Empty Microchannels Research Articles

Taming Taylor-Aris dispersion through chaotic advection

Taylor-Aris dispersion represents an undesired phenomenon in pressure-driven liquid chromatography, often responsible for the unchecked increase of the Height Equivalent of the Theoretical Plate (HETP) when high throughput operating conditions are sought. Previous work on the subject showed how it is possible to contain the augmented dispersion in empty microchannels by inducing cross-sectional velocity components yielding an overall helical structure of the flow streamlines. Here, we explore the possibility of further reducing axial dispersion by devising flow conditions that give rise to chaotic streamlines. A three-dimensional steady flow generated by the combination of a pressure-driven Poiseuille flow and an electroosmotically-induced spatially periodic flow is used as a case study. Brenner’s macrotransport approach is used to predict the axial dispersion coefficient of a diffusing solute in flows possessing regular, partially chaotic and globally chaotic kinematic features. Accurate Lagrangian-stochastic simulations of particle ensembles are used to validate the predictions obtained through Brenner’s paradigm. Our findings suggest that the Taylor-Aris phenomenon can be altogether suppressed in the limit of globally chaotic kinematics. A theoretical interpretation of this outcome is developed by combining macrotransport theory with established results of the spectral approach to mixing in advecting-diffusing chaotic flows.

Brownian Sieving Effect for Boosting the Performance of Microcapillary Hydrodynamic Chromatography. Proof of Concept.

Microcapillary hydrodynamic chromatography (MHDC) is a well-established technique for the size-based separation of suspensions and colloids, where the characteristic size of the dispersed phase ranges from tens of nanometers to micrometers. It is based on hindrance effects which prevent relatively large particles from experiencing the low velocity region near the walls of a pressure-driven laminar flow through an empty microchannel. An improved device design is here proposed, where the relative extent of the low velocity region is made tunable by exploiting a two-channel annular geometry. The geometry is designed so that the core and the annular channel are characterized by different average flow velocities when subject to one and the same pressure drop. The channels communicate through openings of assigned cut-off length, say A. As they move downstream the channel, particles of size bigger than A are confined to the core region, whereas smaller particles can diffuse through the openings and spread throughout the entire cross section, therein attaining a spatially uniform distribution. By using a classical excluded-volume approach for modeling particle transport, we perform Lagrangian-stochastic simulations of particle dynamics and compare the separation performance of the two-channel and the standard (single-channel) MHDC. Results suggest that a quantitative (up to thirtyfold) performance enhancement can be obtained at operating conditions and values of the transport parameters commonly encountered in practical implementations of MHDC. The separation principle can readily be extended to a multistage geometry when the efficient fractionation of an arbitrary size distribution of the suspension is sought.

Combined effects of a microchannel with porous media and transverse vortex generators (TVG) on convective heat transfer performance

The three-dimensional heat transfer and pressure drop of fluid flow within a microchannel with transverse vortex generator and porous medium are numerically investigated. A total of 14 cases with various designs of vortex generator, semi-porous, and completely porous material were studied in detail. Further, the performance is compared between microchannel with different heights and the number of transverse vortex generators, as well as a semi-porous microchannel with a vortex generator. The finite volume method is used to solve the governing equations based on the three-dimensional volume averaging method for single-phase laminar flow. The Darcy-Forchheimer model is applied to solve the flow in a porous medium. The computational domain includes a stainless steel rectangular microchannel with a vortex generator and/or the insertion of porous media. The numerical results indicate that the convective heat transfer coefficient increases with increasing height and the number of transverse vortex generators. Compared to the empty microchannel, the heat transfer coefficient is 12 times higher with a completely filled porous media and 2.6 times higher with eight vortex generators with 12.5% of the channel height. While pursuing a high heat transfer coefficient, the pressure drop of the fluid flow often also increases. Therefore, a thermal performance ratio is defined to normalize the change of heat transfer coefficient and pressure drop. The final combined results show that the microchannel with vortex generator and with top-and-bottom inserted porous media has the highest thermal performance ratio at low and high Reynolds number, respectively. Lastly, detail cross-sectional and down-the-channel flow streamlines and temperature distributions are shown to identify the fundamental mechanism of heat transfer. The findings of this study provide a comprehensive insight into designing an effective microchannel with optimal convective heat transfer and reasonable pressure drop.

