Publications by Year: 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.
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.
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.
Times Cited: 221
Mason, T. G. ; Weitz, D. A. Optical measurements of frequency-dependent linear viscoelastic moduli of complex fluids. Physical Review Letters 1995, 74, 1250-1253. [PDF]
Times Cited: 559
Mason, T. G. ; Bibette, J. ; Weitz, D. A. Elasticity of compressed emulsions. Physical Review Letters 1995, 75, 2051-2054.Abstract
The elastic shear modulus of monodisperse emulsions is shown to exhibit a universal dependence on droplet volume fraction phi when scaled by the Laplace pressure of the droplets, increasing as phi(phi - phi(c)). where phi(c) approximate to 0.635, the value of random close packing of solid spheres. Surprisingly the osmotic pressure required to compress the emulsions to increase phi is nearly the same as the shear modulus over a large range of volume fraction, while the bulk osmotic modulus differs significantly. Models based on the structural disorder of the emulsions are discussed to account for these data.
Times Cited: 222
Lin, B. H. ; Rice, S. A. ; Weitz, D. A. Experimental-evidence for the divergence of a transport coefficient in a quasi-2-dimensional fluid. Physical Review E 1995, 51, 423-429. [PDF]
Times Cited: 8
Ladd, A. J. C. ; Gang, H. ; Zhu, J. X. ; Weitz, D. A. Time-dependent collective diffusion of colloidal particles. Physical Review Letters 1995, 74, 318-321. [PDF]
Times Cited: 48
Ladd, A. J. C. ; Gang, H. ; Zhu, J. X. ; Weitz, D. A. Temporal and spatial dependence of hydrodynamic correlations - Simulation and experiment. Physical Review E 1995, 52, 6550-6572.Abstract
Time-dependent hydrodynamic interactions in a colloidal suspension of hard spheres are studied, both experimentally and through computer simulation. The focus is on the behavior at small wave vectors, which directly probes the temporal evolution of hydrodynamic interactions between nearby particles. The computer simulations show that the time-dependent diffusion coefficient has the same functional form for all wave vectors, with a single characteristic scaling time for each length scale and for each volume fraction. Wave-vector-averaged effective diffusion coefficients, measured experimentally using diffusing wave spectroscopy, also scale to the same functional form. In this case, the scaling time is dependent on both volume fraction and particle size; it decreases sharply with decreasing particle radius, reflecting the greater contribution from smaller wave vectors that is contained in the scattering from the smaller particles. For a direct comparison of simulation and experiment, we simulate the experimentally observed correlation functions, by averaging the wavevector-dependent computer-simulation data with the weighting appropriate to the experimental technique. Although the overall scaling is similar, there are quantitative differences in the simulated and measured relaxation times. We suggest these differences are due to the compressibility of the suspension, and that the resultant pressure waves make an unexpectedly significant contribution to the hydrodynamic interactions.
Times Cited: 39 B
Huang, Z. ; Krall, A. H. ; Weitz, D. A. The density of aggregated polystyrene spheres. Journal of Colloid and Interface Science 1995, 170, 602-603.Abstract
We show that density measurements can provide an alternative method for determining the number and species of ions adsorbed onto colloidal particles. We cause charge-stabilized colloidal dispersions of polystyrene spheres to aggregate by adding various salts. The solvents are mixtures of H2O and D2O in which the colloids are neutrally buoyant after aggregation, which we verify by centrifugation of the samples, By this method we are able to determine the density of the aggregated colloids. We find that the density depends on the species of salt added to initiate the aggregation and can be calculated on the assumption that a cation of the added salt binds with each ionizable group on the surface of the colloidal particles. (C) 1995 Academic Press, Inc.
Times Cited: 0
Gang, H. ; Krall, A. H. ; Weitz, D. A. Thermal fluctuations of the shapes of droplets in dense and compressed emulsions. Physical Review E 1995, 52, 6289-6302.Abstract
We generalize the theory of diffusing-wave spectroscopy (DWS) to include the effects of fluctuations of the amplitudes of the scattered fields. Thus DWS can be used to probe the internal dynamics of flexible particles. We study the thermally induced shape fluctuations of monodisperse emulsion droplets as a function of the droplet volume fraction phi. We find that a droplet's mean-squared deviation from spherical shape increases with phi, while the characteristic rate of relaxation of the shape deformations decreases with phi. Our generalization of the theory of DWS allows us to measure the autocorrelation function of the fluctuating amplitude of the field scattered from a droplet. We use fluid dynamics and scattering theory to calculate this autocorrelation function theoretically for an isolated droplet. The significant contribution of many independent modes of deformation results in a distinctly nonexponential relaxation. The measured behavior agrees with the theory as phi approaches zero. At higher values of phi throughout the range of colloidal liquids we find a surprising scaling behavior, which implies that particle interactions bring about the enhancement and slowing down of shape fluctuations without altering the spectrum of excited deformation modes. We relate the form of the scaling function to the particle radial distribution function. In ''compressed'' emulsions with phi as high as 0.8, shape fluctuations may be the only dynamical behavior that can occur. We suggest that in these systems the amplitude of the shape fluctuations is related to the emulsion's elastic modulus.
Times Cited: 38 B