We measure the dispersion of the longitudinal sound waves in a suspension of solid spheres using Brillouin scattering. We fmd two distinct propagating longitudinal modes when the wavelength of the sound becomes comparable to the sphere diameter. The higher-frequency mode has a velocity intermediate between those of the pure solid and pure liquid phases, and its velocity increases with increasing solid volume fraction. The dispersion curve of this mode has distinct gaps, and the group velocity goes to zero near these gaps. We interpret this mode as a compressional ''citation which propagates through both the liquid and the solid, as expected for a composite medium. The gaps in the dispersion curve result from the very large scattering of the excitation by the spheres, and occur at frequencies where the scattering from a single, isolated sphere is predicted to be a maximim due to a resonance in the sphere. By contrast, the lower-frequency mode has a velocity that is less than those in either the pure solid or the pure fluid. We interpret this mode as a surface acoustic excitation, which propagates between adjacent spheres by means of the exponentially decaying portion of the excitation in the fluid at the surface of the spheres. A summary of a theoretical treatment is also presented.
We study the propagation of sound in complex colloidal systems. By combining Brillouin scattering with ultrasonic techniques, we measure the dispersion in the acoustic propagation over three decades in frequency. Acoustic propagation is sensitive to the bulk compressibility of the medium, and probes new structural and dynamic properties of the colloidal system. We study two colloidal systems. The first is a system of inverted micelles or microemulsions, where the droplet size is significantly smaller than the wavelength of the sound. By measuring the dispersion of the sound velocity as a function of droplet volume fraction, we identify an increased rigidity of the system at high frequencies. The increase in the modulus scales as (phi - phi(c))tau, where phi is the volume fraction of droplets and phi(c) is a critical volume fraction. This is consistent with rigidity percolation. The second system we study consists of a suspension of hard sphere colloids whose diameter is comparable to the wavelength of sound. We measure the dispersion curve for the phonons in this system at different volume fractions of spheres. A new acoustic excitation is found when the wavelength of the sound is comparable to the sphere diameter. This acoustic excitation possesses unusual properties and is attributed to a surface excitation that can propagate coherently between adjacent spheres.
Times Cited: 9 Symp on complex liquid systems Jul 05-10, 1992 Polistena, italy Banca populare polistena; reg calabria; prov reggio calabria; univ messina; hamamatsu photon italia; microcontrole; laserpoint; lasersource; cnr, grp natl struct mat; ciso calabria
We discuss the entension of dynamic light scattering to very strongly scattering media, where the propagation of light is described by the diffusion approximation, allowing the distribution of the light paths to be determined. The temporal evolution of the length of each of these paths, due to the dynamics of the scattering medium, is calculated, and an expression for the temporal autocorrelation function of the intensity fluctuations of the scattered light is obtained. This relates the measured decay of the autocorrelation function to the dynamics of the medium. This technique is called diffusing wave spectroscopy (DWS). To extend its utility, we consider the consequences of interactions between the scattering particles on the light scattering. To illustrate its applications, we consider several examples of new physics that can be investigated using DWS. We study the transient nature of hydrodynamic interactions between a particle and the surrounding fluid. We are able to probe the decay of the velocity correlation function of the particles, and we demonstrate its algebraic decay, with a t(-3/2) rime dependence. We also show that the time-dependent self diffusion coefficient exhibits an unexpected scaling behavior, whereby all the data, from samples of different volume fractions, can be scaled onto a single curve. Finally, we discuss the applications of DWS to the study of the dynamics of foams, and show how it can be used to probe the rearrangement of the bubbles within the foam as they coarsen.
We use ultrasonic attenuation and light scattering to study spatial correlations in the pores of Vycor on filling and draining with hexane. On filling, the hexane initially adsorbs uniformly, but when capillary condensation occurs, vapor microbubbles are formed and persist until the sample is completely full. However, no long-range correlations of the bubbles are observed. By contrast, on drainage, the empty pores exhibit long-range correlations with a fractal dimension of 2.6. This results from the pore connectivity, suggesting that this behavior can be modeled by invasion percolation.
The structure of colloidal crystals formed from suspensions of hard-sphere colloids is studied. The samples are contained in a thin cell. By rocking the samples, we are able to form shear-aligned colloidal crystals with extended, long-range order. The aligned crystals persist after the shear ceases, enabling us to use laser-light crystallography to determine their structure. We observe an unusual ordering, wherein the structure is a nearly perfect single twin of a face-centered cubic crystal, with crystals of different twins formed on each side of the cell. We exploit the coherence of the laser source to observe a speckle-like fluctuation of the intensity in the Bragg peaks. However, this fluctuation occurs only in one scattering direction, and therefore reflects a remnant disorder in the stacking of the hcp planes. We introduce a simple model which accounts for both the nature of the disorder in the stacking as well as the specklelike fluctuations. Our observation of this speckle also confirms recent predictions.
We report the results of static and dynamic evanescent wave light scattering studies of a monolayer of a diblock copolymer, polystyrene-b-polymethylmethacrylate (PS-b-PMMA) with weight averaged molecular weights (M(w)) of 880 000:290 000 supported at the air/water interface. Our studies probe the interfacial structural and dynamic proper-ties of the monolayer on a length scale which is a fraction of the wavelength of light. The static light scattering studies were carried out as a function of polymer surface coverage and temperature; we also report some preliminary data for the dependence of the static structure function on the relative molecular weights of the PS and PMMA blocks. The complementary dynamic light scattering studies were carried out only as a function of surface coverage. Our data suggest that, upon spreading in the air/water interface, PS-b-PMMA (880:290 K) copolymers form thin disklike aggregates containing about 240 molecules. These data are consistent with a model in which each such aggregate is a ''furry disk'' with a dense core consisting of a layer of collapsed PS blocks atop a thin layer of extended PMMA blocks on the water surface and a brushlike boundary of extended PMMA blocks. The data show that the furry disks diffuse freely when the surface coverage is small, but when the surface coverage is large, they are immobile. Our data also suggest that the furry disks can aggregate to form even larger ''islands'' of disks with an extension greater than 20 mum. The static structure function of the assembly of furry disks is well described, over a wide range of surface coverage, by the structure factor of a two-dimensional hard disk fluid modulated by a two-dimensional hard disk form factor.
We study oil in water emulsions when the interaction between the droplets becomes strongly adhesive, causing them to stick together. However, the droplets still retain their integrity and do not coalesce. By using emulsions with droplets that are monodisperse in size, we are able to clearly observe their structure when the emulsions become adhesive. We show that the structure of strongly adhesive emulsions reflects a complex interplay among the strength of the adhesion, the droplet volume fraction, phi, and the time evolution of the adhesion. Initially, strong adhesion of the droplets leads to the formation of an emulsion gel. Moreover, the gel possesses a well-defined characteristic length scale, d(c), as evidenced by an intense ring of small angle light scattering. The characteristic length scale decreases as the droplet volume fraction increases. At low phi, the structure of the emulsion gel is fractal on length scales shorter than d(c), and the measured fractal dimension suggests that the gelation mechanism is controlled by diffusion-limited cluster aggregation. However, at higher phi, the short range structure is more compact, rather than fractal, and a different mechanism must be responsible for the gelation. If the strength of the adhesion is increased still further, the droplets become more deformed, resulting in massive restructuring of the emulsion gel. The structure fractures into independent, more compact flocs, eliminating the overall rigidity of the emulsion gel. These results help rationalize some of the diverse structures that are observed upon flocculation of the more usually studied polydisperse emulsions.