Power consumption and mixing times for liquid mixing in a ribbon mixer

Measurements were taken of power consumption and mixing times for helical ribbon agitators of different pitch and ribbon width under laminar flow conditions in newtonian liquid. Empirical correlations for prediction of the power consumption, the mixing time and the energy required to achieve a prescribed degree of homogeneity are proposed. The results are compared with those of other works.     

Model based design of a microfluidic mixer driven by induced charge electroosmosis

Mixing chemical or biological samples with reagents for chemical analysis is one of the most time consuming operations on microfluidic platforms. This is primarily due to the low rate of diffusive transport in liquid systems. Additionally, much research has focused on detection, rather than sample preparation. In response, we describe a mixer for microfluidic sample preparation based on the electrokinetic phenomenon of induced-charge-electroosmosis (ICEO). ICEO creates microvortices within a fluidic channel by application of alternating current (AC) electric fields. The microvortices are driven by electrostatic forces acting on the ionic charge induced by the field near polarizable materials. By enabling mixing to be turned on or off within a channel of fixed volume, these electronically controlled mixers prevent sample dilution-a common problem with other strategies. A three-dimensional model based on the finite volume method was developed to calculate the electric field, fluid flow, and mass transport in a multi-species liquid. After preliminary experiments, the model was used to rapidly prototype a wide range of designs. A new microfabrication process was developed for devices with vertical sidewalls having conductive metal coatings and embedded electrodes. Mixing experiments were carried out in the devices and the results were compared to the model.     

Longitudinal mixing in fast liquid-phase chemical reactions in a two-phase mixture

The possibility of diminishing the diffusion limitations in the course of fast chemical reactions in liquid-gas systems by varying the reactant ratio and the rate of longitudinal mixing in the liquid phase and at the phase boundary (via changes in the geometry and hydrodynamic parameters of flow in tubular apparatus) was examined.     

Electrochemically driven intramolecular ring closure of a linear hexapyrrole

[24]hexaphyrin(1.0.1.0.0.0) was electrochemically synthesized in good yield starting from a linear hexapyrrolic precursor. This straightforward approach provides an efficient "green" alternative to the reported chemical synthesis, which requires the use of trifluoroacetic acid as the solvent and toxic chromium(VI) salts as the oxidant.     

Three-dimensionally crossing manifold micro-mixer for fast mixing in a short channel length

In this study, we report a neo-conceptive three-dimensionally (3D) crossing manifold micromixer (CMM) embedded in microchannel. Fabricated by sequential processes of photolithography and two photon absorption stereolithography, this leads to a microfluidic system with a built-in micromixer in a site controlled manner. The effectiveness of CMM is investigated numerically and experimentally. Through the numerical simulation, it is estimated that a high mixing ratio of 90% can be obtained even in a channel length shorter than five times the channel width. This compares well with the conventional passive type of micromixers that have a gradual increase in mixing efficiency with the length of the channel. Furthermore, the mixing performance of the realized CMM built-in microchannel is observed by confocal microscopy.     

Flow-induced ordering of particles and flow velocity profile transition in a tube flow of graphene oxide dispersions

We measured the S- and P-order parameters of flow-induced ordered graphene oxide (GO) particles and the flow velocity profiles for a flowing aqueous GO dispersion in a tube, by using an optical method. The order parameters clearly exhibit increasing concentric biaxial ordering as the flow velocity increases, with the exception of a disordered centre. Newtonian to non-Newtonian transition in the flow velocity profile is found, changing from a parabolic shape to a fuller shape at very low Reynolds numbers less than 10. This is attributed to the shear thinning effect (i.e., an ordering-induced reduction in viscosity). In the Newtonian flow, a uniaxial ordering was dominant; whereas a biaxial ordering sharply increased in the non-Newtonian flow, indicating that both the ordering of GO particles and the interparticle interactions influence the flow profile transition.     

Sudden breakdown in linear response of a rotationally driven magnetic microparticle and application to physical and chemical microsensing

In this work, sensing magnetic microparticles were used to probe both the local pH and the viscosity-dependent nonlinear rotational behavior of the particles. The latter resulted from a critical transition marking a driven particle's crossover from phase-locking to phase-slipping with an externally rotating magnetic field, i.e., a sudden breakdown in its linear response that can be used to measure a variety of physical quantities. The transition from simple rotation to wobbling is described both theoretically and experimentally. The ability to measure both chemical and physical properties of a system could enable simultaneous monitoring of chemical and physical interactions in biological or other complex fluid microsystems.     

Experiments on the velocity field of an axially symmetrical air jet in the entrance region of a tube

The development of the turbulent velocity field of a circular air jet in the entrance region of a tube has been explored experimentally. The axial development of the velocity field is illustrated by a sequence of velocity profiles measured along the symmetry lines of the cross section. The entrance length is representable as (L_f/D) Re = 1.96×10⁵ for Re in the range of 8600 ∼ 21000.     

