Silicon Microchannel Research Articles

Single- and Two-Phase Thermal Transport in Microchannels With Embedded Staggered Herringbone Mixers

Improving mixing is an effective means to enhance single- and two-phase heat transfer in microchannels. However, it is challenging to induce since the flow in microchannels is laminar in the most working conditions. We report that heat transfer rate and critical heat flux (CHF) on 1-methoxyheptafluoropropane (HFE-7000) can be significantly enhanced by patterning embed- ded micromixers on the bottom walls in a parallel silicon microchannel array, which consists of five parallel channels (height, width, length: 250 µm × 220 µm × 10 mm). Compared with a plain-wall microchannel array at a mass flux range of 1018 to 2206 kg/m 2 · s and a heat flux range of 10 to 198 W/cm 2 ,s ingle- phase heat transfer rate, two-phase heat transfer rate, and CHF are enhanced up to 221%, 160%, and 61% using microscale staggered herringbone mixers in microchannels, respectively. These mixers consist of 7 or 3.5 Hz with 12 staggered herringbone grooves (50 µm in depth and width) with 90° between two asym- metric arms in each cycle. Its asymmetry is defined in accordance with the off center position of the apex of the herringbone groove. Finally, experimental results suggest that the locations and coverage of the micromixers have significant impacts on both single and two-phase heat transfer in microchannels.(2013-0372)

Ordered-standing nickel hydroxide microchannel arrays: Synthesis and application for highly sensitive non-enzymatic glucose sensors

A non-enzymatic glucose sensor is constructed by using nickel hydroxide (Ni(OH)2) modified silicon microchannel plates (MCP) as the sensing materials. The 3D ordered Si MCP is fabricated by electrochemical etching and the Ni(OH)2 coating are prepared by electroless plating. The nanocomposite electrode exhibits good catalytic activity toward oxidation of glucose in 0.1M KOH. This non-enzymatic glucose sensor boasts a fast amperometric response time of less than 5s, sensitivity of 0.25mAmM−1cm−2, and detection limit of 3.5μM at a signal-to-noise ratio of 3. The sensor shows superior stability, anti-interference capability, and selectivity. The good analytical capability, low cost, and compatibility with silicon technology make the Ni(OH)2/Si MCP electrode promising in amperometric non-enzymatic glucose detection.

Heat transfer performance of a fractal silicon microchannel heat sink subjected to pulsation flow

Abstract Numerical simulation and experiment are conducted to study the heat transfer performance of a symmetrical fractal silicon microchannel network subjected to a pulsation flow. The distributions of pressure drop, temperature and the Nusselt number in the silicon microchannels are presented for different pulsation frequency. Compared to the steady flow case, the influences of pulsation frequency (0–40 Hz) and Reynolds number (1800–2800) on heat transfer enhancement are reported. For the low pulsation frequency (2–10 Hz) and high pulsation frequency (30–40 Hz), the heat transfer rate is higher than that for the moderate frequency (10–20 Hz) at the same Reynolds number. With the increase of the Reynolds number, enhancement factors decrease approximately from 40% to 5% for the investigated frequency range.

Fabrication of nanocomposite electrode based on Bi4−xNdxTi3O12 perovskite supported by silicon microchannel plates for high performance electrochemical capacitors

A novel self-standing Bi4−xNdxTi3O12 electrodes structure based on the three dimensional silicon microchannel plates (Si-MCP) is proposed for electrochemical capacitors. In order to decrease electron transfer barrier and contact resistance, the Ni/Si-MCP compound materials were synthesized as current collectors by electroless plating. The Bi4−xNdxTi3O12 perovskite were distributed on the channel of Ni/Si-MCP compound structure by sol–gel techniques. The morphology and microstructure of as-synthesized mesoporous Bi4−xNdxTi3O12 (BNT) were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD). The electrodes have not only excellent capacitive performances, but also superior electrochemical stability. Owing to the formation of thin film morphology, porous structure, ordered channel and the intimate contact between the electroactive material and the current collector, the BNT/Ni/Si-MCP electrodes provide an enhancement of the kinetics by short transport/diffusion path lengths, timely accompanying of electrolyte and large specific surface areas, thus both high energy density and high power density are achieved. In particular, since the fabrication process is compatible with conventional silicon technology, the structure has immense potential as integrated supercapacitors.

