Initially linear echo-spacing dependence of 1/T2 measurements in many porous media with pore-scale inhomogeneous fields

For a liquid sample with unrestricted diffusion in a constant magnetic field gradient g, the increase R in R2=1/T2 for CPMG measurements is 1/3(taugammag)2D, where gamma is magnetogyric ratio, tau is the half the echo spacing TE, and D is the diffusion constant. For measurements on samples of porous media with pore fluids and without externally applied gradients there may still be significant pore-scale local inhomogeneous fields due to susceptibility differences, whose contributions to R2 depend on tau. Here, diffusion is not unrestricted nor is the field gradient constant. One class of approaches to this problem is to use an "effective gradient" or some kind of average gradient. Then, R2 is often plotted against tau2, with the effective gradient determined from the slope of some of the early points. In many cases, a replot of R2 against tau instead of tau2 shows a substantial straight-line interval, often including the earliest available points. In earlier work [G.C. Borgia, R.J.S. Brown, P. Fantazzini, Phys. Rev. E 51 (1995) 2104; R.J.S. Brown, P. Fantazzini, Phys. Rev. B 47 (1993) 14823] these features were noted, and attention was called to the fact that very large changes in field and gradient are likely for a small part of the pore fluid over distances very much smaller than pore dimensions. A truncated Cauchy-Lorentz (C-L) distribution of local fields in the pore space was used to explain observations, giving reduced effects of diffusion because of the averaging properties of the C-L distribution, the truncation being at approximately +/-1/2chiB0, where chi is the susceptibility difference. It was also noted that, when there is a narrow range of pore size a, over a range of about 40 of the parameter xi=1/3chinua2/D, where nu is the frequency, R2 does not depend much on pore size a nor on diffusion constant D. Examples are shown where plots of R2 vs tau show better linear fits to the data for small tau values than do plots vs tau2. The present work shows that, if both grain-scale and sample-scale gradients are present for samples with narrow ranges of T2, it may be possible to identify the separate effects with the linear and quadratic coefficients in a second-order polynomial fit to the early data points. Of course, many porous media have wide pore size and T2 distributions and hence wide ranges of xi. For some of these wide distributions we have plotted R2 vs tau for signal percentiles, normalized to total signal for shortest tau, again showing initially linear tau-dependence even when available data do not cover the longest and/or shortest T2 values for alltau values. For the examples presented, both the intercepts and the initial slopes of the plots of R2 vs tau increase systematically with signal percentile, starting at smallest R2.     

Shearing behavior of polydisperse media

We study the shearing of polydisperse and bidisperse media with a size ratio of 10. Simulations are performed with a two-dimensional shear cell using contact dynamics. With a truncated power law for the polydisperse media we find that they show a stronger dilatancy and greater resistance to shearing than bidisperse mixtures. To model additives used to control viscosity we introduce so-called “point-like particles”. Even changing the kinematic behavior very little, the point-like particles reduce the force necessary to maintain a fixed shearing velocity.     

Geldart‘sA,B和D类粒子振动流化床空气动力学的研究

主要研究Φ１４８ｍｍ的振动圆柱形流化床的空气动力学，研究了振动强度为１－５ｇ，静止床层高度为１００ｍｍ－２００ｍｍ和属于Ｇｅｌｄａｒｔ‘ｓＡ，Ｂ和Ｄ物料等条件下床层压降，最小流化气速和床层空隙率变化，考察振动条件下振动参数，颗粒物性和床层高度等操作参数对空气动力学的影响，得出固定床段的床层压降的升高和最小流化速度的降低是由于床层空隙率的降低所造成的，并由实验结果关联出床层夺降和最小流化气计算式，同     

Mechanistic study of the sorption properties of OASIS® HLB and its use in solid-phase extraction