Anomalously fast migration of triplet-repeat DNA in capillary electrophoresis with linear polymer solution

We investigated the capillary electrophoretic behavior of single-stranded DNA fragments in methylcellulose solution, and found that triplet-repeat DNA showed anomalously faster mobilities than DNA markers with random sequence. Through the further study on the electrophoretic data, reptation model is proven appropriate to describe the migration of DNA under our experimental conditions. Accordingly, with the equations based on reptation theory, we could obtain the persistence length of DNA fragments and find that these values of triplet-repeat DNAs are larger than that of DNA markers with random sequence, which means the former DNAs are less flexible than the latter ones when they migrate in the electric field. This phenomenon is supposed to result from the characteristic higher-order structure formed by GC base pairs within triplet-repeat DNA, which is further proven by the resumed migration order in accordance with DNA size when the denaturant is added into the sieving matrix.     

Real-time observation of the spin-state mixing process of a micellized radical pair in weak magnetic fields by nanosecond fast field switching

The singlet-triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off-on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S-T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet-triplet and triplet-triplet dephasing rate constants are determined independently to be 2 x 10(8) and 0.2 x 10(8) s(-)1, respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair.     

Comparison of linear and 2D IR spectra in the presence of fast exchange

The few picosecond time scale H-bond making and breaking in the system acetonitrile-methanol dominates the mechanism of vibrational coherence transfer that is evident in the shapes of both the linear and nonlinear IR spectra of the CN group.     

Use of region II of the a/q stability diagram for fast scanning of a linear quadrupole mass spectrometer

For evaluation as a detector for high-speed gas chromatography, a linear quadrupole mass spectrometer was operated in rf only mode by using Region II (a = 0; 7.514 < q < 7.580) of the Mathieu a/q stability diagram. The available power supply and the diameter of the quadrupole rods of the mass spectrometer placed an upper mass limit of ～ m/z 93. Scan rates of 1000 scans/s were obtained with mass spectral peaks resolved over an 80-u range The m/z 91 and 92 ions produced from the electron ionization of toluene are resolved with an R _1/2 of 135. A potential difference between the source and the quadruple mass filter of up to 1000 V was used to accelerate ions into the quadrupole. Broadening of mass-to-charge ratio peaks results from the time constant of the signal amplification rather than the small number of rf cycles the ions experience. The expected loss of sensitivity relative to Region I is observed, and the problem of mass aliasing is discussed.     

Bounds and algorithms for fast exact searches of chemical fingerprints in linear and sublinear time

Chemical fingerprints are used to represent chemical molecules by recording the presence or absence, or by counting the number of occurrences, of particular features or substructures, such as labeled paths in the 2D graph of bonds, of the corresponding molecule. These fingerprint vectors are used to search large databases of small molecules, currently containing millions of entries, using various similarity measures, such as the Tanimoto or Tversky's measures and their variants. Here, we derive simple bounds on these similarity measures and show how these bounds can be used to considerably reduce the subset of molecules that need to be searched. We consider both the case of single-molecule and multiple-molecule queries, as well as queries based on fixed similarity thresholds or aimed at retrieving the top K hits. We study the speedup as a function of query size and distribution, fingerprint length, similarity threshold, and database size |D| and derive analytical formulas that are in excellent agreement with empirical values. The theoretical considerations and experiments show that this approach can provide linear speedups of one or more orders of magnitude in the case of searches with a fixed threshold, and achieve sublinear speedups in the range of O(|D|0.6) for the top K hits in current large databases. This pruning approach yields subsecond search times across the 5 million compounds in the ChemDB database, without any loss of accuracy.     

Magnetically modified bentonite as a possible contrast agent in MRI of gastrointestinal tract

A composite of iron oxide nanoparticles and mineral matrix has been studied by XRD, Mössbauer spectroscopy, and TEM. Magnetite and superparamagnetic magnetite have been identified by Mössbauer spectroscopy in the nanocomposite. A relationship between the hyperfine parameters and iron oxide particle size has been confirmed by TEM. The optimal concentration of “magnetite—bentonite” composite, when the MRI signal is fully reduced, was found for using this composite as a negative contrast agent.     

Interaction of microcrystalline cellulose and water in granules prepared by a high-shear mixer

Microcrystalline cellulose (MCC) granules were prepared by wet granulation using a high-shear mixer. Physical characteristics of the granules were investigated using near IR spectrometry, thermogravimetry and isothermal water vapor adsorption. Near IR spectra of dried MCC granules prepared for various granulation times exhibited different peak intensities at 1428, 1772, and 1920 nm, which were assigned to functional groups of cellulose or water. On isothermogravimetric analysis, the rate of dehydration of water was shown to decrease with granulation time. These results suggest that the physical structure of MCC could change during the granulation process, and the interaction between MCC and water was gradually strengthened. The isothermal water vapor adsorption curves suggested that the amorphous region of MCC would be divided by the strong shear force of the impeller, because the high adsorption ability of intact MCC in the low humidity region was diminished in granules collected following 5 and 10 min of granulation. It was suggested that MCC formed a network which caught water within its structure during the wet granulation process.     

