Liquid elasticity, measures

There are four popular measures of liquid elasticity the first normal stress difference, the extrudate swell, B, the Bagley entrance-exit pressure drop correction, P, and the storage shear modulus, G. For multiphase systems there Is no simple correlation between and G, although the Sprigg s theoretical relation (51) ... 

The fourth measure of liquid elasticity, P, reflects the extenslonal properties rather than elasticity. For polymer blends it is difficult to determine P with a sufficient degree of accuracy. The data Indicate that this parameter Is most sensitive to morphological changes. For example, the degree of droplet coalescence In the Instrument reservoir drastically changes the values of P. ... 

Where E is the elastic, or Young s, modulus with units of N m" or Pa. Such measurements are normally carried out in tension or bending, when the sample is a soft material or a liquid then measurements are normally carried out in shear mode, thus a shear modulus (G) is measured. The two moduli are related to one another by ... 

Dilational elasticity gives the surface tension variation of a liquid surface with respect to the unit fraction area change, and it is a measure of the ability of the surface to adjust its surface tension to an instantaneous stress. As the dilational elasticity measures the ability of the surface to develop surface tension gradient, this property characterizes the film and foam stability. The elasticity of the film is proportional to the dilational modulus from which the film was formed. Films from solutions with higher elasticity have higher... 

Later, the liquid state measuring membrane was changed by an elastic plastic disk. This made the electrode form mechanically more stable. In the early times, well working potassium [16] and ammonium [17] ISEs were made with a silicon rubber matrix. Later, however, plasticized polyvinyl chloride (PVC) matrices became the most popular in ISE fabrication. Their preparation procedure was well worked out and clearly described by Shatkay [18] and Thomas [19], The high molecular weight PVC used in membrane preparation can be dissolved in tetrahydrofurane. In this solution, the plasticizer, the selective ionophore, and other ingredients like special lipophilic salt are dissolved. The solution is transferred into a ring-shaped flat bottomed mold. Care is taken to let the tetrahydro furane (THE) slowly evaporate. In a few days, a circular, flexible membrane is obtained. A disk with appropriate diameter is cut from the membrane and pasted on the tubular electrode body. 

All else being equal, an increase in flow-rate for an elastic liquid, or an increase in the level of elasticity (measured as, say, the first normal-stress difference) produces the effect on the flow pattern in a contraction shown in figure 30. The complex flow pattern shown on the right of the figure is very susceptible to flow instabilities. The onset of these instabilities dictates the maximum flow-rate possible for an extrudate emerging from the end of the die having a smooth, acceptable surface. (See chapter 17 for a discussion of the role of extensional viscosity in this kind of flow.)... 

Pressure defined as force per unit area is usually expressed in terms of familiar units of weight-force and area or the height of a column of hq-uid that produces a like pressure at its base. Process pressuremeasuring devices may be divided into three groups (1) those that are based on the measurement of the height of a liquid column, (2) those that are based on the measurement of the distortion of an elastic pressure chamber, and (3) electrical sensing devices. 

Neutron diffraction is one of the most widely used techniques for the study of liquid structure. In the experiment, neutrons are elastically scattered off the nuclei in the sample and are detected at different scattering angles, typically 3° to 40°, for the purpose of measuring intermolecular structure whilst minimizing inelasticity corrections. The resultant scattering profile is then analyzed to provide structural information. 

By measuring V z), which includes examining the reflectance function of solid material, measuring the phase velocity and attenuation of leaky surface acoustic waves at the liquid-specimen boundary, the SAM can be used indetermining the elastic constants of the material. 

The ratio (p/G) has the units of time and is known as the elastic time constant, te, of the material. Little information exists in the published literature on the rheomechanical parameters, p, and G for biomaterials. An exception is red blood cells for which the shear modulus of elasticity and viscosity have been measured by using micro-pipette techniques 166,68,70,72]. The shear modulus of elasticity data is usually given in units of N m and is sometimes compared with the interfacial tension of liquids. However, these properties are not the same. Interfacial tension originates from an imbalance of surface forces whereas the shear modulus of elasticity is an interaction force closely related to the slope of the force-distance plot (Fig. 3). Typical reported values of the shear modulus of elasticity and viscosity of red blood cells are 6 x 10 N m and 10 Pa s respectively 1701. Red blood cells typically have a mean length scale of the order of 7 pm, thus G is of the order of 10 N m and the elastic time constant (p/G) is of the order of 10 s. 

Non-linear viscoelastic flow phenomena are one of the most characteristic features of polymeric liquids. A matter of very emphasised interest is the first normal stress difference. It is a well-accepted fact that the first normal stress difference Nj is similar to G, a measure of the amount of energy which can be stored reversibly in a viscoelastic fluid, whereas t12 is considered as the portion that is dissipated as viscous flow [49-51]. For concentrated solutions Lodge s theory [52] of an elastic network also predicts normal stresses, which should be associated with the entanglement density. 

In summary, we have commented briefly on the microscopic applications of NMR velocity imaging in complex polymer flows in complex geometries, where these applications have been termed Rheo-NMR [23]. As some of these complex geometries can be easily established in small scales, NMR velocimetry and visc-ometry at microscopic resolution can provide an effective means to image the entire Eulerian velocity field experimentally and to measure extensional properties in elastic liquids non-invasively. 

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

In the elastic floe model, the structural units (which persist at high shear rates) are assumed to be small floes of particles (called floccules) which are characterized by the extent to which the particle structure is able to trap some of the dispersion medium. The degree to which liquid is trapped in the floe is measured by the floe volume ratio, CFF, given by,... 

Measurements suggest that the pressure loss for laminar flow of power law fluids through a sudden contraction is not significantly different from that for Newtonian flow [Skelland (1967)]. This statement applies to inelastic power law fluids in the case of elastic liquids, very high contraction pressure losses occur as discussed in Section 3.10. 

---