Cone-plate rheometers

FIG. 4 Viscosity (cone-plate rheometer, D = 3.23 s1, at 25°C) and clear point of 10% sodium lauryl ether (2 EO) sulfate solutions with 0.5% of added NaCl vs. percentage of dialkanolamide [77],... 

When compared to standard (open cavity) cone-plate or parallel disks rheometers, closed cavity torsional rheometers such as the RPA or the PPA have unique high-strain capabilities, which prompted us to modify the instmment in order to investigate the promises of FT rheometry, as outlined a few years ago by the pioneering works of Wilhelm. The technique consists of capturing strain and torque signals and in using FT calculation algorithms to resolve it into their harmonic components, as detailed below. 

Zero shear viscosities have been determined in solution over a wide range of concentrations with a cone-plate Rheometrics Stress Rheometer. For linear macromolecules, the viscosity is proportional to c below the so called "entanglement concentration", c above c, is proportional to c. However, the viscosity will rise steeply at some concentration below c in the case where particular interconnections are formed at the concentration at which the molecules come into contact with one another. Ideally this will be the overlap threshold c. Below c, the molecules may associate partially but cannot form a network continuous over the entire sample space. Above c, plastic flow will require separation and... 

The cone-plate rheometer. The cone-plate rheometer is often used when measuring the viscosity and the primary and secondary normal stress coefficient functions as a function of shear rate and temperature. The geometry of a cone-plate rheometer is shown in Fig. 2.47. Since the angle Oo is very small, typically < 5°, the shear rate can be considered constant throughout the material confined within the cone and plate. Although it is also possible to determine the secondary stress coefficient function from the normal stress distribution across the plate, it is very difficult to get accurate data. 

A Brookfield Engineering Rheometer, Model DV-III fitted with a Cone/Plate spindle CP-40 was used to measure the viscosity of the polymer samples. The CP-40 was chosen for its relatively small sample volume requirements (0.5 ml). The sample was measured and delivered with a syringe, allowed to equilibrate at temperature for 15 minutes and then rotational force was applied. Data was collected using Brookfield Engineering software... 

All the major manufacturers of viscometers and rheometers have Internet sites that illustrate and describe their products. In addition, many of the manufecturers are offering seminars on rheometers and rheology. Earlier lists of available models of rheometers and their manufacturers were given by Whorlow (1980), Mitchell (1984), and Ma and Barbosa-Canovas (1995). It is very important to focus on the proper design of a measurement geometry (e.g., cone-plate, concentric cylinder), precision in measurement of strain and/or shear rate, inertia of a measuring system and correction for it, as well as to verify that the assumptions made in deriving the applicable equations of shear rate have been satisfied and to ensure that the results provided by the manufecturer are indeed correct. 

Figure 13.19 Schematic representation of a cone-plate rheometer.

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. 

The flow curves can be established for different concentrations and different molar masses of HA samples, and at different temperatures for a better insight into the molecular properties of polymers. Fig. (14) shows results of a series of rheological tests of HA polymers with different molar masses at different concentrations. Fig. (14, left panel) shows the flow curves for three different HA samples with the Mw values of 850 kDa, 600 kDa, and 400 kDa. Fig. (14, right panel) exhibits the flow curves for an HA sample at four different concentrations ranging from 0.11% to 2.16%. The flow curves are obtained by using an AR 2000 stress-controlled rheometer from TA Instruments (New Castle, DE, USA). A cone/plate geometry is used. The rotor was made of the acrylic material, 4 cm of diameter and 1° of cone angle. The measurements were performed 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. 

Fig. 4 Reprinted from [96], (a) Schematics of the confocal rheoscope of the Edinburgh group [96]. The top arrow marks translation of the rheometer head to adjust the geometry gap, the horizontal arrow indicates translation of the arm supporting the objective to image at different radial positions r. (b) Close up of the central part of the rheoscope, similar to the cone-plate imaging system of Derks [111] except that in the latter the lower plate can also be rotated, while in the former the microscope objective radial position r can be varied, (c) Gap profile of a 1° cone-plate geometry, measured in the confocal rheoscope with fluorescent particles coated on both surfaces...

Rheological measurements were performed in shear using a stress controlled rheometer (Carri-Med CSL 100) operating in cone-plate geometry. Each sample is submitted successively to a first frequency sweep in range 10 3-40 Hz under 3% strain, to a creep and recovery test, and finally to a second frequency sweep identical to the first one. The dynamical strain amplitude (3%) and the value of the creep stress (chosen so as to keep the maximum strain below 10%) were set in order to remain within the linear viscoelasticity domain. Creep and creep recovery were recorded during 20 h and 80 h, respectively, times which allowed the steady state to be reached in all cases. A fresh sample was used for each solvent/temperature combination. 

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). 

Viscosity with stational flow. A cone-plate type rotating rheometer (Shimadzu, RM-1, equipped with a reduction gear, RDG-1) was employed. The rate of shear available ranged from 7.48x10" to 74.8/sec. The apparent viscosity at a given rate of shear was calculated from the rate of shear and the observed shear stress. Samples were dissolved in the buffer solution mentioned before at 2 or 4% concentration and measured at room temperature (22+l C). 

Rheological measurements of the silica suspensions were performed using a Paar Physica MCR300 rheometer with a cone-plate geometry. 

The cone-plate rheometer is used when measuring the viscosity and the primary and secondary normal stress coefficient functions as a function of shear rate and... 

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