Rheology and birefringence of Fomblin YR at very high shear rates

The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress, and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 10⁴ s⁻¹, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level.     

Molecular bases of stress relaxation and viscosity of polymers, 1a) New empirical relations

Experiments with several polymers led to new relations which describe stress relaxation at small shears and show the influence of the molecular structure on relaxation. Bubble-free melts of anionically polymerized polystyrenes with the molecular masses (relative molecular mass is referred to as molecular mass throughout this paper)M=6 10² to 1.8 10⁶ g mol^–1 and their blends are studied at several temperatures and at timest>3 ms. As the stress (10² to 10⁶ Pa) deforms the rheometer, a new mathematical method was developed to correct the relaxations. The energy-elastic stress is separated from the entropy-elastic stress. The relaxations of the latter do not confirm the Rouse theory as is proportional to exp (–¦t ^1/2¦) forM<4000. For M>4000 the relaxations deviate from the theoretical functions as well. The initial modulus G(0) then depends onM, and is not proportional to ¦t ^–1/2¦. A new and simple function describes the relaxation of melts with entangled molecules at long times. The influences of concentration and chemical structure on relaxation are formulated for weak intermolecular forces. Published data for the constantD of self-diffusion are used in calculating values ofD for a constant free volume making use of the initial velocity of relaxation and its dependence on temperature. ThenDM ^–1 holds forM<1.3 10⁴ andDM ^–2 for largerM. Correct values of viscosity are calculated from the empirical functions of stress relaxation.Part 2 cf.In memoriam Professor Dr. Drs. h.c. Otto Bayer     

Das rheologische Verhalten von Polymerlösungen: Strömungsverfestigung in Scher- und Dehnströmungen

Zusammenfassung Um das Dehnverhalten auch relativ niedrigviskoser viskoelastischer Fluide praktisch ohne den Einfluß von Vorbeanspruchungen messen zu können, wird ein neuartiges Dehnrheometer eingesetzt. Das Meßverfahren stellt eine Weiterentwicklung der Methode des rohrlosen Hebers (tubeless-syphon) dar, bei dem durch eine rechnergesteuerte Abzugswalze das Material unter definierten Bedingungen entgegen der Wirkung der Schwerkraft nach oben gezogen wird.Die hiermit untersuchten konzentrierten und mäßig verdünnten Polyisobutylen-Lösungen zeigen bei Dehnbeanspruchung ausgeprägte Verfestigungserscheinungen. So werden unter materiell instationären Bedingungen momentane Werte der Dehnviskosität gemessen, die im Falle von konzentrierten Lösungen mehr als 200mal und bei verdünnten sogar mehr als 2000mal größer sind als das Dreifache der Scherviskosität bei verschwindender Beanspruchung. Dabei handelt es sich noch nicht einmal um Gleichgewichtswerte, die Spannungen steigen vielmehr noch genähert exponentiell mit der Dehnung an.Auch im Scherfeld zeigen diese Lösungen Spannungsanomalien, die als Scherverfestigung bezeichnet werden können. Im Falle der verdünnten Lösungen führt das Einsetzen dieser Strömungsverfestigung bei noch unveränderter Schubspannung zunächst zu einer zeitabhängigen Erhöhung der ersten Normalspannungsdifferenz, die aber noch gegen einen stationären Endwert strebt. Eine weitere Steigerung der Schergeschwindigkeit führt dann allerdings zu einem überproportionalen Anwachsen sowohl von Normal- als auch von Schubspannungen, jedoch können sich dann, wie auch bei den konzentrierten Lösungen, keine Gleichgewichtszustände mehr ausbilden.Eine vergleichende Betrachtung des Relaxationsverhaltens in beiden Strömungsformen liefert Hinweise darauf, daß die bei der Dehnbeanspruchung beobachteten Verfestigungseffekte zumindest im Falle der verdünnten Polymerlösungen auf molekulare Assoziationsvorgänge zurückzuführen sind.

