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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 14 (1975), S. 721-728 
    ISSN: 1435-1528
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Zusammenfassung Es wurde das lineare und nicht-lineare Verhalten thixotroper Dispersionen in viskoelastischen Medien experimentell untersucht. Die Systeme bestehen aus Rutil(TiO2)-Teilchen, die in einer 85%igen Lösung von polymerisiertem Leinöl in (monomerem) Leinöl dispergiert sind. Die Volumenkonzentration der dispersen Phase variiert von null bis 23.8%. Bei stationären Fließbedingungen wurden die Gleichgewichts-Viskosität und die erste Normalspannungsdifferenz aufgenommen. Die Viskositätswerte erstrecken sich vomnewtonschen bis zum Potenz-Gesetz-Bereich. Es wurden relative Eigenschaften berechnet, die entweder auf gleiche Schergeschwindigkeiten oder gleiche Schubspannungen bezogen sind. Die Ergebnisse werden im Hinblick auf hydrodynamische und physikochemische Wechselwirkungen diskutiert und mit den vorliegenden Daten anderer viskoelastischer Suspensionen verglichen. Der Beitrag der Thixotropie wird abgetrennt und seine viskoelastische Natur aufgezeigt. An den gescherten Proben werden Schwingungsmessungen durchgeführt, und ihre Ergebnisse werden mit dem Verhalten bei stationären Bedingungen sowohl im linearen als auch im nichtlinearen Konzentrationsbereich verglichen.
    Notes: Summary The linear and nonlinear behaviour of thixotropic dispersions in viscoelastic media has been measured. The systems are composed of rutile TiO2 particles dispersed in a 85% solution of polymerized linseed oil in linseed oil. The volume concentration of the dispersed phase varies between 0 and 23.8%. Under stationary flow the equilibrium viscosities and the first normal stress differences have been recorded. The viscosity data extend from theNewtonian to the power law region. Relative properties have been calculated based upon equal shear rates and also based upon equal shear stresses. The results are discussed in the light of hydrodynamic and physicochemical interactions. A comparison is made with available data on viscoelastic suspensions. The contribution of thixotropy is isolated and its viscoelastic nature is demonstrated. Oscillatory measurements are performed on the sheared samples and the results are compared with the behaviour under stationary flow in the linear as well as in the nonlinear concentration range.
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  • 2
    ISSN: 1435-1528
    Keywords: Key words Immiscible blends ; stress relaxation ; Rayleigh instabilities ; blend morphology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Relaxation has been investigated in immiscible blends that consist of slightly viscoelastic components. Both the shear and normal stresses have been measured after cessation of steady shear flow as well as after transient shear histories. The latter can generate a fibrillar structure which can relax by either retraction or break-up via end-pinching or Rayleigh instabilities. Each of these three relaxation mechanisms is reflected in the shape of the stress curves, from which also the corresponding structural time scales can be deduced. The experimental results have been used to evaluate the Doi-Ohta and Lee-Park models for immiscible blends. The scaling relations by Doi-Ohta are confirmed by the experimental results, but none of the existing models can correctly predict the complex relaxation behaviour observed for a highly deformed droplet phase. In the present study an alternative approach has been proposed. The stress relaxation due to fibril break-up via Rayleigh instabilities has been predicted successfully by combining physical models for the structural changes with the basic approach of the Doi-Ohta model.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 8 (1969), S. 519-525 
    ISSN: 1435-1528
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Zusammenfassung Das rheologische Verhalten von Pigmentdispersionen in viskoelastischen Flüssigkeiten wird mittels dynamischer Messungen im kHz-Bereich untersucht. Die Ergebnisse werden mit stationären Viskositätswerten verglichen. Auf dieser Basis wird die allgemeine Form des Viskositätsspektrums bei Pigmentdispersionen diskutiert. Die Resultate werden auf die spezielle Strömung beim Durchgang durch ein rotierendes Walzenpaar angewandt. Die Mitnahmekraft wird als Funktion der Geschwindigkeit gemessen. Einige theoretische Näherungen werden diskutiert. Es wird vorgeschlagen, als erste Näherung eine nicht-Newtonsche, nichtelastische Flüssigkeit anzunehmen. Als spezielles Beispiel wird dasOstwald-de Waelsche Gesetz benutzt. Aus den experimentellen Ergebnissen läßt sich eine Beziehung zwischen Mitnahmekraft und relativer dynamischer Viskosität ableiten. In einer loglog-Darstellung ergibt sich, wie theoretisch vorausgesagt, eine Gerade. Dennoch zeigen sich Unterschiede zwischen Theorie und Versuch. In diesem Fall würde ein allgemeineres Modell einer nichtelastischen Flüssigkeit zu einer besseren Näherung führen.
