Free-evolution kinetics in a high-swelling polymeric hydrogel.

We employ nuclear magnetic resonance microimaging to study the kinetics of a high-swelling ionic polymer gel. This includes the time evolution of the sphere diameter and also the evolution of the swollen-unswollen boundary. The experimental results for spherical ionic polyacrylamide gels are compared with the predictions of a nonlinear poroelastic theory by numerically solving the equations for the evolution.

Morphology and dynamics of craterlike structures created by recoiling elongated particles.

We study the morphology and dynamics of craterlike structures formed when free-falling, randomly oriented, elongated particles bounce off a flat surface in a single particle scattering mode. The origin of a sharply defined rim with its associated structure, the factors determining the rim diameter, and the scaling of the diameter with impact velocity are examined. The probability distribution of rebounding particle ranges is calculated for a particular example and shown to provide a precursor description of structure formation.

Intercalated water in synthetic fluorhectorite clay.

(7)Li and (1)H nuclear magnetic resonance together with X-ray diffraction measurements in powdered samples and pseudocrystalline films of synthetic fluorhectorite as a function of relative ambient humidity permit to address several aspects of the structure and dynamics of intercalated water molecules. The role of proton exchange as a possibly dominant mechanism of charge transport in the one-water layer regime of hydration is reexamined. The experimental results in Li-fluorhectorite support the result of molecular simulations which predict, for Li-montmorillonite, the existence of an intermediate regime, between one-water layer and two-water layer states.

The isotropic-nematic interface in suspensions of Na-fluorohectorite synthetic clay.

Colloidal suspensions of Na-fluorohectorite synthetic clay platelets in saline water exhibit coexisting isotropic and nematic phases, due to gravitational separation of the polydisperse particles. We study the ordering of the platelets at the interfaces between various coexisting phases. Four different experimental techniques are employed: visual observation of birefringence, synchrotron wide angle and small-angle X-ray scattering, and magnetic resonance imaging.

Fluid imbibition in paper fibers: precursor front.

We employ nuclear magnetic resonance imaging to study water penetration in cylindrical blocks of unsized paper prepared under different molding pressures. From the measured kinetics of the imbibition profiles, we determine the dependence of the effective transport diffusivity upon degree of saturation of the pores by the penetrating fluid. In general, the transport process is found to be non-Fickian and we discuss different methods of data analysis adapted to this situation.

Strongly damped dynamics of nematically ordered colloidal clay platelets in a magnetic field.

The anisotropy of the diffusivity of water molecules, probed via (1)H nuclear magnetic resonance imaging techniques, is used to study the extremely slow dynamics in the nematic phase of synthetic Na-fluorhectorite platelets in aqueous suspension. The anisotropy of the diamagnetic susceptibility of the platelets Deltachi, and the torque experienced in an applied magnetic field, permit one to monitor the time evolution starting from two different initial conditions. The dynamics of the ordered platelets can be modeled by a one-dimensional Fokker-Planck equation, which permits a satisfactory description of the experimental results.

Diamagnetic orientation of a fluid of hard thin disks and anisotropy of the water diffusivity in the nematic phase of a suspension of clay platelets.

Monte Carlo simulations of the diamagnetic orientation of a fluid of hard thin disks in a magnetic field are presented. The particle density and magnetic field dependences of the eigenvalues of the order parameter tensor are calculated in the presence of a wall which promotes nematic order in a definite direction. The existence of a paranematic region is confirmed, and the approach to the Langevin regime at low densities in the paranematic regime is examined.

Anisotropic water diffusion in nematic self-assemblies of clay nanoplatelets suspended in water.

Diffusion-weighted magnetic resonance imaging provides a vivid description of the little understood role played by interfacial interactions with macroscopic bodies in the cooperative self-assembly of clay nanoplatelets suspended in water. The interfacial interaction between hydrophilic glass walls and clay platelets in a Na-fluorhectorite gel can produce, for dilute gels, a face-to-wall anchoring of the platelets that leads to a uniaxial nematic order with platelet faces parallel to the walls but with randomly distributed normals of the faces. The application of a magnetic field perpendicular to the walls transforms this uniaxial order to an extended biaxial nematic order with orthogonal alignment between normals and the field.

High-throughput, non-destructive determination of oil content in intact seeds by continuous wave-free precession NMR.

A new method, based on continuous wave-free precession nuclear magnetic resonance, is proposed as a high-throughput technique for measuring the oil content of intact seeds. The method has the potential to analyze more than 20 000 intact seeds per hour and is shown to be applicable even to mixtures of seeds of different species with similar fatty acid composition.

Water ingress in Y-type zeolite: anomalous moisture-dependent transport diffusivity.

Nuclear magnetic resonance imaging measurements of liquid water ingress in a large number of nonactivated Y-type (Na) zeolite samples prepared under different conditions are reported on. Using an experimental arrangement that permits the application of Boltzmann's transformation of the 1D (one-dimensional) diffusion equation, the spatiotemporal scaling variables required for a collapse of the measured profiles into universal curves revealed subdiffusive behavior in all cases. It is shown that the one-dimensional fractal time diffusion equation constitutes a powerful tool to analyze the data and provides a connection between the moisture dependence of the effective transport diffusivities and the shapes of the universal curves.

Enhanced Overhauser contrast in proton-electron double-resonance imaging of the formation of an alginate hydrogel.

Proton-electron double-resonance imaging (PEDRI) was recently employed to monitor the process of formation of a calcium alginate hydrogel at a field of 16mT. Here, under the same experimental conditions, images obtained through this technique are compared to images obtained by conventional T(2)-weighted method. The results confirm that the image contrast obtained using PEDRI, thanks to the Overhauser effect, exhibits an improved sensitivity with respect to changes in water mobility as previously suggested in the literature.

