Seed bubbles trigger boiling heat transfer in silicon microchannels

The smooth channel surface of microsystems delays boiling incipience in heated microchannels. In this paper, we use seed bubbles to trigger boiling heat transfer and control thermal non-equilibrium of liquid and vapor phases in parallel microchannels. The test section consisted of a top glass cover and a silicon substrate. Microheater array was integrated at the top glass cover surface and driven by a pulse voltage signal to generate seed bubbles in time sequence. Each microheater corresponds to a specific microchannel and is located in the microchannel upstream. Five triangular microchannels with a hydraulic diameter of 100 μm and a length of 12.0 mm were etched in the silicon substrate. A thin platinum film was deposited at the back surface of silicon chip with an effective heating area of 4,500 × 1,366 μm, acting as the main heater for the heat transfer system. Acetone liquid was used. With the data range reported here, boiling incipience was not initiated if wall superheats are smaller than 15°C without seed bubbles assisted. Injection seed bubbles triggers boiling incipience and controls thermal non-equilibrium between liquid and vapor phases successfully. Four modes of flow and heat transfer are identified. Modes 1, 2, and 4 are the stable ones without apparent oscillations of pressure drops and heating surface temperatures, and mode 3 displays flow instabilities with apparent amplitudes and long periods of these parameters. The four modes are divided based on the four types of flow patterns observed in microchannels. Seed bubble frequency is a key factor to influence the heat transfer. The higher the seed bubble frequency, the more decreased non-equilibrium between two phases and heating surface temperatures are. The seed bubble frequency can reach a saturation value, at which heat transfer enhancement attains the maximum degree, inferring that a complete thermal equilibrium of two phases is approached. The saturation frequency is about a couple of thousand Hertz in this study.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

Analysis of laminar-to-turbulent transition for isothermal gas flows in microchannels

In this work the laminar-to-turbulent transition in microchannels of circular cross-section is studied experimentally. In order to single out the effects of relative roughness, compressibility and channel length-to-diameter ratio on the Reynolds number at which transition occurs, experimental runs have been carried out on circular microchannels in fused silica—smooth for all purposes—and in stainless steel (which possess a high surface roughness), with a diameter between 125 and 180 μm and a length of 5–50 cm through which nitrogen flows. For each tube the friction factor has been computed. The values of the critical Reynolds number have been determined plotting the Poiseuille number (i.e., the product of the friction factor, f, times the Reynols number, Re) as a function of the average Mach number between inlet and outlet. The transitional regime was found to start no earlier than at values of the Reynolds number around 1,800–2,000. It has been observed that surface roughness has no effect on the hydraulic resistance in the laminar region for a relative roughness lower than 4.4%, and that friction factor obeys the Poiseuille law, if it is correctly computed taking compressibility into account. It is found that recent correlations for the prediction of the critical Reynolds number in microchannels that link the relative roughness of the microtubes to the critical Reynolds number do not agree with the present results.     

Effect of dispersed phase viscosity on maximum droplet generation frequency in microchannel emulsification using asymmetric straight-through channels

Uniformly sized droplets of soybean oil, MCT (medium-chain fatty acid triglyceride) oil and n-tetradecane with a Sauter mean diameter of d _3,2 = 26–35 μm and a distribution span of 0.21–0.25 have been produced at high throughputs using a 24 × 24 mm silicon microchannel plate consisting of 23,348 asymmetric channels fabricated by photolithography and deep reactive ion etching. Each channel consisted of a 10-μm diameter straight-through micro-hole with a length of 70 μm and a 50 × 10 μm micro-slot with a depth of 30 μm at the outlet of each channel. The maximum dispersed phase flux for monodisperse emulsion generation increased with decreasing dispersed phase viscosity and ranged from over 120 L m⁻² h⁻¹ for soybean oil to 2,700 L m⁻² h⁻¹ for n-tetradecane. The droplet generation frequency showed significant channel to channel variations and increased with decreasing viscosity of the dispersed phase. For n-tetradecane, the maximum mean droplet generation frequency was 250 Hz per single active channel, corresponding to the overall throughput in the device of 3.2 million droplets per second. The proportion of active channels at high throughputs approached 100% for soybean oil and MCT oil, and 50% for n-tetradecane. The agreement between the experimental and CFD (Computational Fluid Dynamics) results was excellent for soybean oil and the poorest for n-tetradecane.     

