Smooth Microchannel Research Articles

Numerical simulation of laminar heat transfer in microchannels with rough surfaces characterized by fractal Cantor structures

The topography of rough surfaces is characterized by using a Cantor set structure. Based on this fractal characterization, a model of laminar heat transfer in rough microchannels is developed and analyzed numerically. The effects of the Reynolds number, relative roughness, and fractal dimension on laminar heat transfer are all investigated and discussed. The results indicate that the local Nusselt numbers after the entrance region are no longer constant but tend to experience fluctuation along the rough microchannels. Differing from the smooth microchannels, the average Nusselt number increases nearly linearly with the Reynolds number and is larger than the classical value. For higher values of relative roughness, the flow-over roughness induces flow separation, which plays an enhancement on laminar convective heat transfer. Moreover, the laminar heat transfer in microchannels is also enhanced by roughness with a larger fractal dimension yielding more frequent variation in the surface profile even though at the same relative roughness.

Morphology and geometry of CO<SUB align=right>2-laser machined PMMA microchannels: influence of molecular weight and number of cut passes

Laser micromachining has been utilised for fabricating channels in polymeric microfluidic devices. The present work focuses on a study of the influence of polymer molecular weight (Mw) on the depth and width of microchannels in polymethyl methacrylate (PMMA) that are machined using a CO2 laser. PMMA of four molecular weights, namely, 96.7 kDa, 120 kDa, 350 kDa and 996 kDa were used. The laser cut was performed at low power ratings of between 0.25 W to 2.5 W and the cutting speeds ranged from 7.0 to 64 mm/s. The depth and width of the machined microchannels varied from 18 μm to 660 μm and 110 μm to 450 μm respectively. The effect of the number of cut passes during laser micromachining on the morphology of the microchannel was considered by subjecting each microchannel up to 7 cut passes. It was demonstrated that microchannels with smooth surfaces could be obtained using CO2 laser machining and that the conditions for obtaining the smooth microchannels depend on the polymer molecular weight.

Role of surface roughness characterized by fractal geometry on laminar flow in microchannels

A three-dimensional model of laminar flow in microchannels is numerically analyzed incorporating surface roughness effects as characterized by fractal geometry. The Weierstrass-Mandelbrot function is proposed to characterize the multiscale self-affine roughness. The effects of Reynolds number, relative roughness, and fractal dimension on laminar flow are all investigated and discussed. The results indicate that unlike flow in smooth microchannels, the Poiseuille number in rough microchannels increases linearly with the Reynolds number, Re, and is larger than what is typically observed in smooth channels. For these situations, the flow over surfaces with high relative roughness induces recirculation and flow separation, which play an important role in single-phase pressure drop. More specifically, surfaces with the larger fractal dimensions yield more frequent variations in the surface profile, which result in a significantly larger incremental pressure loss, even though at the same relative roughness. The accuracy of the predicted Poiseuille number as calculated by the present model is verified using experimental data available in the literature.

Laminar Drag Reduction in Hydrophobic Microchannels

AbstractThe apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30 % were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro‐particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8 % of the centerline velocity, yielding a slip length of approximately 2 μm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three‐dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro‐PIV.

Pressure drop and heat transfer characteristics of boiling water in sub-hundred micron channel

The current work focuses on the pressure drop, heat transfer and stability in two phase flow in microchannels with hydraulic diameter of less than one hundred microns. Experiments were conducted in smooth microchannels of hydraulic diameter of 45, 65 μm, and a rough microchannel of hydraulic diameter of 70 μm, with deionised water as the working fluid. The local saturation pressure and temperature vary substantially over the length of the channel. In order to correctly predict the local saturation temperature and subsequently the heat transfer characteristics, numerical techniques have been used in conjunction with the conventional two phase pressure drop models. The Lockhart–Martinelli (liquid–laminar, vapour–laminar) model is found to predict the two phase pressure drop data within 20%. The instability in two phase flow is quantified; it is found that microchannels of smaller hydraulic diameter have lesser instabilities as compared to their larger counterparts. The experiments also suggest that surface characteristics strongly affect flow stability in the two phase flow regime. The effect of hydraulic diameter and surface characteristics on the flow characteristics and stability in two phase flow is seldom reported, and is of considerable practical relevance.

