▪ Abstract Filament-stretching rheometers are devices for measuring the extensional viscosity of moderately viscous non-Newtonian fluids such as polymer solutions. In these devices, a cylindrical liquid bridge is initially formed between two circular end-plates. The plates are then moved apart in a prescribed manner such that the fluid sample is subjected to a strong extensional deformation. Asymptotic analysis and numerical computation show that the resulting kinematics closely approximate those of an ideal homogeneous uniaxial elongation. The evolution in the tensile stress (measured mechanically) and the molecular conformation (measured optically) can be followed as functions of the rate of stretching and the total strain imposed. The resulting rheological measurements are a sensitive discriminant of molecularly based constitutive equations proposed for complex fluids. The dynamical response of the elongating filament is also coupled to the extensional rheology of the polymeric test fluid, and this can...

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Filament-stretching rheometry of complex fluids

Activated sludge rheology: a critical review on data collection and modelling.

State-of-the-art of polymer tribology

The continuous stretching of viscous liqu ids to form fibers is a primary manufacturing process in the textile industry. The mechanics of certain sheet formation and sheet and wire coating operations are quite similar, as are the mechanics of the formation of glass fibers. Most published experimental and theoretical work has been motivated by applications using polymeric liquids, where the interactions between processing conditions and the complex fluid rheology are of particular interest. The melt spinning process for the manufacture of textile fibers is shown schematically in Figure 1. Molten polymer is extruded through a small hole into cross-flowing ambient air at a temperature below the solidifica­ tion temperature of the polymer. The solidified polymer is taken up at a higher speed than the mean extrusion velocity, resulting in drawing of the filament. The steady-state ratio of extrusion to takeup area is known as the draw ratio (DR); the draw ratio is also equal to the steady-state ratio of takeup to extrusion velocity (to within the approximation that polymer density is independent of temperature). Typical processing variables for the manufacture of poly (ethylene terephthalate) (PET) fiber are shown in Table 1. Commercial PET, a polyester sold under such trade names as Dacron, Terylene, and Fortrel, is a nearly amorphous material as spun, although PET with forty percent crystalline polymer can be produced. Other commercially important melt-spun polymers include Nylon-6, Nylon-66, and polypropylene. The solidified filament is typically subjected

Continuous drawing of liquids to form fibers

A model bearing is described which is 20.0 mm in diameter and 2.5 mm in length; a short bearing of diameter to length ratio eight. The clearance is large (500 μm) and the rotor may be run true or eccentric on its own shaft; in each case the mean load and frictional (tangential) force is measured as the centreline eccentricity is varied. Comparison is made between the lubricating performance of Newtonian and highly elastic liquids; the latter give load enhancement ratios of up to 300 and reductions in coefficient of friction by factors of the order 30. These effects are greatly in excess of those obtained when dealing with bearing of diameter to length ratio close to unity; possible reasons for this are discussed. A Newtonian oil and a polymer-thickened oil are tested in the same way, the latter oil is found to give load enhancement ratios of 1.4 (true rotor) and 3.5 (eccentric rotor) with corresponding reductions of coefficients of friction by factors of 1.5 (true rotor) and 3.0 (eccentric rotor). Such effects had not previously been observed when using oils in the internal cylinder geometry (journal bearing type) although somewhat similar effects have been found in the external cylinder and squeeze film geometries. The rheological properties of the polymer-thickened solutions are measured and the relevance of the results to friction and load bearing discussed.

Load enhancement effects due to polymer thickening in a short model journal bearing

Jet nozzles are described in which liquids are subjected to uniaxial and biaxial stretching motions, shear also being present near the nozzle walls. Jet thrust measurements show that the behaviour of polymer-thickened oils can best be studied by comparison with the behaviour of Newtonian base oils at equivalent Reynolds numbers. These data compare well with measurements previously made in long, straight tubes. Jet thrusts for polymer-thickened oils are lower than those of base oils and show evidence of a threshold point at which the two forms of behaviour deviate. The axial stress in the oil is almost independent of the strain rate, at high strain rates, but is related to axial strain in a simple manner. For 3.5 per cent polymer dissolved in Iranian light base oil, the axial stress is shown to be τ11 = 2000 γG dyn cm−2 for uniaxial strain and τ11 = 2800 γ1.30G dyn cm−2 for biaxial strain, where γG is the measure of strain defined by Green. For a standard Shell Multigrade 20W/50 motor oil the equivalent equation for biaxial strain is τ11 = 3200 γ1.02G dyn cm−2. The stresses developed during biaxial extension are higher than those developed during uniaxial extension. The presence of fully elastic behaviour in polymer-thickened oils at extensional strain rates of the order 500 s−1 and principal extension ratios of only 8 is of potential significance to lubrication technology. The stress rises with the square of the extension ratio and should become large as lubricated surfaces approach. Polymer degradation will commence as the stress rises; it is suggested that further tests be carried out at higher stresses and temperatures.

The flow of polymer thickened motor oils in convergent jet thrust nozzles

Drag reduction in exceptionally dilute polymer solutions

A modification to existing equipment is described which permits continuous squeeze-film flow to be obtained between parallel-sided strips of material rather than between disc-shaped surface. Squeeze film flow is simulated by having liquid move through one of the surfaces via an array of equispaced holes. The Squeeze-film behavior of a Newtonian base oil is first tested at temperatures of 24°C and 55°C. It is shwon that loads are in reasonable agreement with theretical predictions and that end effects (corrected by means of a guard ring) and fluid maldistribution effects are of small proporitons. At the very highest liquid flowrates, the rapid liquid flow through the holes may influence the measured load. The Polymer-thickened oils, representinhg 10 W/30 and 10 W/50 motor oils, are tested at temperatures of 24°C and 55°C. Both oils five marked load enhancement, compared with Newtonian oil under similar flow conditions, at the higher flowrates used. The 10 W/50 oil gives load enhancement of 76 per cent at 55°C, increasing rapidly with the simulated approach velocity. Fluid inertia effects in the squeeze film flow aslo increase the load significantly. The results confirm earlier data using disc-shaped surfaces; load enhancement is greater in the present work on strip squeeze films because the fluid deformation rates are greater (2000 s t-1 in planar extention and 2 x 10 5 s t-1 in shear). It is suggested that the fuel consumption of cars could be improved by the development of elastic, shear stable oils of lower viscosity than those currently in use.

Load enhancement effects by polymer-thickened oils in strip squeeze film flow

Apparatus is described which enables the normal stress-shear rate behaviour of a viscoelastic liquid to be obtained from observations of the thrust of a jet of the liquid issuing from a capillary tube (or slot). Two types of apparatus are mentioned. In one type the jet thrust is obtained from the impact force on a boat which the jet strikes and in the other the reaction force on the jet nozzle is measured.

D.R. Oliver

Papers

Load enhancement effects due to polymer thickening in a short model journal bearing

The flow of polymer thickened motor oils in convergent jet thrust nozzles

Drag reduction in exceptionally dilute polymer solutions

Load enhancement effects by polymer-thickened oils in strip squeeze film flow

Normal stress measurement by means of a jet thrust apparatus