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  • 1
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The elastic constantsc 11 andc 44, and the attenuation of longitudinal phonons along (100)-direction in (NaCN)1−x(KCN)x mixed crystals have been determined by brillouin spectroscopy in the temperature range fromT=10K−300K. Minima inc 44 (T) which determine the freezing tempratureT F in these orientational glasses appear at lower temperatures compared to neutron scattering results. The coupling of molecular reorientations to longitudinal phonons in NaCN-rich smaples shows up in dispersion effects inc 11 (T) and the appearence of a dynamical central peak nearT F .
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  • 2
    ISSN: 1434-6036
    Keywords: PACS. 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures - 83.50.Gd Nonlinear viscoelasticity - 64.70.-p Specific phase transitions - 82.70.-y Disperse systems
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We have investigated the simple shear flow behavior of wormlike micelles using small-angle neutron scattering and mechanical measurements. Ternary surfactant solutions made of cetylpyridinium chloride, hexanol and brine (0.2 M NaCl) and hereafter abbreviated as CPCl-Hex were studied in the concentrated regime, . In a preliminary report (Berret et al. [#!ref16!#]), the discontinuity of slope observed in the shear stress versus shear rate curve was interpreted in terms of first-order phase transition between an isotropic state and a shear-induced nematic state ( transition). At the transition rate, , the solution exhibits a macroscopic phase separation into viscous and fluid layers (inhomogeneous shear flow). Above a second characteristic shear rate, the flow becomes homogeneous again, the sheared solution being nematic only. The neutron patterns obtained in the two-state inhomogeneous region have been re-examined. Based on a consistent analysis of both orientational and translational degrees of freedom related to the wormlike micelles, we emphasize new features for the transition. In the present paper, the shear rate variations of the relative proportions of each phase in the two-state region, as well as the viscosity ratio between isotropic and nematic phases are derived. We demonstrate in addition that slightly above the transition rate, the shear induced nematic phase is already strongly oriented, with an order parameter P 2 = 0.65. The orientational state is that of a nematic flow-oriented monodomain. Finally, from the locations of the neutron scattering maxima for each isotropic and nematic contributions, we evaluate the concentrations for each phase and and derived a dynamical phase diagram of CPCl-Hex, in terms of the stress versus and . According to the classification by Schmitt et al. [#!ref22!#], the transition observed in CPCl-Hex micellar solutions could result from a positive flow-concentration coupling, in agreement with the observed monotonically increasing shear stress in the two-phase region.
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  • 3
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We evaluated already published data on ultrasound absorption and velocity changes below 1 K for a set of 18 different glasses. By uniformly analysing these data according to the tunnelling model we have calculated the standard spectral density $$\bar P$$ of two-level-systems (TLS) and their coupling energyγ 1 andγ 1 to longitudinal and transversal phonons. Contrary to literature, we have not observed any correlation between $$\bar P$$ and the glass transition temperatureT g for our set of 18 glasses. As far asT g is concerned we only found a relation τ min 0.1K withn≃2.5 and where τmin is the shortest TLS-phonon relaxation time at 0.1 K. Since the values for $$\bar P$$ andγ 1,t scatter within half an order of magnitude, we conclude that the TLS parameters of a certain glass are only weakly dependent on bulk properties. We find that the ratioγ t 2 /C 44 (whereC 44 is the shear elastic constant) is a more sensitive parameter for tests on the variation of the TLS spectral density of glasses.
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  • 4
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Dielectric measurements have been performed at 1 and 10 GHz in (KBr)1−x (KCN) x mixed crystals for cyanide concentrationsx=0.25 andx=0.50 in the temperature rangeT=1.7–150 K. The maxima of the real and imaginary parts of the dielectric constant falls into the Arrhenius straight lines obtained at lower frequencies. These dipolar lines cross over the so-called quadrupolar (acoustic) lines, in contradiction with theoretical predictions on the nature of the freezing. We also present a qualitative model which reinterprets the dipolar and quadrupolar freezings in terms of an individual and a collective processes.
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  • 5
    ISSN: 1434-6036
    Keywords: PACS. 83.70.Hq Heterogeneous liquids: suspensions, dispersions, emulsions, pastes, slurries, foams, block copolymers, etc. - 82.70.Dd Colloids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We study the behavior under flow of soft spherical micelles forming a fcc phase at high volume fraction. Due to the size (300 Å) of the elementary objects, the structure can be investigated through X-rays and neutron scattering, at rest and under flow in a Couette cell. Using scattering in two perpendicular directions, different mechanisms of flow are identified. At intermediate shear (around 100 s-1) the system exhibits the so called layer sliding mechanism where two dimensional hexagonal compact planes of spheres align themselves with the Couette cell walls. At lower shear rate, the fcc structure is locally preserved, and the flow is mediated by defects between crystallites. At high shear rate, we observe the melting of the structure and a liquid-like structure factor. Moreover, we were able to use the existence of the layer sliding regime to generate a fcc monocrystal by annealing the satcking faults between the decorrelated planes created by the layer sliding.
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  • 6
    ISSN: 1292-895X
    Keywords: PACS. 83.50.By Transient deformation and flow; time-dependent properties: start-up, stress relaxation, creep, recovery, etc. – 83.50.Qm Thixotropy; thickening flows – 83.85.Ei Optical methods; sonography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. We report on the shear-thickening transition observed in dilute aqueous solutions of cetyltrimethylammonium tosylate (CTAT) at concentrations . We have re-examined the kinetics of the shear-thickening transition using start-up experiments at rates above the critical shear rate . Using simple well-defined protocols, we have found that the transient mechanical response depends dramatically on the thermal and on the shear histories. Using the same protocols, flow birefringence experiments were carried out. The gap of a Couette cell containing the sheared solution has been visualized between crossed polarizers in steady shear conditions, as well as in start-up experiments. We show that the birefringent shear-induced phase starts from the inner cylinder and grows along the velocity gradient direction, as in a shear banding situation. However, around we have not observed a regime of phase coexistence (isotropic and birefringent).
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4896-4903 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using Raman scattering experiments we have investigated the rotational and translational modes in K1−x(NH4)xI mixed crystals for ammonium concentrations x=0.005, 0.07, 0.16, and x=0.46 and in a fully deuterated isomorph with x(D)=0.42. The A1g+4Eg and T2g Raman responses have been measured at atmospheric pressure in the paraelectric plastic phase (T=295 K) and in the orientational glassy phase (T=7 K). In the low-frequency region ν=5–600 cm−1, seven Raman active components could be identified. These bands are entirely due to the admixture of anisotropic NH+4 to pure KI. Their contributions result either from reorientations of the molecules between equivalent directions and rotational-like excitations around these preferential orientations, or from translational modes of the whole Brillouin zone. In this peculiar case, orientational disorder of the ammonium lifts wave vector conservation of the scattering processes. An interpretation of the different Raman allowed transitions is proposed. The composition and polarization dependences as well as the effect of deuteration for these modes are discussed with respect to earlier reports.
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