Effect of gas adsorption on momentum accommodation coefficients in microgas flows using molecular dynamic simulations

The tangential momentum accommodation coefficient (TMAC), usually used in slip boundary conditions in micro-gas flows, is reported to be always less than unity and greatly influenced by temperature and the strength of gas–wall interactions. According to the definitions of accommodation coefficients, a proper statistical algorithm in non-equilibrium molecular dynamics method was described and verified. In planar Poiseuille gas flow in a smooth microchannel, the TMAC were calculated considering both the effects of temperature and gas–wall interaction. In the simulation processes, more gas molecules began to be adsorbed near walls under the condition of stronger gas–wall interaction and lower temperature. The gas adsorption resulted in a longer gas–wall interaction time so that the TMAC increased. While the gas–wall interaction became much stronger, more and more gas molecules were adsorbed to form an explicit layer above the wall. The full coverage of gas molecules on the wall prevented further adsorption; therefore the TMAC did not keep on increasing as the interaction strength continued to increase. Meanwhile, the normal momentum accommodation coefficient (NMAC) was also calculated according to the definition. In the isothermal flow, the average gas momentum normal to the wall was in complete accommodation with the wall, and the NMAC was almost unity in smooth micro channels.

An experimental investigation on pressure drop of steam condensing in silicon microchannels

Experiments are carried out to study the two-phase pressure drop for water vapor condensation in four smooth trapezoidal silicon microchannels having hydraulic diameters of 109 μm, 142 μm, 151 μm, and 259 μm, respectively. It is found that two-phase frictional pressure drops in the microchannels are greatly influenced by the hydraulic diameter, mass flux and vapor quality. The two-phase pressure drop data in microchannels are compared with existing correlations for macro- and mini-channels based on the homogenous model and the separated flow model to determine their applicability to condensing flows in microchannels. A modified correlation for the Matinelli–Chisholm constant, taking into consideration of surface tension and diameter effects, is developed in the form of the Lockhart–Martinelli correlation for the pressure drop in steam condensation in microchannels. The resulting condensation pressure drop correlation equation is within ±15% of the experimental data.

On the eruptive boiling in silicon-based microchannels

This study investigates experimentally eruptive boiling in a silicon-based rectangular microchannel with a hydraulic diameter of 33.7 μm, a width of 99.8 μm and a depth-to-width ratio of 0.203. The microchannel is made of SOI wafer and prepared using bulk micro-machining and anodic bonding. The surface roughness for both the bottom and the side walls was measured using an atomic force microscope. The evolution of the eruptive boiling of water in the smooth microchannel was clearly observed using an ultra high-speed video camera (up to 50,000 frames/s) at mass fluxes of 417 and 625 kg/m 2 s and a heat flux from 14.9 to 372 kW/m 2. It is confirmed that eruptive boiling is a form of rapid bubble nucleation after which the bubble merges with a slug bubble downstream in a short distance or evolve to a slug bubble. The bubble frequency in all of the cases studied is provided. Eruptive boiling may be predicted classically with nano-sized cavities that are consistent with the measured surface roughness.

A Rapid and Low-Cost Procedure for Fabrication of Glass Microfluidic Devices

In this paper, we present a simple, rapid, and low-cost procedure for fabricating glass microfluidic chips. This procedure uses commercially available microscopic slides as substrates and a thin layer of AZ 4620 positive photoresist (PR) as an etch mask for fabricating glass microfluidic components, rather than using expensive quartz glasses or Pyrex glasses as substrates and depositing an expensive metal or polysilicon/amorphous silicon layer as etch masks in conventional method. A long hard-baking process is proposed to realize the durable PR mask capable of withstanding a long etching process. In order to remove precipitated particles generated during the etching process, a new recipe of buffered oxide etching with addition of 20% HCl is also reported. A smooth surface microchannel with a depth of more than 110 mum is achieved after 2 h of etching. Meanwhile, a simple, fast, but reliable bonding process based on UV-curable glue has been developed which takes only 10 min to accomplish the efficient sealing of glass chips. The result shows that a high bonding yield (~ 100%) can be easily achieved without the requirement of clean room facilities and programmed high-temperature furnaces. The presented simple fabrication process is suitable for fast prototyping and manufacturing disposable microfluidic devices.

Enhanced mixing in laminar flows using ultrahydrophobic surfaces

Under laminar, microscale flow conditions, rapid mixing can be difficult to achieve. In these low Reynolds number flows, mixing rates are governed by molecular diffusion, and in the absence of enhanced mixing techniques, mixing lengths and residence times can be much longer than most applications will allow. A number of active mixing techniques have been developed to improve mixing; however, they can be complex to implement and expensive to fabricate. In this paper, we describe a passive mixing method that utilizes a series of ultrahydrophobic surfaces. Our previous experiments have demonstrated that a shear-free air-water interface supported between hydrophobic microridges results in large slip velocities along these ultrahydrophobic surfaces, and significant drag reduction. By aligning the microridges and therefore the air-water interface at an oblique angle to the flow direction, a secondary flow is generated, which is shown to efficiently stretch and fold the fluid elements and reduce the mixing length by more than an order of magnitude compared to that of a smooth microchannel. The designs of the ultrahydrophobic surfaces were optimized through experiments and numerical simulations. A Y-shaped channel was used to bring two streams of water together, one tagged with a fluorescent dye. A confocal microscope was used to measure fluorescence intensity and dye concentration. Quantitative agreement between the experiments and the numerical simulations was achieved for both the flow patterns and degree of mixing. Increasing the angle of the microridges was found to reduce the mixing length up to a critical angle of about 60 degrees , beyond which the mixing length was found to increase with further increases to the angle of the microridge. The mixing enhancement was found to be much less sensitive to changes in microridge width or separation.

