Microrheology of Complex Fluids: From Colloids to Biomaterials

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Project SummaryThe increased demand for knowledge of small-scale behavior is making microrheology a keystep in the understanding of biological systems, the design and use of advanced materials,and the novel applications of already existing materials. Most microrheological experimentsand analyses to date have focused on linear viscoelastic properties, by correlating therandom thermally-driven displacements of small tracers to the complex modulus through ageneralized Stokes-Einstein relation, a process which is relatively well understood but whichis limited in its scope to equilibrium systems. Many systems of practical interest are drivenout of equilibrium and display nonlinear behaviors, but the microrheology work in thisarea has been scarce (conventional macroscale rheometers are used to measure both linearand nonlinear regimes), and the connection between micro and macroscale measurementsis unclear. The need and interest in microscale measurements, whether due to the scarcityof the material or the size of system, makes microrheology an important technology, butunfortunately one which currently lacks fundamental understanding in many aspects.The proposed research examines the relation between micro and macrorheology through theoretical studies, with a particular and strong emphasis on the `active' (driven) and nonlinear regime. This research is best described in terms of applications to colloidal dispersions, and will focus on such systemsbecause they offer very well-defined and well-characterized materials, allowing for comparisonsto macroscale measurements. However, the impact of this research extends beyondcolloidal systems as the theoretical foundation and general conclusions are extendable tomany complex materials, especially biomaterials. Specific issues such as shear thickeningin microrheology, the effect of the size ratio (tracer size to typical medium length scale) onthe `continuum approximation' and on microscale velocity fluctuations, and the interactionsbetween pairs of moving particles leading to structure formation are addressed. This is the first attempt to analyze the nonlinear behavior of materials within the context of microrheologyand provides a fundamental validation of microrheology as a sound technique, critical forits continued application and future growth.Broader Impact: In a broader context, this research will engage PhD students who willbecome experts in colloidal physics and rheology, and who will go on to positions of leadership in industry and/or academia. To aid in the education of future generations of scientists and engineers, a microrheology section for the undergraduatechemical engineering laboratory at Caltech will be developed. To disseminate the researchas widely as possible, in addition to publication in conventional technical journals, a websitewill be maintained with research results that are accessible to the general public. Since thisresearch provides the theoretical foundation for a new experimental technique that haswidespread application in science and technology, its impact is both very broad and deep.