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S Stapf

Publications and source records attributed to S Stapf.

At least 19 recordsLinked to original sources
PubMed2006

Multiecho sequence for velocity imaging in inhomogeneous rf fields.

The unambiguous determination of velocities with spatial resolution in a multiecho PFG NMR sequence strongly depends on the homogeneity of the B1 field. This affects, in particular, the use of surface coils that bear considerable potential for on-line flow monitoring where a fast-imaging sequence can become vital. However, even with most rf coils dedicated for imaging applications, B1 inhomogeneities are sufficiently large to generate severe problems in performing velocity-imaging experiments. In this paper, the use of a combination of different phase cycles in Carr-Purcell sequences is discussed. The suggested phase cycling scheme tolerates large flip angle imperfections arising in inhomogeneous B1 fields, and thus allows acquisition of a maximum number of echoes within a pulse train. The performance of the velocity-imaging sequence is proven by using phantom samples developing known laminar flow patterns.

Journal Article↗
PubMed2005

Internal fluid dynamics in levitated drops by fast magnetic resonance velocimetry.

Fluid motion inside a levitated drop is determined by the interface properties. Momentum transfer through a highly mobile interface results in stationary vortex patterns inside the drop that dramatically enhance mass transfer between both phases, while immobile interfaces suppress internal dynamics. The presence of small amounts of surface-active substances can result in a partial reduction of interface mobility, the so-called rigid cap. The time dependence of internal flow patterns is presented by means of NMR velocity images of levitated drops, and is compared to fast measurements of the velocity distribution along the three orthogonal coordinates.

Journal Article↗
PubMed2005

Visualizing flow vortices inside a single levitated drop.

The internal flow dynamics in single liquid drops, kept in place through levitation by a counterflowing continuous fluid phase in a suitably designed glass cell, is investigated by PFG NMR techniques. The positional stability of the drops was confirmed from series of one-dimensional profiles and was found to be below the spatial resolution of the experiment. Velocity distribution functions (propagators) along all three coordinates were obtained and demonstrated the long-time stability of the internal dynamics in terms of the velocity magnitudes occurring in the systems. Finally, velocity imaging was applied to visualize the internal vortex patterns in the drops either as projections onto different planes or within thin slices of selected orientations. Two different fluid systems were investigated in order to cover the principal cases of rigid and mobile interfaces. Different fast velocity imaging techniques were employed for monitoring the vastly differing velocity ranges of both cases, and the high sensitivity of the internal three-dimensional motion to the cell geometry is demonstrated.

Journal Article↗
PubMed2004

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry.

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.

Journal Article↗
PubMed2004

New approaches to the visualization, quantification and explanation of acid-induced water loss from Ca-alginate hydrogel beads.

The water loss of Ca-alginate hydrogels at pHs below 4.0 was visualized with 1HNMR-imaging by covering a single alginate bead with cyclohexane-d12 in a specially equipped NMR-tube and adding propionic acid at defined concentrations. The exact amount of water expelled from the beads was calculated from their weight loss and correlated with the acid concentrations and pHs within the hydrogel matrix. The maximum water loss of 52% (w/w) occurred at pH 1.0, while only 5% (w/w) of the initial water content were lost at pH 3.6. The analysis of the water collected from several alginate beads for Ca2+ -ions and free polysaccharides led to the assumption that, due to the acid-induced protonation of the carboxyl functions, the ionotropic network is gradually converted to an alginic acid gel structured by H-bonds. This contradicts existing theories explaining the pH-induced water loss by a lower solubility of the alginate chains and decreased repulsion between protonated carboxyl functions, but explains previously reported pH-dependent alterations of mass transport and drug retention of Ca-alginate gels. Thus, the presented experiments enable a more precise and complete view of the acid-induced process within Ca-alginate hydrogels. The transfer to the characterization of other hydrogels is possible and should be advantageous, especially if a calibration of the NMR-measurement could be achieved.