    Notes: Summary The rheological behaviour of pigment dispersions in a viscoelastic liquid have been investigated by dynamic measurements in the kHz range. The results are compared with stationary viscosities. On this basis the general shape of the viscosity spectrum for pigment dispersions is discussed. The results have further been applied to a particular flow problem: passage between rotating rollers. The drag force or “tack” has been measured as a function of speed. Some theoretical approaches are discussed. As a first approximation, it is proposed that a non-Newtonian non-elastic liquid must be assumed. The power law model has been used as an exemple. A correlation has been found experimentally between the drag force and the relative dynamic viscosity. In a double logarithmic plot a straight line is obtained as predicted from a simple power law theory. However, some discrepancies between theory and experiment still exist. A more general non-elastic liquid model would give a better approximation in this case.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 11 (1972), S. 203-209 
    ISSN: 1435-1528
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Summary The rheological behaviour of a thixotropic system has been investigated in terms of the instantaneous structure. The system consists of magnesium silicate fibres, dispersed in a Newtonian. alkyd resin. The application of low frequency oscillatory measurements in the analysis of such materials is discussed. As the characteristic time for thixotropic recovery proved to be several orders of magnitude larger than the average relaxation time, the viscoelastic response of the instantaneous structures can be measured under vibrations of a suitable frequency. Using this technique changes in the relaxation spectrum due to stationary shearing are measured. Equilibrium structures under stationary and oscillatory flow are compared and the similarity principle between both types of measurements is tested for thixotropic systems. The rate of change of thixotropic recovery under rest and under vibrations is considered.
    Notes: Zusammenfassung Das rheologische Verhalten eines thixotropen Systems wurde als Funktion der momentanen Struktur untersucht. Das System besteht aus Magnesiumsilikat-Fasern, in ein Newtonsches Alkydharz dispergiert. Die Anwendung niedrigfrequenter Schwingungsmessungen in der Analyse solcher Materialien wird diskutiert. Es wird gefunden, daß die charakteristische Zeit für den thixotropen Strukturaufbau um Größenordnungen über dem mittleren Wert der viskoelastischen Relaxationszeit liegt. Es ist daher möglich, das elastoviskose Verhalten der momentanen Strukturen mit Schwingungsmessungen zu erfassen. Mittels dieses Verfahrens wurden Änderungen in dem Relaxationsspektrum, hervorgerufen durch stationäre Strömung, gemessen. Gleichgewichtsstrukturen in stationärer und schwingender Strömung wurden bestimmt, und die Analogie zwischen beiden wurde in dem Falle thixotroper Systeme nachgeprüft. Die Geschwindigkeit des Strukturaufbaus in Ruhe und unter Schwingungen wird betrachtet.