Effects of Non-Uniform Heating on the Location and Magnitude of Critical Heat Flux in a Microchannel Heat Sink

Decreasing form factors and diminishing numbers of thermal interfaces and spreading layers in modern, compact electronic packages result in non-uniform heat generation profiles at the chip level being transmitted directly to the heat sinks. An improved understanding of the effects of non-uniform heating on the heat dissipation limits in microchannel heat sinks has become essential. An experimental investigation is conducted to measure the location and magnitude of critical heat flux (CHF) in a microchannel heat sink exposed to a range of non-uniform heating profiles. A 12.7 mm X 12.7 mm silicon microchannel heat sink with an embedded 5 X 5 array of individually controllable heaters is used in the experiments. The microchannels in the heat sink are 240 mm wide and 370 μm deep, and are separated by 110 mm wide fins. The dielectric fluid HFE-7100 is used as the coolant, with an average mass flux in the heat sink of approximately 800 kg/m2s. High-speed visualizations of the flow are recorded to capture the CH...

Superhydrophilic titania wall coating in microchannels by in situ sol–gel modification

An in situ reaction sol–gel method to synthesize superhydrophilic titania films in silicon microchannels at room temperature is reported. Superhydrophilic surface can be realized on TiO2 films with thickness less than 10 nm. The water flow velocity in the TiO2-coated silicon channels reached almost 4 times of the velocity in SiO2-coated channels. The ultra-thin superhydrophilic TiO2 films fabricated by this method show the ability to strongly improve the capillary of microchannels without affecting the morphology of the channel walls, indicating potential applications to biomolecule analysis and surface tension driven microfluidic systems. Due to its low operating temperature, this method is also suitable for polymer microstructures such as PDMS and PMMA microfluidic chips.

A Semi-Analytical Thermal Modeling Framework for Liquid-Cooled ICs

With the development of liquid-cooled integrated circuits (ICs) using silicon microchannels, the study of heat transfer and thermal modeling in liquid-cooled heat sinks has gained interest in the last five years. As a consequence, several methodologies on the thermally-aware design of liquid-cooled 2-D/3-D ICs and multiprocessor system-on-chips (MPSoCs) have appeared in the literature. A key component in such methodologies is a fast and accurate thermal modeling technique that can be easily interfaced with design optimization tools. Conventional fully numerical techniques, such as finite-element methods, do not render themselves to enable such an easy interfacing with design tools and their order of complexity is too large for fast simulations. In this context, we present a new semi-analytical representation for heat flow in forced convective cooling inside microchannels, which is continuous in 1-D, i.e., along the direction of the coolant flow. This model is based on the well-known analogy between heat conduction and electrical conduction, and introduces distributed electrical parameters in the dimension considered to be continuous, resulting in a state-space representation of the heat transfer problem. Both steady state and transient semi-analytical models are presented. The proposed semi-analytical model is shown to have a closed-form solution for certain cases that are encountered in practical design problems. The accuracy of the model has been validated against state-of-the-art thermal modeling frameworks [1] (errors≪ 1%), with 3X speed-up of our proposed modeling framework.

