Microchannel Width Research Articles

Experimental investigations and analytical modeling of multi-pass CO2 laser processing on PMMA

In this research work, microchannels have been fabricated utilizing multi-pass CO2 laser processing on Poly-methyl meth-acrylate (PMMA) substrates. CO2 laser engraving machines are cost effective and less time consuming compared to other tools and methods of fabricating microchannels on PMMA. However, the basic problem of low surface finish of the microchannel walls still restricts thus fabricated product from many potential applications. In this work, experimental and theoretical investigations of multi-pass CO2 laser processing on PMMA have been conducted. A number of experiments were performed to establish the relationship between laser power and scanning speed with microchannel parameters like width, depth, heat affected zone, surface roughness and surface profiles. Experiments were conducted at four different power settings with 50mm/s of constant scanning speed and seven numbers of passes in each setting. Changes in thermo-physical properties of PMMA were observed for as-received PMMA sample and PMMA sample residing in heat affected zone (HAZ) for first pass and secondary passes respectively. Effect of different numbers of passes on microchannel width, depth, HAZ and surface roughness were explored for different power setting. Microchannel profiles resulting from different numbers of passes have been compared. Energy dispersive X-ray analysis was performed to determine elemental composition after each pass. Many advantages of multi-pass processing over single-pass processing were recorded including high aspect ratio, low heat affected zone, smoother microchannel walls and reduced tapering of microchannels. An energy balance based simple analytical model was developed and validated with experimental results for predicting microchannel profiles on PMMA substrate in multi-pass processing. Multi-pass processing was found to be time and cost effective method for producing smooth microchannels on PMMA.

Critical condition for bubble breakup in a microfluidic flow-focusing junction

The critical condition for bubble breakup in a microfluidic flow-focusing junction was studied in this work. The experiments were conducted in a square microchannel of 400μm wide. The critical condition for bubble breakup was investigated by varying the bubble length l0, liquid viscosity μ, velocity of the liquid from the main channel of the flow-focusing junction u1, and velocity of the liquid from the side channels of the junction u2. By analyzing the effects of these factors on the dynamical evolution of gas-liquid interface for bubble deformation and breakup, expressions for describing the bubble deformation and breakup time were established, respectively. On the basis of these two expressions, the critical condition for the bubble breakup in a microfluidic flow-focusing junction was deduced: l0wc=1.5u1u20.75Ca2-0.13, where wc is the width of microchannel, Ca2=u2μ/γ, and γ is the surface tension.

Reliable and high quality adhesive bonding for microfluidic devices

Some issues, such as the removing of air bubbles from bonding interface, microchannel clogging by adhesive, the methods to increase bonding efficiency, as well as the adhesive bonding of a multilayer chip, still limit the use of adhesive bonding in the mass fabrication of microfluidic devices. In this work, an automatic adhesive bonding machine is developed which can obviously increase bonding efficiency and quality as compared with the traditional manually methods. A bonding method which uses a soft pressing head to extrude the air bubbles from bonding interface is presented. The relationship between the adhesive film thickness and the channel clogging is investigated. The adhesive bonding of three types of two-layer microfluidic devices and a five-layer microfluidic device are studied. Results have shown that the uses of the soft pressing head can effectively removing the air bubbles from the bonding interface. The bonding ratios of all devices are above 95%. The channel residual depth, which is the channel depth excluding the protrusion height of the adhesive film within the channel, will increase to follow the increase in microchannel width and adhesive film thickness.

Femtosecond laser micromachining of zirconia green bodies

In this study, we developed micromachining processes using a femtosecond Ti: sapphire laser for zirconia green bodies, i.e., composites comprising zirconia particles and polymer binders. The laser ablation characteristics of the zirconia composites were analysed to optimise the process. In comparison with bulk zirconia, ablation of the zirconia green bodies occurred at lower fluences. In the high-fluence regime, laser irradiation directly ablated the zirconia particles, causing aggregation of resolidified particles. The ablated depth increased with fluence (F) and overlap number (N) initially, but saturated when F and N became larger. Microchannels with a minimum width of 4 µm were fabricated. The microchannel width decreased with decreasing fluence and overlap number. Microholes, microchannels and three-dimensional structures with a size of several tens of micrometres and aspect ratio of 5-15 were successfully fabricated using the zirconia composite.