Alginates↗
PubMed2002

NMR visualization of displacement correlations for flow in porous media.

The temporal correlations of velocities for both water and a water-glycerol mixture flowing through a random packings of monodisperse spherical particles have been investigated using two-dimensional nuclear magnetic resonance methods. By combining various flow rates, fluid viscosities, and bead sizes, a wide range of flow parameters has been covered, the dimensionless Peclet number ranging from 100 to 100 000. The velocity exchange spectroscopy (VEXSY) technique has been employed to measure the correlation between velocities during two intervals separated from each other by a mixing time tau(m). This time is made both large and small compared with the time constant tau(c), required for a fluid element possessing the average flow velocity to cover a distance equal to the characteristic size in the system, the bead diameter. The two-dimensional conditional probability of displacement resulting from the VEXSY method reveals the existence of different "subensembles" of molecules, including a slow moving pool whose displacement is dominated by Brownian motion, an intermediate ensemble whose velocities change little over the mixing time, and a fast flowing ensemble which loses correlation due to mechanical dispersion. We find that that the approach to asymptotic dispersion, as tau(c)/tau(m) increases, depends strongly on the Peclet number, the deviation of the velocity autocorrelation function from a monoexponential Ornstein-Uhlenbeck process becoming more pronounced with increasing Peclet number.

Journal Article↗
PubMed2002

NMR velocimetry of falling liquid films.

First results on NMR velocimetry of falling liquid films are presented. A film of average thickness 1 mm and width 40 mm is sustained by a continuous flow of silicon oil over a vertical plate made from PMMA. The spatial distribution of velocities is measured using a double spin--echo imaging pulse sequence supplemented by a bipolar velocity encoding gradient. Spin density and velocity images as well as two-dimensional velocity maps of different situations, i.e., undisturbed and disturbed falling film flow, are discussed. Experimental and theoretical velocity data for undisturbed film flow are compared.

Journal Article↗
PubMed2002

Spectrally resolved velocity exchange spectroscopy of two-phase flow.

The Velocity EXchange SpectroscopY (VEXSY) technique, which provides a means to correlate macroscopic molecular displacements measured during two intervals separated by a variable mixing period, has been applied for the first time to a system of two-phase flow. The chemical shift difference between water and methyl protons has been exploited to simultaneously determine the probability of displacements, or propagator, of both components in a water/silicone oil mixture flowing through a glass bead pack. The joint two-time probability densities as well as the conditional probabilities of velocities show a clearly distinct dispersion behaviour of both fluids which is a consequence of the different wetting properties of the fluids with respect to the glass surface of the bead pack.

Journal Article↗
PubMed2001

NMR imaging of falling water drops.

The falling water drop is a simple model for studying phenomena related to chemical extraction, where two immiscible phases are dynamically blended to promote the transport of solute molecules from one phase to the other. Convective motion inside the drop significantly influences the extraction efficiency. Whereas optical and tracer methods are model bound or invasive, NMR imaging is noninvasive, direct, and applicable to nontransparent media. The first NMR measurements of a water drop falling through air are reported. It is shown that, in drops from pure water, large-scale convection rolls are observed in contrast to drops with the surface tension lowered by surfactants.

Journal Article↗
PubMed2001

Two-dimensional NMR of velocity exchange: VEXSY and SERPENT.

Two different multidimensional pulsed field gradient sequences are compared which have the purpose of correlating spin displacements in different time intervals with each other. The simplest possible sequence, three-pulse SERPENT, measures displacements in two interleaved time intervals, while in VEXSY, consisting of two independent pairs of gradient pulses separated by a mixing time, displacements during the two encoding intervals are compared to each other. The formalism for both sequences is discussed in q space and in displacement space and common features as well as differences between the two types of experiments are highlighted, employing the particular case of the concurrent VEXSY scheme which allows treatment according to both formalisms.

