Maarten Arnold Rutgers

Triaxial magnetic field gradient system for microcoil magnetic resonance imaging

There is a great advantage in signal to noise ratio (S/N) that can be obtained in nuclear magnetic resonance (NMR) experiments on very small samples (having spatial dimensions ∼100 μm or less) if one employs NMR “micro” receiver coils, “microcoils,” which are of similarly small dimensions. The gains in S/N could enable magnetic resonance imaging (MRI) microscopy with spatial resolution of ∼1–2 μm, much better than currently available. Such MRI microscopy however requires very strong (>10 T/m), rapidly switchable triaxial magnetic field gradients. Here, we report the design and construction of such a triaxial gradient system, producing gradients substantially greater than 15 T/m in all three directions, x, y, and z (and as high as 50 T/m for the x direction). The gradients are switchable within time ∼10 μs and adequately uniform (within 5% over a volume of [600μm3] for microcoil MRI of small samples.

Conducting fluid dynamics experiments with vertically falling soap films

This article gives a detailed description of an apparatus in which flowing soap films are used to perform two dimensional fluid dynamics experiments. We have previously reported scientific findings made with the apparatus, but never carefully described the technique, or its full potential. A brief introduction is given on the nature of soap films as fluids and then all the details necessary for creating robust flowing films are listed. Typical parameters for the system are: flow speeds from 0.5 to 4 m/s, film thickness between 1 and 10 μm, and typical film sizes are 3 m tall and 10 cm wide although films of 20 m tall and 4 m wide have also been made. A vacuum apparatus is also described in which the air drag on the film can be reduced by a factor of 5–10. Finally, a large number of techniques for measuring flow and thickness are outlined and referenced.

Velocity fluctuations in a turbulent soap film: The third moment in two dimensions

Quasi-two-dimensional decaying turbulence is studied in a flowing soap film by measuring the moments of the probability density function P(δv(r)) for the longitudinal velocity differences δv(r) on a scale r. As in three-dimensional (3-D) turbulence, P becomes non-Gaussian with decreasing r. The third moment S3(r)≡〈(δv(r))3〉 is small and negative at small scales, but becomes positive at larger scales. The exact calculation of S3(r) for 2-D homogeneous isotropic turbulence relates this change in sign to the development of the velocity correlation function as the turbulence decays.

Infrared technique for measuring thickness of a flowing soap film

In conducting two-dimensional laminar and turbulence experiments, use of a vertical flowing soap film is often a good choice. However, one of the most frequently encountered and yet highly nontrivial problems is to measure the thickness of the film precisely. We propose a solution to this problem based on the strong absorbtion of infrared light by the water molecules in the film. At λ≃3 μm, a thin sheet of water is essentially opaque. The extinction length of 0.9 μm serves as a precise ruler for gauging the film thickness. Although only the time-averaged, single-point measurements are presented, the technique is general and can be used for multipoint measurements to investigate turbulent driven spatiotemporal fluctuations of the film thickness.