Formulation of monodisperse water-in-oil emulsions encapsulating calcium ascorbate and ascorbic acid 2-glucoside by microchannel emulsification

The aim of this study was to investigate the effects of the emulsifying conditions and emulsifier type on production of water-in-oil (W/O) emulsions encapsulating ascorbic acid derivatives by microchannel (MC) emulsification. The ascorbic acid derivatives added in a dispersed aqueous phase are calcium ascorbate (AA-Ca) and ascorbic acid 2-glucoside (AA-2G). The continuous phase used was decane, soybean oil or their mixture, containing 5% (w/w) tetraglycerin monolaurate condensed ricinoleic acid ester or sorbitan trioleate. A hydrophobized silicon MC array plate (model: MS407) with a channel depth of 7μm was used for MC emulsification. The use of MC emulsification enabled successful encapsulation of AA-Ca and AA-2G in monodisperse W/O emulsion droplets with coefficients of variation (CV) less than 7%. Their average droplet diameter (dav) increased with increasing the continuous-phase viscosity that is similar or higher than the dispersed-phase viscosity. The dav and CV of the resultant monodisperse W/O emulsions were unaffected by the dispersed-phase flow rate below critical values of 1.2–1.6mLh−1 when using decane as the continuous-phase medium.

A dimensional comparison between embedded 3D-printed and silicon microchannels

The subject of this paper is the dimensional characterization of embedded microchannel arrays created using contemporary 3D-printing fabrication techniques. Conventional microchannel arrays, fabricated using deep reactive ion etching techniques (DRIE) and wet-etching (KOH), are used as a benchmark for comparison. Rectangular and trapezoidal cross-sectional shapes were investigated. The channel arrays were 3D-printed in vertical and horizontal directions, to examine the influence of print orientation on channel characteristics. The 3D-printed channels were benchmarked against Silicon channels in terms of the following dimensional characteristics: cross-sectional area (CSA), perimeter, and surface profiles. The 3D-printed microchannel arrays demonstrated variances in CSA of 6.6-20% with the vertical printing approach yielding greater dimensional conformity than the horizontal approach. The measured CSA and perimeter of the vertical channels were smaller than the nominal dimensions, while the horizontal channels were larger in both CSA and perimeter due to additional side-wall roughness present throughout the channel length. This side-wall roughness caused significant shape distortion. Surface profile measurements revealed that the base wall roughness was approximately the resolution of current 3D-printers. A spatial periodicity was found along the channel length which appeared at different frequencies for each channel array. This paper concludes that vertical 3D-printing is superior to the horizontal printing approach, in terms of both dimensional fidelity and shape conformity and can be applied in microfluidic device applications.

3D sensors and micro-fabricated detector systems

Micro-systems based on the Micro Electro Mechanical Systems (MEMS) technology have been used in miniaturized low power and low mass smart structures in medicine, biology and space applications. Recently similar features found their way inside high energy physics with applications in vertex detectors for high-luminosity LHC Upgrades, with 3D sensors, 3D integration and efficient power management using silicon micro-channel cooling. This paper reports on the state of this development.

Radiation hardness tests and characterization of the CLARO-CMOS, a low power and fast single-photon counting ASIC in 0.35 micron CMOS technology

The CLARO-CMOS is a prototype ASIC that allows fast photon counting with 5ns peaking time, a recovery time to baseline smaller than 25ns, and a power consumption of less than 1mW per channel. This chip is capable of single-photon counting with multi-anode photomultipliers and finds applications also in the read-out of silicon photomultipliers and microchannel plates. The prototype is realized in AMS 0.35 micron CMOS technology. In the LHCb RICH environment, assuming 10years of operation at the nominal luminosity expected after the upgrade in Long Shutdown 2 (LS2), the ASIC must withstand a total fluence of about 6×10121MeVneq/cm2 and a total ionizing dose of 400krad. A systematic evaluation of the radiation effects on the CLARO-CMOS performance is therefore crucial to ensure long term stability of the electronics front-end. The results of multi-step irradiation tests with neutrons and X-rays up to the fluence of 1014cm−2 and a dose of 4Mrad, respectively, are presented, including measurement of single event effects during irradiation and chip performance evaluation before and after each irradiation step.

