Measuring the Viscosity of Non-Newtonian Fluids

Measuring the Viscosity of Non-Newtonian Fluids The exact start and end poin ts of the ramp can be decided from the ramp performed in Basic Protocol 1. A stress of 1/lOth to l/100th of the apparent yield stress to the nearest order of magnitude should be set. 

The method of ultrasound Doppler velocimetry (UDV) [9] was proposed to measure the viscosity of non-Newtonian fluids over a wide range of shear rates and in a short period of time. This is a noninvasive, nondisturbing, quick, and accurate procedure. The distribution of shear-stress can be found by pressure drop. At a radial position, the ratio of shear-stress to shear rate, by definition, yields the viscosity at that point. Thus, for the shear rate range in the flow, viscosity values can be obtained by means of only one online experiment. This is a method known in the literature as pointwise rheological measurement [10,11]. 

This equation suggests that the determination of the viscosity of non-Newtonian fluids with plate-plate geometry requires that InM first be plotted against InKj . Consequently, the determination of the shear-dependent viscosity requires that the torque be measured at different shear rates. The value of the viscosity is then determined with the value of the local slope in conjunction with Eq. (13.84). If the fluid obeys a power law, then... 

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). 

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). 

Most of the studies on heat transfer, with fluids have been done with Newtonian fluids. However, a wide variety of non-Newtonian fluids are encountered in the industrial chemical, biological, and food processing industries. To design equipment to handle these fluids, the flow property constants (rheological constants) must be available or must be measured experimentally. Section 3.5 gave a detailed discussion of rheological constants for non-Newtonian fluids. Since many non-Newtonian fluids have high effective viscosities, they are often in laminar flow. Since the majority of non-Newtonian fluids are pseudoplastic fluids, which can usually be represented by the power law, Eq. (3.5-2), the discussion will be concerned with such fluids. For other fluids, the reader is referred to Skelland (S3). 

Gla.ss Ca.pilla.ry Viscometers. 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 Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed 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 instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

Chapter HI relates to measurement of flow properties of foods that are primarily fluid in nature, unithi.i surveys the nature of viscosity and its relationship to foods. An overview of the various flow behaviors found in different fluid foods is presented. The concept of non-Newtonian foods is developed, along with methods for measurement of the complete flow curve. The quantitative or fundamental measurement of apparent shear viscosity of fluid foods with rotational viscometers or rheometers is described, unithi.2 describes two protocols for the measurement of non-Newtonian fluids. The first is for time-independent fluids, and the second is for time-dependent fluids. Both protocols use rotational rheometers, unit hi.3 describes a protocol for simple Newtonian fluids, which include aqueous solutions or oils. As rotational rheometers are new and expensive, many evaluations of fluid foods have been made with empirical methods. Such methods yield data that are not fundamental but are useful in comparing variations in consistency or texture of a food product, unit hi.4 describes a popular empirical method, the Bostwick Consistometer, which has been used to measure the consistency of tomato paste. It is a well-known method in the food industry and has also been used to evaluate other fruit pastes and juices as well. 

Capillary viscometers are ideal for measuring the viscosity of Newtonian fluids. However, they are unsuitable for non-Newtonian fluids since variations in hydrostatic pressure during sample efflux results in variations in shear rate and thus viscosity. This unit contains protocols for measuring the viscosity of pure liquids and solutions (see Basic Protocol) and serums from fruit juices and pastes (see Alternate Protocol). 

The constant of proportionality in equation 2.10 is the viscosity of the liquid tf). Some fluids, such as water, olive oil and sucrose solutions obey this equation and are said to be Newtonian. Their viscosity does not depend on the velocity gradient, i.e. how fast the liquid is sheared - known as the shear rate, More complex fluids (e.g. solutions of polymers) have a viscosity that does depend on the shear rate. Such fluids are called non-Newtonian . Many complex fluids, for example tomato ketchup and ice cream mix, become less viscous when they are sheared and are described as shear-thinning . Tapping the bottom of the bottle applies shear to the ketchup, which becomes less viscous and flows more easily onto your plate. Other fluids, such as a concentrated solution of cornstarch or quicksand, become more viscous (i.e. they are shear-thickening ). Experiment 7 in Chapter 8 gives some examples of non-Newtonian fluids. A single viscosity is not sufficient to describe the flow properties of non-Newtonian liquids and if a viscosity is stated, the shear rate at which it was measured must also be given. 

By changing the size of the spindles, the rotational viscometer can measure the viscosity of any fluid substance. Additionally, due to the fact that the apparatus has the ability to fluctuate shear rate, the identification of a Newtonian (a fluid having a viscosity that is independent of the shear rate) or non-Newtonian fluid is also possible. Additionally, in the case... 

The flow behavior of non-Newtonian fluids is usually described by expressing either shear rate or viscosity as a function of shear stress. Absolute viscometers, either capillary or rotational, are used to perform the necessary measurements. In the capillary viscometer, the flow rate is measured as a function of applied pressure. Apparent viscosities calculated by means of Poiseuille s relation [Eq. (9)], are converted to true viscosities using the Weissen-berg-Rabinowitsch correction... 

In the proceeding sections of this chapter, the concepts of Newtonian and non-Newtonian fluids were explored. Measuring the viscosity of a slurry mixture is recommended for ho-... 

The use of capillary-like viscometers is complicated by the effective sUp of non-Newtonian fluid-suspended material, which tends to move away from the wall, leaving an attached layer of liquid. The result is a reduction in the measurements of effective viscosity. Therefore, it is often recommended to conduct such tests in a number of tubes of different diameters. 

Dynamic (absolute) viscosity, or the coefficient of absolute viscosity rf), is the measure of internal friction of the Hquid (viscosity) at shear stress (r) between the layers of non-turbulent fluid (Newtonian fluid) moving in straight parallel Hnes, which is given by the equation ... 

There is some equipment to be used for viscosity measurement which broadly classified into two categories dynamic and kinematic viscometer. A dynamic viscometer is one of the shear rate can be controlled and measured (rotational viscometer). It is the only typ>e of viscosity measurement that is relevant to fluids where the viscosity is related to the shear rate (non-Newtonian fluids). A kinematic viscometer is where the shear rate can neither be controlled nor measured, for example capillary viscometer. 

A constant is often determined from measurements with a Newtonian oil, particularly when the caUbrations are suppHed by the manufacturer. This constant is vaUd 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 instmment constant can save a great deal of time and effort and increase accuracy because end and edge effects, sHppage, turbulent interferences, etc, are included. 

The measurements are carried out at preselected shear rates. The resulting curves are plotted in form of flow-curves t (D) or viscosity-curves ti (D) and give information about the viscosity of a substance at certain shear rates and their rheological character dividing the substances in Newtonian and Non-Newtonian fluids. 

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