Continuous wave MRI diffusion study of water in bentonite clay

Three-layered clay minerals such as montmorillonite (bentonite) exhibit very short transverse relaxation times. This is especially true for samples with relatively low water contents in the region of 20% to 30%, which is the water content typically used in environmental technology applications (e.g., as a mineral liner material for landfills). The diffusion of water in samples with such short transverse relaxation times can be measured with NMR by observing the moisture gradients or isotope tracer fronts propagating through appropriately prepared samples by means of continuous wave MRI. The first results from such studies on bentonite clays are presented in this paper.     

The monitoring of water and water vapour diffusion into clay by impedance spectroscopy

The diffusion of water vapour into Bayard montmorillonite and the diffusion/flow of liquid water into Friedland clay were examined with impedance spectroscopy as a first step towards the development of an impedance based sensor system for the in situ monitoring of clay-based barriers for waste containment. By impedance measurements the diffusion of water vapour into clay and the leaching of liquid water through clay barriers can be monitored time-dependently and parameters can be derived from the data that can be used to predict the speed of diffusion/flow through the barrier. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

MR microscopy of water diffusion tensor in biological systems

Diffusion tensor imaging (DTI) of the control and mechanically injured spinal cord of a ratin vitro andin vivo are reported.In vitro experiments were done on a home-built 6.4 T magnetic resonance microscope. Results for formaline-fixed samples show significant differences in diffusion tensor components between gray matter and white matter of the control spinal cord. Moreover, it is shown that already 6–10 min after the injury DTI can detect changes in water diffusion in areas extending far beyond the region of primary tissue damage.In vivo experiments were performed using a 9.4 T Magnex magnet and Bruker Medspec imaging system. Good-quality DTI images, free from motion artifacts were obtained. Results from control samples confirm differences in water diffusion between white matter and gray matter, observedin vitro. In vivo experiments show that characteristic changes in water diffusion observedin vitro 6–10 min after injury are preserved 60–360 min after injury, without significant alteration during this time.     

Energy diffusion in hard-point systems

We investigate the diffusive properties of energy fluctuations in a one-dimensional diatomic chain of hard-point particles interacting through a square-well potential. The evolution of initially localized infinitesimal and finite perturbations is numerically investigated for different density values. All cases belong to the same universality class which can be also interpreted as a Levy walk of the energy with scaling exponent γ=3/5. The zero-pressure limit is nevertheless exceptional in that normal diffusion is found in tangent space and yet anomalous diffusion with a different rate for perturbations of finite amplitude. The different behaviour of the two classes of perturbations is traced back to the “stable chaos" type of dynamics exhibited by this model. Finally, the effect of an additional internal degree of freedom is investigated, finding that it does not modify the overall scenario.     

Strong-coupling diffusion in relativistic systems

Different from the early universe, heavy-ion collisions at very high energies do not reach statistical equilibrium, although thermal models explain many of their features. To account for nonequilibrium strong-coupling effects, a Fokker-Planck equation with time-dependent diffusion coefficient is proposed. A schematic model for rapidity distributions of participant baryons is set up and solved analytically. The evolution from SIS via AGS and SPS to RHIC energies is discussed. Strong-coupling diffusion produces double-peaked spectra in central collisions at the higher SPS momentum of 158 A.GeV/c and beyond.     

Incommensurate diffusion in confined systems

Molecular simulations corroborate the existence of the disputed window effect, i.e., an increase in diffusion rate by orders of magnitude when the alkane chain length increases so that the shape of the alkane is no longer commensurate with that of a zeolite cage. This window effect is shown to be characteristic for molecular sieves with pore openings that approach the diameter of the adsorbate. Furthermore, the physical compatibility between the adsorbate and the adsorbent has a direct effect on the heat of adsorption, the Henry coefficients, the activation energy, and the frequency factors.     

Thermal diffusion in disperse systems

This paper discusses a theory for a new effect, the migration of solid dispersed particles initiated by a nonuniform temperature field. The reason for the motion is the inhomogeneity of the properties of a thin protective layer around a particle. The example of ionic dispersion shows that the sign of the coefficient of thermodiffusion depends on the magnitude of the electrostatic potential at the particle surface and the thickness of the Debye layer and that the coefficientis larger than the values known for molecular systems by a factor of 100 to 10000. In contrast to molecular systems, in disperse systems thermodiffusion should play a much more important role. Zh. éksp. Teor. Fiz. 115, 1721–1726 (May 1999)     

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toD _t =3·10^–14 m²s^–1 using a¹H-NMR technique applying static field gradients up to 53T m^–1 In a range of more than two decades theD _t values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureT _g . A change of mechanism is found for translational diffusion atT _c 1.2T _g whereD _t is proportional to the inverse shear viscosity ^–1 atT>T _c butD _t with =0.75 atT<T _c . Rotational correlation times determined by²H-NMR stimulated echo techniques in deuterated OTP remain proportinal to ^–1 down toT _g . Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.     