Magnetic Resonance Spectroscopy↗
PubMed2000

NMR characterization of the pore structure and anisotropic self-diffusion in salt water Ice

NMR imaging and one- and two-dimensional self-diffusion propagator measurements of the liquid phase in salt water ice are presented. The properties of the network of brine-filled pores are found to depend on the growth conditions of the ice. Two types of samples are compared: (a) shock-frozen ice produced in the probe in situ and (b) ice grown over several hours under controlled conditions. By shock-freezing, an ice structure could be produced which featured streak-like porous channels of diameters of up to 300 &mgr;m allowing almost unrestricted self-diffusion along one preferential axis but reduced diffusivities in the remaining directions. In ice grown under controlled conditions, the pore sizes are near the resolution limit of the imaging experiment of typically 50 &mgr;m. For this type of samples, strongly non-Gaussian self-diffusion propagators are obtained, indicating restricted self-diffusion on rms scales of 30 &mgr;m. Common to all samples was the observation of highly anisotropic self-diffusion. One- and two-dimensional propagators are compared in order to estimate the degree of anisotropy and the size of the restrictions. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Two-dimensional PFG NMR for encoding correlations of position, velocity, and acceleration in fluid transport

A generalized approach to obtain two-dimensional maps of spatial particle coordinates and their derivatives with respect to time by PFG-NMR employing multiple gradient pulses is presented. A sequence of n magnetic field gradient pulses makes it possible, after independent stepping of each pulse and subsequent Fourier transformation, to plot the spin density distribution in coordinate space at n times and along the respective directions of the gradient pulses. In particular, two gradient pulses of effective area k(1) and k(2) separated by a time interval Delta lead to a plot of the combined two-time probability density, W(2)(r(1), 0; r(2), Delta), to find a particle at a coordinate r(1) at t = 0 and at r(2) at t = Delta. A conventional experiment for measuring transport properties by simultaneous stepping of the gradients under the condition k(1) = -k(2) is equivalent to a projection onto the secondary diagonal in the [r(1), r(2)] plot. The main diagonal represents an average position between the two timepoints t = 0 and t = Delta, so that a rotation of the coordinate plot by an angle of 45 degrees allows one to correlate the displacement R = r(2) - r(1) with the averaged position r parallel to the gradient direction. While an average velocity during the time interval Delta can be defined as &vmacr; = R/Delta, an extension toward acceleration and higher order derivatives is straightforward by modification of the pulse sequence. We discuss this concept by application to flow through a circular and a narrowing pipe (confusor), respectively, the experimental results of which are compared to numerical simulations. Copyright 2000 Academic Press.

Journal Article↗
PubMed1999

Time Correlations in Fluid Transport Obtained by Sequential Rephasing Gradient Pulses.

We present a basic experiment by which the evolution of the displacement probability density (propagator) of static or flowing fluid in N successive time intervals is obtained by single labeling, coupled with multiple rephasing events during the course of a pulsed field-gradient sequence. We term this type of sequence SERPENT: SEquential Rephasing by Pulsed field-gradients Encoding N Time-intervals. Realizations of the SERPENT experiment for the case N = 2 which include spin echo, stimulated echo, and Carr-Purcell pulse sequences are suggested. They have in common a spatial spin-labeling of the initial magnetization by a gradient of area q0, followed by successive rephasing via gradients q1 and q2 at times t = Delta1 and t = Delta2, respectively, where q0 + q1 + q2 = 0. A two-dimensional Fourier transform with respect to q1 and q2 gives directly the joint probability density W2(R1, Delta1; R2, Delta2) for displacements R1 and R2 in times Delta1 and Delta2, respectively. q1 and q2 may be in arbitrary directions. Assuming R1 ||R2, the correlation coefficient rhoR1,R2 then reflects the time-history of the fluctuating velocities. The behavior of the cross moment can be obtained from either a full two-dimensional or a set of one-dimensional SERPENT measurements. Experimental results are presented for water flowing through a bed of packed glass beads. While Delta1 is appropriately chosen to sample the short-time velocity field within the system, increasing Delta2 clearly shows the loss of correlation when the average fluid element displacement exceeds the bead diameter. Copyright 1999 Academic Press.

