Microrheology: a review of the method and applications Pietro Cicuta* a and Athene M. Donald* b DOI: 10.1039/b706004c A set of local mechanical probes has been developed over the last ten years, allowing a kind of dynamical mechanical testing known as microrheology. This paper provides a short introductory review of these methods of performing rheology, comparing them to conventional rheometry, and highlighting the major advantages. The authors also share their outlook on some of the most promising and fastest developing areas that are being studied though microrheology, in the areas of biophysics and soft matter. 1. Introduction Rheology is an interdisciplinary subject, spread between different communities including chemical engineers, physicists, material scientists and chemists. 1 It is also remarkable as a subject of extreme industrial importance; a very wide range of technologies, from paints to foods, from oil recovery to processing of plas- tics, all rely heavily on understanding the flow of complex fluids. ‘‘Simple’’ (Newtonian) fluids are characterized by a viscosity and have a negligible elasticity. ‘‘Simple’’ (Hookean) solids do not flow, and are characterized by an elastic modulus. These two limiting behaviors clearly cannot describe a vast number of soft materials that are both viscous and elastic over the timescale at which they are probed.{ Traditionally, viscoelastic materials have been studied with mechanical rheometers, in various deformation geometries depending on the extent of strain and the magnitude of moduli to be measured. Microrheo- logy is a term that does not describe one particular technique, but rather a number of approaches that attempt to overcome some serious limitations of traditional bulk rheology, such as the range of frequency and moduli that can be probed, the sample size and hetero- geneity, and cost. The ‘‘micro-’’ in the term refers to the size of the stress/strain probe, which is typically a micron-sized colloidal particle, but also indicates that this type of rheology can be carried out on very small volumes, of the order of a micro-litre. The advantages offered by microrheology approaches, summarized in Table 1 and described in greater detail below, have made these measurements very popular over the past decade, and have opened up new fields of investiga- tion. The unconventional geometry and conditions encountered in microrheology experiments have also raised a number of theoretical challenges that are still not fully resolved. In this short review paper our aims are limited to: a) providing an overview for newcomers to this topic, and b) giving our personal perspective of where this technique is proving most important, and where the most promis- ing future developments lie. There are other recent reviews that can serve as extensive references to the microrheology methods and the spectrum of applica- tions, in particular ref. 2 and 3. Details of the fluid-dynamics aspects of microrheology are presented in ref. 4. These references are recommended to a Cavendish Laboratory and Nanoscience Center, University of Cambridge, JJ Thomson Avenue, Cambridge, UK CB3 0HE. E-mail: pc245@cam.ac.uk b Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, UK CB3 0HE. E-mail: amd3@cam.ac.uk Pietro Cicuta Dr Pietro Cicuta obtained his PhD in 2003 at the Cavendish Laboratory, University of Cambridge. After holding an Oppenheimer Research Fellowship, he was appointed as a Lecturer in Physics in October 2006. His research focuses on liquid interfaces and membranes, and applying new rheological tools to the study of complex fluids and biological materials. Athene M. Donald Professor Athene Donald obtained her PhD in 1977 at the Cavendish Laboratory, University of Cambridge. After 4 years at Cornell University in the US, she spent 2 years in the Materials Science Department in Cambridge before returning to the Cavendish Laboratory, where she became a Professor in 1998. She was elected a Fellow of the Royal Society in 1999. Her research spans many aspects of soft matter, including problems of biological relevance. { Even the ‘‘simple’’ behavior of Newtonian or Hookean systems is always confined to some frequency range. HIGHLIGHT www.rsc.org/softmatter | Soft Matter This journal is ß The Royal Society of Chemistry 2007 Soft Matter, 2007, 3, 1449–1455 | 1449