Torsion wire

Coaxial (Concentric Cylinder) Viscometer, The eadiest and most common type of rotational viscometer is the coaxial or concentric cylinder instmment. It consists of two cylinders, one within the other (cup and bob), keeping the specimen between them, as shown in Figure 27. The first practical rotational viscometer consisted of a rotating cup with an inner cylinder supported by a torsion wire. In variations of this design the inner cylinder rotates. Instmments of both types ate useful for a variety of apphcations. 

The MacMichael viscometer is probably the most straightforward rotatioaal viscometer. The outer cup rotates and the inner cylinder is suspended from a torsion wire. The drag on the inner cylinder is measured as degree of twist on the wire. Wires of different stiffness are available, and the maximum viscosity is ca 10 mPa-s. The shear rate range is limited, ca 2-12, but with modification, higher shear rates can be attained. The iastmment is best... 

Experimentally, the method is capable of good precision. A so-called chainomatic balance has been used to determine the maximum pull, but a popular simplified version of the tensiometer, as it is sometimes called, makes use of a torsion wire and is quite compact. Among experimental details to mention are that the dry weight of the... 

The float-torsion wire assembly is the pressure-sensing system in the film balance. The flexible barriers that connect the float to the edges of the tray must be considered part of this mechanism. As with gas pressure determinations, it is essential that the system be leakproof. For this reason the float and flexible barriers must always be hydrophobic, that is, not wetted by the aqueous substrate. If these surfaces were wet by water, the possibility of surfactant transferring to them —that is, a leak in the system —would be enhanced. Thin pieces of mica... 

Surface pressures were measured by the conventional horizontal float-torsion wire assembly. A strain gage bridge, F (Statham G10 B), senses the surface pressure in the film balance from one arm of the torsion... 

For measurements on polymer melts, an apparatus of the concentric cylinder type can be used. The internal cylinder of such an apparatus is preferably suspended between two torsion wires. One of them is fixed with its lower end in the bottom of the unit, the other is twisted sinusoidally with the prescribed angular frequency at its upper end. Phase difference and ratio of amplitudes of the oscillations of the upper wire end and of the internal cylinder are measured. From these measurements the dynamic shear moduli, as defined above, can be deduced, when the inertia of the system is taken into account. Such an apparatus has been developed by Den Otter (26) at this Institute, making use of earlier experiences, as made by several other authors. (See e.g. ref. [27, 28 and 29).]... 

In principle, the shear modulus could be measured using test pieces strained in torsion and in engineering practice components, such as torsion discs and bushes, do operate in this mode. However, it is not common practice to test rubber in this manner except as a low temperature test (see Chapter 15) when a strip test piece is twisted by means of a torsion wire. The instrument traditionally used is not really accurate enough for precise measurement of modulus at room temperature but it would seem reasonable to suppose that an accurate instrument could be devised. 

Few details of the apparatus are given in ISO 4663, it is simply stated that means shall be provided to measure frequency to 1% ( 5% in a transition region), amplitude to 1% and, for method C, the supplied energy to 2%. It is suggested that a moment of inertia of about 0.03 gm is suitable for the inertia member which may be a disc or rod. For methods B and C the torsion wire should be of such dimensions that its restoring torque is not more than 25% of the total restoring torque due to the test piece and suspension. BS 903 (equivalent to method B of ISO 4663) suggests that moments of inertia between 50 and 500 g cm are suitable and states that the tensile strain on the test piece should be between 0 and 5%. The British... 

ISO 4663 gives no advice as to the relative merits of the three methods it specifies. Method C, which is not strictly a free vibration method, removes the difficulties associated with changing amplitude through the course of the test but at the expense of a rather more complex apparatus. When the inertia member is supported by a torsion wire, as in method B, the tensile strain in the test piece can be controlled to a low level by means of counterweights. 

It was of interest to compare the results obtained with the FRAP technique with those obtained with classical surface rheological techniques. Our detailed knowledge of properties of solutions of /3-lg containing Tween 20 made this an ideal system on which to compare the methods. Firstly, surface shear viscosity measurements were performed on the Tween 20//3-lg system [47] using a Couette-type torsion-wire surface rheometer as described previously [3,48]. All the experiments were carried out at a macroscopic n-tetradecane-water interface at a fixed protein concentration of O.Olmg/ml. In the absence of Tween 20, the surface shear... 

