Viscosity equilibrium flow tests

Basic Protocol 2 Measuring Viscosity with Equilibrium Flow Tests HI. 2.6... 

Basic Protocol 2 is for time-dependent non-Newtonian fluids. This type of test is typically only compatible with rheometers that have steady-state conditions built into the control software. This test is known as an equilibrium flow test and may be performed as a function of shear rate or shear stress. If controlled shear stress is used, the zero-shear viscosity may be seen as a clear plateau in the data. If controlled shear rate is used, this zone may not be clearly delineated. Logarithmic plots of viscosity versus shear rate are typically presented, and the Cross or Carreau-Yasuda models are used to fit the data. If a partial flow curve is generated, then subset models such as the Williamson, Sisko, or Power Law models are used (unithi.i). 

A good diagnostic for creep and stress relaxation tests is to plot them on the same scales as a function of either compliance (J) or modulus (G), respectively. If the curves superimpose, then all the data collected is in the linear region. As the sample is overtaxed, the curves will no longer superimpose and some flow is said to have occurred. These data can still be useful as a part of equilibrium flow. The viscosity data from the steady-state part of the response are calculated and used to build the complete flow curve (see equilibrium flow test in unit hi.2). 

Another method for measuring uniaxial extensional viscosity is by bubble collapse. A small bubble is blown at the end of a Ciq>illary tube placed in the test fluid (see Figure 7.6.1). It comes to equilibrium with the surrounding pressure and surface tension. Then at time r = 0 the pressure inside the bubble is suddenly lowered or the surrounding pressure increased. The decrease in bubble radius with time is recorded. If the deformation is reversed (i.e., the pressure inside the bubble is suddenly increased), the growing bubble radius can be used to give the equibiaxial viscosity. This flow appears to be less stable and has not been studied as a rheometer. 

If a logarithmic ramp is performed, then the data should not be fit with linear models (unit m.i). These data should be plotted as viscosity versus shear rate on logarithmic axes and the Carreau-Yasuda or Cross models (or subsets) should be used instead. It is unlikely that the zero-shear plateau will be seen in these types of tests. For a complete flow curve, the equilibrium tests described in Basic Protocol 2 should be used. 

The surface area expansion process in Figure 3.5 must obey the basic thermodynamic reversibility rules so that the movement from equilibrium to both directions should be so slow that the system can be continually relaxed. For most low-viscosity liquids, their surfaces relax very rapidly, and this reversibility criterion is usually met. However, if the viscosity of the liquid is too high, the equilibrium cannot take place and the thermodynamical equilibrium equations cannot be used in these conditions. For solids, it is impossible to expand a solid surface reversibly under normal experimental conditions because it will break or crack rather than flow under pressure. However, this fact should not confuse us surface tension of solids exists but we cannot apply a reversible area expansion method to solids because it cannot happen. Thus, solid surface tension determination can only be made by indirect methods such as liquid drop contact angle determination, or by applying various assumptions to some mechanical tests (see Chapters 8 and 9). 

Rheological measurements are performed so as to obtain a test fluid s material functions. Under viscometric flows we have seen that the shear viscosity and the primary and secondary normal stress differences suffice to rheologically characterize the fluid. If the flow field is extensional and the material is able to attain a state of dynamic equilibrium, then one measures the extensional viscosity otherwise, we measure the extensional viscosity growth or decay functions. In this section, we will examine steady and dynamic shear plus uniaxial extensional tests, since these make up the majority of routine rheological characterization. 

This figure shows that the x value determines not only the t s value, but the flow regime as well. Actually, if Xr < 30 min., OBUA shows a non-Newtonian liquid type flow (t depends on y), but if Xr > 30 min. the system behaves like a Newtonian liquid. For the problem studied it is very important that within the range of 0 < Xr < 30 min., the t s absolute values increase with the increase of Xr, while within Xr > 30 min. the stationary viscosity does non depend on Xf. Relaxation time, where ps is not dependent on Xr, is assumed as eritieal value Xr " . It has the same physical meaning as Xex ", i.e., this is the time to reaeh equilibrium aeeording to viscosity test. 

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