Experimental observations and lattice Boltzmann method study of the electroviscous effect for liquid flow in microchannels

The existing experimental data in the literature on hydrodynamics for liquid flow in microchannels are analyzed and the reasons causing the diversities are discussed and summarized. The present experimental data for deionized water flow in glass microtubes with diameters ranging from 50 to 530 µm show that the friction factors and transition Reynolds numbers from laminar to turbulent flow are in good agreement with the conventional theoretical predictions. However, the friction factors in stainless steel microtubes with diameters of 119 and 172 µm are much higher than the conventional theoretical predictions. This discrepancy is attributed to the large surface relative roughness or dense roughness distribution in the stainless steel tubes. Numerical simulations taking into account the electroviscous effect are carried out by using the lattice Boltzmann method. The simulation results show that the electroviscous effect does not play a significant role in the flow characteristics for channel dimensions of the order of microns and hence it can be neglected in engineering applications for moderate electrical conductivity of the liquid and conductivity of the walls. From the literature review and the present test data, it is validated that for liquid flow in smooth microchannels the conventional theoretical prediction for flow characteristics should still be applied.

Numerical simulation of stacked microchannel heat sink with mixing-enhanced passive structure

The paper is focused on the investigation of numerical simulation of stacked two-layer microchannel heat sink with enhanced mixing passive microstructure. In contrast to the smooth microchannel studies in the literature, the microchannel with embedded passive microstructure is chosen. The computational fluid dynamics (CFD) will be used to simulate the flow and heat transfer in a stacked two-layer microchannels with multiple MEMS easy-processing passive microstructures. To simulate the conjugated heat transfer among the heatsink and fluid, the three-dimensional conjugated model is used to solve this problem. The important parameters (e.g. the ratio of embedded structure height to microchannel height and fluid property) are investigated. The ratio of embedded structure height to microchannel height is changed from 0.13 to 0.26. The microchannel Reynolds number is fixed at 14.8. The stacked microchannel with passive structures has better performance than the smooth microchannels.

Electroosmotic flow in microchannels with prismatic elements

Fundamental understanding of liquid flow through microchannels with 3D prismatic elements is important to the design and operation of lab-on-a-chip devices. In this paper, we studied experimentally and theoretically the electroosmotic flow (EOF) in slit microchannels with rectangular 3D prismatic elements fabricated on the bottom channel wall. The average electroosmotic velocity measured by the current-monitoring technique was found lower than that in a smooth microchannel. This velocity reduction becomes larger in microchannel with larger but less number of the prisms even though the space taken by the prisms are identical. The velocity distribution and streamlines on two typical horizontal planes in the microchannel are measured and visualized by a particle-based technique. These experimental observations are in good agreement with the numerical simulation. The comparison of streamlines near the prisms in the pressure-driven flow with that in the EOF showed that the EOF was more sensitive to the local geometry.

Visualization of convective boiling heat transfer in single microchannels with different shaped cross-sections

Convective boiling in transparent single microchannels with similar hydraulic diameters but different shaped cross-sections was visualized, along with simultaneous measurement of the local heat transfer coefficient. Two types of microchannels were tested: a circular Pyrex glass microtube (210 μm inner diameter) and a square Pyrex glass microchannel (214 μm hydraulic diameter). A 100-nm-thick semi-transparent ITO/Ag thin film sputtered on the outer wall of the microchannel was used for direct joule heating of the microchannel. The flow field visualization showed semi-periodic variation in the flow patterns in both the square and circular microchannels. Such variation was because the confined space limited the bubble growth in the radial direction. In the square microchannel, both the number of nucleation bubbles and the local heat transfer coefficient increased with decreasing vapor quality. The corners acted as active nucleation cavities, leading to the higher local heat transfer coefficient. In contrast, lack of cavities in the smooth glass circular microchannel yielded a relatively smaller heat transfer coefficient at lower vapor quality. Finally, the heat transfer coefficient was higher for the square microchannel because corners in the square microchannel acted as effective active nucleation sites.