Heat transfer intensification in microchannel by induced-charge electrokinetic phenomenon: a numerical study

In this numerical investigation, the induced-charge electrokinetic phenomenon is used to intensify the convective heat transfer rate in the microchannel. The electrically conductive obstacles are placed in the microchannel for inducing the vortices and subsequently, enhancing the mixing of fluid and convective heat transfer rate. The results gained by the current numerical simulation are benchmarked with the other data available in the literature to ensure about the precision of the numerical solver. The influences of several parameters, including the length, location, and number of conductive obstacles and the electric field intensity, on the convective heat transfer rate are studied. The outcomes indicate that the convective heat transfer coefficient achieved for the case of the microchannel equipped with the electrically conductive obstacle is about 18 times larger as compared with the case of the empty microchannel without placing the electrically conductive obstacle. It is suggested to install the conductive obstacle near the inlet of the microchannel to achieve a higher heat transfer rate. The convective heat transfer coefficient increases up to 215.4% inside the microchannel as length of the conductive obstacle is increased from 15 to 35 µm. Finally, the normalized convective heat transfer coefficient improves about 743.4% when the electric field strength is boosted from 20 to 60 V cm−1.

Diverse nature of femtosecond laser ablation of poly(L-lactide) and the influence of filamentation on the polymer crystallization behaviour

Over the past few years we have witnessed growing interest in ultrafast laser micromachining of bioresorbable polymers for fabrication of medical implants and surface modification. In this paper we show that surface structuring of poly(L-lactide) with 300 fs laser pulses at 515 and 1030 nm wavelength leads to formation of defects inside the polymer as a result of laser beam filamentation. Filament-induced channels have diameter around 1 μm and length of hundreds of micrometers. SEM images of microchannels cross-sections are presented. The influence of wavelength and pulse spacing on bulk modification extent was investigated and parameters limiting filamentation were determined. We show that filamentation can be used for controlling properties of PLLA. The presence of filament-induced modifications such as empty microchannels and pressure wave-induced stress lead to increased ability of polymer to crystallize at lower temperature. Crystallization behaviour and crystal morphology after laser treatment was investigated in details using different analytical techniques such as WAXD, DSC and FTIR/ATR. Hydrolytic degradation experiment was performed. Presented method can be applied for controllable, spatially distributed modification of polymer crystallinity, crystalline phase structure and hydrolytic degradation profile.

A Microfluidically Reconfigurable Dual-Band Slot Antenna With a Frequency Coverage Ratio of 3:1

Reconfiguration of filled and empty microchannels is used to realize a switchable dual-band slot antenna whose both bands can be controlled independently. Reactive loading effect of Galinstan bridges is used to shift down each slot antenna's frequency of operation. Using five microchannels results in a 5-bit reconfigurable antenna. Any combination of frequencies within the ranges of 1.8-3.1 GHz (first band) and 3.2-5.4 GHz (second band) can be obtained. This translates into a discrete tuning ratio of ~ 1.7:1 for both bands and overall ratio of 3:1 (1.8-5.4 GHz) for the antenna. The antenna circuit board is realized using common printed circuit board (PCB) techniques, and the microchannels' polydimethylsiloxane (PDMS) substrate is fabricated using 3-D printing technologies. The circuit board and the PDMS substrates are bonded to each other using a thin layer of spin-coated PDMS. The performance of the antenna is measured, and close agreement between the simulation and measurement is observed.

Dielectric Impedance Spectroscopy in Flexible Polymer Microchip: Towards the Non Contact Biosensors

