Viscosity instrumentation Newtonian fluids

Houskaet al. (1998) determined the relationships for five sensory methods of oral and non-oral viscosity evaluation between viscosity scores and instrumentally measured dynamic viscosity for Newtonian fluid foods of low and medium viscosities. From those relationships, the effective shear rates for the five the sensory tests were estimated. Highest shear rates were predicted for viscosity perception by compression of samples between tongue and palate, and the lowest for pouring the fluid foods from a teaspoon. Mixing with a teaspoon, slurping and swallowing exhibited nearly... 

The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test liquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, approx 0.4 16,000 mm /s. However, external pressure can be applied to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instruments 1-15 Pa or 10-150 d5m/cm if operated by gravity only. The rate of shear can be as high as 20,000 s based on a 200-800 s efflux time. 

The membrane viscometer must use a membrane with a sufficiently well-defined pore so that the flow of isolated polymer molecules in solution can be analyzed as Poiseuille flow in a long capillary, whose length/diameter is j 10. As such the viscosity, T, of a Newtonian fluid can be determined by measuring the pressure drop across a single pore of the membrane, knowing in advance the thickness, L, and cross section. A, of the membrane, the radius of the pore, Rj., the flow rate per pore, Q,, and the number of pores per unit area. N. The viscosity, the maximum shear stress, cr. and the velocity gradient, y, can be calculated from laboratory measurements of the above instrumental parameters where Qj =... 

Capillary viscometers are useful for measuring precise viscosities of a large number of fluids, ranging from dilute polymer solutions to polymer melts. Shear rates vary widely and depend on the instruments and the liquid being studied. The shear rate at the capillary wall for a Newtonian fluid may be calculated from equation 18, where Q is the volumetric flow rate and r the radius of the capillary the shear stress at the wall is rw = rApf2L. 

A constant is often determined from measurements with a Newtonian oil, particulady when the calibrations are supplied by the manufacturer. This constant is valid only for Newtonian specimens if used with non-Newtonian fluids, it gives a viscosity based on an inaccurate shear rate. However, for relative measurements this value can be useful. Employment of an instrument constant can save a great deal of time and effort and increase accuracy because end and edge effects, slippage, turbulent interferences, etc, are included. 

The relationship between viscosity, angular velocity, and torque for a Newtonian fluid in a concentric cylinder viscometer is given by the Maigules equation (eq. 26) (21,146), where M is the torque on the inner cylinder, h the length of the inner cylinder, Q the relative angular velocity of the cylinder in radians per second, R the radius of the inner cylinder wall, Rr the radius of the outer cylinder wall, and k an instrument constant. 

The on-line measurement of viscosity under plant conditions poses particular difficulties. This is due to the wide range of viscosities that can occur within a process plant, to the difficulty of obtaining reliable measurements (particularly for non-Newtonian fluids) and to the accuracy that is often required (e.g. better than within 1 per cent for lubricating oils). Variables which can affect the measured viscosity are the temperature, pressure and rate of flow of the sampled stream— quite apart from the normal errors that can occur in any similar instrument (e.g. due to variations in supply voltage and frequency, sample contamination, sample not being representative of the bulk fluid, etc.). Automatic temperature compensation is always required and, in the case of multiphase systems, the difficulty of obtaining a representative sample is considerable (see Section 6.9). In this instance... 

Continuous viscometers based upon the Couette principle are able to measure the viscosity of both Newtonian and non-Newtonian fluids over a wide range (Table 6.7). A typical instrument of this type is illustrated in Fig. 6.40<5J). 

A rheological instrument such as a viscometer can be used to evaluate t and 7 and hence obtain a value for the shear viscosity, 17. Examples of Newtonian fluids are pure gases, mixtures of gases, pure liquids of low molecular weight, dilute solutions, and dilute emulsions. In some instances, a fluid may be Newtonian at a certain shear-rate range but deviate from Newton s law of viscosity under either very low or very high shear rates (2). 

Work continues on developing equations for interfacial viscosity, where both fluids are liquids, and for non-Newtonian surface behavior, where ps varies with stress. In all such cases the deep channel geometry considerably simplifies these derivations. As these analyses are developed, the promise of this arrangement as a basic instrument should improve. 

An alternative procedure, to ensure no external force is applied to the powder bed by the vaned paddle, is to place the compacted sample on a balance and when the paddle is immersed in the powder to raise the vaned head slowly until the balance reading is zero. This dynamic method of bulk powder characterisation is allied to the rheological method for measurement of the viscosity of non-Newtonian fluids and suspensions. Commercial instruments based on the WSL cohesion tester are now available in the form of the FT4 Powder Rheometer (Freeman Technology) and the Stable Micro Systems Powder Flow Analyser (Stable Micro Systems). 

Figure 5 (p. 32) shows the Ostwald (power law) regression for a nearly Newtonian fluid having a viscosity of 250 Pa s (250,000 cP). It would take considerably more torque capability in an instrument to test the fluid at a shear rate of 1000 sec (a 30-hp rheometer would be quite expensive). Because the correlation to the Ostwald model is good (99.628, in this case), we can use the model to estimate the viscosity at 500 sec and 1,000 sec. The values were 246.725 Pa-s and 246.090 Pa-s, respectively, seen at the end of the results where stress and rate are also shown. The Ostwald n and k values are shown as constants 1 and 0 the n is 0.9962 while the k is 252.489 Pa s. 

---