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A new extensional rheometer is used in order to measure the extensional (elongational) behaviour of low viscosity viscoelastic liquids with only a very small influence of previous deformations. The measuring technique represents a further development of the tubeless-syphon, in which a computer-controlled roller draws the material upwards against gravitational forces in a well defined manner.The concentrated and moderately diluted polyisobutylene solutions investigated here show a pronounced extension hardening in extensional flows. Under transient conditions apparent values of extensional viscosities were measured, which for concentrated solutions were more than 200 times larger than three times the zero-shear-rate viscosity, and for dilute solutions even more than 2000 times larger. These large values occurred before equilibrium conditions were reached, the stresses at this point increasing almost exponentially with the deformation.These solutions also exhibit stress anomalies in shear flows, which can analogously be described as shear hardening. In case of the dilute solutions the beginning of the shear hardening produces a time-dependent increase to a constant value for the first normal-stress difference, while the shear stress immediately reaches its constant value. A further increase in the shear rate then leads to an over-proportional growth of the normal as well as the shear stresses, however, as for concentrated solutions, an equilibrium state is no longer reached.A comparison of the relaxation properties in both types of flow suggests that the hardening effect in extensional flows can be attributed to molecular aggregation, at least for the dilute polymer solutions.

Erster Teil einer von der Abteilung Chemietechnik der Universität Dortmund unter dem Titel Der Einfluß der Strömungsform auf das Fließverhalten viskoelastischer Fluide genehmigten Dissertation; auszugsweise vorgetragen auf den Jahrestagungen der Deutschen Rheologischen Gesellschaft in Aachen vom 5.–7. März 1979 und in Berlin vom 11.–13. Mai 1981.     

Rheo-optic flow-induced crystallisation of polypropylene and polyethylene within confined entry–exit flow geometries

This paper reports experimental observations on the way polypropylene (PP) and high-density polyethylene (HDPE) can crystallise during flow. Both a deep and a shallow slit geometry were chosen for the rheo-optical study. Preliminary linear viscoelastic rheological tests enabled the temperature window for quiescent crystallisation to be established. Flow-induced crystallisation (FIC) studies were performed in a temperature regime above the normal quiescent crystallisation conditions. In the case of HDPE, FIC occurred during flow at the sidewalls of the slit and in localised regions downstream and the processing pressure increased during the piston movement. In the case of PP, flow-induced crystallisation was generally observed after flow cessation, and the processing pressure did not change during flow. For PP, FIC also occurred preferentially at the walls in the form of elongated crystallites, but the fibres gradually emerged after flow cessation. The difference in the FIC behaviour was attributed to differences in both the rheology and the crystal growth kinetics of the two materials at the particular super-cooling used.     

On the viscosity of magnetic fluid with low and moderate solid fraction

The design of a pressurized capillary rheometer operating at prescribed temperature is described to measure the viscosity of magnetic fluids （MFs） containing Fe3O4 magnetic nanoparticles （MNPs）. The equipment constant of the rheometer was obtained using liquids with predetermined viscosities. Experimentally measured viscosities were used to evaluate different equations for suspension viscosities. Deviation of measured suspension viscosities from the Einstein equation was found to be basically due to the influence of spatial distribution and aggregation of Fe3O4 MNPs. By taking account of the coating layer on MNPs and the aggregation of MNPs in MFs, a modified Einstein equation was proposed to fit the experimental data. Moreover, the influence of external magnetic field on viscosity was also taken into account. Viscosities thus predicted are in good agreement with experimental data. Temperature effect on suspension viscosity was shown experimentally to be due to the shear-thinning behavior of the MFs.     

Surface tension driven jet break up of strain-hardening polymer solutions

Experimental studies attempting to ascertain the influence of viscoelasticity on the atomization of polymer solution are often hindered by the inability to decouple the effect of shear thinning from the effect of extensional hardening. Here, the influence of viscoelasticity on the jet break up of a series of non-shear-thinning viscoelastic fluids is quantified. Previous characterization using an opposed-nozzle rheometer identified the critical extensional rates for strain hardening of these model fluids. The strain hardening fluids exhibit a beads-on-string structure with reduction or elimination of satellite drops. Capillary instabilities grow on the filaments connecting the spheres and eventually break the filaments up into a string of very small drops about one order of magnitude smaller than the satellite drops formed by a Newtonian fluid with the same shear viscosity, surface tension, and density. These results confirm that strain hardening is the key rheological property in jet break up and that the critical extensional rate of a fluid is pertinent in determining the final characteristics of break up. Results suggest that the opposed-nozzle rheometer does probe extensional behavior in the range of extensional rates that are relevant to jet break up, providing a tool to roughly predict jet break up.     