Journal Article↗
PubMed1997

Multiple nonlinear stimulated echoes.

Three-pulse sequences in the presence of magnetic field gradients at high magnetic fields produce multiple nonlinear stimulated echoes (NOSE) at times ntau1 after the third pulse, where n is an integer and tau1 the interval between the first two pulses. These phenomena are due to the demagnetizing field produced by the spatial modulation of the nuclear magnetization arising in the sample after the first two pulses. The theory is presented and compared with experiments. The dependence of the NOSE amplitudes on the flip angles and on the pulse intervals is described. Implications for multidimensional NMR experiments based on sequences of three or more pulses in the presence of field gradients are discussed.

Dimethylpolysiloxanes↗
PubMed1996

Self-diffusion in fluids in porous glass: confinement by pores and liquid adsorption layers.

Diffusion coefficients of 10 different polar and nonpolar liquids filled in porous glasses with mean pore diameters of 4 or 30 nm were determined with the aid of the NMR field-gradient technique. In the time scale of these experiments (0.3 to 500 ms) diffusion coefficients were found to be time independent. Within the experimental error, no influence of the polarity of the adsorbate can be stated. The diffusion coefficients of all investigated fluids in glass with 4 and 30 nm pores were reduced by factors of 0.17 and 0.63, respectively, relative to the bulk values. This relatively weak reduction can be explained by considering the known porosities of the adsorbents. The second objective of this study was to examine the diffusion behaviour below the melting point of adsorbates in porous glass. Fluids confined in pores do not freeze at the bulk freezing temperatures. In this respect, two phases must be distinguished. A maximal two monolayer thick film adsorbed on the inner surfaces does not crystallize at all, whereas the "free" fraction of the fluid in the pores freezes at reduced temperatures according to the Gibbs-Thompson relation. The nonfrozen surface layers form a network in which self-diffusion can be investigated. Experiments have been carried out with cyclohexane. A reduction factor of 0.06 was found relative to the extrapolated values of the entirely unfrozen fluid in porous glass with a mean pore diameter of 30 nm. It is, thus, demonstrated that molecules in adsorption layers virtually retain their translational degrees of freedom along the surfaces. The lowering of the diffusivity is mainly due to the geometric restriction rather than to the interaction with the surface.

Diffusion↗
PubMed1996

Investigation of molecular order and dynamics in liquid crystals confined in porous media using the dipolar-correlation effect on the stimulated echo.

A new application of the stimulated echo pulse sequence is presented that permits the elucidation of molecular order and dynamics in a time scale between about 100 microseconds and the spin-lattice relaxation time. The technique exploits the influence of dipolar coupling on the quotient of the stimulated and primary echoes produced by the standard three 90 degrees-pulse sequence. Results obtained for a nematic liquid crystal in bulk and confined in porous glass (mean pore diameter 4 nm) are compared. In both cases the echo amplitude quotient oscillates as a function of the pulse spacing. In a bulk nematic crystal these oscillations originate from strong unaveraged dipolar interactions and directly reflect the molecular order in the material. In porous glass a real nematic order is absent. In this case, the oscillations can be attributed to spin exchange between inequivalent protons. Exchange rates are estimated.

Crystallography↗
PubMed1996

Field-cycling NMR relaxometry of liquids confined in porous glass: evidence for Levy-walks.

The frequency dependence of the longitudinal relaxation time T1 (v) of different liquids confined to microporous glass features a distinct difference between polar and nonpolar adsorbates. This finding is explained by "weak" and "strong" adsorption, respectively. The T1 dispersion of polar adsorbates can be described with the aid of Lévy walk statistics of "bulk-mediated surface diffusion" recently suggested by Bychuk and O'Shaughnessy.

Diffusion↗