Extensional flow techniques

L. R. Schmidt, A Special Mold and Tracer Technique for Studying Shear and Extensional Flows in a Mold Cavity during Injection Molding, Polym. Eng. Sci., 14, 797 (1974). 

In addition, stagnation point (it is a point at which the net magnitude of velocity is zero, such as at the midpoint of a cylinder or a sphere in cross flow) devices, such as the dough roll mill, cross slot flow, and opposing jet devices can be used in which an area of intense extensional flow is created without the need to sustain a continuous filament. The latter technique is illustrated in Figure 3-27. Clark (1997) obtained extensional viscosity data on syrups and food gums dispersions using creation of... 

One technique for measurement of extensional flow that has been used to study various doughs is that of Cogswell (1972, 1978) for entrance flows. The analysis is based on several assumptions (Padmanabhan and Bhattacharya, 1993) (1) The flow is isothermal and creeping (negligible inertial effects), (2) the fluid is incompressible and has a pressure-independent viscosity, (3) the shear viscosity follows the power law model, t]a = Ky" (4) there is no slip at the edge of the converging profile, and (5) that the entrance pressure drop (Ape) in converging flow from a circular barrel in to a circular die can be considered to be made up of that due to shear (Ape,s) and extensional flow (Ape,E) ... 

For evaluation of DUEVs, with less shear viscosity contribution to the total response, the modified stagnation technique is the procedure of preference. For practical applications the vacuum-suction filament-draw technique is probably the more valuable because the deformation rates in many applications are not solely extensional in nature. The comments regarding the velocity gradients in the tubeless siphon (41) are appropriate to the fiber-suction approach flow in a tubeless siphon approximates extensional flow in a sense that the largest components of the velocity gradient tensors are diagonal ones. ... 

Where a fluid is not spinnable the various orifice flow techniques, which involve pressure drop measinements across a contraction [Binding, 1988, 1993], can provide a means of estimating the extensional-viscosity behaviour of shear-thinning polymer solutions. 

In practice the technique is beset by experimental difficulties, such as the need to measine extremely small pressures. In addition, the difficulty of maintaining adequately lubricated flows requires the provision of large sample volumes. The application of converging flows for determining the extensional flow behaviom of polymer melts has been reviewed [Rides and Chakravorty, 1997]. 

In this chapter, we describe different techniques of generating extensional flows and also of measuring the stresses and rates of strain. Some of these methods give fundamental quantitative information about polymer behavior while others yield only qualitative or overall results. Yet other stretching techniques require the use of a rheological model for data interpretation. Available results are summarized, and brief comments are made regarding the validity and use of constitutive equations. 

New approaches and techniques have been proposed to characterize peculiar aspects of L.C. phases or amphiphilic-based systems. The director reorientation of a side-chain liquid crystalline polymer was observed under extensional flow using a 4-roll mill placed in the magnet of a NMR spectrometer. A steric obstruction model was proposed to predict charge-induced molecular... 

In Chapter 4 it was explained that the linear elastic behavior of molten polymers has a strong and detailed dependency on molecular structure. In this chapter, we will review what is known about how molecular structure affects linear viscoelastic properties such as the zero-shear viscosity, the steady-state compliance, and the storage and loss moduli. For linear polymers, linear properties are a rich source of information about molecular structure, rivaling more elaborate techniques such as GPC and NMR. Experiments in the linear regime can also provide information about long-chain branching but are insufficient by themselves and must be supplemented by nonlinear properties, particularly those describing the response to an extensional flow. The experimental techniques and material functions of nonlinear viscoelasticity are described in Chapter 10. 

We have seen that rheometers capable of accurate measiuements of extensional flow properties are limited to use at low Hencky strain rates, usually well below 10 s . In order to reach higher strain rates, the drawdown of an extruded filament ( melt spinning ) and the converging flow into an orifice die or capillary have been used to determine an apparent extensional viscosity . Since the stress and strain are not imiform in these flows, it is necessary to model the flow in order to interpret data in terms of material functions or constants. And such a simulation must incorporate a rheological model for the melt under study, but if a reliable rheological model were available, the experiment would not be necessary. This is the basic problem with techniques in which the kinematics is neither controlled nor known with precision. It is necessary to make a rather drastically simplified flow analysis to interpret the data in terms of some approximate material function. 

A.J. Frank, "Elongational Testing of Polymer Melts Using Uniaxial Extensional Flow and Lubricated Squeezing Flow Techniques," Conf. Engineering Rheol., London, 1983. 

Machine processing exposes dispersions not only to shear but also to tensile stresses (extensional flow). Because of the lack of commercially available test equipment, studies of the extensional flow of polymer dispersions are stUl in their infancy. Little use is also made of viscoelastic techniques where the sample is subjected to low-amplitude oscillatory shear and the amplitude and phase of the oscillating stress is measured (usually as a function of the frequency of the oscillation). 

---