University of Cambridge > > Extra Theoretical Chemistry Seminars > Diffusion and viscosity in complex liquids at all lengthscales

Diffusion and viscosity in complex liquids at all lengthscales

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Water-based polymer solutions appear in a wide variety of systems and industrial processes and products (e.g. biological cells, food production processes, paints, and personal care products). Their viscosity can be larger by several orders of magnitude than that of water. However, because water is the dominant fraction in these solutions, small objects of sub-nanometer size should experience only the viscosity of water while diffusing in the solutions. On the other hand, large objects, much greater than the polymer size, should experience the large macro-viscosity of the solution. It follows immediately that the coefficient of viscosity depends on the length-scale at which it is probed: viscosity should change from the value for water at the nano-scale to a large macro-viscosity at the macroscale. A number of questions are still open in this context: what is the length-scale for which we observe a crossover from nanoviscosity to macroviscosity? What is the relation between nanoviscosity determined from the diffusion of nanoprobes and macroviscosity measured by standard rheometers? Is the Stokes–Sutherland-Einstein relation valid for nanoprobe diffusion in polymer solutions? Using fluorescence correlation spectroscopy (FCS), photon correlation spectroscopy (PCS), capillary electrophoresis (CE), NMR , SANS and rheology data, we show how the viscosity changes from the value for water at the molecular scale to the large macroviscosity in complex liquids. We determined the viscosity experienced by nanoprobes (of sizes from 0.28 to 190 nm) in PEG (flexible polymer) solutions, aqueous micellar solution of hexaethylene-glycol-monododecyl-ether (C12E6 – nonionic surfactant) and fd virus solutions and determined the scaling laws relating nanoviscosity to the macroviscosity. In polymer solutions the crossover length scale is set by the radius of gyration and in micellar solutions by the length of micelles.

This talk is part of the Extra Theoretical Chemistry Seminars series.

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