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.
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
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.
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.
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].
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
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.
A liquid-glass transition was observed experimentally in a new system, an oil-in-water emulsion. Dynamic light scattering was employed to obtain the intermediate scattering function f(q,t) for a range of volume fractions phi and scattering vectors q. The results are compared with predictions of the mode coupling theory. While the usual idealized version of the theory provides accurate fits to the data on the Liquid side of the transition, fits for volume fractions near the transition and in the glass phase were found to require the extended version, presumably due to an additional decay mechanism related to the deformability of the oil droplets. [S1063-651X(99)04901-6].
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].
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.
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.
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.
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.
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.
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.
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].
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.
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.