滚筒内非等粒径二元颗粒体系增混机理研究

提出了在圆形滚筒内设置十字形内构件的增混方式,并采用离散单元方法对设置不同大小内构件的滚筒内非等粒径二元颗粒体系的混合进行了数值模拟试验.通过模拟结果重点分析了内构件对混合的影响,讨论了内构件的尺寸对混合效果的作用,分析和探讨了滚筒内构件对二元颗粒体系的增混机理.研究发现,当滚筒内无内构件时,对流、扩散和离析三种作用机制对颗粒体系的混合和分离都起到了重要作用;当滚筒内含内构件时,颗粒的混合则只受到颗粒对流和扩散机制的作用,而颗粒的离析效应得到了很大程度的抑制.十字形内构件很大程度上会破坏滚筒内的自由表面流,从而使发生在自由表面流中的颗粒分离不能发生,最终可有效地增加颗粒之间的混合.对于采用在滚筒内设置十字形内构件的方式来增加颗粒间的混合,存在一个最优的内构件尺寸,内构件过小或过大都不利于颗粒间的混合. 关键词： 分离 混合 离散单元法     

Anomalies in avalanche mixing of granular material at half filling

Two fractions of granular material mix in a drum rotating slowly about its longitudinal axis arranged horizontally. The drum is filled not to full capacity, and mixing occurs at each moment of time only on the free surface of material. It is shown that in a half-filled drum the material will never reach uniformly mixed state. Near this critical point, the inverse characteristic mixing time is proportional to δ ² ln(|δ|⁻¹), where δ is the deviation from the half-fill level. Fiz. Tverd. Tela (St. Petersburg) 39, 1479–1483 (August 1997)     

Failure time and critical behaviour of fracture precursors in heterogeneous materials

The acoustic emission of fracture precursors, and the failure time of samples of heterogeneous materials (wood, fiberglass) are studied as a function of the load features and geometry. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. We find that the time interval δt between events (precursors) and the energy ɛ are power law distributed and that the exponents of these power laws depend on the load history and on the material. In contrast, the cumulated acoustic energy E presents a critical divergency near the breaking time τ which is E∼ . The positive exponent γ is independent, within error bars, on all the experimental parameters. Received 31 July 2001 and Received in final form 17 December 2001     

Two-particle dispersion in model two-dimensional velocity fields

We consider two-particle dispersion in a velocity field, where the relative two-point velocity scales according to v ²(r) ∝r ^α and the corresponding correlation time scales as τ(r) ∝r ^β, and fix α = 2/3, as typical for turbulent flows. We show that two generic types of dispersion behavior arize: For α/2 + β < 1 the correlations in relative velocities decouple and the diffusion approximation holds. In the opposite case, α/2 + β > 1, the relative motion is strongly correlated. The case of Kolmogorov flows corresponds to a marginal, nongeneric situation. In this case, depending on the particular parameters of the flow, the dispersion behavior can be rather diffusive or rather ballistic. Received 13 March 2001     

Interaction force between incompatible star-polymers in dilute solution

We consider a low-density assembly of spherical colloids, such that each is clothed by L end-grafted chemically incompatible polymer chains either of types A or B. These are assumed to be dissolved in a good common solvent. We assume that colloids are of small size to be considered as star-polymers. Two adjacent star-polymers A and B interact through a force F originating from both excluded-volume effects and chemical mismatch between unlike monomers. Using a method developed by Witten and Pincus (Macromolecules 19, 2509 (1986)) in the context of star-polymers of the same chemical nature, we determine exactly the force F as a function of the center-to-center distance h. We find that this force is the sum of two contributions F _e and F _s. The former, that results from the excluded volume, decays as F _e∼A _L h ^-1, with the L -dependent universal amplitude A _L∼L ^3/2. While the second, which comes from the chemical mismatch, decays more slowly as F _s∼χB _L h ^{-1 - τ}, where τ is a critical exponent whose value is found to be τ 0.40, and χ is the standard Flory interaction parameter. We find that the corresponding L-dependent universal amplitude is B _L∼L ^{3 + τ /2}. Theses forces are comparable near the cores of two adjacent star-polymers, i.e. for h∼h _c∼a (a is the monomer size). Finally, for two star-polymers of the same chemical nature (A or B), the force F that simply results from excluded-volume effects coincides exactly with F _e, and then the known result is recovered. Received 2 October 2000 and Received in final form 24 January 2001     

Anomalous dynamical light scattering in soft glassy gels

We compute the dynamical structure factor S(q,τ) of an elastic medium where force dipoles appear at random in space and in time, due to “micro-collapses” of the structure. Various regimes are found, depending on the wave vector q and the collapse time θ. In an early time regime, the logarithm of the structure factor behaves as (qτ)^3/2, as predicted in (L. Cipelletti et al., Phys. Rev Lett. 84, 2275 (2000)) using heuristic arguments. However, in an intermediate-time regime we rather obtain a (qτ)^5/4 behaviour. Finally, the asymptotic long-time regime is found to behave as q ^3/2τ. We also give a plausible scenario for aging, in terms of a strain-dependent energy barrier for micro-collapses. The relaxation time is found to grow with the age t_w, quasi-exponentially at first, and then as t _w ^4/5 with logarithmic corrections. Received 15 April 2002     

c-axis tunneling on YBCO films

We have measured I(V) characteristics of c-axis planar tunnel junctions on Y₁Ba₂Cu₃O _{7 - δ} films. Our results and their analysis provide experimental support for the importance of the two-dimensional character of the YBCO band structure, and a method to measure the ratio between the Fermi energy of YBCO and the barrier height. The analysis is based on the relation between the linear conductance background, related to the inelastic tunneling component, and the zero bias conductance, related to the elastic one. Received 24 September 2000 and Received in final form 15 November 2000     