Roll manufacturing of flexible microfluidic devices in thin PMMA and COC foils by embossing and lamination

Microfluidics on foil is gaining momentum due to a number of advantages of employing thin films combined with the capability of cost-effective high-volume manufacturing of devices. In this work, ultra-thin, flexible Y-microreactors with microchannels of 100 μm width and 30 μm depth were fabricated in thermoplastic polymer foils. The fluidic pattern was hot roll embossed in 125 μm thick poly-methyl-methacrylate (PMMA) and 130 μm thick cyclic-olefin-copolymer (COC) films using a dry-etched microstructured silicon wafer as a flat embossing tool in a laminator. The sealing of the channels was performed with two different techniques, one based on lamination of SU8 dry film resist (DFR) and the other one based on spin-coated poly-dimethylsiloxane (PDMS). Testing of the interconnected microreactor was carried out using two dye colorant solutions to demonstrate mixing.     

Gas–liquid dynamics at low Reynolds numbers in pillared rectangular micro channels

Most heterogeneously catalyzed gas–liquid reactions in micro channels are chemically/kinetically limited because of the high gas–liquid and liquid–solid mass transfer rates that can be achieved. This motivates the design of systems with a larger surface area, which can be expected to offer higher reaction rates per unit volume of reactor. This increase in surface area can be realized by using structured micro channels. In this work, rectangular micro channels containing round pillars of 3 μm in diameter and 50 μm in height are studied. The flow regimes, gas hold-up, and pressure drop are determined for pillar pitches of 7, 12, 17, and 27 μm. Flow maps are presented and compared with flow maps of rectangular and round micro channels without pillars. The Armand correlation predicts the gas hold-up in the pillared micro channel within 3% error. Three models are derived which give the single-phase and the two-phase pressure drop as a function of the gas and liquid superficial velocities and the pillar pitches. For a pillar pitch of 27 μm, the Darcy-Brinkman equation predicts the single-phase pressure drop within 2% error. For pillar pitches of 7, 12, and 17 μm, the Blake-Kozeny equation predicts the single-phase pressure drop within 20%. The two-phase pressure drop model predicts the experimental data within 30% error for channels containing pillars with a pitch of 17 μm, whereas the Lockhart–Martinelli correlation is proven to be non-applicable for the system used in this work. The open structure and the higher production rate per unit of reactor volume make the pillared micro channel an efficient system for performing heterogeneously catalyzed gas–liquid reactions.     

Replication of precise polymeric microlens arrays combining ultra-precision diamond ball-end milling and micro injection molding

A simple and novel combination of ultra-precision diamond ball-end milling and micro injection molding technique is described to produce precise microlens arrays out of polycarbonate (PC), polymethylmethacrylate (PMMA) as well as polystyrene (PS). The microlens arrays consist of 100 lenses in a 10 × 10 array with a lens radius of 273 μm, a lens diameter of 300 μm and a lens depth of 45 μm. Pitch between the lenses is fixed at 800 μm. The injection molding parameters were optimized to get precise microlens geometries with low surface roughness. The results show a precise diamond milled mold insert and injection molded microlens arrays with minor deviations in radius and surface roughness of the microlenses, particularly for microlens arrays out of PMMA.     

Selective adsorption of solvents in a multiscale device

We have incorporated microspheres, from 50 to 80 μm in diameter, of periodic mesoporous organosilica (inner surfaces up to 1,000 m²/g and pore sizes in the nanometre range) with two types of organic functionalities (benzene and ethane bridges, respectively) inside microstructured channels (each 200 μm wide and 100 μm deep) and, exemplarily, monitored by Raman microscopy (Raman spectroscopy through microscope optics) that the temperature characteristics of the adsorption–desorption equilibria of benzene and ethanol vary significantly with the type of organic functionality of the microspheres and the pore morphology. The integration of this class of nanostructured material into devices by means of microchannels is a promising novel approach to, among others, substance separation in analytics, micro process engineering, and micro chemistry.     

Blood flow driven by surface tension in a microchannel

This study aims to identify distinct blood flow characteristics in a microchannel at different sloping angles. The channel is determined by a bottom hydrophilic stripe on a glass substrate and a fully covered hydrophobic glass substrate. The channel has a height of 3 μm, and a width of 100 μm. It is observed that increasing the sloping angle from −90° (downward flow) to 90° (upward flow) increases the blood flow rate monotonically. These peculiar behaviors on the micro scale are explained by a dynamic model that establishes the balance among the inertial, surface tension, gravitational, and frictional forces. The frictional force is further related to the effective hematocrit. The model is used to calculate the frictional force, and thus the corresponding hematocrit, which is smaller when the blood flows upward, reducing the frictional force.     

