Surface Microchannels Research Articles

Fabrication of microchannels: A review

Microchannels are primarily used in biomedical devices and microfluidic applications. Fabrication of microchannels has always been a tough task using conventional manufacturing technologies. Various types of materials are in use for fabricating microchannels in different types of applications including metals, polymers and ceramics. A number of methods are in use for fabricating different types of microchannels. These processes include both conventional and nonconventional fabrication techniques such as micromilling, lithography, embossing processes and laser ablation processing. During the recent years, some hybrid techniques have also been developed for fabrication of microchannels. This survey of various literatures reveals a broad spectrum of different processes used for fabricating microchannels. Currently, laser micromachining has been evolved as a potential technology for fabricating microchannels. Laser processing has been proved to be the most time efficient and clean. In this article, fabrication processes for creating microchannels have been reviewed with special emphasis on laser micromachining. This article mostly addresses the fabrication techniques for creating surface microchannels.

Femtosecond laser microstructuring for polymeric lab‐on‐chips

This paper provides an overview of femtosecond laser microfabrication in polymeric materials, with emphasis on lab-on-chip applications. Due to the nonlinear interaction of femtosecond laser pulses with polymers, laser-induced modifications are localized to the focal volume, enabling high resolution patterning in 3D. Femtosecond laser microfabrication offers unmatched versatility in fabricating surface microchannels and diffractive optics by means of laser ablation, buried optical waveguides and micro-optics through refractive index modification and complex 3D microstructures in photoresists by two-photon polymerization. Femtosecond laser microfabrication technology opens the door to fabricating integrated lab-on-chip devices with a single tool.

Predicting the onset of condensate droplet departure from a vertical surface due to air flow—Applications to topographically-modified, micro-grooved surfaces

Air-flow induced water droplet movement on micro-patterned aluminum surfaces consisting of parallel grooves tens of microns in width and depth is considered, and a simple model for calculating the critical air flow force for water droplet departure from vertical surfaces is presented. This model which considers the combined effect of the gravitational, surface tension, and air flow forces on the droplet was found to provide reasonably accurate agreement with experimental data for water droplets 1–25μL in size. In this method, the user must specify the advancing and receding contact angles of the surface, droplet volume, and micro-channel width and depth (if applicable). When applied to surfaces of varying wettability, the model was shown to calculate the actual critical air velocity to within 15.2% for 89% of the droplet volumes analyzed. For surfaces with parallel micro-channels aligned vertically with gravity and no surface coating, the critical air velocity for water droplet departure was found to be more than 180% higher than the baseline surface (i.e. no micro-channels or coating)—a behavior that was also observed experimentally. Moreover, for micro-grooved samples with a PDMS coating, the required air velocity for movement across the channels was 1.0–1.2m/s larger than the air velocity required for movement along the channels and only slightly lower than the baseline case. These results suggest that topographically-modified, anisotropic surfaces may be useful in air cooling applications where both heat and mass transfer occur and where the surface micro-channels are aligned parallel to gravity to not only assist in condensate drainage but to also resist the carryover of condensate into the occupied space.

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m 2 s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm 2. The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.

Methodology for Calculating the Volume of Condensate Droplets on Topographically Modified, Microgrooved Surfaces

Liquid droplets on micropatterned surfaces consisting of parallel grooves tens of micrometers in width and depth are considered, and a method for calculating the droplet volume on these surfaces is presented. This model, which utilizes the elongated and parallel-sided nature of droplets condensed on these microgrooved surfaces, requires inputs from two droplet images at ϕ = 0° and ϕ = 90°--namely, the droplet major axis, minor axis, height, and two contact angles. In this method, a circular cross-sectional area is extruded the length of the droplet where the chord of the extruded circle is fixed by the width of the droplet. The maximum apparent contact angle is assumed to occur along the side of the droplet because of the surface energy barrier to wetting imposed by the grooves--a behavior that was observed experimentally. When applied to water droplets condensed onto a microgrooved aluminum surface, this method was shown to calculate the actual droplet volume to within 10% for 88% of the droplets analyzed. This method is useful for estimating the volume of retained droplets on topographically modified, anisotropic surfaces where both heat and mass transfer occur and the surface microchannels are aligned parallel to gravity to assist in condensate drainage.

