Rotating rheometer

The measuring modes of rotation rheometers can be divided in the following manner ... 

Another typical example of inhomogeneity in rheometry is the oxidation of a polymer in a rotational rheometer in which a disk-shaped sample is held between metal fixtures. The oxygen enters the sample through the free surface (at the outer diameter) and diffuses radially inwards. The result is a radial gradient in properties which changes with time. If the reaction with oxygen results in... 

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. 

Figure HI. 1.1 Common fixtures used for viscosity measurements with rotational rheometers. Cross-sectional views are shown on the left and external views are shown on the right.

The drive system of a rotational rheometer is likely to be optimized in one of two ways depending upon its preferred mode of operation. The most common form of rheometer is a controlled-rate (controlled-speed) device. This configuration is also used in most viscometers and has been around for decades. A shear rate is applied to a rotor by the motor controlling the viscometer s speed. The rotor is normally a flat plate or cylindrical cup. The stator is thus a cone or plate for the first two geometries or a cylindrical bob for the third (Figure HI. 1.1). The stator is linked to the rotor via the sample, which acts to couple the input signal like an automobile transmission. Thus, the torque on the stator when measured by a transducer is used to derive the shear stress in the sample. 

This section describes common steps designed to measure the viscosity of non-Newtonian materials using rotational rheometers. The rheometer fixture that holds the sample is referred to as a geometry. The geometries of shear are the cone and plate, parallel plate, or concentric cylinders (Figure HI. 1.1). The viscosity may be measured as a function of shear stress or shear rate depending upon the type of rheometer used. 

Sample, stored in an appropriate clean container (sealed if solvent loss is an issue) Controlled-rate or controlled-stress rotational rheometer with Computer control and appropriate software for instrument control, data acquisition, and model fitting... 

Load a sample on a controlled-rate or controlled-stress rotational rheometer and equilibrate to the test temperature (see Strategic Planning). 

Data from a rotational rheometer using cone-and-plate or concentric cylinders should agree within limits of -2% to 5% for Newtonian samples and -10% for non-Newtonian time-in-dependent samples. Time-dependent samples can be orders of magnitude apart. 

Figure H3.1.4 Structure recovery after loading a sample of a grease or fat on a cone-and-plate fixture on a rotational rheometer. G was measured over a period of 17 hr at constant oscillatory stress, frequency, and temperature.

Dynamic rotational rheometer with appropriate test fixture Computer with software package to control rheometer... 

Attach an appropriate test fixture to a dynamic rotational rheometer and connect rheometer inline with a computer. 

Recent advances in dynamic rotational rheometers are of growing importance in food analyses for several reasons. (1) The measurement minimizes the destruction of the material. (2) The time required for a measurement is reasonably short in comparison with chemical or physical changes in the material. (3) The viscoelasticity of gels is characterized by determining G and G" in the linear viscoelastic (LVE) region no other method gives dynamic moduli values. 

Rotational rheometer (unithi.i e.g., Bohlin Instruments, Chandler Engineering) controlled stress (for applied step shear stress) or controlled strain (for applied step shear strain) with appropriate software for rheometer control, data acquisition, and data analysis... 

Electrorheological properties of fluids, in this case PS/MWCNT suspensions in silicone oil, can be demonstrated in the form of flow curves obtained from measurements on a rotational rheometer in the absence and presence of external electric field (21). The flow curves expressing shear stress vs. shear rate dependence are... 

Various instruments are used to study the texture of gels. Some empirical instruments are in use in the food industry but a more scientific approach is to use rheometry. A rotation rheometer is not an appropriate instrument to study gels since the rotation destroys the gel. A method... 

P.R.Soskey, H.H.Winter, Large step shear strain experiments with parallel disk rotational rheometers, J. Rheol. 2S (1984), 625-645. 

Creep tests can be performed with steady rotating viscometers. Conventional forced-oscillation measurements can also be performed with ordinary shear geometries such as plate-plate or plate-cone systems in rotating rheometers. 

Rheology is a powerful method for the characterization of HA properties. In particular, rotational rheometers are particularly suitable in studying the rheological properties of HA. In such rheometers, different geometries (cone/plate, plate/plate, and concentric cylinders) are applied to concentrated, semi-diluted, and diluted solutions. A typical rheometric test performed on a HA solution is the so-called "flow curve". In such a test, the dynamic viscosity (q) is measured as a function of the shear rate (7) at constant strain (shear rate or stress sweep). From the flow curve, the Newtonian dynamic viscosity (qo), first plateau, and the critical shear rate ( 7 c), onset of non-Newtonian flow, could be determined. 

Moreover, rotational rheometers can be used in dynamic oscillatory mode, frequency sweep, to assess the elastic G module as well as the viscous G" module and the correlated phase angle 6, as a function of the frequency co. G and G" allow to study the viscoelastic behaviour of HA macromolecules. Fig. (15) shows the frequency sweep curves (G, G", and tg(5) vs. the frequency co) for an HA sample (Mw=1350 kDa, polydispersity index D=1.6, concentration c = 2%) at 20 °C. 

Both strain- and stress-controlled rotational rheometers are widely employed to study the flow properties of non-Newtonian fluids. Different measuring geometries can be used, but coaxial cylinder, cone-plate and plate-plate are the most common choices. Using rotational rheometers, two experimental modes are mostly used to study the behavior of semi-dilute pectin solutions steady shear measurements and dynamic measurements. In the former, samples are sheared at a constant direction of shear, whereas in the latter, an oscillatory shear is used. 

Although the flow in a capillary rheometer is regarded as pressure-driven flow, namely, the shear is generated by the pressure difference along the capillary length, in rotational rheometers the shear is generated between a moving and a fixed surface [9]. 

Steady Shear Viscosity and Dynamic Viscosity Data Neat HDPE rheology data fairly well correspond to each other when obtained by both capillary and rotational rheometers. This actually means that HDPE melt obeys the Cox-Merz rule [26]. The... 

Wood flour as a filler decreases elasticity of polyethylene [33], In other words, the shear stresses in the system of polyethylene highly filled with wood flour dominated over normal stresses. This was shown using a parallel-plate rotational rheometer under steady shearing conditions. The decreased elasticity was expressed as a decrease of the stress ratio with increasing wood flour loading at the same shear rate. 

Rheometer Any instrument designed for the measurement of non-Newtonian as well as Newtonian viscosities. The principal class of rheometer consists of the rotational instruments in which shear stresses are measured, and a test fluid is sheared between rotating cylinders, plates, or cones. Various types of rotational rheometers are concentric cylinder, cone-cone, cone—plate, double cone—plate, plate—plate, and disc (16). 

ABSTRACT A rotational benchtop Rheometer with vane spindles can be used to measure the static yield stress behavior of materials. By running at different rotational speeds, the Rheometer data can be equated with the viscoelastic information determined by an oscillating rheometer. The rotational Rheometer offers a less expensive method suitable for Quality Control needs. 

KEYWORDS yield stress, viscoelastic, rotational rheometer... 

Polymer Characterization. Viscosity. Solution viscosities of the NaAMPS copolymers were determined with a Cannon-Ubbelohde four-bulb shear dilution viscometer (size 100). Solution viscosities of the associative polymers were determined with a Contraves low-shear rotational rheometer at a shear rate of 1.28 s" All viscosity studies were conducted in the absence of surfactant at temperatures in the range of 25-45 °C and [NaCl] ranging from 0 to 0.684 M. 

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