Microrheology of giant-micelle solutions

– Optical tracer-microrheology has been applied to study the rheological properties of a concentrated surfactant solution. Under the chosen conditions these surfactants selfassemble to form giant polymer-like micelles, resulting in a strongly viscoelastic liquid. A novel approach to the analysis of the local dynamic properties of tracer particles is presented, based on a combination of singleand multi-speckle diffusing wave spectroscopy (DWS). With this technique we could significantly extend the accessible frequency range to the key rheological properties as given by the loss and storage modulus, G′′(ω) and G′(ω). A study of the highfrequency range, not accessible to standard techniques, shows good overall agreement with conventional models. However, we find a distinct temperature dependence of the characteristic modulus G0 incompatible with the simple picture of a single relaxation process. Introduction. – In recent years significant progress has been made in the development of modern optical techniques to study and characterize the rheological properties of complex fluids. These techniques have been first applied in fundamental research on complex systems from colloids and emulsions to biopolymers [1–3]. More recently they have become available to both industrial and applied researchers. For example, it has been shown that optical microrheology can be successfully used to characterize ceramic slurries and green bodies as well as colloidal suspensions and biopolymer gels, such as yogurt [4–7]. The underlying concept is to study small (colloidal) particles embedded in the system under study. In this case the particles can either be artificially introduced, which is then called tracer-microrheology, or can be part of the system itself, e.g., as in the case of colloidal gels. By analyzing the thermal motion of the particles it is possible to obtain quantitative information about the loss and storage moduli, G′′(w) and G′(w), over an extended range of frequencies, not accessible to standard rheometers [1, 2]. One of the most popular techniques to study the motion of the particles is diffusing wave spectroscopy (DWS), an extension of standard photon correlation spectroscopy (PCS) to turbid media. Here the analysis of (multiply) scattered laser light is used to determine the time evolution of the probe particle mean-square displacement [1–3, 8]. DWS allows access to a broad range of time scales which results in a very large frequency range, typically 1 to 10 Hz, as we will show later in the text. Despite the broad use of DWS-based tracer-microrheology (see ref. [2]) there is still a lack of quantitative information about the validity of the fundamental assumptions of the microrheology approach itself, which is based on an ad hoc assumption (∗) E-mail: [email protected]