Oscillation viscometer

An oscillation viscometer employs a vibrating surface in contact with a fluid of interest (Figure 6.8) [365]. If such a surface is made to vibrate at its natural fre-... 

Figure 6.9 A submersible oscillation viscometer for in situ viscosity measurements in a continuous process vessel. From Schramm [92]. Copyright 1989, The Petroleum Society.

Oscillation viscometers Oscillating viscosity tester Newtonian... 

Figure 6.8 An oscillation viscometer s vibrating surface in contact with a fluid. (Adapted from Reference [26]. Copyright (1972), reproduced with permission Nametre Co.)...

The surface viscosity varies significantly along the isotherm and across monolayer phase boundaries. Addition of subphase metal ions increases the surface viscosity drastically, as was recently reinvestigated [36]. Recently, microscopy methods have been used to image velocity profiles of different monolayer phases flowing through a narrow channel, such as used in the canal viscometer [37], The two main methods used to study monolayer viscosity are the canal viscometer and the oscillating disc method [8,9]. 

A body of a cylindrical or spherical shape is suspended in a melt and oscillating rotational motion is fed to it. A schematic drawing of a viscometer is shown in Fig. 23. This initial oscillation is gradually attenuated by the viscosity resistance. The viscosity is obtained as an absolute value from the logarithmic decrement of the swings of the pendulum s oscillation. Since the sample melt can be completely closed in this method, this is the best method for a melt of high temperature. 

Torklep and 0ye measured the viscosity of NaCl by this method, which has been adopted as the standard (see Section 1.2). The accuracy of the working equation is better than 0.1% for the cylinder geometry, hquid densities, and liquid viscosities used by these authors. They claim that the remarkable theoretical analysis of oscillating-body viscometers given by Kestin and Newell supersedes all former, less accurate theories. All the results were once suspect owing to possible solid impurities in the liquid. 

Figure 23. Schematic drawing of oscillating cylinder viscometer. (Reprinted from Ref 124. Crown Copyright 1992. Reproduced by permission ofthe Controller of Her Majesty s Stationary Office.)...

The logarithmic decrement of the oscillations of a pendulum consisting of a crucible containing a test liquid is measured by the oscillating cup method. A schematic drawing of the oscillating cup (vessel) viscometer is shown in Fig. 24. 

Rheological measurements, 21 702-758 techniques in, 21 739-747 viscometers, 21 725-739 Rheological oscillation experiments,... 

Physics Viscometer, quartz-crystal oscillator Viscosity... 

The Weissenbeig Rheogoniometer (49) is a complex dynamic viscometer that can measure elastic behavior as well as viscosity. It was the first rheometer designed to measure both shear and normal stresses and can be used for complete characterization of viscoelastic materials. Its capabilities include measurement of steady-state rotational shear within a viscosity range of 10-1 —13 mPa-s at shear rates of 10-4 — 104 s-1, of normal forces (elastic effect) exhibited by the material being sheared, and of an oscillatory shear range of 5 x 10-6 to 50 Hz, from which the elastic modulus and dynamic viscosity can be determined. A unique feature is its ability to superimpose oscillation on steady shear to provide dynamic measurements under flow conditions all measurements can be made over a wide range of temperatures (—50 to 400°C). 

In rotational viscometers thixotropy shows up as a progressive decrease in torque as time increases (until the infinite-time properties are reached) at constant rotational speed. Care must be taken, however, not to confuse the initial oscillation of a viscometer cup or bob, which is due to inertial forces, with thixotropy. Conversely, rheopectic behavior shows progressively increasing torques at constant rotational speeds. 

Before leaving this discussion, it is important to note that other forms of Peclet numbers are also possible and may be more appropriate depending on the type of convective influence studied. For example, in the case of oscillatory flows (as in oscillatory viscometers), it is more useful to define the Peclet number as (Rfa/D), where co is the frequency of oscillation. Regardless of the particular definition, the general significance of the Peclet number remains the same, i.e., it compares the effect of convection relative to diffusion. 

If a fully compounded thermosetting rubber is subjected to a plasticity measurement at a high enough temperature and for long enough, it will cure and, consequently, there is not always a clear distinction between a plasticity test and a test for scorch or rate of cure. For example, the Mooney viscometer is used to measure scorch, i.e. the onset of vulcanisation, and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure as well as the increase in stiffness as curing takes place. 

With rubbers a similar situation is met, but now with the aid of a rotation viscometer. The Mooney viscosity is measured as the torque needed to rotate two parallel plates, between which the rubber mass is present, with respect to each other. This provides a rough indication of the viscosity, and thus of the molar mass. This measurement can also be used to characterize the vulcanization behaviour under vulcanization conditions the increase of the Mooney viscosity indicates the onset of network formation. When the network develops further, a continuous rotation can, of course, no longer be applied because the viscosity increases unlimitedly therefore an oscillation method is mostly used with a cone-and-plate geometry. Initially, the viscosity is being measured, and later on the build-up of the E-modulus of the network. Another characterization of the viscosity of unvulcanized rubbers is the Hoekstra method. The rubber is present between two parallel plates, which are moved towards each other with a certain speed the force needed to do so is an indication of the viscosity. 

Sieben, A., Cellulase and other hydrolytic enzyme assays using an oscillating tube viscometer. Anal Biochem 1975, 63,(1), 214-9. 

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