Thermal diffusivity and nuclear spin relaxation: a continuous wave free precession NMR study.

Continuous wave free precession (CWFP) nuclear magnetic resonance is capable of yielding quantitative and easily obtainable information concerning the kinetics of processes that change the relaxation rates of the nuclear spins through the action of some external agent. In the present application, heat flow from a natural rubber sample to a liquid nitrogen thermal bath caused a large temperature gradient leading to a non-equilibrium temperature distribution. The ensuing local changes in the relaxation rates could be monitored by the decay of the CWFP signals and, from the decays, it was possible to ascertain the prevalence of a diffusive process and to obtain an average value for the thermal diffusivity.

Concentration-dependent diffusivity and anomalous diffusion: a magnetic resonance imaging study of water ingress in porous zeolite.

Magnetic resonance imaging is employed to study water ingress in fine zeolite powders compacted by high pressure. The experimental conditions are chosen such that the applicability of Boltzmann's transformation of the one-dimensional diffusion equation is approximately satisfied. The measured moisture profiles indicate subdiffusive behavior with a spatiotemporal scaling variable eta=x/t(gamma/2) (0 View Article and Find Full Text PDF

Distant-dipole field in liquids and diffusion: a perturbative approach.

A perturbative approach is employed to solve the Bloch-Torrey equations in the presence of distant-dipole fields in nuclear magnetic resonance. The procedure, although only carried out to first order in the perturbation parameter a=1/k2Dtaud, could, in principle, be generalized to higher orders. Here D is the diffusivity, taud the dipolar demagnetization time, and k is the wave vector of the spatial modulation of magnetization produced by the magnetic field gradient.

Fast and simultaneous measurement of longitudinal and transverse NMR relaxation times in a single continuous wave free precession experiment.

The purpose of this communication is to describe a method for rapid and simultaneous determination of longitudinal (T1) and transversel (T2) relaxation times, based on a single continuous wave free precession (CWFP) experiment which employs RF pulses with a pi/2 flip angle. We analyze several examples, involving nuclei such as 1H, 31P, and 19F, where good agreement with T1 and T2 measurements obtained by traditional methods is apparent. We also compare with the more time-consuming steady-state free precession (SSFP) method of Kronenbitter and Schwenk where several experiments are needed to determine the optimum flip angle.

Intermolecular double-quantum coherences in two-dimensional spectra of binary mixtures in solution. The role of diffusion.

Proton NMR two-dimensional 2-D spectra of binary mixtures, obtained with the correlation spectroscopy revamped by asymmetric Z gradient echo detection pulse sequence, were employed to test various assumptions usually adopted to describe the role of diffusion in intermolecular double quantum coherences. When two molecular species, with significantly different diffusivities, are considered, the relative amplitudes of the peaks, and their widths, furnish a stringent test that unveils some inadequacies in standard approximations.

Low field intermolecular double-quantum coherence imaging via the Overhauser effect.

Intermolecular double-quantum coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented.

Enhanced migration and ionic transport through membranes.

The effect of Coulomb forces upon transport enhancement of mobile ions, in the presence of slowly migrating charged polymeric chains, is investigated in a nonequilibrium regime brought about by a semipermeable membrane and a chemical reaction. By means of a numerical solution of the Nernst-Planck-Poisson equations, we predict the size of the effect, the conditions for positive or negative enhancement, and the dependence upon all relevant parameters. The limitations of the description of migration enhancement by an effective diffusion coefficient are also established.

Monte Carlo simulations of non-Fickian water transport in a saturated porous gel.

A Monte Carlo algorithm, which incorporates tensile effects as well as diffusion, is proposed. It provides a description of water transport in a drying porous gel close to saturation permitting an interpretation of magnetic resonance imaging profiles. Boltzmann's transformation of the one-dimensional diffusion equation is employed to examine the onset of a non-Fickian transport regime caused by the collective motion of diffusers associated with tensile forces.

Tensile water transport in a porous gel.

A Monte Carlo simulation model, which incorporates the effect of tensile forces as well as diffusion, is proposed to explain the behavior of water transport in a saturated porous gel. The algorithm is able to account for the puzzling moisture profiles, which were first observed by conventional magnetic resonance imaging and, more recently, by Overhauser imaging.

Flow sensitivity and coherence in steady-state free spin precession.

Steady-state free precession (SSFP) of nuclear spins in the presence of a magnetic field gradient is known to be very sensitive to flow. We present a theoretical and experimental study of flow sensitivity in a regime where the spacing of the radio-frequency pulses is extremely short compared with the free induction decay time and the relaxation times. Under these rather drastic conditions, a truly continuous wave free precession (CWFP) regime is established, in which, unlike standard SSFP, a large degree of coherence is preserved.

Quantitative analysis using steady-state free precession nuclear magnetic resonance.

The use of steady-state free precession nuclear magnetic resonance (NMR) for quantitative analysis in low magnetic field is investigated and shown to exhibit substantial advantages compared to more conventional NMR methods. With only minor additional requirements, the technique permits a considerable increase in signal-to-noise ratio for a given acquisition time. The experimental conditions needed for implementation and optimization of the acquisition parameters are explored and shown to be easily accessible with unsophisticated equipment.

Mobility and free radical concentration effects in proton-electron double-resonance imaging.

The dependence of the enhancement of proton-electron double-resonance images upon the mobility of the proton bearing molecules, of the concentration of free radicals, and of the pulsed saturating RF power is studied in a magnetic field of 16 mT. The data exhibit a behavior which, in the potentially interesting region of small free radical concentration, may differ substantially from the high-concentration regime depending upon experimental conditions. The results permit a clearer understanding of the factors determining enhancement and contrast in images obtained by dynamic nuclear polarization.