Sound Propagation in Complex Fluids.

In this thesis the sound propagation in complex fluids is studied. The complex fluid systems we study include AOT micelles, AOT microemulsions, a quaternary microemulsion and PMMA hard sphere colloids. We show that the sound propagation is modified by the properties of the dispersed phase in the system, the rigidity and the size of the droplets, and the interaction between the droplets. Therefore, measurements of the sound velocity as a function of the droplet concentration and the frequency allow us to study the contribution of the structural correlations to the elastic behavior of the system. In the micelle and microemulsion systems, the size of the spherical droplets are small compared with the wavelength of the sound. In the AOT micelles and mircoemulsions, an attractive interaction between the droplets leads to the formation of short-lived connecting network. Due to the rigidity of the dispersed phase, the network can support shear, which contributes to the longitudinal elastic modulus of the system. Therefore, measurements results of the sound velocity as a function of the droplet volume fraction in the MHz-GHz frequency regime indicate a pronounced viscoelastic behavior. We find that the additional increase of the elastic modulus at high frequency due to the rigidity of the networks exhibits a power-law scaling, which suggests a dynamic rigidity percolation in the system. The variation of the strength of the attractive interaction, achieved by changing the oil solvent or by varying the droplet size in the microemulsions, leads to a change in the viscoelastic behavior, since the characteristic time of the interaction between droplets is changed. By contrast, in the quaternary microemulsion, the repulsive interaction between droplets is dominant. We find that increasing the repulsive interaction by adding ionic charges to the system has no effect on the observed elastic behavior. The measurements of the sound velocity as a function of droplet volume fraction in the MHz-GHz frequency regime indicate that the elastic behavior in this microemulsion system may be strongly affected by the interaction between alcohol and water molecules, since it exhibits a behavior that is reminiscent of a binary mixture of water and alcohol. In the PMMA hard sphere colloids, the wavelength of the sound is comparable to the size of the solid spheres. We find two novel propagating acoustic excitations. One has a velocity intermediate between that of the fluid and the solid phases, and is interpreted as an acoustic wave propagating through the composite medium of fluid and solid spheres. The second has a velocity slower than both that of the solid and the fluid phases and is interpreted as a propagating interfacial wave.