Elongational viscometers

Typical output Extensional stress as a fimction of extensional strain rate/ time. 

Exercise Eind the Internet websites of the most prominent viscometer and 

Exercise Imagine you are working in the following areas, and discuss the kinds of rheometers you would hke to use in setting up a laboratory to study asphalt, blood flow, cosmetics, chocolate. 

Normal Force (to give Ni) Oscillation (to give G, G ) C ontrolled Stress (to give J(t)) 

Haake - RT20, RS75/150 Haake - RheoWave 1, RS 75/150 Haake - RSI, RS 75/150 

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Currently Performed Experiments. Flow experiments with equipment similar to the Schlumberger elongational viscometer have been attempted, but encountered many difficulties an elongational cross cell system (same flow as the Taylor four-roll-mill) is being developed Shear flow has been studied more extensively in a Couette viscometer (see Ref. for review). A periodic oscillatory uniaxial deformation has been recently started on PDMS rubbers . Up to now the range of available time... 

A rather complete survey of the entire field of viscometry, including the mathematical relationships applicable to various types of instruments, has been made by Philippoff (P4). The problem of slip at the walls of rotational viscometers has been discussed by Mooney (M15) and Reiner (R4). Mori and Ototake (M17) presented the equations for calculation of the physical constants of Bingham-plastic materials from the relationship between an applied force and the rate of elongation of a rod of such a fluid. ... 

Strictly speaking, the viscosity rj, measured with shear deformation viscometers, should not be used to represent the elongational terms located on the diagonal of the stress and strain rate tensors. Elongational flows are briefly discussed later in this chapter. A rheologist s... 

Initially several researchers measured the effective viscosity of bulk foam using rotational or capillary viscometers (1, 49) with the hope of applying their results to porous media. On the basis of the earlier discussion of foam morphology in porous media, such data are inappropriate (50). Interaction of elongated bubbles and pore-spanning lamellae with pore walls determines the effective viscosity of the flowing portion of foam. Such interactions are simply not mirrored in bulk foam viscometry. 

Figure 9.14 a) Schematic cross section of viscometer, b) Fiber elongation method. 

There are several direct methods of measurement of yield stress. The constant stress rheometer is most frequently used to determine value in shear. Dzuy and Boger [1983, 1985] used a rotational vane viscometer. Yield stresses in compression can be calculated from the unrelaxed stress values in parallel plate geometry. Its value in elongation has been directly measured as the critical stress value below which no sample deformation was observed during 30 minutes of straining in an extensional rheometer. 

The most widely used viscometers are based on measurements of the rate of elongation of a fiber of known dimensions under a known load. This method can be used for viscosities ranging from 10 to 10 Pa s. This method is also used for the determination of the Littleton softening and annealing reference points. Since the method requires formation of a long fiber for a specimen, it is well suited for many easily-worked commercial glasses, but difficulties in the formation of good fibers from many experimental compositions often limit the use of this method for basic research studies. 

The determination of viscosity from the couple exerted on concentric cylinders is used in many viscometers. For silicate glasses, a wide range of techniques are used for measuring viscosity. For low viscosities (1 Pa s to 1 MPa s), a rotating spindle technique, based on the above principle, is often used. For higher viscosities, techniques based on fiber elongation, compression of a cylinder and be im-bending are utilized. 

It should, however, be noted that there exist some hints that bicontinuous microemulsions behave elastically. This has been assumed to be due to the differences observed in measuring viscosities once in a Couette flow and in the other case by a capillary viscometer. Here it was observed that the values obtained with the capillary viscometer are markedly higher. It has been suggested that in capillary flow a component of elongational flow is observed and that in this type of flow elastic components can be observed much earlier than in shear flow [106,107]. 

Mergules viscometer Fiber elongation Beam bending... 

FIGURE 21.5 Schematic illustration of instruments used to measure viscosity (a) a viscometer (b) the fiber elongation method. 

Tail . ( ) Highest boiling solvent fraction. (2) Elongated, somewhat pointed extension of the lower portion of the rising bubble in a bubble tube viscometer, characteristic of a varnish or resin solution that is near or approaching gelation or which has a peculiar rheological characteristic. 

The static spectrum can be prepared from the decay of stress a during time t of shear in shear viscometers or that of the tensile force / during time t of elongation. Because the spectrum expresses the change in the number of pseudo-links of various sizes at the relaxation time t, t is replaced by the time for deformation t in the static spectrum, and then... 

Although most viscometric and rheological studies are carried out in simple shear flows such as rotational viscometers, real flows experienced by real liquids are very often extensional (stretching or elongational) in nahire, and for some liquids there can be a very large difference between their shear and extensional viscosities. 

The rheology of K30-water solutions are characterized by measurements with rotational and capillary viscometers (Fig. 19.1 left). The measurements have been conducted by Schroder et al. [6]. The elongational deformation mechanisms of PVP-water solutions have been determined by a CaBER-device (Fig.19.1 right). 

Fig. 8 plots resistance factor vs. flux during injection of many polymer solutions into a 5120-md porous polyethylene core. For polymer concentrations even as low as 25 ppm (one-eighth the value of C ), shear thickening was evident. In contrast, viscosity vs. shear rate showed Newtonian behavior, with a value very close to 1 cp. As described in the literature (Durst et al. 1982), the elongational flow field associated with porous media can accentuate viscoelastic behavior that is not apparent from a pure shear field. This observation provides our first reason to doubt that the transition from Newtonian behavior to shear-thinning behavior in a viscometer correlates directly with the onset of shear thickening in porous media. 

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