Biomedical subjects

S Stapf

Publications and source records attributed to S Stapf.

24 records · Page 2Linked to original sources

PubMed1994

Microstructure of porous media probed by NMR techniques in sub-micrometer length scales.

It is shown that field-cycling NMR relaxation spectroscopy in combination with pulsed-gradient spin-echo diffusion studies especially in the supercon fringe field version are suitable techniques for the investigation of length scales of porous media in the range 10 A to 10 microns. Data for water adsorbed in fine particle agglomerates, porous glass and ceramics are reported. An orientational structure factor is introduced permitting the characterization of hydrated surfaces on the basis of reorientations mediated by translational displacements of the adsorbed molecules. Known lengths such as the mean pore or particle size have been reproduced in this way. In length scales beyond these structural elements, the geometry of the internal surfaces can be discussed in terms of wavenumber-space fractals.

Diffusion↗

PubMedDate not supplied

Multi-gradient pulse investigations of fluid transport in porous media.

NMR pulsed field gradient (PFG) experiments employing the application of n gradient pulses k(1) ellipsis k(n) are discussed in a general way as an n-fold encoding of position at successive times. The experiments are then represented by a sampling of n-dimensional k-space, K(n). Various parameters of motion can be derived from the evolution of correlations within the n-dimensional position (r-)space, R(n), which is the Fourier conjugate space to K(n). A wide class of NMR experiments may be described by this formalism, where the dimension of the experiment is often reduced by imposing conditions to the free variables. This is demonstrated for the case of displacement measurements where the condition summation operatork(i) = 0 is met. The two simplest pulse sequences which allow one to correlate displacements at two different times with each other are presented. While the three-pulse version of SERPENT encodes displacements in two interleaved time intervals Delta(1) and Delta(2), the four-pulse VEXSY experiment includes a mixing time tau(m) in between both encoding intervals Delta. The behaviour of fluid transport subject to external pressure through a model porous system is demonstrated by means of numerical simulations of SERPENT and VEXSY experiments for water flowing through a packed bed of monosized spherical particles. Displacements parallel (Z) and perpendicular (X) to the main flow direction are determined and the 2-D joint probability densities and the conditional probabilities are discussed along with the correlation coefficients related to the displacements at different encoding times. It is shown that all possible correlations between Z and X(2) in VEXSY decay with time constants comparable to the average time needed for a fluid molecule to cover one bead diameter, while a negative correlation is observed between transverse (X) displacements which is explained by molecules flowing along streamlines which follow the circumference of the spherical particles. Correlations for displacements during the different times in SERPENT generally decay much slower and provide complementary information about the evolution of displacements with time.

Computer Simulation↗

PubMedDate not supplied

Field-cycling NMR relaxometry of molecules undergoing Lévy walks at the surface of fine particles and porous glass.

Slow surface dynamics of solvents confined in the pore space of porous glass and fineparticles was studied by proton field-cycling relaxometry. Molecular reorientations mediated by translational displacements are shown to be an important low-frequency T1-relaxation mechanism in porous media at low nuclear magnetic resonance (NMR) frequencies in the strong adsorption limit. This is revealed by the pronounced T1-dispersion in this case while a flat one is observed for weak adsorption. The following nanoporous materials were examined in this respect: fineparticles of ZnO,-TiO2, porous glass. Silanized surfaces lead to a somewhat steeper T1-dispersion of non-polar cyclohexane than the native, i.e. polar, surfaces. On the other hand, there is no strong influence of the surface groups on the T1(omega) slope of (polar) acetone. The results are discussed with reorientations mediated by translational displacements enhanced by Lévy walks on the surface.

Cyclohexanes↗

PubMedDate not supplied

Diffusion and relaxation in interface layers of crystals in nanoporous glass.

Below the freezing point, experimental data provide evidence for the existence of a non-frozen interface layer (NFL) between the crystal in the pore and the pore walls. The molecules in this layer are effectively confined in a quasi-two-dimensional space that forces them to reorient when moving along the curved pore surface. Through this relaxation mechanism, the translational mobility in the unfrozen phase reveals itself in a pronounced frequency dependence of the spin-lattice relaxation time. The experimentally observed drop of the self-diffusion coefficients by more than one order of magnitude below the phase transition can be interpreted in terms of geometric restrictions as well as interactions with the surface.

Alkanes↗

PubMedDate not supplied

Molecular dynamics and order of microconfined liquid crystals.

A new technique based on the dipolar-correlation effect was applied in combination with field-cycling-relaxometry to study ordering effects and slow director fluctuations in a nematic liquid crystal confined in porous glasses. Both methods demonstrate a strong influence of geometrical confinements on the distribution of director fluctuation modes. The mean-squared fluctuation estimated from the dipolar-correlation effect decreases exponentially with decreasing pore diameter. The critical mean pore size for the onset of bulk behaviour was found to be of the order of 120 nm. Frequency dependences of spin-lattice relaxation times exhibit sudden sharp deviations from the square root law at frequencies below the MHz-range. These changes are assumed to reflect the lack of long wavelength fluctuations in the spectrum of director fluctuation modes due to finite pore sizes.

Crystallization↗

PubMedDate not supplied

The characterisation of fluid transport in porous solids by means of pulsed magnetic field gradient NMR.

The determination, by pulsed field gradient (PFG) nuclear magnetic resonance (NMR), of the probability distributions (propagators) of displacements for fluids undergoing transport by both flow and self-diffusion within porous solids is outlined. The nature of the observed propagator, P delta (Z), for the transport of a single aqueous phase through an outcrop sandstone (Fontainebleau) is described. Recent measurements of the propagators for both aqueous and oil phase flow in the limiting saturation states of irreducible water (Swi) and residual oil (Sor) in the same sample are illustrated through the use of difference propagators. These are shown to emphasise the regions of the propagators most affected by the presence of the second, stationary, phases in these limiting saturation conditions. Measurement of the propagators for both oil and aqueous phases undergoing simultaneous flow are also described for the same sandstone sample and the effect of increasing Swi on the nature of the oil flow is briefly discussed. Finally, a new two-dimensional (2-D) experiment is introduced which measures the propagator P delta (X, Z). This is the joint probability for displacements X and Z in time delta. Some preliminary observations of these two-dimensional propagators are shown for single-phase flow in the Fontainebleau sandstone sample, where Z and X are, respectively, displacements in the axial and radial direction for the cylindrical sample for which the pressure gradient is along Z and where bulk radial flow is constrained to be zero.

Computer Graphics↗