Origin of apparent viscosity in yield stress fluids below yielding

TLDR

Debate persists over whether yield‑stress fluids behave as solids below the yield stress or flow, with proponents noting a rapid viscosity rise and opponents observing a plateau that mimics a very high‑viscosity Newtonian fluid. The study aims to show that the apparent low‑stress Newtonian viscosity is an artifact of non‑steady‑state experiments. The authors performed experiments on four materials with three rheometers, five geometries, and two measurement methods to test this hypothesis. Measurements up to 10⁴ s revealed that the apparent Newtonian viscosity grows without bound, confirming that a yield stress exists and separates flowing states from solid‑like states with infinite steady‑state viscosity.

Abstract

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!