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  • 5
    ISSN: 1435-1528
    Keywords: Liquid crystalline polymers ; hydroxypropylcellulose ; dynamic mechanical properties ; lyotropic polymer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Some unusual rheological features of a liquid crystalline solution of hydroxypropylcellulose (HPC) in water have been investigated. Measurements have been performed by using a variety of different apparatuses with cone and plate geometries. Particular attention has been devoted to the experimental procedures, including the use of different sealing techniques, which are necessary to avoid solvent evaporation during the very long transients. Shear fracture effects, and their dependence on the type of sealing agents have also been studied. In steady shear, the HPC solution shows some rheological features which are common to other lyotropic systems, such as a three-region viscosity curve, and a double sign change in the first normal stress difference vs shear rate curve. The structural changes which take place after cessation of shear flow have been investigated by following the evolution of the dynamic moduli as a function of the time elapsed after the shear flow is stopped. It was found that the rate of the previously applied shear strongly affects both the kinetics and the asymptotic, long time values of the dynamic properties. Possible explanations for such behavior in terms of microstructure evolution are presented and discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 33 (1994), S. 165-174 
    ISSN: 1435-1528
    Keywords: Colloidal suspensions ; bimodal dispersion ; viscosity ; dynamic moduli ; steric stabilization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract In order to investigate the effect of the particle size distribution on the rheological properties of concentrated colloidal dispersions both steady-state shear and oscillatory measurements have been performed on well-characterized bimodal dispersions of sterically stabilized PMMA particles. Replacing a minor amount of large particles by small ones in a concentrated dispersion, keeping the total effective volume fraction constant, decreases the viscosity quite drastically. On the other hand, replacing a small amount of small particles by big ones hardly effects the viscosity at all. This behavior can be attributed to the deformability of the stabilizing polymer layer. A procedure is proposed to calculate the limiting viscosities in a bimodal colloidal dispersion starting from the characteristics of the monodisperse systems. A good agreement has been obtained between the calculated values and the experimental results. The linear viscoelastic properties of the concentrated dispersions have been investigated by means of oscillatory measurements. The plateau values of the storage modulus for the bimodal dispersions decrease with an increasing fraction of the coarse particles. By substituting the bimodal dispersion by an equivalent monodisperse system the storage modulus can be superimposed on the values for the monodisperse suspensions when plotted as a function of the mean interparticle distance.
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  • 7
    ISSN: 1435-1528
    Keywords: Key words Liquid crystalline polymers ; Larson-Doi model ; Flow- induced texture ; Transient recoil
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The mesoscopic models for the rheological properties of liquid crystalline polymers proposed by Larson and Doi in 1991 and Kawaguchi and Denn in 1999 are based on phenomenological expressions that describe the evolution of the defect density and the contribution of the “texture” to the stress. In the present work, we attempt to assess some of these assumptions by monitoring how the energy stored in the texture of liquid crystalline materials evolves during shear flows. For that purpose, strain recovery is measured as a function of the applied strain for flow reversal and intermittent flow. Solutions of poly-benzylglutamate in m-cresol, hydroxypropylcellulose in water and a nematic surfactant solution are used as model systems. Although the behaviour is described qualitatively by the model, discrepancies between the predictions and the experiments are observed, especially when the shear history includes rest periods.
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  • 8
    ISSN: 1435-1528
    Keywords: Key words Immiscible blends ; Flow-induced morphology ; Elastic recovery ; Blend rheology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Due to the interfacial tension, immiscible blends can show an elastic recovery that is substantially larger than that of their pure components. Here it is attempted to relate the elastic recovery after steady shear flow to the underlying morphology. On the one hand, the predictions of the Palierne and the Doi-Ohta models are calculated for the flow conditions during recoil. On the other hand, systematic recoil experiments after steady state shearing have been performed on a model blend. As the component polymers hardly show any recoil under the stresses applied in these tests, the measured recovery can be attributed completely to the action of the interface. Comparison of the model predictions with the experimental results shows that the recoverable strain can be derived quantitatively from the linear Palierne theory. Although the droplet deformation remained limited during the preshear, the retardation time predicted by this model has to be multiplied by the aspect ratio of the droplet phase to the power 2/3 to describe the experiments. For conditions in which the material does not show an intrinsic length scale, particular scaling relations as derived from the Doi-Ohta theory are found to apply also to recoil.
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  • 9
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 253 (1975), S. 618-619 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] In mechanics, thixotropy is another typical example. It describes materials whose resistance to flow is lowered when subjected to mechanical agitation. In such a system the shear stress, T, will normally not be in phase with a shear rate, y, that is varied periodically. Hence a i-y plot will show a ...
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