Electroosmotic pump based on asymmetric silicon microchannel membranes

This paper is devoted to the design and characterization of an electroosmotic pump based on asymmetric microchannel silicon membranes. A pronounced dependence of the pump flow rate on the structural asymmetry of microchannels was first found in experiments using deionized water. Pump flow rate was determined as a function of the applied voltage and the orientation of the matrix with respect to the volume of water pumped. An analytical description of the spatial structure of the microchannel matrices is proposed, which makes it possible to more accurately relate the structural and transport characteristics of the device. The data were used to calculate the zeta potential of the deionized water-silica-silicon system. It is assumed that the observed effect can be used as the basis for designing electroosmotic micropumps for modern bioanalytical micro- and nanofluidic systems.

Impedance study of adsorption phenomena on three-dimensional nano-nickel electrode deposited on silicon microchannel plate

Three-dimensional nano-nickel electrodes fabricated on silicon microchannel plates by electroless deposition are characterized by electrochemical impedance spectroscopy. Compared to the nickel sheet, adsorption of cations and anions is detected due to the larger surface area of the porous nickel structure and three-dimensional architecture of the silicon microchannel plates. Adsorption is sensitive to the frequency and increasing step of the external voltage and different desorption rates of 0.003 and 0.027 A/cm2 are observed from cations and anions, respectively. The experimental results indicate that adsorption is responsible for the anomalous increase of the impedance in the low frequency region.

Cybotactic behavior in the de Vries smectic-A* liquid-crystal structure formed by a silicon-containing molecule.

We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.

Contact line characteristics of liquid–gas interfaces over grooved surfaces

Surface wetting is an important phenomenon in many industrial processes including micro- and nanofluidics. The wetting characteristics depend on the surface tension forces at the three-phase contact line and can be altered by introducing patterned groove structures. This study investigates the effect of the grooves on the transition in the wetting behavior between the Cassie to Wenzel regimes. The experiments demonstrate that the wettability on a patterned surface depends on the spacing factor (S = channel depth/channel width). The spacing factor influences the contact angle, contact angle hysteresis, and the transition characteristics between the Cassie and Wenzel states. It was noted that under certain conditions (S > 1) the droplet behaved as a Cassie droplet, while exhibiting Wenzel wetting the rest of the time on the silicon microchannels tested. This criterion was used to design the groove structures on the sidewall of the proton exchange membrane fuel cell gas channel to remove the water effectively. The water coming from the land region into the gas channel is pulled by the grooves to the top wall where the airflow aided in its removal. Also, the contact angles measured on the surfaces were compared with the classical models that use wetted area, and the contact line model that uses the three-phase contact line length. It was found in our experiments that the contact line model predicts the contact angle on the patterned groove surfaces more accurately than the classical models.

Enhanced Performance of Oxidation of Rosalva (9-decen-1-ol) to Costenal (9-decenal) on Porous Silicon-Supported Silver Catalyst in a Microstructured Reactor

The use of metal-assisted HF chemical etching as a convenient technique to produce a few microns thick porous layer in silicon microchannels was demonstrated. Gas phase selective oxidation of rosalva to its aldehyde (costenal) was performed in glass/silicon microstructured reactors at temperatures of 375–475 °C on silver catalyst which was deposited on both porous and flat silicon surface by sputter-coating. The effects of temperature (375–475 °C), rosalva concentration (1.17%–3.43%), O2 to rosalva ratio (0.5–20) and residence time on the reaction were investigated. The reactivity of rosalva on the porous silicon supported silver was 5.7–6.4 times higher than on the thin film silver catalyst at 450 °C. Furthermore, activation energy for the porous silicon supported silver was lower. Isothermal conditions in the microreactors allowed high conversion and selectivity to be achieved in a wide range of temperature and oxygen concentration. At typical reaction conditions (1.75% rosalva, O2/rosalva = 3, residence time 18 ms and 450 °C), conversion of 97% and selectivity of 95% to costenal was achieved, corresponding to a turnover frequency of 268 h−1.