Investigation of thermal conductivity and rheological properties of vegetable oil based hybrid nanofluids containing Cu–Zn hybrid nanoparticles

Vegetable oils (Ground nut) are being investigated to serve as a possible substitute for non-biodegradable mineral oils, which are currently being used as metal-cutting fluids in machining processes. In this study, thermophysical properties of hybrid nanofluids (vegetable oil) to be used as metalworking cutting fluids are investigated. In-situ synthesis of copper (Cu) and zinc (Zn) combined hybrid particles is performed by mechanical alloying with compositions of 50:50, 75:25, and 25:75 by weight. Characterizations of the synthesized powder were carried out using X-ray diffraction, a particle size analyzer, FE-SEM, and TEM. Hybrid nanofluids with all the three combinations of hybrid nanoparticles were prepared by dispersing them into a base fluid (vegetable oil). The thermophysical properties, such as thermal conductivity and viscosity, were studied for various volume concentrations and at a range of temperatures. Experimental results have shown enhancement in thermal conductivity in all cases and also an increase in viscosity. The enhancement in viscosity is similar in all three combinations, as the particle size and shape are almost identical. The enhancement in thermal conductivity is higher in Cu–Zn (50:50), resulting in better enhancement in thermal conductivity due to the Brownian motion of the particles and higher thermal conductivity of the nanoparticles incorporated.     

A melt rheometer with integrated small angle light scattering

A stress-controlled commercial rheometer has been equipped with small-angle light scattering (SALS) instrument working in a range of wave-vectors between 0.5 and 4.2 m^–1 By a specially designed optical system the scattered light is focused directly on the square chip of a CCD-detector with high dynamical range allowing quantitative two-dimensional (2 D) intensity measurements of polarized and depolarized scattering. The rheo SALS system is suited for simultaneous fast measurements of 2D SALS and rheological material functions of polymer melts and liquid crystals. As a first application results on the influence of shear rate on the late stage of spinodal decomposition of a polybutadiene/polyisoprene blend are reported. A second application concerns a creep experiment in the liquid crystalline phase of a lyotropic non-ionic surfactant with simultaneous measurement of the time-dependent compliance and associated changes of the 2D intensity patterns of the polarized and depolarized scattering.     

The rheology of suspensions of vinylon fibers in polymer liquids. I. Suspensions in silicone oil

Summary The steady shear flow properties of suspensions of vinylon fibers in silicone oil were measured by means of a cone-plate type rheometer. Three kinds of vinylon fibers used had no distributions of length and were more flexible than glass fibers and the like. The content of the fibers ranged from 0 to 7 wt.%. Shear viscosity, the first normal-stress difference, yield stress, and relative viscosity were discussed. Shear viscosity and relative viscosity increased with the fiber concentration and the aspect ratio, and depended upon the shear rate. The applicability of Ziegel's equation of viscosity for fiber suspensions was investigated. The first normal-stress difference increased with the fiber concentration, aspect ratio, and shear rate and its relative increase was much larger than for shear stress and viscosity depending on the properties of the characteristic time, The yield stress could be determined by Casson plots for large aspect ratio fiber suspensions even in low concentration comparing with the suspensions of spherical particles or powder. The influence of the flexibility of the fibers for the rheological properties of the fiber suspensions can not be ignored.With 12 figures and 2 tables     

Unsteady rheometry: can we characterize weak gels with a controlled stress rheometer?

This paper is concerned with the influence of apparatus inertia effects in controlled stress rheometry. As evidenced on creep experiments, the coexistence of apparatus inertia and viscoelasticity leads to a coupling frequency. For weak gels, this coupling frequency is typically between 1 and 100 Hz. Therefore, frequency sweeps around and above this coupling frequency also corresponds to an effective shear stress sweep evolution due to a non-trivial resonant effect. In other words, frequency sweep experiments are not made at constant shear stress. The detailed modelling and analysis of this inertia effect on a typical weak gel shows a clear and fundamental limitation for its characterization using a controlled stress rheometer. Also, alternative approaches to standard rheometer software analysis are proposed to take this coupling effect into account. This paper was presented at the 3rd Annual Rheology Conference, AERC 2006, April 27-29, 2006, Crete, Greece.