In recent years, development of miniaturization technology was increasing due to well known advantages and the crucial need for technology to get the micrometer scale measurements and to improve sensitivity of detection methods [1]. Among the various microsystems developed for “laboratory on chip”, non contact microelectrodes in microsystems is a very interesting method for detection because of the absence of direct contact between the electrodes and the solution microchannel [2]. The present work describes new approaches for non contact impedance device which permit to reach a best understanding of this specific transduction. Measurements were carried out through a Polyethylene Terephthalate (PET) microchannel photoablated having a trapezoidal cross-section shape, depth of 45 µm, width of 100 µm and a length of 1.4 cm. The distance separation between both microchannels is 120 µm center to center. The microband of PET is carried out in the following way: microchannels are coated with gold nanoparticules with an average size of 19 nm a standard deviation of 2 nm. The detection is achieved using hydride carbon ink/gold nanoparticules electrodes, thermally laminated by a polyethylene terephthalate/polyethylene terephthalate (PET/PET) layer with thickness of 35 µm at 135°C and a pressure 2 bar. The distance separation in the PET band between the two planar microelectrodes and the main microchannel is equal to 5 µm and the detection surface area per microelectrode is 88 µm x 100µm [3]. Firstly, the study was carried out by dielectric impedance spectroscopy measurement in alternative mode (ac) through the two embedded microelectrodes with the empty microchannel at frequency range from 1 MHz to 1 Hz and 0.1 V amplitude. Then, experiments were performed in the same conditions with streaming electrolyte in the microchannel in order to have the electrical behaviour of the overall microsystem and to extract the specific microchannel response. A procedure for eliminating the contribution of the surrounding polymer on the global impedance response has been established. After correction, the impedance diagrams exhibit a loop from very high frequencies to medium frequencies (1MHz to 100 Hz) and a capacitive behavior at low frequencies. This operation allows a clear observation of solution microchannel impedance such as, resistance, capacitance and diffusion phenomenon in microchannel. As a consequence, the diameter of the loop is inversely proportional to the microchannel conductivity when the streaming electrolyte concentration increases. We also found that the capacitive behaviour at the interface PET / microchannel is clearly influenced by conductivity and the flow rate. Secondly, the modeling of the non contact transduction is based on a system of partial differential equations representing the phenomena occurring in the microsystem. Solving these equations, using the methods of finite element resolutions with Comsol Multiphysics, allow get back to the values of current and potential in the microsystem. The model validation is obtained by comparison with experimental results. Study on dielectric behavior of the PET by impedance and modeling show predominance of an interfacial contribution on PET resistivity variation. This latter observation underlines that resistivity cannot be neglected, because the overall measurement in the microsystem will also depend on PET resistivity change. From the proposed model, which is also depends on interfacial impedance, microchannel conductivity and PET impedance have to be taken into account for a better understanding of global experimental data at fixed frequencies. Finally, the bioanalytical application concerns experiments conducted at a fixed frequency (200 Hz) in the interfacial impedance region to monitor the proteins adsorption onto the polymer surface. This will be illustrated with results obtained for protein adsorption monitoring into the microchannel. Protein need not to be labelled, as in optical biosensor, even if they need to be attached to the polymer surface coupled with the microelectrodes when a biomolecular interaction occurs. Modeling the microchip interface using an appropriate equivalent circuit permits to extract the value of the interfacial capacitance for ultralow protein concentration. The promising results obtained with this methodology make it a competing method in comparison with other transductions for biosensors developments. [1] J. Gamby, J.-P Abid, H. H. Girault, J. Am. Chem. Soc., 2005, 127(38), 13300. [2] J. Gamby, J.-P Abid, B. Tribollet, H. H. Girault, Small, 2008, 4, 802. [3] M. Kechadi, J. Gamby, L. Chaal, H. H. Girault, B. Saidani, B. Tribollet, Electrochim. Acta., 2013, 105, 7.

Free Contact Microchannel Impedance Through Two Antiparallel Planar Microelectrodes

The present work describes new results using a contact free impedance device, which permits to reach a best understanding of the free contact transduction. Measurements were carried out through a polyethylene terephthalate (PET) microchannel photoablated. Impedance measurement of empty microchannel at different PET thickness separating the two electrodes demonstrate that the recorded impedance is not changed whatever the PET thickness. The result shows that the polyethylene (PE) film lamination has an effect on the impedance measurement in empty microchannel. Then impedances diagrams in the filled microchannel were plotted by subtracting impedance contribution in empty microchannel from the global microdevice response. The Nyquist diagram of filled microchannel exhibits a loop diameter from high to medium frequencies, and a capacitive behavior at low frequencies. The loop diameter obtained is inversely proportional to the microchannel conductivity. Modeling impedance of microsystem highlighted the effect of microchannel rough surface phenomena.

Oxidative steam reforming of methane to synthesis gas in microchannel reactors

Oxidative steam reforming of methane to synthesis gas (syngas) over an alumina supported bimetallic Pt–Rh catalyst was comparatively investigated in coated and packed microchannel reactors. In the first configuration, thin layers of catalysts are coated on opposite walls of a single microchannel, while the second one is described by particulate catalysts packed into an empty microchannel of dimensions identical with the first one. Both geometries are compared on the basis of methane conversion and CO selectivity measured at different values of parameters, namely reaction temperature (773–923 K), molar steam-to-carbon (S/C = 0–3.0) and oxygen-to-carbon (O2/C = 0.47–0.63) ratios in the feed, and contact time (0.36–0.71 mg min cm−3). Although methane conversions are found to be comparable, the coated catalyst gave significantly higher CO selectivities than the packed counterpart in the whole parameter range. Increase in all of the parameter values led to improvement in methane conversion, while CO selectivity increased only with temperature and contact time. Molar H2/CO ratios obtained in the coated microchannel reactor are found to vary between 1.0 and 3.0 which are at least three times smaller than those produced in the packed microchannel reactor. Catalyst deactivation is not detected in both configurations. Stable operation up to 72 h over coated microchannel verified mechanical and chemical stability of the Pt–Rh coating that produced syngas with H2/CO ratio of 2.12 at temperatures lower than employed in industrial reformers. Different flow distribution properties of coated and packed microchannels seem to play roles in affecting the product distribution.

