Yield stress: A time-dependent property and how to measure it

Abstract

This paper reviews the different aspects of the yield stress phenomenon and attempts a synthesis of knowledge. Yield stress can be probed using constant shear stress or shear rate. The magnitude of the result depends on the time allowed to determine whether the sample has developed continuous flow or has ceased flowing. It is closely associated with creep, stress growth and thixotropic breakdown and recovery, and the characteristic times of these transient responses play a part in yield stress measurement. In thixotropic fluids, yield stress is a function of structure and hence of time. In simple thixotropy, the yield stress derived from the equilibrium flow curve is the same as that for the fully built-up structure. But in many materials, the static yield stress obtained after prolonged rest is much higher than the dynamic yield stress from the equilibrium fiow curve. This is associated with the phenomenon in which the equilibrium flow curve bends upwards as the shear rate is reduced to very low values. The paper also reviews the many methods that can be used to measure yield stress. It is pointed out that the choice of observation time or shear rate to use should be related to the characteristic time of the flow process to which the result is to be applied. Examples discussed are solids suspension capability of fluids, levelling and sagging, pipeline flow and start-up power requirement of mixers.

Observations on the rheological response of alkali activated fly ash suspensions: the role of activator type and concentration

“…In both cases there is a 380 stress plateau at lower shear rates (< 1/s) indicating an apparent yield stress (Cheng 1986;Barnes 1999), 381 which is preceded by a linear portion at relatively constant slope from shear rates of about 10 to 100/s. 382…”

Development of Bio-Oil Emulsion to Allow Realistic Testing of Oil Spill Control Equipment

“…In fact, it is a common occurrence at British Society of Rheology meetings, as well as international conferences, that there is heated discussion as to whether yield stress exists at all. Some asserted that yield stress exists as a well-defined quantity Whether the yield stress actually exists or not, it is too important an industrial and engineering parameter to disregard, but it must be realised that absolute values of yield stress cannot be obtained as its magnitude may vary according to the experimental or practical conditions (Cheng, 1986).…”

Experimental Investigation on Yield Stress of Water-in-Heavy Crude Oil Emulsions in Order to Improve Pipeline Flow

“…The yield stress is one of the most important rheology characteristics since the fluids cannot be transported when external force is less than its yield stress. [1][2][3] Here, the yield stress can be defined as a limit stress below which the sample behaves like a solid, but above the value of yield stress, the application of stress undergoes unlimited deformation to cause the fluid flow. [4][5][6] To start the heavy crude oil flow in pipeline, the theoretical minimal pressure drop (DP min ) could be given by the following correlation [7] :…”