Diffusing wave spectroscopy for investigating emulsions: I. Instrumental aspects

Diffusing Wave Spectroscopy (DWS) is a technique used to characterize microscopic structure and rheological properties of turbid samples. This is achieved by analyzing the time evolution of coherence speckles of light that has been multiply scattered within the sample. One attractive feature of DWS is that it is non-invasive, and robust. For this reason, DWS has been selected as a diagnostic tool for an experiment module uploaded onboard the International Space Station (ISS) and devoted to the study of soft matter dynamics -related phenomena, among which, the basic mechanisms responsible for emulsions and foams aging. Moreover, the sensitivity of DWS to tiny motions and to fast time scales allows to extend measurements beyond the limits of usual rheological techniques, of micro-imaging and related techniques. However, the accuracy obtainable by DWS measurements is often limited. Here we revise different experimental approaches to the characterization of emulsions by DWS. We provide quantification of the accuracy of the correlation functions and we discuss how this affects the determination of the mechanical moduli, providing an objective criterion to establish the time window over which DWS results are reliable. These concepts are applied to discuss a laboratory prototype and to address the design of the DWS diagnostics suitable for the emulsion studies to be performed onboard the ISS.