Planar microcoil-based microfluidic NMR probes

Microfabricated small-volume NMR probes consisting of electroplated planar microcoils integrated on a glass substrate with etched microfluidic channels are fabricated and tested. 1H NMR spectra are acquired at 300 MHz with three different probes having observed sample volumes of respectively 30, 120, and 470 nL. The achieved sensitivity enables acquisition of an 1H spectrum of 160 microg sucrose in D2O, corresponding to a proof-of-concept for on-chip NMR spectroscopy. Increase of mass-sensitivity with coil diameter reduction is demonstrated experimentally for planar microcoils. Models that enable quantitative prediction of the signal-to-noise ratio and of the influence of microfluidic channel geometry on spectral resolution are presented and successfully compared to the experimental data. The main factor presently limiting sensitivity for high-resolution applications is identified as being probe-induced static magnetic field distortions. Finally, based on the presented model and measured data, future performance of planar microcoil-based microfluidic NMR probes is extrapolated and discussed.     

Transient flow near a rotating disk

Summary The solution of the time dependent flow due to the impulsive starting of a single infinite disk from rest is obtained numerically for the entire history of the transient. The primary tangential velocity exhibits a single overshoot of its steady value while the growth of the secondary flows is monotonic. The overshoot is seen to be a direct consequence of the lag in the development of the secondary flows. An analytical solution is obtained for a related linearized problem: The angular velocity of an infinite disk, initially rotating with an infinite environment, is perturbed. The oscillatory decays to the steady state, which occur in both unbounded and bounded linearized analyses, are discussed in relation to the overshoot in the impulsively started disk problem.     

Über die Cycloaddition von Dithiocyan und Trithiocyan mit Hexafluoraceton und Folgeprodukte aus der Spaltung der Schwefel-Schwefel-Bindung mit elementarem Chlor im Dithiocyan-Hexafluoraceton-Addukt

Cycloaddition of Dithiocyanogen and Trithiocyanogen with Hexafluoroacetone and Products of the Cleavage of the Sulfur-Sulfur Bond in Dithiocyan-Hexafluoroacetone Adduct by Elemental Chlorine The reaction of dithiocyanogen and trithiocyanogen with hexafluoroacetone (HFA) leads to the cycloaddition products (SCN)₂ · 4 HFA 1 and S(SCN)₂ · 4 HFA 2 . These are the first reactions of (SCN)₂ and S(SCN)₂ without cleavage of the S—S bonds. Elemental chlorine cleaves in 1 the S? S bond and 3 is formed. In 3 the chlorine atom can be replaced by the rests —CN 4 , (C₆H₁₁)₂N— 5 , and n-C₄H₉– 6. 6 eliminates one molecule of HFA by warming up and the four-membered ring 7 is formed. The compounds were characterized on the basis of mass and nmr spectra.     

Instability waves and the origin of bubbles in fluidized beds: Part 1: Experiments

Statistically significant measurements of the propagation properties of instability waves in a two-dimensional liquid fluidized bed are reported. Visual and quantitave measurements show that although the waves experience an initially exponential growth in amplitude, the ultimate state of motion exhibited is that of the complicated formation and destruction of cylindrical bubble-like structures. Expected values of their amplitude, frequency, and velocity are measured, and preliminary scaling laws are proposed. The implication is that bubbles in gas-fluidized beds are a result of the same instability of the state of uniform fluidization.     

Stability of a radially bounded rotating fluid heated from below

The stability of a bounded rotating cylinder of fluid heated from below is treated mathematically under the assumptions of stationary onset and axisymmetry. Critical Rayleigh numbers are computed by Galerkin's method as a function of the Taylor number and cylinder aspect ratio for Taylor numbers,10⁶. The constraining effect of the side walls is shown to decrease with either increasing or increasing radius/height ratios. For>10⁶, most cylinders, excluding extremely tall ones, will appear infinite in horizontal extent as far as stability characteristics are concerned. The form of the motion at onset is discussed in relation to previous work.     

