No steady state flows below the yield stress. A true yield stress at last?

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 10,000 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!