Tunable elastic stiffness with microconfined magnetorheological domains at low magnetic field

Surface micropatterning enhances the interfacial sliding resistance of magnetorheological fluid at low magnetic field (10–35 mT). Fluid is confined to surface microchannels, resulting in the formation of spatially aligned magnetic domains. The channels are supported by a pair of overlapping ribbons, which, along with the surrounding fluid, are enclosed in a soft elastomer. The embedded elastomer represents an electromagnetic alternative to current methods of active stiffness control that are based on principles of gel hydration, particle jamming, and pneumatics.

Subcooled convective boiling in structured surface microchannels

Experiments are performed to investigate subcooled flow boiling, on side wall cavities with angles of 60°, 90° and 120° with bottom-wall asymmetrical heating, for a microchannel heat sink containing 75 parallel 100 µm × 200 µm channels. The heated surface is made of a Cu metal sheet with/without 2 µm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water + 1 vol.% MCNT additive solution, FC-72 fluids over a mass velocity range of 820–1600 kg m−2 s, inlet temperatures of 15 (8.6), 25 (13.6), 44 (24.6) and 64 °C (36.6 °C) for DI water (FC-72) and heat fluxes up to 800 W cm−2. Flow morphologies, boiling incipience, two-phase heat transfer coefficients and critical heat fluxes are obtained and presented. Both cavities' structured and sputtered diamond ultrathin film surfaces in nanofluid flow boiling in a microchannel are shown to significantly enhance heat transfer through the promotion of bubble ebullitions and to reduce boiling incipience. A significant increase in critical heat flux (CHF) is also found.

Squeezing Flow of a Power Law Fluid Between Grooved Plates

Surface microchannels can accelerate squeezing flow of a fluid in a thin gap between parallel plates. Using a computational model and restricting attention to power law fluid, a parametric study is made of the influence of channel size for a certain family of channel patterns, with results tabulated for general use. An approximate heat conduction model is used to estimate the effect of channel size and pattern on resistance to heat flow normal to the fluid layer.

Transition Criteria for Entropy Reduction of Convective Heat Transfer from Micropatterned Surfaces

This paper develops an entropy-transition number for characterizing irreversibilities of external flow past micropatterned surfaces with controlled surface roughness. It is shown that embedded surface microchannels can reduce flow irreversibilities below a classical boundary-layer limit, due to drag reduction of slip-flow conditions within the microchannels. A surface-irreversibility ratio establishes the proportion of entropy production of slip-flow relative to no-slip conditions. Then an entropy-transition number is defined to characterize flow regimes in which the ratio decreases below unity, thereby indicating conditions in which surface micropatterning has an overall beneficial impact on energy conversion efficiency. Results are presented for various micropatterned surfaces. It is shown that slip-flow conditions within the embedded surface microchannels can overcome the additional friction irreversibility of more surface area, reducing the total entropy production up to 20% below the boundary-layer flow without microchannels. At lower Reynolds numbers, it is shown that the irreversibility ratio decreases (for example, down to 0.79) at a Reynolds number of 1200. The newly defined surface-irreversibility and entropy-transition numbers are shown to provide useful new parameters to characterize the boundary-layer irreversibilities of convective heat transfer.

Entropy-based surface microprofiling for passive near-wall flow control

In this paper, embedded surface microchannels with varying geometrical profiles are developed and applied to drag reduction and near-wall flow control along a surface. Local slip-flow conditions and a temperature discontinuity are encountered within the microchannels along the flat surface. Diverging sections of each microchannel affect both the near-wall pressure and velocity distributions, thereby influencing the boundary layer separation and entropy production. This impact of surface microchannels is characterized with respect to entropy production of friction and thermal irreversibilities. The total entropy production is expressed in terms of the microchannel base and exit angles, depth and other geometrical parameters that characterize the microprofiled surface. Numerical results are presented for the optimized geometrical configuration of the microchannel profiles. Applications of the microprofiling technique are presented for near-wall flow control along a helicopter engine cooling bay surface.

