8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P9.12Tue 611:23-14:00Complex fluids flows in sub-microchannelsAmandine Cuenca 1 and Hugues Bodiguel 11 LOF (UMR5258 Rhodia, CNRS, Bordeaux1), 178, Avenue du Dr Schweitzer, 33608,Pessac, FranceSub-microscale flows have recently attracted the attention of both experimentalists and theorists.We focus on the flow properties of high molecular weight polymer solutions below the micrometerscale. Velocity measurement is an issue at sub-microscale, mainly due to the lack of sensitivity ofusual detection techniques. We use a non-invasive technique based on fluorescence photobleachingto evaluate velocity from direct visualization. A strip of photobleached molecules is createdand flows in the channel, which gives access to the mean velocity. The strip is also spreadingin time due to the molecular diffusion and the Taylor-Aris hydrodynamic dispersion. The evolutionof the effective diffusion coefficient with the Péclet number quantitatively follows the lawD eff = D mol (1 + Pe 2 /210). Therefore, we measure flow velocities and diffusion coefficients.This technique is applied to measure complex fluids viscosity in sub-microchannels. We studypressure-driven flows of high molecular weight polyacrylamide solutions at various concentrationsand for several channel heights. Experimental data lead to pressure/flow rate relations, whichgive the effective viscosities. For channel height below 1µm, the effective viscosity is significantlylower than the bulk one at similar shear rate, up to an order of magnitude in 400nm height channel.For high molecular weight polymers and concentrated solutions, a stronger reduction is observed.By correlating the data at different thicknesses, we obtain both the slippage velocity and the rheologicalflow curve of the solutions. We compare measurements at sub-microscale to the bulk onesusing PIV (Particle Image Velocimetry) for direct slipping velocity evaluation or a rheometer toobtain the flow curve. For high molecular weight polymers, the effective viscosity reduction is notonly due to slippage, but also to a modification of the rheological properties at sub-microscale.12