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

P9.16Tue 611:23-14:00From stress induced fluidization processes toHerschel-Bulkley behavior in simple yield stress fluidsThibaut Divoux 11 ENS de Lyon, 46 allée d’Italie, 69001, LYON, FranceStress-induced fluidization of a simple yield stress fluid, namely a carbopol microgel, is addressedthrough extensive rheological measurements coupled to a time and spatially resolved velocimetry.This technique allows us to rule out any bulk fracture-like scenario during the fluidization process.On the contrary, we observe that the transient from solidlike to liquidlike behavior under aconstant shear stress σ successively involves creep deformation, total wall slip, and shear bandingbefore a homogeneous steady state is reached. Interestingly, the total duration of this transientregime scales as the following power law τ f ∝ 1/(σ − σ c ) β , where σ c stands for the yield stressof the gel, and β is an exponent which only depends on the microgel properties and not on thegap width or on the boundary conditions. Together with recent experiments under imposed shearrate [1], this scaling law suggests a route to rationalize the phenomenological Herschel-Bulkley(HB) power-law classically used to describe steady-state rheology of simple yield stress fluids.In particular, the steady state HB exponent is interpreted here as the ratio of two fluidizationexponents extracted separately from the transient fluidization processes observed respectively forcontrolled shear rate and shear stress experiments.[1] Divoux et al. , Phys. Rev. Lett. 104, 208301 (2010).16