Anomalous dynamical light scattering in soft glassy gels

We compute the dynamical structure factor S(q,τ) of an elastic medium where force dipoles appear at random in space and in time, due to “micro-collapses” of the structure. Various regimes are found, depending on the wave vector q and the collapse time θ. In an early-time regime, the logarithm of the structure factor behaves as (qτ)^3/2, as predicted in L. Cipelletti, S. Manley, R.C. Ball, D.A. Weitz, Phys. Rev. Lett. 84, 2275 (2000) using heuristic arguments. However, in an intermediate-time regime we rather obtain a (qτ)^5/4 behaviour. Finally, the asymptotic long-time regime is found to behave as q ^3/2τ. We also give a plausible scenario for aging, in terms of a strain-dependent energy barrier for micro-collapses. The relaxation time is found to grow with the age t _w, quasi-exponentially at first, and then as t _w ^4/5 with logarithmic corrections. Received 23 July 2001     

The Z = 82 shell closure in neutron-deficient Pb isotopes

Recent mass measurements show a substantial weakening of the binding-energy difference δ_2p(Z, N) = E(Z - 2, N) - 2E(Z, N) + E(Z + 2, N) in the neutron-deficient Pb isotopes. As δ_2p is often attributed to the size of the proton magic gap, it might be speculated that reduction in δ_2p is related to a weakening of the spherical Z = 82 shell. We demonstrate that the observed trend is described quantitatively by self-consistent mean-field models in terms of deformed ground states of Hg and Po isotopes. Received: 25 October 2001 / Accepted: 28 February 2002     

Band structure of <Superscript>146</Superscript>Ce studied through γ-γ angular correlation measurements

The β-decay of ¹⁴⁶La was studied using the on-line isotope separator KUR-ISOL. Gamma-gamma angular correlation measurements were performed with a 4-Ge detectors system. Spin assignments of three levels were made: 3⁺ for the 1576.5 keV level, 4⁺ for the 1627.1 keV level and 5⁺ for the 1810.2 keV level. The mixing ratios (E2/M1) were deduced to be δ_183.2= 0.25 ± 0.08, δ_638.9= 0.33 ± 0.05, δ_959.0= 1.19^+0.16 _−0.14, δ_1015.9= 5.4^+3.1 _−1.5 and δ_1318.1= 6.5^+1.7 _−1.1. These were compared to the calculated values obtained in three cases involving different Majorana force parameter values. The band structure of ¹⁴⁶Ce is discussed based on the results of calculation using the IBM-2 theory. Received: 20 September 1999 / Revised version: 8 February 2000     

Granular flows in a rotating drum: the scaling law between velocity and thickness of the flow

The flow of dry granular material in a half-filled rotating drum is studied. The thickness of the flowing zone is measured for several rotation speeds, drum sizes and beads sizes (size ratio between drum and beads ranging from 47 to 7400). Varying the rotation speed, a scaling law linking mean velocity vs. thickness of the flow, v∼h^m, is deduced for each couple (beads, drum). The obtained exponent m is not always equal to 1, the value previously reported for a drum in litterature, but varies with the geometry of the system. For small size ratios, exponents higher than 1 are obtained due to a saturation of the flowing zone thickness. The exponent of the power law decreases with the size ratio, leading to exponents lower than 1 for high size ratios. These exponents imply that the velocity gradient of a dry granular flow in a rotating drum is not constant. More fundamentally, these results show that the flow of a granular material in a rotating drum is very sensible to the geometry, and that the deduction of the “rheology” of a granular medium flowing in such a geometry is not obvious.     

Femtosecond ultraviolet pulses generated using noncollinear optical parametric amplification and sum frequency mixing

We demonstrate the generation of ultraviolet 33-fs pulses with a shot-to-shot energy fluctuation of less than 3% using sum frequency mixing of visible pulses of a noncollinear optical parametric amplifier with sub-40-fs pulses of a 1-kHz Ti:sapphire-amplified system. The pulses are transform-limited (ΔνΔτ=0.36) and tunable in the range from 315 nm to 355 nm with energy above 1 μJ (2.6 μJ at 330 nm). Received: 21 July 2000 / Published online: 8 November 2000     

Irreversible deposition of extended objects with diffusional relaxation on discrete substrates

