Rotational viscometers, rheological measurements

Frequently used single-point viscosity tests in the starch plant are orifice pipettes,56 orifice funnels,57 the Hot Scott viscometer, and various methods to determine alkaline fluidity.58 For absolute measurements of the rheological properties, rotating viscometers with coaxial cylinders are used.59 The paper industry uses mainly the Brookfield viscometer and the Hercules viscometer for determining shear-dependent viscosity, pseudoplasticity, and thixotropy. Oscillatory and capillary viscometers are used for more detailed viscosity characterization, such as yield value, elastic properties, and viscoelasticity.60... 

Instrumental quality control before, during, and after manufacture is one area to which food rheology makes important contributions. For example, the measurement of apparent viscosity and yield stress of ketchup helps to predict how well tomato ketchup drains from a bottle. A number of tests have been developed using either basic rheological instruments (rotational viscometer, capillary viscometers, etc.) or instruments simulating the situation in which the rheological properties are of importance (Bostwick... 

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

In systems with suspended solids, rheologic measurements are difficult to perform owing to settling in the measurement devices. Conventional methods for measuring rheologic properties (cone-and-plate, concentric cylinder, and rotating-bob viscometers) do not produce accurate and reliable data for some solid suspensions. 

For pumping ease, the initial Be should be very low, preferably < 30 Be, which can be measured by the consistometer. The rheological behavior can be measured by means of a rotational viscometer, which consists of an outer sleeve and an inner drum, both rotating at different speeds. The outer sleeve is rotated at a constant speed, which causes a torque on the inner dmm that can be measured on a dial. Starting from 600 rpm, the rotor speed is lowered successively at 20 s intervals, and the measurements of the torque are taken at the end of each period. Typically the speed is lowered in steps of 6 rpm, and the results are represented graphically. 

In the last decade of the nineteenth century, Maurice Couette invented the concentric cylinder viscometer. This instrument was probably the first rotating device used to measure viscosities. Besides the coaxial cylinders (Couette geometry), other rotating viscometers with cone-plate and plate-plate geometries are used. Most of the viscometers used nowadays to determine apparent viscosities and other important rheological functions as a function of the shear rate are rotating devices. 

Rotational Viscometers. Rotational viscometers are the most widely used instruments for the measurement of the rheological properties of a fluid (e.g., a pure liquid, emulsion, or suspension). The test fluid is placed in a gap formed by either two coaxial rotating cylinders, two flat discs, or a flat disc and a cone. The major advantages of the rotational viscometers are... 

Parallel Plate Viscometer, This instrument resembles the cone and plate viscometer, except that it has a flat horizontal rotating plate in place of the cone. The shear rate within the narrow gap of the two plates is not as uniform as for the cone and plate viscometer. The limiting shear rates for the parallel plate viscometer are similar to those of the cone and plate instrument. This type of a viscometer is suitable for rheological measurements of suspensions and emulsions. 

Several researchers reported viscoelastic behavior of yeast suspensions. Labuza et al. [9] reported shear-thinning behavior of baker s yeast (S. cerevisiae) in the range of 1 to 100 reciprocal seconds at yeast concentrations above 10.5% (w/w). The power law model was successfully applied. More recently, Mancini and Moresi [10] also measured the rheological properties of baker s yeast using different rheometers in the concentration range of 25 to 200 g dm. While the Haake rotational viscometer confirmed Labuza s results on the pseudoplastic character of yeast suspension, the dynamic stress rheometer revealed definitive Newtonian behavior. This discrepancy was attributed to the lower sensitivity of Haake viscometer in the range of viscosity tested (1.5 to 12 mPa s). Speers et al. [11] used a controlled shear-rate rheometer with a cone-and-plate system to measure viscosity of... 

Dodd [68] measured the rheological properties of interfacial films in a semiquanti-tative manner by employing an interfacial shear rotational viscometer to study crude-water interfaces with NaCl, acid, and basic additives in the water phase. He concluded that the film must be comprised of naphthenic acids, naphthenic acid soaps, and naphthenic acid anions, in combination with resins, asphaltenes, and waxes. Furthermore, the acidic species must desorb from the interface under basic conditions and partition into the aqueous phase, rendering the interface considerably less rigid. Subsequent researchers have shown that acidic asphaltenes are more effective at emulsion stabilization than their neutral counterparts. 

Various instruments are available to measure the viscosity of polymer melts and solutions, and more generally their rheological behavior, which include capillary and rotational viscometers. The former can be used to measure parameters such as shear viscosity, melt fracture, and extensional viscosity, which are important for many polymer processes. The latter type of device can be used in either steady or oscillatory mode, thus providing a measure of the viscosity as well as viscoelasticity (G", G, and tan 5) as a function of frequency and temperature. 

Consequently, the rheological measurements of MPSs should be carried out such that the dimension of the flow channel is significantly larger than the size of the flow element. For example, the relative viscosity, jjr, of diluted spherical suspensions measured in a capillary instrument depends on the (d/D) factor, where 7) is the sphere diameter and d that of the capillary—for d 107), the error is around 1% [Happel and Brenner, 1983]. Thus, if 1% error is acceptable, the size of the dispersion should be at least 10 times smaller than the characteristic dimension of the measuring device (e.g., diameter of a capillary in capillary viscometers, distance between stationary and rotating cylinders or plates). Following this recommendation is not always possible, which lead to the decline and fall of continuum mechanics [Tanner, 2009]. 

The steady-state shear flow properties in the low shear rate region and the dynamic functions were measured using a rotational viscometer (cone-plate type, RGM151-S, Nippen Rheology Kiki Co., Ltd., Japan). The cone radius R was 21.5mm, the gap between the central area of the cone and plate H was kept at 175p.m, and the cone angle 0 was 4°. The measurements were carried out at 200°C Steady state shear properties (shear viscosity //, and the first normal stress difference Ni) as well as dynamic functions (storage and loss moduli G, G", respectively. 

Rheological measurements were carried out with a computer-controlled Rheometer MCI (Paar Physica) rotational viscometer, with a Z4 concentric cylindrical measurement system at 25 0.1 °C. The complete flow and viscosity curves of the samples were plotted, and the yield value and the size of the thixotropic area were determined. 

Dynamic rheological measurements will be briefly described. In dynamic measurements an oscillatory microscopic strain is given and the corresponding stress is measured. This method gives information on viscoelastic behavior even in the region where no viscous flow takes place. The measurements can be carried out by using the rotating cylinder viscometer or the cone and plate viscometer. 

---