Visualization study of flow condensation in hydrophobic microchannels

A visualization study on flow condensation in hydrophobic rectangular silicon microchannels with hydraulic diameter of approximately 150 μm is conducted. Thin Au film with thickness of 200 nm is sputtered on channel surfaces to create a hydrophobic surface with an equilibrium contact angle of approximately 96°. In addition to traditional droplet flow, droplet‐annular compound flow, droplet‐injection compound flow, and droplet‐bubble/slug compound flow are also observed. The results indicate that injection location is postponed, and injection frequency increases with increasing inlet vapor Reynolds number and condensate Weber number. An empirical correlation of the injection location and injection frequency are presented and discussed. In particular, for a larger inlet vapor Reynolds number, the injection flow is closer to the channel outlet and the condensation heat transfer is enhanced. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1182–1192, 2014

Fabrication of highly ordered porous nickel oxide anode materials and their electrochemical characteristics in lithium storage

The structure and electrochemical properties of silicon microchannel plates (MCP)-supported NiO nanocomposites (NiO/Si-MCP) synthesized by silicon micromachining, electroless plating, and thermal annealing are investigated as anodes in lithium ion batteries. Galvanostatic charge and discharge results indicate that the NiO/Si-MCP is capable of delivering a higher capacity than the bare nickel-oxide film. At a 1C current, the NiO/Si-MCP nanocomposite film shows an enormous first discharge capacity of about 3190mAg−1 and charge capacity of 1977mAg−1. After 15 cycles, the NiO/Si-MCP nanocomposite retains a reversible capacity of 1531mAg−1 with 63.7% of the capacity maintained in the 2nd cycle. The lithium storage capacity is maintained at ∼880mAhg−1 after 50 discharge/charge cycles and it is much larger than that of NiO and its composites. The enhanced electrochemical performance of the highly ordered three-dimensional materials is attributed to the synergistic effects offered by the silicon microchannel plates in the nickel oxide film subsequently facilitating electrolyte penetration, diffusion, and migration. The structure is promising anode materials in lithium-ion batteries.

Local measurement of flow boiling heat transfer in an array of non-uniformly heated microchannels

As electronics packages become increasingly thinner and more compact due to size, weight, and performance demands, the use of large intermediate heat spreaders to mitigate heat generation non-uniformities are no longer a viable option. Instead, non-uniform heat flux profiles produced from chip-scale variations or from multiple discrete devices are experienced directly by the ultimate heat sink. In order to address these thermal packaging trends, a better understanding of the impacts of non-uniform heating on two-phase flow characteristics and thermal performance limits for microchannel heat sinks is needed. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse peak-heating conditions spanning across the entire heat sink area. The investigation is conducted using a silicon microchannel heat sink with a 5×5 array of individually controllable heaters attached to a 12.7mm×12.7mm square base. The channels are 240μm wide, 370μm deep, and separated by 110μm wide fins. The working fluid is the dielectric fluorinert liquid FC-77, flowing at a mass flux of approximately 890kg/m2s. High-speed visualizations of the flow are recorded to observe the local flow regimes. Despite the substrate beneath the microchannels being very thin (200μm), significant lateral conduction occurs and must be accounted for in the calculation of the local heat flux imposed. For non-uniform heat input profiles, with peak heat fluxes along the streamwise and transverse directions, it is found that the local flow regimes, heat transfer coefficients, and wall temperatures deviate significantly from a uniformly heated case. These trends are assessed as a function of an increase in the relative magnitude of the nonuniformity between the peak and background heat fluxes.

An Ionic Liquid Based Strain Sensor for Large Displacements

Abstract A novel ionic liquid based strain sensor is fabricated for high strain measurements (up to 10% and higher). A tubular silicone microchannel with a diameter of 310 μm and a length of 5 mm is filled with an ionic liquid (1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). The final sensor has an outer diameter of 650 μm and a length of 15 mm. When the channel is axially loaded, geometrical deformations change the electrical properties of the liquid channel. 4-point impedance spectroscopy with the integrated circuit AD5933 from Analog Devices and a dedicated galvanostatic front-end is performed to characterize this piezoresistive effect. The highest sensitivity is observed in the frequency range from 10 to 25 kHz, with corresponding gauge factors between 2 and 2.5 for an elongation of 10%.