Experimental and theoretical analysis of defocused CO2 laser microchanneling on PMMA for enhanced surface finish

The poor surface finish of CO2 laser-micromachined microchannel walls is a major limitation of its utilization despite several key advantages, like low fabrication cost and low time consumption. Defocused CO2 laser beam machining is an effective solution for fabricating smooth microchannel walls on polymer and glass substrates. In this research work, the CO2 laser microchanneling process on PMMA has been analyzed at different beam defocus positions. Defocused processing has been investigated both theoretically and experimentally, and the depth of focus and beam diameter have been determined experimentally. The effect of beam defocusing on the microchannel width, depth, surface roughness, heat affected zone and microchannel profile were examined. A previously developed analytical model for microchannel depth prediction has been improved by incorporating the threshold energy density factor. A semi-analytical model for predicting the microchannel width at different defocus positions has been developed. A semi-empirical model has also been developed for predicting microchannel widths at different defocusing conditions for lower depth values. The developed models were compared and verified by performing actual experiments. Multi-objective optimization was performed to select the best optimum set of input parameters for achieving the desired surface roughness.

Design and Feasibility Analysis of Microscale Bumped Channel With Supersonic Flow for Electronics Cooling

In this paper, microchannel heat sink cooling device utilizing supersonic gas flow with isentropic expansion has been designed and analyzed. The objective is to study the feasibility of the supersonic microchannel with bumped section design for electronics cooling. Numerical simulations on the designed microchannel flows and heat transfer analysis have been conducted (in conjunction with a supersonic air flow inside a heated fin array) and compared with experimental results in this paper. The performance of the designed microchannel is also compared with traditional Laval nozzles for the feasibility analysis. Under 350 and 330 K inlet temperature conditions, the calculated Nu number is found higher than pure through channel flows. The supersonic expanding flow is successfully generated with lower heat transfer temperatures and a simpler geometric shape. The design of 100– $300~\mu \text{m}$ is found to have highest heat transfer for integrated microchannel width selection. For electronics cooling tests, the newly designed supersonic channel is found sufficient for a cooling capacity around 1.4 MW/ $\text{m}^{2}$ . In addition, the supersonic flow and heat transfer characteristics are also discussed into detail in this paper. [2016-0037]

Effects of Size of Microchannels on Thermo-Electrical Performance of an Internally Cooled Li-Ion Battery Cell

Thermal management of Li-ion batteries utilizing internal cooling method is the promising way to keep these batteries in an appropriate temperature range and to improve the temperature uniformity. In this study, three-dimensional transient thermal analysis was carried out to investigate the effects of size of embedded microchannels inside the electrodes on the thermal and electrical performances of a Li-ion battery cell. Based on the ratio of the width of microchannels to the width of the cell, different cases were designed; from the ratio of 0 (without any microchannels) to the ratio of 0.5. The results showed that increasing the size of the microchannels from the width ratio of 0 to the width ratio of 0.5 can reduce the maximum temperature inside the battery cell up to 11.22 K; it can also improve the temperature uniformity inside the battery cell. Increasing the electrolyte flow inlet temperature from 288.15 K to 308.15 K can enhance the temperature uniformity inside the battery and the cell voltage up to 33.20% and 2.79%, respectively. Increasing the electrolyte flow inlet velocity from 1 cm/s to 10 cm/s can reduce the maximum temperature inside the battery cell up to 8.09 K.

Machining Complex Three-Dimensional Nanostructures With an Atomic Force Microscope Through the Frequency Control of the Tip Reciprocating Motions

A novel method relying on atomic force microscope (AFM) tip based nanomachining is presented to enable the fabrication of microchannels that exhibit complex three-dimensional (3D) nanoscale floor surface geometries. To achieve this, reciprocating lateral displacements of the tip of an AFM probe are generated, while a high-precision stage is also actuated to move in a direction perpendicular to such tip motions. The width and length of microchannels machined in this way are determined by the amplitude of the tip motion and the stage displacement, respectively. Thus, the processing feed can be changed during the process as it is defined by the combined control of the frequency of the tip reciprocating motions and the stage speed. By employing the built-in force feedback loop of conventional AFM systems during such operations, the variation of the feed leads to different machined depths. Thus, this results in the capability to generate complex 3D nanostructures, even for a given normal load, which is set by the AFM user prior to the start of the process. In this paper, the fabrication of different microchannels with floor surfaces following half triangular, triangular, sinusoidal, and top-hat waveforms is demonstrated. It is anticipated that this method could be employed to fabricate complex nanostructures more readily compared to traditional vacuum-based lithography processes.