Linear stability of a draining film squeezed between two approaching droplets

In the present paper we analyze the effect of infinitesimal non-axisymmetric perturbations in determining the critical gap thickness at which a draining, finite radius thin-film becomes unstable. The film is part of the suspending fluid trapped between two approaching deformable drops under the action of a flow field. We carry out a linear stability analysis in the context of a quasi-static approximation where the rate of growth of the disturbances is assumed to be much faster than the rate of film drainage. An analytical solution is derived for the model in the special case of a uniformly thick film, for two types of perturbation: fixed-end and free-end. It is shown, for this special case, when the hydrodynamic force pushing the drops together from the external flow is constant, that the four most unstable disturbances are of the free-end kind, associated with the lowest frequency modes of azimuthal variation in the film thickness. Higher modes are stabilized by surface tension. Our analysis also shows that adopting the unretarded form of the van der Waals disjoining pressure yields results similar to the analysis when electromagnetic retardation effects are included in the calculation. A second case is analyzed where the film is also of uniform thickness but its lateral extent and the gap thickness are both time-dependent. This case was included to extend the predictions to glancing drop-collisions where the external hydrodynamic force is time-dependent. We find that there is a maximum capillary number below which the film becomes unstable, and that there is range of angles in the trajectory where the film becomes unstable, but that outside this range the film is stable.     

Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip

A germanium (Ge) strip waveguide on a silicon (Si) substrate is integrated with a microfluidic chip to detect cocaine in tetrachloroethylene (PCE) solutions. In the evanescent field of the waveguide, cocaine absorbs the light near 5.8 μm, which is emitted from a quantum cascade laser. This device is ideal for (bio-)chemical sensing applications.     

A high current density DC magnetohydrodynamic (MHD) micropump

This paper describes the working principle of a DC magnetohydrodynamic (MHD) micropump that can be operated at high DC current densities (J) in 75-microm-deep microfluidic channels without introducing gas bubbles into the pumping channel. The main design feature for current generation is a micromachined frit-like structure that connects the pumping channel to side reservoirs, where platinum electrodes are located. Current densities up to 4000 A m(-2) could be obtained without noticeable Joule heating in the system. The pump performance was studied as a function of current density and magnetic field intensity, as well as buffer ionic strength and pH. Bead velocities of up to 1 mm s(-1) (0.5 microL min(-1)) were observed in buffered solutions using a 0.4 T NdFeB permanent magnet, at an applied current density of 4000 A m(-2). This pump is intended for transport of electrolyte solutions having a relatively high ionic strength (0.5-1 M) in a DC magnetic field environment. The application of this pump for the study of biological samples in a miniaturized total analysis system (microTAS) with integrated NMR detection is foreseen. In the 7 T NMR environment, a minimum 16-fold increase in volumetric flow rate for a given applied current density is expected.     

Dynamic response of geometrically constrained vapor bubbles

We consider a two-dimensional model of a vapor bubble between two horizontal parallel boundaries held at different temperatures. When the temperatures are constant, a steady state can be achieved such that evaporation near the contact lines at the hot bottom plate is balanced by condensation in colder areas of the interface near the top. The dynamic response of the bubble is probed by treating the case of time-dependent wall temperatures. For periodic modulations of the wall temperature the bubble oscillates about the steady state. In order to describe such time-dependent behavior we consider the limit of small capillary number, in which the effects of heat and mass transfer are significant only near the contact lines at the bottom plate and in a small region near the top. When the bottom temperature is modulated and the top temperature is held fixed, the amplitude of forced oscillations is constant for low-frequency modulations and then rapidly decays in the high-frequency regime. When the top temperature is modulated with fixed bottom temperature, the dynamic-response curve is flat in the low-frequency regime as well, but it also flattens out when the frequency is increased. This shape of the response curve is shown to be the result of the nonmonotonic behavior of the thickness of the liquid film between the bubble interface and the top plate: when the temperature is decreased, the film thickness increases rapidly, but then slowly decays to a value which is smaller than the initial thickness. The dynamic response is also studied as a function of the forcing amplitude.