Viscoelastic liquids

The difference between solids and liquids is found in the magnitude of D. Liquids, which relax in small fractions of a second, have small D. Solids have a large D. A sufficient lime span can reduce the Deborah number of a solid to unity, and impact loading can increase D of a liquid. Viscoelastic materials are best characterized under conditions in which D lies within a few decades of unity. 

It should be noted that Eq. (9,10) is similar for solid and liquid viscoelastic... 

Illustrative curves showing these inequalities for liquid viscoelastic systems are represented in Figures 9.2 and 9.3. 

Perfectly elastic solid Perfectly viscos liquid Viscoelastic system... 

Values of kLa in bubble columns decrease with increasing values of liquid viscoelasticity. In viscoelastic liquids, relatively large bubbles mingle with a large number of very fine bubbles less than 1 mm in diameter, whereas most bubbles in water are more uniform in size. As very fine bubble contribute less to mass transfer, the kLa values in viscoelastic liquids are smaller than in inelastic liquids. 

LIQUID VISCOELASTICITY UNDER RARE-GAS INFUSION. KNOLLMAN G C HAMAMOTO A S MILES D 0 J CHEM PHYS... 

Figure 25 shows that the viscoelastic behaviour strongly depends on nanotube dispersion. When the nanotubes are supposed to be well dispersed in an isotropic configuration, the variation of the complex shear modulus (under 5 % strain) shows a rheological behaviour close to the viscoelastic behaviour of the PDMS matrix, i.e. a liquid viscoelastic behaviour. After the aggregation process, the viscoelastic behaviour of the composite shows a solid-like behaviour, at least in the frequency window used in the present study, due to the formation of the CNT network from dynamic CNT aggregation. 

Because of their complex structure the mechanical behavior of polymeric materials is not well described by the classical constitutive equations Hooke s law (for elastic solids) or Newton s law (for viscous liquids). Polymeric materials are said to be viscoelastic inasmuch as they exhibit both viscous and elastic responses. This viscoelastic behavior has played a key role in the development of the understanding of polymer structure. Viscoelasticity is also important in the understanding of various measuring devices needed for rheometric measurements. In the fluid dynamics of polymeric liquids, viscoelasticity also plays a crucial role. " Also in the polymer-processing industry it is necessary to include the role of viscoelastic behavior in careful analysis and design. Finally there are important connections between viscoelasticity and flow birefringence. ... 

Polymers owe much of their attractiveness to their ease of processing. In many important teclmiques, such as injection moulding, fibre spinning and film fonnation, polymers are processed in the melt, so that their flow behaviour is of paramount importance. Because of the viscoelastic properties of polymers, their flow behaviour is much more complex than that of Newtonian liquids for which the viscosity is the only essential parameter. In polymer melts, the recoverable shear compliance, which relates to the elastic forces, is used in addition to the viscosity in the description of flow [48]. 

Gent, A.N. and Schultz, J., Effect of wetting liquids on the strength of adhesion of viscoelastic materials. J. Adhes., 3, 281-294 (1972). 

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

It seems that results rather close in their meaning should be obtained, provided that monodisperse polymers are taken as dispersion media. Investigations into dynamic (viscoelastic) properties of such liquids with highly active filler have shown that [6],... 

Though experimental data on suspensions of fibers in Newtonian dispersion media give more or less regular picture, a transition to non-Newtonian viscoelastic liquids, as Metzner noted [21], makes the whole picture far or less clear. Probably, the possibility to make somewhat general conclusions on a longitudinal flow of suspensions in polymer melts requires first of all establishing clear rules of behavior of pure melts at uniaxial extension this problem by itself has no solution as yet. 

Bubbles are formed instantaneously. This conclusion made in [33] is based on estimates taken from earlier works [37]. As seen from the above cited works by S. E. Sosin et al., this is not always true viscoelastic liquids under triaxial stretching stress are not destroyed instantly. The existence of an induction period may produce a considerable effect on foam growth kinetics upon free foaming, when pressure is lowered instantaneously from P > Pcr to P < Pcr in a melt with dissolved gas. However, it would appear that microfaults in polymer melts, which are caused by factors... 

Rheology is the science that deals with the deformation and flow of matter under various conditions. The rheology of plastics, particularly of TPs, is complex but understandable and manageable. These materials exhibit properties that combine those of an ideal viscous liquid (with pure shear deformations) with those of an ideal elastic solid (with pure elastic deformation). Thus, plastics are said to be viscoelastic. 

Many of the new plastics, blends, and material systems require special, enhanced processing features or techniques to be successfully injection molded. The associated materials evolution has resulted in new plastics or grades, many of which are more viscoelastic. That is, they exhibit greater melt elasticity. The advanced molding technology has started to address the coupling of viscoelastic material responses with the process parameters. This requires an understanding of plastics as viscoelastic fluids, rather than as purely viscous liquids, as is commonly held... 

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