Microrheology of solutions embedded with thread-like supramolecular structures David Lopez-Diaz and Rolando Castillo * Received 17th February 2011, Accepted 28th March 2011 DOI: 10.1039/c1sm05274h A family of methods uses colloidal particles as a mechanical probe for deforming the medium in conjunction with a procedure to trace the movement of the particles to get rheological information in a very wide frequency range. All of them are under the heading of microrheology. In the last decade, they have been developed up to the point of being a useful tool for understanding the structure and the dynamics of solutions with embedded thread-like supramolecular structures. This is the case of wormlike micellar solutions, which is the main interest of the paper. Here the impact of microrheology has been essential, providing structural information not easily obtained by other methods. Microrheology has also made an important contribution to the understanding of other threadlike system, as in the case of F-actin or fd virus solutions; they will also be discussed. 1. Introduction Many of the diverse material properties observed in fluid soft materials are related to the complex supramolecular structures embedded in them, which introduce complex dynamics, usually described with multiple characteristic lengths and time scales that can be assessed by different methods. One of the most popular and frequently used methods is rheology, which is employed to examine a broad variety of materials ranging from paints, polymeric formulations, food, biomaterials, surfactant solutions and personal care products, just to mention a few. The rheo- logical response of soft matter materials can be linear or non- linear depending on the applied stress. Nonlinearity is usually a sign of structural rearrangement in the system by the applied stress or deformation. For systems close to thermodynamic equilibrium, there is always a linear response regime for small enough applied strain or stress. Here, in soft materials, one of the most important properties is the shear modulus, G, which connects the deformation and flow of materials in response to applied stresses, s ¼ ð t N dt 0 Gðt  t 0 Þ _ g. Here, s is the shear stress and _ g is the shear rate. In contrast with other materials, like Instituto de F ısica, Universidad Nacional Aut  onoma de M exico, P. O. Box 20-364, Mexico, D. F. 01000. E-mail: rolandoc@fisica.unam.mx David Lopez-Diaz Dr David Lopez-Diaz obtained his Doctor in Science (Chem- istry) degree at the Universidad de Salamanca, in Spain in 2007. He is finishing a postdoctoral position at the Institute of Physics at the National Univer- sity of Mexico. His research focuses on the study of molec- ular structure in complex fluids, and particularly in wormlike micellar solutions. Rolando Castillo Professor Rolando Castillo obtained his Doctor in Science (Physics) degree at the National University of Mexico in 1986, where he obtained a permanent position at the Institute of Physics. His research has been focused on simple liquids and complex fluids, and in the past years, he has been focused on rheological properties of wormlike micellar solutions, pattern formation in monolayers, and self-assembled systems. He is the organizer of the Mexican Network in Soft Matter of the Mexican National Council of Science and Technology. 5926 | Soft Matter , 2011, 7, 5926–5937 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2011, 7, 5926 www.rsc.org/softmatter REVIEW Downloaded by University of California - Riverside on 08 August 2011 Published on 09 May 2011 on http://pubs.rsc.org | doi:10.1039/C1SM05274H View Online