Theoretical and experimental characterization of a low-Reynolds number split-and-recombine mixer

A mixing device based on the split-and-recombine (SAR) principle is characterized using both theoretical and experimental methods. The theoretical model relies on solving a 1D diffusion equation in a frame of reference comoving with the flow, thus avoiding the usual numerical artefacts related to the prediction of high-Péclet number mixing. It accounts both for the hydrodynamic focusing of the flow inside the mixing channel and the nontrivial flow topology. The experimental technique used for quantifying the degree of mixing utilizes two initially transparent salt solutions that form a colored compound in a fast chemical reaction. The degree of mixing is derived from the average color saturation found at specific positions along the mixing channel. The data obtained from the theoretical model are in reasonable agreement with the experiments and underline the excellent performance of the SAR mixer, with a mixing length growing only logarithmically as a function of Péclet number.     

Sensible drinking limits

A large number of prospective population studies from many countries have described a J- or U-shaped relation between alcohol intake and mortality. Both heavy drinkers and abstainers are at a higher risk of dying from all causes than individuals with light to moderate alcohol intake. This makes information to the public about sensible drinking limits more complex than, eg, that concerning smoking. The present paper aims at identifying upper thresholds for harmless alcohol intake. The review is mainly based on epidemiological evidence concerning somatic morbidity and mortality. It is concluded that the present Danish recommendations--14 drinks per week for women and 21 drinks per week for men--should be maintained. It is emphasized that these limits apply to adults who are at no risk of dependency and that they do not apply to pregnant women. Information about a potentially beneficial effect of a moderate alcohol intake should be reserved for individuals already at risk of coronary events.     

Cracking in stabilized austenitic stainless steel piping of German boiling water reactors—characteristic features and root cause

Cracks have been found in the welds of piping systems made from stabilized austenitic stainless steels in German boiling water reactors (BWR). In the course of the intensive failure analysis metallographic examinations, microstructural investigations by electron microscopy, corrosion experiments and welding tests have been performed. The results show that cracking under the given medium conditions is due to intergranular stress corrosion cracking (IGSCC) in those parts of the heat affected zone (HAZ) which are overheated during welding and where solution of titanium carbides and subsequent precipitation of chromium carbides and depletion of chromium along the affected grain boundaries could occur.     

Effect of sputtering power on friction coefficient and surface energy of co-sputtered titanium and molybdenum disulfide coatings and its performance in micro hot-embossing

Si micromolds are common for fabrication of polymer-based microfluidic devices by hot-embossing because of the well established fabrication methods for Si, e.g., deep reactive ion etching, for favorable surface finish and accuracy. The problems with low yield, poor reproducibility, premature failure and limited lifetime of a Si micromold are induced by high friction and surface adhesion generated during demolding. Therefore, Titanium (Ti) and molybdenum disulfide (MoS₂) coatings were deposited on Si micromolds via magnetron co-sputtering at various combinations of target powers to improve its surface properties. Coating composition, crystallographic orientation, roughness, critical load, hardness, friction coefficient and surface energy were measured by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy, scratch testing, nanoindentation, ball-on-disc tribometry and the contact angle method respectively. A statistical design of experiment matrix was used to investigate the effect of the Ti and MoS₂ target powers on the friction coefficient and surface energy of the coatings. From this designed experiment, it was observed that increasing MoS₂ target power was associated with increasing surface energy and decreasing friction coefficient and target powers had statistically significant effects on these parameters. Crystallinity, roughness and hardness of the coatings increased with increasing Ti concentration. A mathematical model of the effects of Ti and MoS₂ target powers on the friction coefficient and surface energy of the coatings has been fit to the experimental results using the response surface method. Uncoated and MoS₂–Ti coated Si micromolds were used in hot-embossing for a comparative study on replication performance of uncoated and various coated micromolds. Hotembossed PMMA microstructures showed that coating improve replication performance of Si micromolds. Si micromold coated with co-sputter of Ti and MoS₂ at power of 300 and 75 W respectively, showed better replication quality among the selected target powers.     