Fabrication of metallic micromolds by laser and electro-discharge micromachining

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper. The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet. The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate. The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing. To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS). The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra. The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4. Experimental results are discussed and suggestions for improvement are presented.     

Capillarity-driven dynamics of water–alcohol mixtures in nanofluidic channels

We investigated the spontaneous capillarity-driven filling of nanofluidic channels with a thickness of 6 and 16 nm using mixtures of ethanol and water of variable composition. To improve the visibility of the fluid, we embedded metal mirrors into the top and bottom walls of the channels that act as a Fabry–Pérot interferometer. The motion of propagating liquid–air menisci was monitored for various concentrations in transmission with an optical microscope. In spite of the visible effects of surface roughness and different affinity of water and ethanol to the channel walls, the dynamics followed the classical t ^1/2—dependence according to Lucas and Washburn. While the prefactor of this algebraic relation falls short of the expectations based on bulk properties by 10–30%, the relative variation between mixtures of different composition follows the expectations based on the bulk surface tension and viscosity, implying that—despite the small width of the channels and the large surface-to-volume ratio—specific adsorption or chemical selectivity effects are not relevant. We briefly discuss the impact of surface roughness on our experimental results.     

Design of microfluidic channels separated by an ultra-thin free-standing dielectric membrane

A microfluidic (MF) surface plasmon polariton sensor featuring a gold Mach–Zehnder interferometer on an ultra-thin (20–35 nm) dielectric membrane is described. While the presence of the membrane is required to maintain a near mirror symmetry of the dielectric properties of the medium on either side of the interferometer, it is a source of unique challenges in the MF system design. The pressure required to drive the fluid flow in microchannels causes deflection whose value depends on the membrane’s residual stress in the low pressure range and on its modulus at the higher pressure range. Depending on the empirical membrane strength which would meet the required equipment reliability, narrow fluidic channels may require tight dimensional tolerances to maintain the pressure difference across the membrane below a critical value. With wider channels (≥100 μm) dimensional tolerances are relaxed even with relatively weak membranes.     

Novel U-shape gold nanoparticles-modified optical fiber for localized plasmon resonance chemical sensing

A novel fiber-optic localized plasma resonance (FO-LPR) sensor composed of a U-shape optical fiber was proposed and demonstrated in this study. The U-shape optical fiber was fabricated by a femtosecond laser micromachining system. The dimensions of the U-shape zone were 100 μm in depth measured from the surface of the polymer jacket layer, 80 μm in width in the jacket layer, 60 μm in width in the cladding layer. The total length is 5 mm. After laser annealing treatment, the average surface roughness was 205.8 nm as determined by Atom Force Microscope (AFM). The exposed surface of the U-shape fiber was modified with self-assembled gold nanoparticles to produce the FO-LPR sensor. The response of the sensor shows that the signal increases linearly with increasing refractive index. The sensor resolution of the sensor was determined to be 1.06 × 10⁻³ RIU.     

Pressure-sensitive channel chip for visualization measurement of micro gas flows

We have developed a new pressure sensing tool named pressure-sensitive channel chip (PSCC) by combining the pressure-sensitive paint (PSP) technique with the poly(dimethylsiloxane) (PDMS) micro-molding technique. The PSP technique based on the oxygen quenching of luminescence is a potential diagnostic tool for pressure measurement of micro gas flows. However, the application of PSP to micro scale measurement is very difficult, because the thickness and the surface roughness of conventional PSPs cannot be neglected compared with the characteristic length of micro channels, and the spatial resolution is not enough for micro scale measurements due to the aggregations of luminophore. PSCC is fabricated with PDMS containing a pressure-sensitive luminophore; thus PSCC is a micro channel which itself works as a pressure “distribution” sensor. A micro converging-diverging nozzle with the throat width of 120 μm was demonstrated. The pressure distribution on the nozzle surface was successfully obtained by PSCC without the shortcomings of conventional PSPs.     

An integrated flow-cell for full sample stream control

In this study, we present a novel three-dimensional hydrodynamic sheath flow chip that allows full control of a sample stream. The chip offers the possibility to steer each of the four side sheath flows individually. The design of the flow-cell exhibits high flexibility in creating different sample stream profiles (width and height) and allows navigation of the sample stream to every desired position inside the microchannel (vertical and horizontal). This can be used to bring the sample stream to a sensing area for analysis, or to an area of actuation (e.g. for cell sorting). In addition, we studied the creation of very small sample stream diameters. In microchannels (typically 25 × 40 μm2), we created sample stream diameters that were five to ten times smaller than the channel dimensions, and the smallest measured sample stream width was 1.5 μm. Typical flow rates are 0.5 μl/min for the sample flow and around 100 μl/min for the cumulated sheath flows. The planar microfabricated chip, consisting of a silicon–glass sandwich with an intermediate SU-8 layer, is much smaller (6 × 9 mm2) than the previously presented sheath flow devices, which makes it also cost-effective. We present the chip design, fluidic simulation results and experiments, where the size, shape and position of the sample stream have been established by laser scanning confocal microscopy and dye intensity analysis.     