Local measurement of flow boiling in structured surface microchannels

Experiments were conducted to investigate flow boiling in 200 μm × 253 μm parallel microchannels with structured reentrant cavities. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass velocities ranging from G = 83 kg/m 2 s to G = 303 kg/m 2 s and heat fluxes up to 643 W/cm 2. Comparisons of the performance of the enhanced and plain-wall microchannels were performed. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation. The structured surface was also shown to significantly reduce the boiling inception and to enhance the critical heat flux.

Bubble Dynamics During Boiling in Enhanced Surface Microchannels

An experimental investigation is presented on flow boiling of deionized water in 227-mum hydraulic diameter microchannels with reentrant type cavities. Key features of nucleate boiling are discussed. Active nucleation site density, bubble frequency and departure diameter, and flow patterns over mass velocities ranging from 41 kg/m2-s to 302 kg/m2-s and heat fluxes ranging from 28 to 445 W/cm2 are studied. Similarities and differences with results obtained on large-scale systems and unenhanced microchannels are discussed

Surface micro-grooves for near-wall exergy and flow control: application to aircraft intake de-icing

This paper develops a new surface micro-profiling technique for reducing exergy losses and controlling near-wall flow processes, particularly for anti-icing of a helicopter surface. Fabrication of embedded surface microchannels entails surface etching with KOH and XeF2 gas, so that the interspersed microchannels can be assembled into a surface layer of silicon. Testing of the micro-profiled surfaces is performed with particle image velocimetry in a water tunnel. Experimental data indicate that converging open microchannels lead to certain differences of flow patterns on the downstream side of an engine cooling bay. Furthermore, exergy losses for external flow past the parallel embedded microchannels are shown to be lower than previous benchmark results without microchannels. Analytical results are presented for these losses of available energy and exergy destruction. Reduced drag of slip-flow conditions within each microchannel offsets the added friction irreversibility of larger surface area. By altering the near-wall flow patterns, it is anticipated that embedded surface microchannels can provide a useful new approach for dealing with flow-related problems of aircraft icing.

Embedded converging surface microchannels for minimized friction and thermal irreversibilities

Embedded converging surface microchannels are developed as effective new ways of reducing entropy production in boundary layer flow with convective heat transfer. A similarity solution with slip-flow boundary conditions is developed to find the spatial distributions of velocity, temperature, wall shear stress and wall heat flux. Mixed Knudsen number conditions along the surface require both slip-flow and no-slip formulations to be combined in an integral solution. Predicted results from the method of EBSM (entropy based surface micro-profiling) are presented for the optimized microchannel aspect ratio, spacing between adjacent microchannels and the angles characterizing the converging microchannels.

A low-temperature IC-compatible process for fabricating surface-micromachined metallic microchannels

In this paper, a low-temperature integrated-circuit (IC)-compatible process for fabricating metallic microchannels is described. Arrays of 1-100 metallic microchannels have been fabricated on silicon and glass substrates. The process can be extended to many planar substrate materials including polymers and ceramics. The microchannels are formed using microelectro-formed metals. The microchannels demonstrated in this paper use nickel as the structural material and gold as the surface coating on the inside walls of the microchannels. The inner dimensions of the individual microchannels fabricated to date range from 30 /spl mu/m to 1.5 mm in width, 0.5 mm to several centimeters in length, and 5-100 /spl mu/m in thickness. The wall thickness ranges from 5 to 50 /spl mu/m. The microchannel fabrication technology enables the fabrication of surface microchannels with a relatively large cross-sectional area. The metallic microchannels can be fabricated to extend from the substrate edge. Interfacing schemes are given for attaching external pressure feeds.