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  • 1
    ISSN: 1434-3916
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A new technique has been developed to position the cup rigidly into the acetabulum during total hip joint replacement. After positioning the flanged cup on the acetabulum by means of screws, a low-viscosity cement is injected through a hole in the rim of the cup. A comparison with the standard cementing technique was made on the basis of in vitro experiments. For this purpose a mold was constructed to produce castings simulating the human acetabulum. The distribution of the cement, the pressure developed in the acetabulum, and the penetration of the cement are compared for the two cementing techniques.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic response of liquid crystalline polymers at high shear rates has been studied by measuring shear and normal stresses during transients following a step in the shear rate. Stepwise increase and decrease as well as flow reversals have been investigated. A solution of poly(benzylglutamate) (PBG) in m-cresol is used in the experiments. One purpose of the work is to perform a comparison with the theoretical predictions that apply to the case of spatially uniform situations. The latter are expected to prevail at high shear rates, when the tumbling character of the nematic phase is effectively suppressed. The results indicate profound differences with respect to previous experiments at lower shear rates. A somewhat surprising result is that the behavior at high shear rates appears to be dominated by the viscous, rather than the elastic stress.
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  • 3
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1435-1528
    Keywords: Key words Polymeric liquid crystals ; anisotropy ; fillers ; string phases ; negative normal stresses
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The effect of fillers on the flow curves of polymeric liquid crystals is investigated. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose (HPC) in water are used. By reducing the HPC concentration an isotropic solution can be prepared. It serves as a reference to isolate the effect of the isotropic/anisotropic structure of the suspending medium on the rheological behaviour. Suspensions in the isotropic solution behave as expected for filled viscoelastic matrices in general. In the anisotropic medium the shear rate rather than the shear stress seems to govern the changes in the relative viscosity. This behaviour is clearly different from isotropic viscoelastic media. The most dramatic effect however is that even small amounts of particles eliminate or drastically shift the region of negative normal stress differences. As far as the structure is concerned, microscopic observations show that particles align in anisotropic as well as in isotropic media. At rest or at relatively low shear stresses the liquid crystalline structure is, in the present case, hardly affected by the presence of the particles. If anything, it becomes more homogeneous.
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  • 6
    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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  • 7
    ISSN: 1435-1528
    Keywords: Phase behavior ; rheo-optics ; second normal stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Mechanical and optical rheometric measurements are reported on solutions of polystyrene dissolved in dioctyl phthalate, a solution that can undergo an apparent phase separation upon the application of shear. Solutions prepared using three molecular weights ranging from one to four million were studied. Time-temperature superposition was observed to apply for these solutions up to and including the onset of an apparent shear thickening of the steady shear and first normal stresses. Optical measurements employing turbidity and scattering dichroism determined that concentration fluctuations were enhanced by flow and grew parallel to the vorticity axis below the critical velocity gradient for the onset of the apparent shear thickening effect. Prior to the onset of thickening, the fluctuations were observed to rearrange and orient parallel to the flow direction. Second normal stress difference measurements indicate these solutions have a negative ratio of the second to the first normal stress differences. It is interesting to point out that the ratio tends to zero in the vicinity of the shear rate range at which shear thickening occurs.
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  • 8
    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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  • 9
    ISSN: 1435-1528
    Keywords: Mechanical rheometry ; polarimetry ; polystyrene ; segmental orientation ; flow-induced phase separation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Experiments combining mechanical rheometry with polarimetry (birefringence and scattering dichroism) have been conducted on a 6% solution of polystyrene (1.86x106 molecular weight) in dioctyl phthalate. Birefringence is used to measure the extent of segmental orientation, whereas the dichroism is sensitive to orientation and deformation of concentration fluctuations associated with the process of flow-induced phase separation. The results indicate that these fluctuations grow predominately along the neutral (or vorticity axis) of a simple shear flow. At higher rates of shear, orientation in the flow direction is favored. The transition in orientation direction is accompanied by time-dependent behavior in the optical properties of the solution during shear and the onset of shear thickening of the viscosity and the first normal stress difference coefficient.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 33 (1994), S. 317-321 
    ISSN: 1435-1528
    Keywords: Rheo-optics ; birefringence ; multiple-order retardation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract An improved procedure is proposed to solve the problem of measuring strong birefringence on flowing samples which have a retardation δ larger than π/2 The procedure can be applied to devices which are based on the modulation of the polarization vector with a high-speed rotating half-wave plate. It uses two optical configurations which have different responses to the retardation. By combining the information obtained from the two configurations, one can easily and accurately determine the actual retardation of the sample, irrespective of the quadrant in which the measured retardation is located, provided the sample is isotropic. This technique can also overcome the problem of large errors occurring in the vicinity of δ = (2m+1)π/2 due to the limited sensitivity around these values of δ. The use of the technique is illustrated with measurements on an isotropic solution of poly(benzyl glutamate) in m-cresol, which is strongly birefringent during shear flow.
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