Investigation of catalyst performance and microstructured reactor configuration for syngas production by methane steam reforming

This work involves comparative investigation of two catalytic microchannel configurations for synthesis gas generation by methane steam reforming. In the first configuration, catalyst is in the form of a layer which is coated onto the microchannel wall, while the second one is defined by the packing of particulate catalyst into an empty microchannel of the same dimension. Comparison between these geometries is carried out for four different catalysts – Rh, Ru, Pd, Ni – all of which are supported on alumina, and involves testing of different reaction temperatures (873, 923, 973, 1023 K) and steam-to-carbon ratios (2.5, 3.0, 3.5) for each catalyst through a parametric approach. Compared with the packed version, wall-coated microchannel geometry leads to higher methane conversions and syngas production rates as well as lower H 2/CO ratios in the entire parameter and catalyst range investigated. Rh is the best catalyst in terms of activity and H 2 and CO throughputs, while Ni gives the highest hydrogen selectivity. The results indicate the possibility of reducing H 2/CO ratio in methane steam reforming by the use of reformers characterized by wall-coated microchannels.

Impact of Conduit Geometry on the Performance of Typical Particulate Microchip Packings

This work investigates the impact of conduit geometry on the chromatographic performance of typical particulate microchip packings. For this purpose, high-performance liquid chromatography (HPLC)/UV-microchips with separation channels of quadratic, trapezoidal, or Gaussian cross section were fabricated by direct laser ablation and lamination of multiple polyimide layers and then slurry-packed with either 3 or 5 microm spherical porous C8-silica particles under optimized packing conditions. Experimentally determined plate height curves for the empty microchannels are compared with dispersion coefficients from theoretical calculations. Packing densities and plate height curves for the various microchip packings are presented and conclusively explained. The 3 microm packings display a high packing density irrespective of their conduit geometries, and their performance reflects the dispersion behavior of the empty channels. Dispersion in 5 microm packings correlates with the achieved packing densities, which are limited by the number and accessibility of corners in a given conduit shape.

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.

Three-dimensional microfabrication by femtosecond pulses in dielectrics

We report femtosecond laser microfabrication in thin films of polymeric photoresists by (i) direct laser writing and (ii) holographic recording techniques. With (i) we have fabricated empty microchannels in polymethylmethacrylate; the channel width closely follows the intensity distribution in the focal spot of a high numerical aperture lens in accordance with scalar and vectorial Debye theory. With (ii) we have fabricated free-standing two-dimensional periodic structures in SU8 photoresist. The structures contain cylindrical pillars with noticeable ripple-like modulation of their diameter along their axes. This modulation results from the interference between the waves incident on the sample and back-reflected from the glass substrate or air, and is explained using an appropriate numerical modeling.

Techniques to preserve soluble surface components in birch pollen wall: a scanning and transmission electron microscopic study.

The exine of birch pollen was examined by scanning and transmission electron microscopy in the native state and after fixation in different aqueous fixatives: glutaraldehyde + OsO4; glutaraldehyde + cetylpyridinium chloride (CPC) + OsO4; glutaraldehyde + cuprolinic blue (CB); and periodate + lysine + paraformaldehyde (PLP). The native pollen exine showed a thin (3-5-nm) border of electron-dense material lining the tectum and electron-dense material within microchannels and bacula cavities. Fixation with the addition of CPC resulted in a voluminous surface coat surrounding the pollen grain, but empty microchannels and bacula cavities. After fixation with the addition of CB, there was a thin surface coat, whereas microchannels and bacula cavities were partially filled with electron-dense material. The other fixatives led to empty microchannels and bacula cavities. There was no surface coat on the pollen grain. However, after all fixation procedures, a thin electron-dense border of the tectum remained visible. Concerning the electron-dense material filling microchannels and bacula cavities in the native pollen grain, the results obtained in the present study suggest that it is either completely lost (after conventional and PLP fixation) or, after fixation with a precipitating additive, partially (CB) or completely (CPC) solubilized and precipitated on the surface of the pollen grain as a surface coat.