Diffusion of particles or molecules in a fluid is an essential manifestation of thermal energy. It is seen in the familiar brownian motion of dust particles in a fluid or a gas, and it ensures the mixing of different molecules in a fluid. So mixing, at the shortest length scales, results from diffusion rather than convection. This is behind a standard method for measuring molecular diffusion coefficients: a sharp concentration gradient is established between two fluids, and the decay of this gradient as the two fluids mix determines the diffusion coefficient of one fluid in the second. Observers look in a direction perpendicular to the gradient (that is, with the interface edge-on), and the results are interpreted assuming a smooth relaxation of the concentration gradient. But that may not be valid: on page 262 of this issue, a team from Milan report unexpected spatial fluctuations in the concentration of two fluids mixing by diffusion.

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.

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.

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.

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.

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.

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.

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.