Viscous behaviour

A fluid of density 1,2. x I03 kg/m3 flows down an inclined plane at 15 to the horizontal. If the viscous behaviour is described by the relationship ... 

There are various test methods, one being the De Mattia Flex Test method which is suitable for rubbers that have reasonably stable stress-strain properties, at least after a period of cycling, and do not show undue stress softening or set, or highly viscous behaviour. The results obtained for some thermoplastic rubber should be treated with caution if the elongation at break is below,... 

The final main category of non-Newtonian behaviour is viscoelasticity. As the name implies, viscoelastic fluids exhibit a combination of ordinary liquid-like (viscous) and solid-like (elastic) behaviour. The most important viscoelastic fluids are molten polymers but other materials containing macromolecules or long flexible particles, such as fibre suspensions, are viscoelastic. An everyday example of purely viscous and viscoelastic behaviour can be seen with different types of soup. When a thin , watery soup is stirred in a bowl and the stirring then stopped, the soup continues to flow round the bowl and gradually comes to rest. This is an example of purely viscous behaviour. In contrast, with certain thick soups, on cessation of stirring the soup rapidly slows down and then recoils slightly. 

As noted in Chapter 1, purely viscous behaviour corresponds to A = 0, while purely elastic behaviour is approached as A — It will now be... 

Thus purely viscous behaviour is approached as Aw 0 and purely elastic behaviour as Ao> — o°. 

The fluid s relaxation time A is the characteristic time of the fluid and, for oscillatory shearing, cu 1 can be taken as a measure of the characteristic time of the flow process, so De = A to. Thus, viscous behaviour occurs when the Deborah number is low, reflecting the fact that the fluid is able to relax. When the Deborah number is high, elastic behaviour is observed because the fluid is unable to relax sufficiently quickly. 

This is strong evidence for assuming that dispersions of ideal hard spheres would be expected to show a transition in the viscous behaviour between

short time selfdiffusion coefficient Z)s. This still shows a significant value after the order-disorder transition. The problem faced by the rheologist in interpreting hard sphere systems is that at high concentrations there is... 

It is obvious that this model is not sufficiently general, since it does not describe the well-known non-Newtonian viscous behaviour of fluid polymer systems. However, this fact is considered as less important for the moment. It appears that present knowledge of shear and normal stresses is insufficient even at very low shear rates. The purpose of the present report is mainly to contribute to this matter. 

According to the theory of linear elastico-viscous behaviour (47) the steady-state shear viscosity t] and the steady-state shear compliance Je depend in the following way on the shear relaxation modulus G (t), where t is here the time of the relaxation experiment ... 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

In many processes based on extrusion the material is subjected to further manipulation after leaving the die—as examples, by stretching or casting on chill rolls in the manufacture of film. In all such cases it is essential that an extrudate withstand the forces applied to it and not tear—in other words, while there should be some strength and elasticity the main requirement is that the molecules of which it is comprised can flow relative to each other (in this sense its viscous behaviour is the most important feature). 

As long as stresses and rates of shear are proportional we Ccui speak of linear viscous behaviour. We shall not consider situations and systems where linearity is not satisfied. For the elastic equivalent, see [3.6.6], linearity is also supposed to apply. Fluids obeying linear viscous behaviour are called Newton fluids. Only one material parameter, the viscosity 77, is needed to define their rheological behaviour. 

Plaizier-Vercammen JA. Viscous behaviour of laponite XLG, a synthetic hectorite and its use in pharmaceutical dispersions. [In Dutch]. Farmaceutisch Tijdschrift voor Belgie 1994 71(4—5) 2—9. 

Rheology is concerned with the flow and/or deformation of matter under the influence of externally imposed mechanical forces. Two limiting types of behaviour arc possible. The deformation may reverse spontaneously (relax) when the external force is removed this is called elastic behaviour and is exhibited by rigid solids. The energy used in causing the deformation is stored, and then recovered when the solid relaxes. At the other extreme, matter flows and the flow ceases (but is not reversed) when the force is removed this is called viscous behaviour and is characteristic of simple liquids. The energy needed to maintain the flow is dissipated as heat. Between the two extremes arc systems whose response to an applied force depends on the lime-scale involved. Thus pitch behaves as an elastic solid if struck but flows if left for years on a slope. Similarly, a ball of Funny Putty , a form of silicone rubber, bounces when dropped on a hard surface, when the contact time is a few milliseconds, but flows if deformed slowly on a time-scale of seconds or minutes. Systems of this kind are said to be visco-elastic. The precise nature of the observable phenomena depends on the ratio of the time it takes for the system to relax to the time taken to make an observation. This ratio is called the Deborah number (De) ... 

Newtonian fluids are characterised by pure linear viscous behaviour. When a load is applied they display a linear change in shear over time, and there is a linear relationship between shear rate and stress, i.e. dynamic viscosity is independent of shear rate. When the load is removed, the shear remains completely preserved. 

Concrete has a viscous behaviour when it is loaded with a constant stress it shows a strain that increases with time. Conventionally an elastic deformation is considered when it occurs during application of the load, while subsequent deformation is attributed to creep. It is possible to define a modulus of elasticity for concrete that can be evaluated with short-term tests [9]. Similarly as for the tensile strength, empirical formulae are available that give an approximate correlation of the modulus of elasticity with the compressive strength [1,9]. A dynamic modulus can also be estimated with non-destructive tests that measure the rate of propagation of ultrasonic vibrations through concrete [1],... 

A further observation which applies to the parallel plate situation is that the linear velocity profile applies regardless of the rheological behaviour of the material and thus this idealized mixing behaviour is independent of rheology. Of course, in situations when the velocity field does vary with material rheology the viscous properties of the liquids become relevant. For example in the coaxial cylinder the velocity profile will depend upon the viscous behaviour of the fluid and the results in Figure 11.3 are only valid for Newtonian liquids. 

Brockleburst.B., and Young, R.N., 1994 FluoteaoeDoe anisotropy decays and viscous behaviour of 2-nwthylteinhydrofliran, /. Chem. Soe., Faraday Trans. 90i271-278. 

S. A. Dunn, Viscous Behaviour of Silica with Tungsten Inclusions, Ceram. Bull. 47, 554-559 (1968). 

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