Arnold diffusion in large systems

A new regime of Arnold diffusion in which the diffusion rate has a power-law dependence on the perturbation strength is studied theoretically and in numerical experiments. The theory developed predicts this new regime to be universal in the perturbation intermediate asymptotics, the width of the latter increasing with the dimensionality of the perturbation frequency space, particularly in large systems with many degrees of freedom. The results of numerical experiments agree satisfactorily with the theoretical estimates. Zh. éksp. Teor. Fiz. 112, 1132–1146 (September 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Hyperbolic diffusion in chaotic systems

We consider a deterministic process described by a discrete one-dimensional chaotic map and study its diffusive-like properties. Starting with the corresponding Frobenius-Perron equation we derive an approximate evolution equation for the probability distribution which is a partial differential equation of a hyperbolic type. Consequently, the process is correlated, non-Markovian, non-Gaussian and the information propagates with a finite velocity. This is in clear contrast to conventional diffusion processes described by a standard parabolic diffusion equation with an infinite velocity of information propagation. Our approach allows for a more complete characterisation of diffusion dynamics of deterministic systems.     

Theory of diffusion in premelting systems

Summary The diffusion in premelting films and surfaces is described by the Fokker-Planck equation with a periodic potential whose amplitude depends on the temperature and vanishes at the bulk melting temperature. The self-part of the dynamic structure factor is calculated and the results are compared with neutron scattering data from CH₄ premelting films adsorbed on MgO and with atom scattering data from the (110) surface of Pb. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Deterministic diffusion in almost integrable systems

Diffusion processes are usually associated with randomness in the system. In this paper we show that deterministic diffusion processes can also occur in systems with zero entropy that mimic chaos with any precision without being mathematically chaotic. A random walk model is used to predict the behavior of the diffusion coefficient. (c) 1996 American Institute of Physics.     

Decay and diffusion in hierarchical systems

We investigate the combined effects of decay and diffusion in a hierarchically organized system, i.e. in an ultrametric space. The long time asymptotics for the law of disappearance of particles is found and the conditions that the disappearance is diffusion-controlled are determined. The diffusional spreading of particles in the presence of decay is analyzed.     

Turbulent-like diffusion in complex quantum systems

We study a quantum particle propagating through a “quantum mechanically chaotic” background, described by parametric random matrices with only short range spatial correlations. The particle is found to exhibit turbulent-like diffusion under very general situations, without the a priori introduction of power law noise or scaling in the background properties.     

Geometrical restrictions of water diffusion in aqueous protein systems. A study using NMR field-gradient techniques

Geometrical restrictions of water diffusion in different aqueous protein systems were studied using two versions of the NMR field gradient technique. The samples were aqueous systems of bovine serum albumin, gelatin and horse myoglobin at concentrations ranging from diluted solutions to almost dry powders being only partly hydrated. Hydrated protein aerogels were produced by the aid of a special preparation procedure and studied in addition. The experiments referred to the, temperature and concentration dependences of the water diffusion coefficient above and below the free-water freezing temperature. The diffusion coefficient within clusters of overlapping hydration shells is reduced by one order of magnitude compared with that of bulk water. Geometrical restrictions manifest themselves (a) by the obstruction effect observed at low protein concentrations, (b) by the topologically two-dimensional diffusion in the network of overlapping hydration shells, (c) by the percolation threshold appearing at about 15%_b.w. water and (d) by the anomalous diffusion behaviour concluded from the protein aerogel study.     

Manifestation of Arnol'd diffusion in quantum systems

We study an analog of the classical Arnol'd diffusion in a quantum system of two coupled nonlinear oscillators one of which is governed by an external periodic force with two frequencies. In a classical model this very weak diffusion happens in a narrow stochastic layer along the coupling resonance and leads to an increase of the total energy of the system. We show that quantum dynamics of wave packets mimics, up to some extent, global properties of the classical Arnol'd diffusion. This specific diffusion represents a new type of quantum dynamics and may be observed, for example, in 2D semiconductor structures (quantum billiards) perturbed by time-periodic external fields.     

Nonlinear diffusion of water molecules in glycerol

The dependence of the concentration of water molecules in water vapor brought into contact with glycerol surface was measured in a time window of 10⁻¹–10³ s. An obvious nonlinearity of water molecule diffusion was found in a short time approximation (within a minute). A proposal is made with regard to the convective character of diffusion due to heat release during absorption.