nonlinear in gamma at the yield strain, gamma(y). The phi dependence of gamma(y) is independent of alpha and exhibits a minimum near the critical volume fraction, phi(c) approximate to 0.635, associated with the random close packing of monodisperse spheres. We show that the yield stress, tau(y), increases dramatically as the volume fraction increases above phi(c); tau(y) also scales with the Laplace pressure, sigma/alpha, where sigma is the interfacial tension. For comparison, we also determine the steady shear stress over a wide range of strain rates, gamma. Below phi approximate to 0.70, the flow is homogeneous throughout the sample, while for higher phi, the emulsion fractures resulting in highly inhomogeneous flow along the fracture plane. Above phi approximate to 0.58, the steady shear stress exhibits a low strain rate plateau which corresponds with the yield stress measured with the oscillatory technique. Moreover, tau(y) exhibits a robust power law dependence on gamma with exponents decreasing with phi, varying from 2/3 to 1/2. Below phi approximate to 0.58, associated with the colloidal glass transition, the plateau stress disappears entirely, suggesting that the equilibrium glassy dynamics are important in identifying the onset of the yield behavior. (C) 1996 Academic Press, Inc.
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Times Cited: 204

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