Experimental investigation of water flow in smooth and rough silicon microchannels

Water flow in smooth rectangular cross-section microchannels and microchannels with discrete rectangle roughness elements was experimentally investigated. The microchannels were microfabricated on silicon wafers and the hydraulic diameter ranged from 153 µm to 191 µm. The rectangle roughness elements mounted on the sidewall of the microchannel were 50 µm high and 50 µm wide. The global pressure drop and velocity field were measured for Reynolds numbers between 100 and 2300. For the smooth rectangular microchannels, the friction factor of the laminar flow agreed with the value predicted by classical theory. The transition from laminar to turbulent flow occurred at about Re = 2100. For the microchannels with roughness elements, the friction factor approaches the conventional value for Re < 900 and is higher than the conventional value when Re > 900. It indicates that the presence of roughness elements leads to the early transition in the microchannels. The streamwise mean velocity profiles and turbulent intensities obtained from microscopic particle image velocimetry (micro-PIV) show that the transition from laminar flow to turbulent flow occurred for Reynolds numbers ranging between 900 and 1100. This result was consistent with the experimental data of the friction factor. The experimental results of flow fields also suggest that the flow became fully developed turbulence for Re > 1400 in the rough microchannel. In the microchannel with roughness elements, large scale coherence structures are observed in the flow fields for Re < 2000.

Experimental characterization of water flow through smooth rectangular microchannels

This article presents experimental results obtained in water flows through smooth rectangular microchannels. The experimental setup used in the present study enabled the investigation of both very small length scales (21–4.5μm) and a wide range of Reynolds numbers (0.1–300). The evolution of the friction coefficient was inferred from pressure drop versus flow-rate measurements for two types of water with different electrical conductivities. The channels were made of a silicon engraved substrate anodically bonded to a Pyrex cover. In these structures, pressure losses were measured internally with micromachined Cu–Ni strain gauges. When compared to macroscale correlations, the results demonstrate that in smooth silicon-Pyrex microchannels larger than 4μm in height, the friction law is correctly predicted by the Navier-Stokes equations with the classical no-slip boundary conditions, regardless of the water electrical conductivity (&amp;gt;0.1μScm−1).

Fully developed laminar flow and heat transfer in smooth trapezoidal microchannel

Abstract Numerical analysis of fully developed laminar slip flow and heat transfer in trapezoidal micro-channels has been studied with uniform wall heat flux boundary conditions. Through coordinate transformation, the governing equations are transformed from physical plane to computational domain, and the resulting equations are solved by a finite-difference scheme. The influences of velocity slip and temperature jump on friction coefficient and Nusselt number are investigated in detail. The calculation also shows that the aspect ratio and base angle have significant effect on flow and heat transfer in trapezoidal micro-channel.

Chaotic flow and efficient mixing in a microchannel with a polymer solution.

Microscopic flows are almost universally linear, laminar, and stationary because the Reynolds number, Re, is usually very small. That impedes mixing in microfluidic devices, which sometimes limits their performance. Here, we show that truly chaotic flow can be generated in a smooth microchannel of a uniform width at arbitrarily low Re, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating three-dimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable to the microchannel width, the flow behavior is fully compatible with that in a macroscopic channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.

Laminar Liquid Flow Through Silicon Microchannels

The present work deals with the analysis of the fully developed laminar flow through silicon microchannels. The main integral flow parameters, such as the Poiseuille number f Re, the momentum flux correction factor, the kinetic energy correction factor, the asymptotic incremental pressure drop number and the approximate value of the hydrodynamic entrance length are numerically evaluated for trapezoidal and double-trapezoidal cross sections of the silicon microchannels. The results are quoted in tabular and in graphic form as a function of the microchannel aspect ratio. Finally, very simple polynomial representations of the integral flow parameters are given. These numerical data are a useful tool for technicians and designers involved in micro-fluidic applications and it is demonstrated that these results can be used (instead of or before a CFD simulation approach) for a first evaluation of the pressure drop for liquid flows through smooth microchannels having a hydraulic diameter greater than 30 μm.

Planar cmos compatible process for the fabrication of buried microchannels in silicon, using porous-silicon technology

This work presents a new method for the fabrication of buried microchannels, covered with porous silicon (PS). The specific method is a two-step electrochemical process, which combines PS formation and electropolishing. In a first step a PS layer with a specific depth is created at a predefined area and in the following step a cavity underneath is formed, by electropolishing of silicon. The shape of the microchannel is semi-cylindrical due to isotropic formation. The method allows accurate control of the dimensions of both PS and the cavity. The formation conditions of the PS layer and the cavity were optimized so as to obtain smooth microchannel walls. In order to obtain stable structures the area underneath the PS masking layer was transformed into n-type by implantation, taking advantage of the selectivity of PS formation between n- and p-type silicon. With this technique, a monocrystalline support for the PS layer is formed on top of the cavity. Various microchannel diameters with different thickness of capping PS layer were obtained. The process is CMOS compatible and it uses only one lithographic step and leaves the surface of the wafer unaffected for further processing. A microfluidic thermal flow sensor was fabricated using this technology, the experimental evaluation of which is in progress.