From dissipative particle dynamics scales to physical scales: a coarse-graining study for water flow in microchannel

In the present work, dissipative particle dynamics (DPD) simulation of simple flows is studied based on coarse-graining parameter. Reference scales of DPD are expressed in terms of physical units and DPD parameters and equations are expressed in terms of Reynolds number and apparent Peclet number. DPD parameters for a given coarse-graining are calculated by matching the density and viscosity of water and Reynolds number of the flow. The formulation is applied to water flow in microchannels of height 5 and 10 μm and tested for a wide range of coarse-graining parameter varying from 10⁷ to 10⁹. The results are in a good agreement with the continuum formulation and simulated the correct hydrodynamics of water flow in microchannels. By inspecting the microscopic detail of the interaction between the DPD particles, it is found that diffusivity is low for high coarse-graining parameter, which results in higher values of Schmidt number. Parameters are tested within the continuum assumption. It is shown that correct Schmidt number can be achieved using small coarse-graining parameter. Also, it is observed that low diffusivity or high Schmidt number does not affect the hydrodynamics of water.     

Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars

Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with high-speed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the non-uniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 μm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.     

Assessment of multispectral ATSR2 stereo cloud-top height retrievals

The Along-Track Scanning Radiometer 2 (ATSR2) instrument has a dual view capability that allows for stereo height retrievals. Stereo heights were retrieved for selected scenes over the United Kingdom Chilbolton Facility for Atmospheric and Radio Research and the United States Atmospheric Radiation Measurement program Southern Great Plains site from 1997 to 2000. Stereo cloud-top heights obtained with the 11 μm and 1.6 μm channels of ATSR2 were compared with ground-based millimeter-wave cloud radar measurements at both sites. On average ATSR2 11 μm channel cloud-top height retrievals were 350 m higher than those from radar with a standard deviation of 1 km. This difference increased with decreasing cloud-top height. One major problem found in the 11 μm channel cloud-top height retrievals was poor delineation between surface (i.e., clear) and low-altitude cloud pixels, though this tends to lower cloud-top heights rather than raise them. The ATSR2 1.6 μm channel stereo cloud-top heights had large discrepancies compared to radar because of the 1.6 μm channel sensitivity to a lower layer in the case of multilayer clouds or the terrain in the case of optically thin single-layer clouds. For single-layer clouds, the agreement between the ATSR2 1.6 μm channel and the radar cloud-top heights was similar to that between ATSR2 11 μm channel and radar cloud-top heights.     

Micromachining of passive planar micromixer on poly (methyl methacrylate) substrate with excimer laser ablation

Excimer laser ablation technique was introduced into this work to fabricate a passive planar micromixer on the PMMA substrate. T-junction shaped and width-changed S-shaped microchannels were both designed in this micromixer to enhance mixing effect. The mixing experiment of distilled water and Rhodamine B with injection flow rate of 500 and 1,500 μm/s validates the mixing effectivity of this micromixer, and indicates the feasibility of excimer laser ablation in the microfabrication of μ-TAS device.     

A numerical hydrodynamic and thermal characterization of an inter-strata liquid cooling solution for 3D ICs

A novel integrated thermal management solution is proposed to alleviate hot spots in a contemporary 3D IC architecture. The solution employs a series of integrated microchannels, interconnected through each stratum by through silicon fluidic vias (TSFVs), and permits the transfer of heat, via a coolant, from hot to cold zones. This microfluidic system is driven by an integrated AC electrokinetic pump embedded in the channel walls. Recent advancements in electrokinetic micropump technology have allowed greater increases in fluid velocity (mm/s) while operating within the voltage constraints of a 3D IC. This paper presents a 2D simulation of an electrokinetic micropump operating at V_pp = 1.5 V in a 40 μm channel and examines its velocity profile for six frequencies in the range 100 ≤ ω ≤ 100 MHz. An optimum frequency of 100 kHz was established within this range and this was further examined with a constant heat flux of 186 W/cm2 imposed on the wall for an inlet fluid temperature of 40°C. Temperature profiles are presented at the channel-silicon interface and compared with theory.