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High frequency linear rheology of complex fluids measured from their surface thermal fluctuations

Reference : Basile Pottier, Allan Raudsepp, Christian Frétigny, François Lequeux, Jean-François Palierne and Laurence Talini, J. Rheol 57, 441 (2013).

A new contactless method to characterize the linear viscoelastic properties of complex fluids. We have developed an optical technique that measures the surface fluctuations of a free surface. Elastic and loss moduli of complex fluids are inferred from the measured power spectrum density of thermal noise, with the implicit use of Kramers-Kronig relations. The technique, besides being non invasive, provides rheological data in a large frequency range and at vanishing strains. It is therefore especially well suited for the rheological characterization of complex fluids. We have thus measured the viscoelastic modulus of supramolecular polymer solutions in a frequency range extending up to 6 decades. Comparison of our measurements with rheometric data at low and high frequency shows a very good agreement. Using supramolecular polymer solutions of different natures, exhibiting or not surface viscoelasticity, we furthermore show that the technique provides a characterization of the bulk properties of the fluids.