Class Transition (Tg) and Second-Order Transitions. Tg and viscoelastic responses of the elastomers and polyblends were measured using a torsion wire type apparatus consisting of a modification of the Gehman twist technique for elastomers and also using a duPont thermal analyzer. 

In this method the force required to detach a ring from a surface or interface is measured either by suspending the ring from the arm of a balance or by using a torsion-wire arrangement (du Noiiy tensiometer). 

The surface pressure of the film is determined by measuring the force which must be applied via a torsion wire to maintain the float at a fixed position on the surface (located optically) and dividing by the length of the float. For precise work, the surface balance is enclosed in an air thermostat and operated by remote control. With a good modern instrument, surface pressures can be measured with an accuracy of 0.01 mN m1. 

In the first of these techniques an approximation to uniform rate of shear throughout the sample is achieved by shearing a thin film of the liquid between concentric cylinders. The outer cylinder can be rotated (or oscillated) at a constant rate and the shear stress measured in terms of the deflection of the inner cylinder, which is suspended by a torsion wire (Figure 9.2) or the inner cylinder can be rotated (or oscillated) with the outer cylinder stationary and the resistance offered to the motor measured. 

The du Noiiy ring surface tension method involves slowly raising a platinum ring through a liquid until it detaches from the surface (Figure 3.8, Lower). The force at the point of detachment is measured using a balance or torsion wire. If F is the force on the ring,... 

Figure 3.19 Langmuir film (or surface) balance for the study of monolayers. The float (F) is attached to a torsion wire (T), that is used to measure the force on the float caused by the expanding surface film contained between it and the barrier (B). The area available for the film can be adjusted by repositioning the barrier. From Leja [91]. Copyright 1982, Plenum Press.

One satisfactory type of apparatus is illustrated diagrammatically in Mg. 7. There is a trough on three levelling screws (not shown), filled to the brim with water an instrument with two torsion wires for measuring... 

The surface pressure was balanced and measured, in the earlier instruments, by a simple bell crank balance in which weights were hung in a pan. Nowadays torsion wires are nearly always used. These instruments have either vertical or horizontal wires, and the position of the float is indicated by a mirror. As a rule the instruments are used as null instruments, i.e. the twist that must be applied to the torsion wire, to bring the float back to the zero position it had before any film was put on, is measured. 

Two very sensitive instruments have been constructed with vertical torsion wires. Marcelin s instrument2 has a vertical wire placed to one side of the trough if it is combined with Guastalla s vaselined threads to block the gaps at the ends of the float, it gives good results. 

In the Gallenkamp balance [64,65] the pan is placed below a sedimentation chamber with an open bottom and the whole assembly is placed in a second chamber filled with sedimentation liquid so that all the powder falls on to the pan. The weight settled is determined from the deflection of a torsion wire, and either, the run continues until all the powder has settled out of suspension, or a second experiment is carried out to determine the supernatant fraction. Problems arise during the charging operation with leakage into the clear water reservoir and particle adhesion to the premixing tube. 

In the Sartorius balance [66-68] the pan is suspended in the suspending liquid and a correction has to be applied for the particles which fall between the rim of the pan and the sedimentation vessel. In this instrument, when 2 mg of sediment has deposited, electronic circuitry activates a step-by-step motor which twists a torsion wire to bring the beam back to its original position. A pen records each step on a chart. The manufacturers suggest that about 8% of the powder does not settle on the pan. Leschonski [69] and Leschonski and Alex [70] reported losses of between 10% and 35%, depending on the fineness of the powder the difference was attributed to the pumping action of the pan as it re-balances. 

The force at the barrier may be measured directly by a calibrated torsion wire that is mechanically attached to the barrier. However, nowadays the surface tensions at both sides of the barrier are measured Independently using one of the appropriate methods in sec. 1.8, mostly by the static Wilhelmy plate technique. The latter method has the advantage that einy leakage of monolayer material across the barrier can be easUy detected. 

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