Summary The solvation parameter model is used to characterize the sorption properties of the porous polymer Oasis^? HLB for solid-phase extraction with water and water-methanol mixtures as a sample solvent. Increasing solute size and electron lone pair interactions favor retention from water. Oasis^? HLB is not competitive with water for dipole-type and hydrogen-bond interactions, which result in lower analyte retention. The selectivity of Oasis^? HLB is different to porous graphic carbon (Hypercarb^?), a conventional poly (styrene-divinylbenzene) porous polymer sorbent (PLRP-S 100) and two silica-based, octadecylsiloxane-bonded sorbents with a high and a low carbon loading, respectively. Because of selectivity differences no single sorbent is ideal for the extraction of analytes possessing a wide range of polar interactions. Oasis^? HLB is preferred for the extraction of low molecular mass and polar compounds, PLRP-S 100 for the extraction of higher molecular mass compounds of moderate polarity, and the silica-based octadecylsiloxane sorbent with a high carbon loading is the best compromise for the extraction of compounds that cover a wide polarity range. For methanol-water mixtures as a sample solvent PLRP-S 100 is the best general choice with Oasis^? HLB preferred for the extraction of strong hydrogen-bond acids. Hypercarb^? is shown to have favorable retention properties for solid-phase extraction with the except for its low surface area.     

Porous polymers coated with stationary phases. Contribution of the support to retention behaviour in gas chromatography

Summary The gas chromatographic behaviour of a series of compounds of different polarity on columns of Ethofat 60/25 on Chromosorb P HMDS (an inactive support) and Chromosorb 101 (an active support) was investigated. To observe how the support influences the chromatographic process, we separated the contribution to the retention volume of the interaction phenomena associated with the stationary phase, the solid support and the corresponding interfaces. In some cases the Chromosorb 101 exerts an influence on the process even with high percentages (>10%) of stationary phase. The values of the experimental partition constants of a series of compounds and the factors that contribute to adsorption on these columns are calculated.     

INSTANTANEOUS SHRINKING AND LOCALIZATION OF FUNCTIONS IN Yλ （m,p,q,N） AND THEIR APPLICATIONS

61. IntroductionAs well knoWn, the solutionS of semilinear Parabolic equations at = An -- ac, 0 < q < 1have the ISS (the thetantaneous shrinking of the support) ~erty (see 11--5]).Such PrOPerty is d~d as follows.Defiallion 1.1. A hoction u E La(Q) is sold to haw the ISS p~rty, ijjor augr > 0, there hots a POsiboe n~ber R = R(T) such thatu(z, t) = 0for a. e. (z,t) e (R" \ Ba) x (T, co), tDhere AN is the N-dimensional Euclidean space,and Q = AN X (0, co).UP to the preseDt, there…     

Making nanometer thick silica glass scaffolds: an experimental approach to learn about size effects in glasses

A convenient method for the synthesis of very well defined porous silica glasses using ionic liquids as templates is presented. Depending on template concentration, these systems form a homologous series of mesoporous systems with diverse shapes, with the pores having constant thickness of about 2.4 nm. These nanostructures allow the analysis of the two-dimensional behavior of glasses, either from a liquid to be embedded in the pore or of the silica glass forming the wall. For the walls, the third reduced dimension can be varied in a systematic fashion. This approach is exemplified by analyzing the static glass structure of 2D-silica by WAXS.     

Probability distribution of inherent states in models of granular media and glasses

The present paper develops a Statistical Mechanics approach to the inherent states of glassy systems and granular materials by following the original ideas proposed by Edwards for granular media. We consider three lattice models (a diluted spin glass, a system of hard spheres under gravity and a hard-spheres binary mixture under gravity) introduced to describe glassy and granular systems. They are evolved using a “tap dynamics” analogous to that of experiments on granular media. We show that the asymptotic states reached in such a dynamics are not dependent on the particular sample history and are characterized by a few thermodynamical parameters. We assume that under stationarity these systems are distributed in their inherent states satisfying the principle of maximum entropy. This leads to a generalized Gibbs distribution characterized by new “thermodynamical” parameters, called “configurational temperatures” (related to Edwards compactivity for granular materials). Finally, we show by Monte Carlo calculations that the average of macroscopic quantities over the tap dynamics and over such distribution indeed coincide. In particular, in the diluted spin glass and in the system of hard spheres under gravity, the asymptotic states reached by the system are found to be described by a single “configurational temperature”. Whereas in the hard-spheres binary mixture under gravity the asymptotic states reached by the system are found to be described by two thermodynamic parameters, coinciding with the two configurational temperatures which characterize the distribution among the inherent states when the principle of maximum entropy is satisfied under the constraint that the energies of the two species are independently fixed. Received 19 March 2002 and Received in final form 14 June 2002