AN ECDM APPROACH FOR THE DEVELOPMENT OF 3D MICROCHANNELS ON GLASS SUBSTRATES

Microchannels proved to be most favorable component for microfluidic applications. Electrochemical discharge machining provides cost effective solution to produce micro profiles on nonconductive and conductive materials regardless of their hardness, brittleness and surface reflectivity. The present study discusses the effect of applied voltage, electrolyte concentration and workpiece feed rate during fabrication of microchannels on glass substrates. The output parameters studied were material removal rate and microchannel width over cut. The optimum combination of applied voltage (= 50V), electrolyte concentration (= 20%) and feed rate (=16mm/min.) yields the best accuracy of the micro channels.

CO2 Laser Microchanneling Process: Effects of Compound Parameters and Pulse Overlapping

PMMA (Polymethyl methacrylate) is commonly used in many microfluidic devices like Lab-on-a-chip devices, bioanalytical devices etc. CO2 lasers provide easy and cost effective solution for micromachining needs on PMMA. Microchannels are an integral part of most of these microfluidic devices. CO2 laser beams have been successfully applied by many authors to fabricate microchannels on PMMA substrates. Laser beam power and scanning speed are the most important laser input parameters affecting the output parameters like microchannel depth, width and heat affected zone (HAZ). The effect of these individual parameters on output parameters are well known and already elaborated by many authors. However, these output parameters can more significantly be described by some compound parameters (combination of direct input laser parameters) like laser fluence, specific point energy, interaction time and P/U (power/scanning speed) ratio. The explanation of effect of these compound parameters was not found in earlier researches. In this work, several experiments were carried out to determine the effects of these compound parameters on output parameters i.e. microchannel width, depth and heat affected zone. The effect of pulse overlapping was also determined by performing experiments at different pulse overlaps and with two different energy deposition settings. The concept of actual pulse overlapping has been introduced by considering actual beam spot diameter instead of using theoretical beam diameter. Minimum pulse overlapping was determined experimentally in order to ensure smooth microchannel edges.

Quantitative visualization of asymmetric gas flow in constricted microchannels by using pressure-sensitive paint

Asymmetric flow in constricted microchannel devices was quantitatively investigated using a pressure-sensitive paint (PSP) technique. For microchannel devices with constriction ratios of 2 : 1 and 5 : 1, detailed pressure maps for the region around the constriction structure were obtained and enabled visualization of the flow field. Symmetric flow was observed in the microchannel device with a constriction ratio of 2 : 1 at the Reynolds number range 2–165. In the microchannel with a constriction ratio of 5 : 1, a deflected flow pattern was clearly identified from PSP measurements at Reynolds numbers exceeding 107. Furthermore, PSP measurements showed a pressure difference of up to 2.5 kPa between the two lateral locations corresponding to y = ±0.15 W (W is the microchannel width) downstream of the constriction at a Reynolds number of 279. The pressure difference resulted from asymmetric bifurcation of the flow.

Rapid and Automated Measurement of Milk Adulteration Using a 3D Printed Optofluidic Microviscometer (OMV)

