A Kinetic Theory for Particulate Systems with Bimodal and Anisotropic Velocity Fluctuations

Observations of bubbles rising near a wall under conditions of large Reynolds and small Weber numbers have indicated that the velocity component of the bubbles parallel to the wall is significantly reduced upon collision with a wall. To understand the effect of such bubble-wall collisions on the flow of bubbly liquids bounded by walls, a model is developed and examined in detail by numerical simulations and theory. The model is a system of bubbles in which the velocity of the bubbles parallel to the wall is significantly reduced upon collision with the channel wall while the bubbles in the bulk are acted upon by gravity and linear drag forces. The inertial forces are accounted for by modeling the bubbles as rigid particles with mass equal to the virtual mass of the bubbles. The standard kinetic theory for granular materials modified to account for the viscous and gravity forces and supplemented with boundary conditions derived assuming an isotropic Maxwellian velocity distribution is inadequate for describing the behavior of the bubble-phase continuum near the walls since the velocity distribution of the bubbles near the walls is significantly bimodal and anisotropic. A kinetic theory that accounts for such a velocity distribution is described. The bimodal nature is captured by treating the system as consisting of two species with the bubbles (modeled as particles) whose most recent collision was with a channel wall treated as one species and those whose last collision was with another bubble as the other species. The theory is shown to be in very good agreement with the results of numerical simulations and provides closure relations that may be used in the analysis of bidisperse particulate systems as well as bounded bubbly flows

Finite-Weber-Number Motions of Bubbles Through a Nearly Inviscid Liquid

A method is described for computing the motion of bubbles through a liquid under conditions of large Reynolds and finite Weber numbers. Ellipsoidal harmonics are used to approximate the shapes of the bubbles and the flow induced by the bubbles, and a method of summing flows induced by groups of bubbles, using a fast multipole expansion technique is employed so that the computational cost increases only linearly with the number of bubbles. Several problems involving one, two and many bubbles are examined using the method. In particular, it is shown that two bubbles moving towards each other in an impurity-free, inviscid liquid touch each other in a finite time. Conditions for the bubbles to bounce in the presence of non-hydrodynamic forces and the time for bounce when these conditions are satisfied are determined. The added mass and viscous drag coefficients and aspect ratio of bubbles are determined as a function of bubble volume fraction and Weber number

Field-induced charge-density-wave transitions in the organic metal α-(BEDT-TTF)₂KHg(SCN)₄ under pressure

Successive magnetic-field-induced charge-density-wave transitions in the layered molecular conductor α-(BEDT-TTF)₂KHg(SCN)₄ are studied in the hydrostatic pressure regime, in which the zero field chargedensity-wave (CDW) state is completely suppressed. The orbital effect of the magnetic field is demonstrated to restore the density wave, while the orbital quantization induces transitions between different CDW states at changing the field strength. The latter appear as distinct anomalies in the magnetoresistance as a function of field. The interplay between the orbital and Pauli paramagnetic effects acting, respectively, to enhance and to suppress the CDW instability is particularly manifest in the angular dependence of the field-induced anomalies

Angle-dependent magnetoresistance in the weakly incoherent interlayer transport regime

We present comparative studies of the orientation effect of a strong magnetic field on the interlayer resistance of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ samples characterized by different crystal quality. We find striking differences in their behavior which is attributed to the breakdown of the coherent charge transport across the layers in the lower quality sample. In the latter case, the nonoscillating magnetoresistance background is essentially a function of only the out-of-plane field component, in contradiction to the existing theory.Comment: 4 pges, 3 figure

Staggered Spin Order of Localized pi-electrons in the Insulating State of the Organic Conductor kappa-BETS)2Mn[N(CN)2]3

Magnetic properties of the conduction pi-electron system of kappa-BETS)2Mn[N(CN)2]3 have been probed using 13C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T~23K is shown to be accompanied by ordering of the pi-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ~0.5 kbar, the pi-spin system maintains the properties of the metallic state down to 5K.Comment: 13 pages, 4 figure

Shubnikov–de Haas oscillations and electronic correlations in the layered organic metal κ-(BETS)₂Mn[N(CN)₂]₃

We present magnetoresistance studies of the quasi-two-dimensional organic conductor κ-(BETS)₂Mn[N(CN)₂]₃, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. Under a moderate pressure of 1.4 kbar, required for stabilizing the metallic ground state, Shubnikov–de Haas oscillations, associated with a classical and a magnetic-breakdown cyclotron orbits on the cylindrical Fermi surface, have been found at fields above 10 T. The effective cyclotron masses evaluated from the temperature dependence of the oscillation amplitudes reveal strong renormalization due to many-body interactions. The analysis of the relative strength of the oscillations corresponding to the different orbits and of its dependence on magnetic field suggests an enhanced role of electron-electron interactions on flat parts of the Fermi surface

Fullerene Black: Relationship between Catalytic Activity in n-alkanes Dehydrocyclization and Reactivity in Oxidation, Bromination and Hydrogenolysis

The reactivity of fullerene black in oxidation (by air oxygen or ions MnO4–or Cr2O7 2– in solution), bromination (by Br2 or (C4H9)4NBr3) and hydrogenolysis (without hydrogenation catalyst) are studied. The dehydrocyclization of n-alkanes over fullerene black is realized via the monofunctional mechanism, i.e. the dehydrogenation and cyclization stages proceed on the same catalytic center. The addition of alumina to the catalyst transforms dehydrocyclization mechanism to bifunctional one, when fullerene black acts as dehydrogenation agent. Reactivity studies and ESR spectroscopy data for initial and annealed fullerene black show the presence in fullerene black structure of both non-conjugated multiple and dangling bonds. Nonconjugated bonds determine catalytic activity and reactivity of fullerene black. They are localized in amorphous part of fullerene black. Technological aspects of fullerene black as alkanes dehydrocyclization catalyst are discussed