Fluids, nature viscoelastic

Viscoelasticity or Rheology The study of materials whose mechanical properties have characteristics of both solid and fluid materials. Viscoelasticity is a term often used by those whose primary interest is solid mechanics while rheology is a term often used by those whose primary interest is fluid mechanics. The term also implies that mechanical properties are a function of time due to the intrinsic nature of a material and that the material possesses a memory (fading) of past events. The latter separates such materials from those with time dependent properties due primarily to changing environments or corrosion. All polymers (fluid or solid) have time or temperature domains in which they are viscoelastic. 

Non-Newtonian fluids have both viscous and elastic properties, and they are called viscoelastic fluids. An example is so-called "silly putty," which is made from poly-dimethyl-siloxane (silicone). It flows like a liquid out of the container, but when it forms a ball, it behaves as elastic, i.e., it bounces back. The crucial factor determining the viscous and elastic behavior is the time period of the force applied short force pulse leads to elastic response, whereas long-lasting force causes flow. The viscoelasticity in polymers is due to shear-induced entanglements and nonlinear behavior of tire chains, coils. A well-known natural viscoelastic material is for, example, the egg white, which springs back when a shear force is released. A polymer resembles both liquid and solids. 

The Weissenbetg Rheogoniometer is well suited to research on homogeneous viscoelastic fluids and elastic melts. For oscillatory shear a second motor-drive mechanism is added. This allows the use of 60 frequencies in the range of 7.6 x 10 to 40 Hz at ampHtudes between 2 x 10 and 3 X 10 rad. An electronic circuit improves the precision of oscillatory measurements, particularly at frequencies neat the natural resonance frequency of the instmment itself (298). 

Figures 4 and 5 give a broad indication of the relevant biomechanical properties of a number of flow sensitive biomaterials. In the case of the data shown in Fig. 5, the surface mechanical properties are lumped into a single measure of the surface integrity. Admittedly, in view of what has been said in the introduction about the viscoelastic nature of the wall material, the information given in Figs. 4 and 5 are oversimplistic. The data in Fig. 5 are based on reported critical minimum stresses (often expressed in terms of the mean bulk fluid stresses) at which physical damage is first observed. Figure 6 gives an indication of the...

We can get a first approximation of the physical nature of a material from its response time. For a Maxwell element, the relaxation time is the time required for the stress in a stress-strain experiment to decay to 1/e or 0.37 of its initial value. A material with a low relaxation time flows easily so it shows relatively rapid stress decay. Thus, whether a viscoelastic material behaves as a solid or fluid is indicated by its response time and the experimental timescale or observation time. This observation was first made by Marcus Reiner who defined the ratio of the material response time to the experimental timescale as the Deborah Number, Dn-Presumably the name was derived by Reiner from the Biblical quote in Judges 5, Song of Deborah, where it says The mountains flowed before the Lord. ... 

This section summarizes results of the phenomenological theory of viscoelasticity as they apply to homogeneous polymer liquids. The theory of incompressible simple fluids (76, 77) is based on a very general set of ideas about the nature of mechanical response. According to this theory the flow-induced stress at any point in a substance at time t depends only on the deformations experienced by material in an arbitrarily small neighborhood of that point in all times prior to t. The relationship between stress at the current time and deformation history is the constitutive equation for the substance. 

The most straightforward rheological behaviour is exhibited on the one hand by Newtonian viscous fluids and on the other by Hookean elastic solids. However, most materials, particularly those of a colloidal nature, exhibit mechanical behaviour which is intermediate between these two extremes, with both viscous and elastic characteristics in evidence. Such materials are termed viscoelastic. 

The models described so far provide a qualitative illustration of the viscoelastic behaviour of polymers. In that respect the Maxwell element is the most suited to represent fluid polymers the permanent flow predominates on the longer term, while the short-term response is elastic. The Kelvin-Voigt element, with an added spring and, if necessary, a dashpot, is better suited to describe the nature of a solid polymer. With later analysis of the creep of polymers, we shall, therefore, meet the Kelvin-Voigt model again in more detailed descriptions of the fluid state the Maxwell model is being used. 

As the fluid is cooled, the relaxation times of the system become comparable with (Awi)"1, and the viscoelastic nature of polymer liquids must be taken into account. The longitudinal velocity becomes (13) ... 

Hyaluronic acid is found in most connective tissues including human vitreous, skin, synovial fluid and umbilical cord. One of its biological roles is its ability to contain large amounts of water in intercellular space. It can thus locate cells in a jelly-like matrix and resistance of tissue towards infection may partly depend on this property. The intercellular matrix has a viscoelastic nature [75-77] and is thought to have a role in absorbing shock. Hyaluronic acid can exist in the form of aggregates [81,82] or a network structure [78-80], Such a network offers a filter or macromolecular sieve-like resistance to other molecules as well as cells [83,84],... 

There are many cases in which other techniques have been applied to biphasic systems in order to establish the nature of mixing. For example, fluorescence microscopy of DPPC monolayers containing 2% of a fluorescent probe have shown the coexistence of solid and fluid phases of DPPC at intermediate pressures (Weis, 1991). Similar results have been achieved with a variety of other phospholipids using the same technique (Vaz et al., 1989). The recent application of laser light scattering to this area (Street et al., unpublished data) has yet to produce any conclusive evidence, but the future for this particular technique is also promising. It also provides information about the viscoelastic properties of the monolayer and how these are affected by the inclusion of penetration enhancers. 

Hyaluronic acid is a naturally occurring polysaccharide that is widely distributed in body tissues and intracellular fluids, including the aqueous and vitreous humour, synovial fluid, and in the ground substance that surrounds cells (1). It is a high-molecular weight substance originally developed for use as a vitreous replacement. Although 98% of the product consists of water, it is very viscoelastic. 

In the rheological structure of most food systems there is a viscous element present, and the deformation curves are often highly influenced by the rate of the imposed strain. This is due to the fact that the material relaxes (or flows) while tested under compression and the resultant deformation of this flow is dependent on the nature of the viscous element (Szczesniak, 1963 Peleg and Bagley, 1983). In the viscoelastic food systems, where during processing it is caused to oscillate sinusoidally, the strain curve may or may not be a sine wave. In cases when a periodic oscillatory strain is applied on a food system like fluid material, oscillating stress can be observed. The ideal elastic solid produces a shear stress wave in phase with... 

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