Molecular velocity auto-correlation of simple liquids observed by NMR MGSE method

The velocity auto-correlation spectra of simple liquids obtained by the NMR method of modulated gradient spin echo show features in the low frequency range up to a few kHz, which can be explained reasonably well by a $t^{-3/2}$ long time tail decay only for non-polar liquid toluene, while the spectra of polar liquids, such as ethanol, water and glycerol, are more congruent with the model of diffusion of particles temporarily trapped in potential wells created by their neighbors. As the method provides the spectrum averaged over ensemble of particle trajectories, the initial non-exponential decay of spin echoes is attributed to a spatial heterogeneity of molecular motion in a bulk of liquid, reflected in distribution of the echo decays for short trajectories. While at longer time intervals, and thus with longer trajectories, heterogeneity is averaged out, giving rise to a spectrum which is explained as a combination of molecular self-diffusion and eddy diffusion within the vortexes of hydrodynamic fluctuations.Comment: 8 pages, 6 figur

Shared ATM networks and banking competition

In this paper we study the incentives of banks to share their Automatic Teller Machines (ATMs) when they are competitors in the market for deposits. We construct a stylized model of banking competition which emphasises the distinctive features of ATM compatibility. We find that in equilibrium either a strict subset of banks share their ATMs or total incompatibility prevails. We also derive the implications for ATM compatibility of withdrawal fees, interchange bank fees, entry, and depositor switching costs. Finally, we investigate the normative implications of our model and draw some policy conclusions. © 1994.Authors acknowledge financial support from the Fundacion del Banco de Bilbao-Vizcaya, and Matutes from the Ministerio de Education y Ciencia through DGICYT grants PB89-0075 and PB-0340.Peer Reviewe

One and Two-dimensional NMR to evaluate the performance of consolidants in porous media with a wide range of pore sizes: Applications to cultural heritage

The bone volume fraction, defined as the Bone-Volume-To-Total-Volume ratio (BV/TV), is an important parameter in the assessment of the bone's micro-structure, and is related to the fracture risk in osteoporosis. Although Magnetic Resonance Imaging (MRI) can assess bone micro-architecture parameters invivo, high-field whole body MRI scanners are hardly suitable for wide-scale screening campaigns. Recently, a new NMR application to assess in laboratory the BV/TV of trabecular bone (TB) has been proposed using low-field single-sided NMR. This paper presents an improvement of this technique that allows detection of only the signal from the marrow in the TB suppressing signals from other tissues, which may be present in the sensitive volume of single-sided scanners in a real scenario. The improvement was achieved by filtering signal acquisition by molecular diffusion. Experiments based on an ad-hoc designed Diffusion Weighted T1-T2 pulse sequence have demonstrated that muscle and cartilage signals can be suppressed by using diffusion weighting. On the basis of those experiments, a 1D measurement was established to allow correct estimation of the BV/TV of a TB sample also in the presence of muscle tissue in the sensitive volume of the NMR-scanner. The experiments presented, carried out with the NMR-MOUSE PM10 and the NMR-MOLE, exploiting diffusion-weighted pulse sequences, may pave the way for the in-vivo BV/TV evaluation using single-sided scanners