Strategies for size reduction of microreactors by heat transfer enhancement effects

One of the likely aims of reactor miniaturization in the field of chemical production and energy generation is to increase the conversion to the desired product and the selectivity of the process through better control of heat and mass transfer. In addition to the effects related to miniaturization, a further increase of the transfer coefficients is achieved by applying microstructuring techniques. In this context, three different approaches for heat transfer enhancement in miniaturized reaction systems are presented. The ideas put forward rely on entrance flow effects, inertial flows in meandering channels, and suppression of axial heat conduction. Among these ideas the entrance flow effect, realized by an arrangement of microfins with a typical dimension of a few hundred micrometers, provides the most efficient heat transfer. It is found that a heat transfer enhancement of at least one order of magnitude can be achieved compared to unstructured channels. On this basis, a miniaturized heat-exchanger reaction system is investigated, where a kinetic model of an endothermic, heterogeneously catalyzed gas-phase reaction is used. The miniaturized heat-exchanger reactor, both with and without heat transfer enhancement, is subsequently benchmarked against conventional fixed-bed technology. It is shown that, for the reaction system under study, a substantial reduction of the required amount of catalyst can be achieved in microsystems.     

Magnetophoretic mixing for in situ immunochemical binding on magnetic beads in a microfluidic channel

Functionalized magnetic beads offer promising solutions to a host of micro-total analysis systems ranging from immunomagnetic biosensors to cell separators. Immunochemical binding of functional biochemical agents or target biomolecules serves as a key step in such applications. Here we show how magnetophoretic motion of magnetic microspheres in a microchannel is harnessed to promote in situ immunochemical binding of short DNA strands (probe oligonucleotide) on the bead surface via streptavidin–biotin bonds. Using a transverse magnetic field gradient, the particles are transported across a co-flowing analyte stream containing biotinylated probe oligonucleotides that are labeled with a Cy3-fluorophore. Quantification of the resulting biotin–streptavidin promoted binding has been achieved through fluorescence imaging of the magnetophoretically separated magnetic particles in a third stream of phosphate buffered saline. Both the experimental and numerical data indicate that for a given flow rate, the analyte binding per bead depends on the flow fraction of the co-flowing analyte stream through the microchannel, but not on the fluid viscosity. Parametric studies of the effects of fluid viscosity, analyte flow fraction, and total flow rate on the extent of binding and the overall analyte separation rate are also conducted numerically to identify favorable operating regimes of a flow-through immunomagnetic separator for biosensing, cell separation, or high-throughput applications.     

Bronchial inhalation challenge by plethysmography in asthmatic children

A new technique for breath-by-breath controlled bronchial inhalation test is described, using the body plethysmographic method. The values of airway resistance (Raw) and of thoracic gas volume at resting expiratory level (TGV) have been compared to the data of total pulmonary flow resistance (R1), measured with the intraesophageal catheter method, and of functional residual capacity (FRC). The dynamic lung compliance (C1 dyn) was calcualted, too; the mechanical parameters of breathing (R1; Raw; C1 dyn) have been investigated on the same breathing cycles before, during and after respiratory challenges in symptom-free asthmatic children. Regarding the breath-to-breath variation of the parameters, a respiratory challenge is usually considered as positive, if the R1 or Raw values increase 100% or more. Out of 39 assays, "positive" challenges were observed in 14 casis regarding R1 values and in 11 cases regarding Raw values. "Doubtful" tests - i.e. +50% less than or equal to R1 or Raw less than 100% - were observed in 5 assays regarding R1 and in 7 cases regarding Raw. A decrease of more than 40% of C1 dyn was observed in 12 out of the 19 'positive' or 'doubtful-positive' challenges; the largest spontaneous breath-to-breath variation being +/- 25%. Such a decrease of C1 dyn is suggested as a sign of peripheral bronchial obstruction. An increase of more than 25% of TGV was observed in 11 out of 19 assays, considered "positive" or "doubtful-positive". A correlation being observed between C1 dyn decrease and TGV increase this latter parameter is suggested to be an additional sign of bronchial obstruction. Beside the practical problem of the interpretation of a respiratory challenge, these results are discussed from the phatophysiological point of view.