Synchronized diffusive-wave spectroscopy: Principle and application to sound propagation in aqueous foams

Abstract : We present an experimental method to measure oscillatory strains in turbidmaterial. Thematerial is illuminatedwith a laser, and the speckle patterns are recorded. The analysis of the deformations of the optical path lengthshows that the speckle patterns are modulated at the strain frequency. By recording those patterns synchronouslywith the strain source, we are able to measure the amplitude and the phase of the strain. This method is testedin the specific case of an aqueous foam where an acoustic wave propagates. The effects of material internaldynamics and heterogeneous deformations are also discussed.