We study the acoustic properties of AOT micelles and microemulsions by measuring the velocity of sound as a function of droplet volume fraction and of frequency, using Brillouin scattering and ultrasonic techniques. These weakly attractive interparticle interactions lead to the formation of short–lived networks of droplets, which can support shear and result in a pronounced increase in the speed of sound at high frequencies. These networks do not persist for sufficiently long times to affect the sound velocity at lower frequencies. The increased rigidity due to the networks exhibits a percolation behavior with volume fraction, while the frequency dependence of the interactions depends on hydrocarbon chain length of the solvent. Increasing the water concentration results in an increase in interaction energy, but a decrease in the intrinsic droplet rigidity, which affects the magnitude and frequency dependence of the contribution of the microemulsion networks to the elastic constant of the suspension. A consistent picture of the viscoelastic behavior of these suspensions is obtained.
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