ABSTRACT
We report on the smooth transition between gradient-banded velocity profiles with a sharp interface and curved velocity profiles, both resulting from strong shear-thinning dispersions of concentrated xanthan (a highly charged poly-saccharide). Pronounced shear-banded flow, where two extended shear-bands are separated by a relatively sharp interface, is observed in a limited range of shear rates, at very low ionic strength and at a high concentration, using heterodyne light scattering to measure spatially resolved velocity profiles. The width of the interface between the coexisting shear-bands broadens to span a sizable fraction of the gap of the shear cell, either by changing the shear rate, by lowering the concentration, or by increasing the ionic strength. The broadening results in a smooth transition to highly curved velocity profiles and is connected to a disappearing flow birefringence. Thus, these experiments show that the classic shear-banding instability can give rise to highly curved velocity profiles, due to the existence of broad interfaces between the bands, with an extent of the order or larger than the cell gap width. This observation may aid to resolve the ongoing dispute concerning shear-banding of highly entangled polymeric systems, suggesting that the curved velocity profiles that have been observed in the past are in fact shear-banded states with an unusually broad interface.
ABSTRACT
This corrects the article DOI: 10.1103/PhysRevE.95.032603.
ABSTRACT
The shear flow dynamics of linear and branched wormlike micellar systems based on cetylpyridinium chloride and sodium salicylate in brine solution is investigated through rheometric and scattering techniques. In particular, the flow and the structural flow response are explored via velocimetry measurements and rheological and rheometric small-angle neutron scattering (SANS) experiments, respectively. Although all micellar solutions display a similar shear thinning behavior in the nonlinear regime, the experimental results show that shear banding sets in only when the micelle contour length L[over ¯] is sufficiently long, independent of the nature of the micellar connections (either linear or branched micelles). Using rheometric SANS, we observe that the shear banding systems both show very similar orientational ordering as a function of Weissenberg number, while the short branched micelles manifest an unexpected increase of ordering at very low Weissenberg numbers. This suggests the presence of an additional flow-induced relaxation process that is peculiar for branched systems.
ABSTRACT
We report on a high pressure cell with six optical windows which can be used up to 2 kbars for laser light scattering applications at scattering angles of 45 degrees , 90 degrees , and 135 degrees of liquid samples in a temperature range between -20 and 150 degrees C. The pressure transmitting medium is compressed nitrogen. The window material used is SF57 NSK, a glass with an extremely low stress optical coefficient in the order of about 10(-5) which allows thus to maintain the plane of polarization even under the action of high pressure. In order to demonstrate the functioning of the cell we show Rayleigh-Brillouin spectra of poly(methylphenylsiloxane) at different polarizations and pressures.
ABSTRACT
The effect of intramolecular relaxations on the damping of longitudinal and transverse phonons was studied in poly(methylphenylsiloxane) (PMPS) and poly(ethylmethylsiloxane) (PEMS) polymers by means of Brillouin spectroscopy. It is shown that studies of the polarized and depolarized Brillouin spectra as functions of temperature and pressure allow for the separation of the contributions of the internal and structural relaxations to the damping of longitudinal and transverse phonons, respectively. In polymers with intramolecular relaxations these processes contribute not only to the damping of longitudinal phonons, according to theoretical predictions, but also transverse phonons, in contradiction to the theory.
ABSTRACT
Isotropic Brillouin spectra of the two chemically similar van der Waals glass forming liquids, 1,1(')-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) and 1,1(')-bis(p-methoxy-phenyl)cyclohexane (BMPC) and ortho-terphenyl (OTP), were studied in a broad temperature and pressure range in order to characterize the effect of internal relaxations on the damping of longitudinal phonons. Such relaxations are present in BMPC, while in BMMPC and OTP they are strongly hindered. The authors show that in BMPC (with strong internal relaxations) the damping (broadening) of longitudinal phonons (Brillouin peaks) is much stronger than in BMMPC and OTP (with weaker internal relaxations). The contributions of the internal and structural relaxations to the phonon damping can be separated using high pressure, due to their very different pressure dependences. They show that internal relaxations strongly contribute to the damping of longitudinal phonons at all temperatures and should be taken into account in theoretical models describing the Brillouin spectra of supercooled liquids.
ABSTRACT
In some polymers, in addition to the usual structural alpha relaxation, a slower alpha' relaxation is observed with a non-Arrhenius temperature dependence. In order to understand better the molecular origin of this alpha' relaxation in poly(methylphenylsiloxane) (PMPS) we have studied, for the first time, the pressure dependence of its relaxation time, together with the usual temperature dependence, by means of dynamic light scattering (DLS). For the same material the alpha relaxation was also studied by means of DLS and dielectric spectroscopy (DS) in broad temperature and pressure ranges. We find that the temperature dependence of both alpha and alpha' relaxation times, at all pressures studied, can be described by a double Vogel-Fulcher-Tammann (VFT) law. The pressure dependence of the characteristic temperatures Tg (glass transition temperature) and T0 (Vogel temperature) as well as the activation volumes for both alpha and alpha' processes are very similar, indicating, that both relaxation processes originate from similar local molecular dynamics. Additionally, for both alpha and alpha' relaxations the combined temperature and pressure dependences of the relaxation times can be described using a parameter Gamma=rhon/T with the same value of the exponent n.
Subject(s)
Computer Simulation , Glass/chemistry , Nanotechnology , Siloxanes/chemistry , Materials Testing , Molecular Conformation , Pressure , Scattering, Radiation , Spectrum Analysis , TemperatureABSTRACT
The propagation of sound waves in suspensions of hard sphere colloids is studied as a function of their volume fraction up to random close packing using Brillouin light scattering. The rich experimental phonon spectra of up to five phonon modes are successfully described by theoretical calculations based on the multiple scattering method. Two main types of phonon modes are revealed: Type A modes are acoustic excitations which set up deformations in both the solid (particles) and the liquid (solvent) phases; for type B modes the stress and strain are predominantly localized near the interface between the solid particles and the surrounding liquid (interface waves). While the former become harder (increase their effective sound velocity) as the particle volume fraction increases the latter become softer (the corresponding sound velocity decreases).
ABSTRACT
We study the slow dynamics of salol by varying both temperature and pressure using photon correlation spectroscopy and pressure-volume-temperature measurements, and compare the behavior of the structural relaxation time with equations derived within the Adam-Gibbs entropy theory and the Cohen-Grest free volume theory. We find that pressure-dependent data are crucial to assess the validity of these model equations. Our analysis supports the entropy-based equation, and estimates the configurational entropy of salol at ambient pressure approximately 70% of the excess entropy. Finally, we investigate the evolution of the shape of the structural relaxation process, and find that a time-temperature-pressure superposition principle holds over the range investigated.
ABSTRACT
The phonon propagation in lamellar nanostructures formed via self-assembling of short styrene-b-isoprene (SI) as well as of its more incompatible styrene-b-(ethylene-alt-propylene) (SEP) counterpart was studied by inelastic x-ray scattering. Irrespective of the physical state of the block copolymers, a single acoustic phonon was observed in SI (ordered and disordered) and SEP (ordered). At GHz frequencies, inelastic light scattering from the same samples revealed very small dispersion in the sound phase velocity but a short phonon lifetime.
