Origin of apparent viscosity in yield stress fluids below yieldingP. C. F. Møller1, A. Fall2 and D. Bonn1, 2
1 École Normale Supérieure, Laboratoire de Physique Statistique - Paris, F-75231 France, EU
2 van der Waals-Zeeman Institute, University of Amsterdam - 1018 XE Amsterdam, The Netherlands, EU
received 8 April 2009; accepted in final form 20 July 2009; published August 2009
published online 24 August 2009
For more than 20 years it has been debated if yield stress fluids are solid below the yield stress or actually flow; whether true yield stress fluids exist or not. Advocates of the true yield stress picture have demonstrated that the effective viscosity increases very rapidly as the stress is decreased towards the yield stress. Opponents have shown that this viscosity increase levels off, and that the material behaves as a Newtonian fluid of very high viscosity below the yield stress. In this paper, we demonstrate experimentally (on four different materials, using three different rheometers, five different geometries, and two different measurement methods) that the low-stress Newtonian viscosity is an artifact that arises in non–steady-state experiments. For measurements as long as 104 seconds we find that the value of the “Newtonian viscosity” increases indefinitely. This proves that the yield stress exists and marks a sharp transition between flowing states and states where the steady-state viscosity is infinite —a solid!
83.60.-a - Rheology: Material behavior.
83.60.La - Viscoplasticity; yield stress.
83.60.Pq - Time-dependent structure (thixotropy, rheopexy).
© EPLA 2009