Communication: drift velocity of Brownian particle in a periodically tapered tube induced by a time-periodic force with zero mean: dependence on the force period

We study the drift of a Brownian particle in a periodically tapered tube, induced by a longitudinal time-periodic force of amplitude ∣F∣ that alternates in sign every half-period. The focus is on the velocity dependence on the force period, which is usually considered not tractable analytically. For large ∣F∣ we derive an analytical solution that gives the velocity as a function of the amplitude and the period of the force as well as the geometric parameters of the tube. The solution shows how the velocity decreases from its maximum value to zero as the force period decreases from infinity (adiabatic regime) to zero. Our analytical results are in excellent agreement with those obtained from 3D Brownian dynamics simulations.     

Scale up of the mixer of a mixer-settler model used in a uranium solvent extraction process

Scale up relations were obtained for the mixer of a box type mixer-settler used in an uranium extraction process from chloridric leaches. Three box type mixers of different sizes and with the same geometry were used for batch and continuous-flow experiments. The correlations between the extraction rate and the specific power input,D/T ratio(=turbine diameter/mixer width) and residence time were experimentally determined. The results showed that the extraction rate increases with the power input at a constantD/T ratio equal to 1/3, remaining however, independent from the mixer size for a specific value of the power input. This behaviour was observed for power input values ranging from 100 to 750 W/m³.     

Influence of the tube surface on atomization behavior in a graphite tube furnace

Summary Sensitivity is compared for a number of elements in tubes for electrothermal atomic absorption spectrometry made from pyrolytic graphite coated and uncoated polycrystalline electrographite and from glassy carbon. The best atomization efficiency is always obtained in a pyrolytic graphite coated tube, independent of the volatility of the element and the atomization mechanism involved. The pronounced sticking properties of metal atoms to active sites of uncoated polycrystalline electrographite and glassy carbon are believed to be responsible to this behavior. This theory is supported by the observed shifts in peak maxima, peak broadening, and the higher atomization temperatures that have to be applied in these tubes. For elements forming stable carbides, these effects can be enhanced by chemical reactions with the tube material.

---

Einfluß der Rohroberfläche auf das Atomisierungsverhalten in einem Graphitrohrofen

Zusammenfassung Für eine Reihe von Elementen wird die Empfindlichkeit in Rohren für die elektrothermische Atomabsorptionsspektrometrie aus pyrolytisch beschichtetem und unbeschichtetem polykristallinem Elektrographit und aus glasartigem Kohlenstoff verglichen. Die wirksamste Atomisierung wird stets in pyrolytisch beschichteten Graphitrohren erzielt, unabhängig von der Flüchtigkeit des Elements und von dem Atomisierungsmechanismus. Wir nehmen an, daß die ausgeprägten Hafteigenschaften von Metallatomen an aktiven Stellen von polykristallinem Elektrographit und glasartigem Kohlenstoff für dieses Verhalten verantwortlich sind. Diese Theorie wird gestützt von den beobachteten Verschiebungen der Peakmaxima, Peakverbreiterungen und den höheren Atomisierungstemperaturen, die in diesen Rohren verwendet werden müssen. Für Elemente, die stabile Carbide bilden, können diese Effekte durch chemische Reaktionen mit dem Rohrmaterial noch verstärkt werden.

     

Cross-streamline migration of a semiflexible polymer in a pressure driven flow

Experiments and simulations on single α-actin filaments in the Poiseuille flow through a microchannel show that the center-of-mass probability density across the channel assumes a bimodal shape as a result of pronounced cross-streamline migration. We reexamine the problem and perform Brownian dynamics simulations for a bead-spring chain with bending elasticity. Hydrodynamic interactions between the pointlike beads are taken into account by the two-wall Green tensor of the Stokes equations. Our simulations reproduce the bimodal distribution only when hydrodynamic interactions are taken into account. Numerical results on the orientational order of the end-to-end vector of the model polymer are also presented together with analytical hard-needle expressions at zero flow velocity. We derive a Smoluchowski equation for the center-of-mass distribution and carefully analyze the different contributions to the probability current that causes the bimodal distribution. As for flexible polymers, hydrodynamic repulsion explains the depletion at the wall. However, in contrast to flexible polymers, the deterministic drift current mainly determines migration away from the centerline and thereby depletion at the center. Diffusional currents due to a position-dependent diffusivity become less important with increasing polymer stiffness.