Shear Stress Measurements of Non-Spherical Particles in High Shear Rate Flows

The behavior of liquid-solid flows varies greatly depending on fluid viscosity; particle and liquid inertia; and collisions and near-collisions between particles. Shear stress measurements were made in a coaxial rheometer with a height to gap ratio (b/r0) of 11.7 and gap to outer radius ratio (h/b) of 0.166 that was specially designed to minimize the effects of secondary flows. Experiments were performed for a range of Reynolds numbers, solid fractions and ratio of particle to fluid densities. With neutrally buoyant particles, the dimensional shear stress exhibits a linear dependence on Reynolds number: the slope is monotonic but a non-linear function of the solid fraction. Though non-neutrally buoyant particles exhibit a similar linear dependence at higher Reynolds numbers, at lower values the shear stress exhibits a non-linear behavior in which the stress increases with decreasing Reynolds number due to particle settling

Effective Action and Schwinger Pair Production in Scalar QED

Some astrophysical objects are supposed to have very strong electromagnetic fields above the critical strength. Quantum fluctuations due to strong electromagnetic fields modify the Maxwell theory and particularly electric fields make the vacuum unstable against pair production of charged particles. We study the strong field effect such as the effective action and the Schwinger pair production in scalar QED.Comment: RevTex 6 pages, no figure; Proceedings of APCTP Winter School on Black Hole Astrophysics 2008, Jan 24-29, 200

Theory of Thermodynamic Stresses in Colloidal Dispersions at the Glass Transition

We discuss the nonlinear rheology of dense colloidal dispersions at the glass transition. A first principles approach starting with interacting Brownian particles in given arbitrary homogeneous (incompressible) flow neglecting hydrodynamic interactions is sketched. It e.g. explains steady state flow curves for finite shear rates measured in dense suspensions of thermosensitive core-shell particles consisting of a polystyrene core and a crosslinked poly(N-isopropylacrylamide)(PNIPAM) shell. The exponents of simple and generalized Herschel Bulkley laws are computed for hard spheres.Comment: 3 pages, 1 figure; contribution to The XVth International Congress on Rheology, August 3-8, 2008, Monterey, California; submitted to J. Rheo

Direct Atomistic Modelling of Deformed Polymer Glasses

We use molecular-dynamics computer simulations to explore the influence of thermal and mechanical history of typical glassy polymers, atactic polystyrene (PS) and (bis)phenol A polycarbonate (PC), on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers. Energy partitioning reveals essential differences between mechanical and thermal rejuvenation. All results are qualitatively interpreted by considering the ratio's of relevant timescales: for cooling down, for deformation, and for intrinsic segmental relaxation

Molecular Mobility of Soft Segment of Polyurethane Elastomers under Elongation

Abstract. In this study, we investigated molecular mobility of a soft segment in the poly(oxypropylene) glycol (PPG), 4,4\u27-diphenylmethane diisocyanate (MDI) and 1,4-butane diol (BD)-based polyurethane elastomers (PUE) with and without elongation by dynamic viscoelastic property measurement and pulse nuclear magnetic resonance (NMR) measurement. The peak position of the loss tangent (tand) curves shifted to the lower temperature region with increasing elongation. In the pulse NMR measurement, the long spin-spin relaxation time {T2) component appeared at -18.0 (e = 0) and -26.0 °C (e =1.5), respectively, with increasing temperature. Since this temperature seems to be related to the glass transition temperature (Jg) of the soft segment in the PUE, it is likely to consider that the Tg decreased with increasing strain. These results might be attributed that the size of cooperative motion during the glass transition decrease due to the orientation of the soft segment, and the soft segment phase approach to a pure phase on account of the extraction of the hard segment from the soft segment phase.THE XV INTERNATIONAL CONGRESS ON RHEOLOGY: The Society of Rheology 80th Annual Meeting : Monterey (California), 3?8 August 200

A sliding plate melt rheometer incorporating a shear stress transducer /

In currently used shear rheometers, shear stress is inferred from a measurement of either total force or total torque. These methods are subject to experimental errors due to uncontrolled flow at the sample boundaries. Such errors can be avoided by measuring the shear stress locally, in the region of controllable flow, using a shear stress transducer. A new sliding plate rheometer for molten plastics has been developed to incorporate a recently developed shear stress transducer. The rheometer operates at temperatures up to 250$sp circ$C. Static and dynamic calibrations showed that the shear stress transducer sensitivity is stable and that its frequency response is suitable for the study of molten plastics. This rheometer was equipped with a computer controlled servohydraulic linear actuator, which provided wide flexibility in shear history selection. Digital data acquisition and signal processing enabled the use of the discrete Fourier transform for nonlinear viscoelastic property determination. Important differences were observed between the locally measured shear stress and values inferred from total force in both large amplitude oscillatory shear and in reciprocating exponential shear tests. For these property measurements, free boundary errors can dominate the dynamics of total force measurements