Random sequential adsorption with diffusional relaxation of extended objects both on a one-dimensional and planar triangular lattice is studied numerically by means of Monte Carlo simulations. We focus our attention on the behavior of the coverage θ(t) as a function of time. Our results indicate that the lattice dimensionality plays an important role in the present model. For deposition of k-mers on 1D lattice with diffusional relaxation, we found that the growth of the coverage θ(t) above the jamming limit to the closest packing limit θ_CPL is described by the pattern θ_CPL-θ(t) ∝E_β[-(t/τ)^β], where E_β denotes the Mittag-Leffler function of order β∈(0,1). In the case of deposition of extended lattice shapes in 2D, we found that after the initial “jamming", a stretched exponential growth of the coverage θ(t) towards the closest packing limit θ_CPL occurs, i.e., θ_CPL - θ(t) ∝exp [-(t/τ)^β]. For both cases we observe that: (i) dependence of the relaxation time τ on the diffusion probability P_dif is consistent with a simple power-law, i.e., τ∝P_dif ^-δ; (ii) parameter β depends on the object size in 1D and on the particle shape in 2D.     

旋转筒内黑索今和铝颗粒混合与分离的计算机模拟

采用分子动力学方法对旋转圆筒内黑索今和铝二元颗粒体系进行模拟.模拟结果表明,混合均匀度与填料方式无关.当圆筒壁无挡板时,圆筒的旋转可以使黑索今和铝颗粒发生相分离现象,当有挡板时,可以增加混合效应.另外,对上述混合与分离现象给予了一定的理论解释.     

Relativistic diffusion model

Evidence is presented that diffusion drives colliding many-particle systems at relativistic energies from the initial δ–functions in rapidity towards the equilibrium distribution. Analytical solutions of a linear Fokker-Planck equation represent rapidity spectra for participant protons in central heavy-ion collisions at SPS-energies accurately. Thermal equilibrium in the interaction region is not attained, nonequilibrium features persist and can account for the broad rapidity spectra. Received: 24 November 1998 / Revised version: 20 February 1999     

Discrete particle simulation of radial segregation in horizontally rotating drum: Effects of drum-length and non-rotating end-plates

The radial segregation phenomena of a mixture of two different size grains in a horizontal rotating drum are studied by DEM simulations. The grano-dynamics of radial segregation phenomena is examined as a function of the axial length and the friction between grains and not-rotating end-plates of the drum. The results indicate that, in the longer drums, the radial segregation ratio is higher and the friction on the end-plates shows little effect. Whereas in the shorter drums, the radial segregation is very slow or negligible; however, decreasing the friction on non-rotating end-plates increases the segregation ratio. If we increase the friction further (greater than the frictions between the grain-grain and the grains and the inner wall), the segregation ratio drops in the longer drums while in the shorter drums mixing is seen instead. The cause of these phenomena lies in the mechanism of diffusion in granular flows due to shearing strain by the end-plates. For more roughened end-plates, this shearing activity increases the granular temperature of the system and only the mixing can be observed instead of the segregation.     

Aeolian transport of sand

The airborne transport of particles on a granular surface by the saltation mechanism is studied through numerical simulation of particles dragged by turbulent air flow. We calculate the saturated flux q_s and show that its dependence on the wind strength u_* is consistent with several empirical relations obtained from experimental measurements. We propose and explain a new relation for fluxes close to the threshold velocity u_t, namely, q_s=a(u_*-u_t)^α with α≈2. We also obtain the distortion of the velocity profile of the wind due to the drag of the particles and find a novel dynamical scaling relation. We also obtain a new expression for the dependence of the height of the saltation layer as function of the strength of the wind.     

Erasure effect of the reading beam on the decay process of χ(2) in all-optical poling

We have studied the effect of the reading beam on the decay process of the photoinduced second-order susceptibility χ⁽²⁾ in both the guest-host system (disperse red 1-doped polymethyl methactrylate) and the side-chain system (disperse red 19-functionalized polyimide polymer). Both experimentally and theoretically, we have proved that the reading beam has the erasure effect on the decay process, that is, χ⁽²⁾ decays faster in the high intensity than in the low intensity of the reading beam. Furthermore, the relationship between the time constant τ_B of the slow component of the decay process and the intensity of the reading beam I_1.06 is derived from the theoretical equations, which is fitted well with the experimental results. Based on the fitting parameters, we also compare, quantitatively, the thermal-induced orientational diffusion of trans- and the quantum efficiency of trans-to-cis photoisomerization in the two systems. Received: 8 September 1999 / Revised version: 14 March 2000 / Published online: 16 August 2000     

The lifetime of the soliton in the improved Davydov model at the biological temperature 300 K for protein molecules

We study the effects of quantum fluctuations and thermal perturbations on the lifetime of the soliton in the improved Davydov model proposed by us with two-quanta and with an added interaction. By using quantum perturbation theory, we compute the soliton lifetime for a wide ranges of parameter values relevant for protein molecules. The lifetime of the new soliton at the biological temperature 300 K is of the order of 10^-10 second or τ/τ≥ 500 for parameters appropriate to α-helical protein molecules. This shows clearly that the new soliton in the improved model is a viable mechanism for the bio-energy transport in the α-helix region of proteins. Received 7 January 1999 and Received in final form 16 August 2000