Publications

1998
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
Liu, A. J. ; Ramaswamy, S. ; Mason, T. G. ; Gang, H. ; Weitz, D. A. Anomalous viscous loss in emulsions. Physical Review Letters 1996, 76, 3017-3020.Abstract
We propose a model for concentrated emulsions based on the speculation that a macroscopic shear strain does not produce an affine deformation in the randomly close-packed droplet structure. The model yields an anomalous contribution to the complex dynamic shear modulus that varies as the square root of frequency. We test this prediction using a novel light scattering technique to measure the dynamic shear modulus, and directly observe the predicted behavior over six decades of frequency and a wide range of volume fractions.
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Times Cited: 73
Lacasse, M. D. ; Grest, G. S. ; Levine, D. ; Mason, T. G. ; Weitz, D. A. Model for the elasticity of compressed emulsions. Physical Review Letters 1996, 76, 3448-3451.Abstract
We present a new model to describe the unusual elastic properties of compressed emulsions. The response of a single droplet under compression is investigated numerically for different Wigner-Seitz cells. The response is softer than harmonic, and depends on the coordination number of the droplet. Using these results, we propose a new effective interdroplet potential which is used to determine the elastic response of a monodisperse collection of disordered droplets as a function of volume fraction. Our results are in excellent agreement with recent experiments. This suggests that anharmonicity together with disorder are responsible for the quasilinear increase of G and Pi observed at phi(c).
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Times Cited: 108
1995
Weitz, D. A. ; Ladd, A. J. C. Scaling of transient hydrodynamic interactions in concentrated suspensions - Reply. Physical Review Letters 1995, 75, 2253-2253.
Times Cited: 2
Page, J. H. ; Liu, J. ; Abeles, B. ; Herbolzheimer, E. ; Deckman, H. W. ; Weitz, D. A. Adsorption and desorption of a wetting fluid in vycor studied by acoustic and optical techniques. Physical Review E 1995, 52, 2763-2777.Abstract
We study the adsorption and desorption of hexane in porous Vycor, as the ambient vapor pressure is varied, through sorption isotherm, ultrasonic velocity and attenuation, and light scattering measurements. On adsorption, we show that the fluid fills the pore space uniformly until capillary condensation occurs; however, small, randomly distributed, vapor bubbles remain, as detected by a large increase in the attenuation of the ultrasound. On desorption, the mismatch in the index of refraction between the empty pores and the surrounding filled pores leads to intense scattering of light that reveals the presence of long-range correlations in the pore space. These correlations have a fractal dimension of 2.6, which is very near the value predicted for invasion percolation. Finally, we also investigate the time dependence of the changes in the adsorbed fluid mass and use these measurements to identify three distinct regimes with vastly differing mechanisms for mass transport. The results presented here provide information on the differences in pore-space correlations on filling and drainage, and highlight the critical role of the connectivity of the pores to the surface in determining the desorption behavior.
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Times Cited: 34 A
Page, J. H. ; Schriemer, H. P. ; Bailey, A. E. ; Weitz, D. A. Experimental test of the diffusion-approximation for multiply scattered sound. Physical Review E 1995, 52, 3106-3114.Abstract
We have critically tested the application of the diffusion approximation to describe the propagation of ultrasonic waves through a random, strongly scattering medium. The transmission of short ultrasonic pulses has been measured through a concentrated suspension of glass beads immersed in water. The transmitted sound field is found to exhibit temporal fluctuations with a period determined by the width of the incident pulse. Provided that appropriate boundary conditions are used to account for the reflectivity of the interfaces, the time dependence of the ensemble-averaged transmitted intensity is shown to be well described by the diffusion equation. This enables us to determine both the diffusion coefficient for the sound waves as well as the inelastic absorption rate. The consistency of these results is established by varying the experimental geometry; while the transmitted pulse shape changes markedly, the values for the diffusion coefficient and absorption rate obtained through a description using the diffusion approximation remain unchanged. We have also measured the absolute transmitted intensity of the sound as the sample thickness is varied; this provides an accurate measure of the transport mean free path and thus also the energy transport velocity. These results convincingly demonstrate the validity of using the diffusion approximation to describe the propagation of sound waves through strongly scattering media.
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Times Cited: 55 B
Mason, T. G. ; Weitz, D. A. Linear viscoelasticity of colloidal hard-sphere suspensions near the glass-transition. Physical Review Letters 1995, 75, 2770-2773.Abstract
The frequency-dependent viscoelastic shear modulus of concentrated suspensions of colloidal hard spheres is shown to be strongly modified as the volume fraction approaches the glass transition. The elastic or storage component, G', becomes larger than the viscous or loss component, G''. The frequency dependence of G' develops a plateau while that of G'' develops a minimum. We propose a physical model to account for these data, using a description of the glasslike behavior based on mode-coupling theory, and a description of the high-frequency behavior based on hydrodynamic flow calculations.
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Times Cited: 221

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