170,160 research outputs found
Advances in understanding the relationship between rock wettability and high-frequency dielectric response
No full textPrevious studies have shown that wettability has an effect on the complex permittivity of rock in a wide frequency interval. At low frequencies ( < 10 kHz), the dielectric behaviour is primarily controlled by the connectivity of the water phase. At higher frequencies (10 kHz-10 GHz), the governing parameter is the shape of the water inclusions, i.e. large-surface inclusions, typical of a water wet system, and small-surface inclusions, generally associated with oil wet situations, show markedly different electrical responses. Maxwell-Wagner-Sillars polarisation governs these high-frequency effects, which are relatively independent of the level of water saturation of the system, allowing direct and easier determination of wettability. Several experiments carried out on synthetic porous media and reservoir rocks made strongly oil wet using a sylanisation process have been published by the authors [Rev. Inst. Fr. Pet. 53 (1994) 771; Bona, N., Rossi, E., Capaccioli, S., 1999a. Wettability and dielectric constant. Proc. of the 1999 Int. Symp. of the Society of Core Analysts, Golden, Colorado, paper SCA 9925; Bona, N., Rossi, E., Capaccioli, S., 1999b. Electrical measurements in the 100 Hz-10 GHz frequency range for efficient rock wettability evaluation. Proc. of the 1999 SPE Int. Symp. on Oilfield Chemistry, Houston, paper SPE 50720, 207-217; J. Phys. D: Appl. Phys. 33 (2000) 1036]. This paper summarizes the results obtained on "fresh-state" rocks with preserved wettability. Two reservoirs were studied. One is a fractured oil reservoir, with a permeability so low that it cannot be investigated using the conventional wettability test techniques. The other is a carbonate reservoir, with Arnott test results available. While the second reservoir was found to be strongly water wet, the first showed the coexistence of water wet and oil wet zones, which were clearly recognised by two distinct relaxation peaks in the dielectric spectra. Air-mercury pore size distribution measurements and CryoSEM analyses were also carried out. All the collected data were used to build a consistent model of the behaviour of the rock. (C) 2002 Elsevier Science B.V. All rights reserved
Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)
No full textLocal segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy and analyzed in combination with the equation of state obtained from PVT measurements. Significant variations of glass transition temperature and fragility with molecular weight are observed. In accord with the general properties of glass-forming materials, single molecular weight dependent scaling exponent gamma is sufficient to define the mean segmental relaxation time tau(alpha) and its distribution. This exponent can be connected to the Gruneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties ("dynamic crossover") are observed as a function of both temperature and pressure, with tau(alpha) serving as the control parameter for the crossover. At longer tau(alpha) another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ionic conductivity. (C) 2007 American Institute of Physics
La bottega degli apprendimenti. Tra multiculturalizzazione del lavoro, arene trasformative e ricerca educativa
No full textIl contributo presenta i risultati di un progetto di ricerca di Dottorato interessato a indagare dove, quando e a quali condizioni le persone che vivono in un quartiere ad alto tasso multietnico apprendono a gestire le differenze etnicoculturali che lo caratterizzano. Ancorandosi ad un framework teorico che fa riferimento a) alle teorizzazioni sull’apprendimento informale e trasformativo, b) alla prospettiva del multiculturalismo quotidiano e c) alle più recenti teorizzazioni sull’ipotesi del contratto, oggetto della ricerca sono stati gli apprendimenti che emergono a seguito di esperienze di incontro e contatto che originano dalla frequentazione di negozi gestiti da commercianti con background migratorio. In questa prospettiva, il piccolo negozio etnico di quartiere è stato individuato come caso emblematico di quei contesti che elicitano apprendimenti di tipo informale, attraverso processi di microsocializzazione e di contatto, tra individui che non condividono gli stessi riferimenti etnico-culturali
Change of caged dynamics at T-g in hydrated proteins: Trend of mean squared displacements after correcting for the methyl-group rotation contribution
No full textThe question whether the dynamics of hydrated proteins changes with temperature on crossing the glass transition temperature like that found in conventional glassformers is an interesting one. Recently, we have shown that a change of temperature dependence of the mean square displacement (MSD) at T-g is present in proteins solvated with bioprotectants, such as sugars or glycerol with or without the addition of water, coexisting with the dynamic transition at a higher temperature T-d. The dynamical change at T-g is similar to that in conventional glassformers at sufficiently short times and low enough temperatures, where molecules are mutually caged by the intermolecular potential. This is a general and fundamental property of glassformers which is always observed at or near T-g independent of the energy resolution of the spectrometer, and is also the basis of the dynamical change of solvated proteins at T-g. When proteins are solvated with bioprotectants they show higher T-g and T-d than the proteins hydrated by water alone, due to the stabilizing action of excipients, thus the observation of the change of T-dependence of the MSD at T-g is unobstructed by the methyl-group rotation contribution at lower temperatures [S. Capaccioli, K. L. Ngai, S. Ancherbak, and A. Paciaroni, J. Phys. Chem. B 116, 1745 (2012)]. On the other hand, in the case of proteins hydrated by water alone unambiguous evidence of the break at T-g is hard to find, because of their lower T-g and T-d. Notwithstanding, in this paper, we provide evidence for the change at T-g of the T-dependence of proteins hydrated by pure water. This evidence turns out from (i) neutron scattering experimental investigations where the sample has been manipulated by either full or partial deuteration to suppress the methyl-group rotation contribution, and (ii) neutron scattering experimental investigations where the energy resolution is such that only motions with characteristic times shorter than 15 ps can be sensed, thus shifting the onset of both the methyl-group rotation and the dynamic transition contribution to higher temperatures. We propose that, in general, coexistence of the break of the elastic intensity or the MSD at T-g with the dynamic transition at T-d in hydrated and solvated proteins. Recognition of this fact helps to remove inconsistency and conundrum encountered in interpreting data of hydrated proteins that thwart progress in understanding the origin of the dynamic transition. (C) 2013 AIP Publishing LLC
What can we learn by squeezing a liquid?
No full textRelaxation times tau(T,v) for different temperatures, T, and specific volumes, v, collapse to a master curve vs Tu(gamma), with gamma a material constant. The isochoric fragility, m(V), is also a material constant, inversely correlated with gamma. From these experimental facts, we obtain a three-parameter function that accurately fits tau(T,v) data for several glass-formers over the supercooled regime, without any divergence of tau below T(g). Although the values of the three parameters depend on the material, only gamma significantly varies; thus, by normalizing material-specific quantities related to gamma, a universal power law for the dynamics is obtained
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