In developing countries like India, adulteration in the milk consumed by the population presents stern implications as tarnishing of the same poses serious issues, such as health deterioration, corruption, and so on. There are many adulterants that are added to milk, including water, flour, starch, and even urea, in quantitative measures making it undetectable. There are many devices in the market to measure adulteration in milk but most of them are bulky, require large sample volume, and need a technical operator for working. In the recent decade, microfluidics has emerged as a huge market for biomedical research. It has paved the pathway for a quick, robust, and plug-and-play device for various applications. This paper describes a low-cost, durable, and simple optofluidic microviscometer fabricated by the stereolithography technique. The device operation is based on the linear relationship between dynamic viscosity and channel width derived from the flow of two immiscible fluids inside a channel. The principle of operation is based on the modified Hagen–Poiseuille flow equation. The working principle is the viscosity-dependent capture of the microchannel width by the fluids flowing inside the microchannel under the laminar flow based on the pressure gradient between the inlets and the outlet. In this paper, around 60 milk samples with various adulteration ratios of various adulterants ranging from 1% to 10% have been tested. A best fit curve for every adulterant was defined, and the device was found to be accurate enough to measure the entire range of adulteration ratios with a high accuracy of 0.95.

Dimensional optimization of a micro-channel heat sink using Jaya algorithm

In this paper, a recently proposed optimization algorithm named as ‘Jaya algorithm’ is used for the dimensional optimization of a micro-channel heat sink. Two case studies are considered and in both the case studies two objective functions related to the heat transfer and pressure drop, i.e. thermal resistance and pumping power, are formulated to examine the performance of the micro-channel heat sink. In the first case study, two non-dimensional design variables related to the micro-channel depth, width and fin width are chosen and their ranges are decided through preliminary calculations of three-dimensional Navier–Stokes and energy equations. In the second case study, three design variables related to the width of the micro-channel at the top and bottom, depth of the micro-channel and width of fin are considered. The objective functions in both the case studies are expressed in terms of the respective design variables and the two objectives are optimized simultaneously using the proposed Jaya algorithm. The results are compared with the results obtained by the TLBO algorithm and a hybrid multi-objective evolutionary algorithm (MOEA) and numerical analysis. The results obtained by the application of Jaya algorithm are found much better than those reported by the other approaches.

Fluid Flow and Heat transfer Characteristics within a Rectangular Microchannel Array of Different Manifold Shapes Modelization and Optimization Using CFD and Response Surface Methodology

Different manifold shapes of microchannel device are investigated for the pressure drop characteristics under the effect of varying microchannel depth, width and spacing. The response surface methodology is applied to optimise the operating conditions of different microchannel systems. Four operating parameters, the channel width, depth, spacing and the inlet Reynolds number are considered in this study. The individual effects of these operating parameters and the combined effects of multiple operating conditions on pressure drop characteristics are examined using CFD. The semi-elliptical manifold shapes results the high level of fluid flow uniformity within microchannels and lower pressure drop. The maximum percentage reduction in the pressure drop across the microchannel device is 70%, obtained for semi-elliptical manifold shape as compared to other. The heat transfer analysis is also carried out in the extent of the study.

Effect of entrained air in abrasive water jet micro-machining: Reduction of channel width and waviness using slurry entrainment

The erosion produced by abrasive water jets is a result of the complex interaction of the three-phase jet (water-air-abrasive) and the target. The objective of the present work was to isolate the effect of the entrained air in abrasive water jet micro-machining (AWJM) by comparing milled surfaces made by AWJM with those produced by high-pressure slurry jet micro-machining (HASJM) while maintaining a constant particle velocity. An existing model developed for abrasive particle velocities in AWJM was modified and used to predict the particle velocity in HASJM, and then verified using a double disc apparatus (DDA). The model allowed prediction of the conditions required to achieve average particle velocities of ~255m/s using the two systems with the same 38µm garnet particles. Under this condition of equal particle velocity, there was a very large reduction in the centerline waviness, Wa, of micro-channels made in SS316L and Al60661-T6 using HASJM; typically 3.4 times smaller waviness than found in channels made with AWJM using the same dose of particles. This was found to be mainly due to the much larger variation in abrasive flow rate in AWJM brought about by the air/particle entrainment system, rather than any fundamental change in erosion mechanism due to the impact of the nonhomogeneous three phase air/particle/water in AWJM. The centerline roughness, Ra, was approximately the same in both processes at a traverse velocity of Vt=4572mm/min and a nozzle angle of 90°. For both processes, Wa and Ra increased with an increase in pressure and abrasive particle dose, and decreased with an increase in nozzle angle. For micro-channels of a given depth, the widths of those made with HASJM were 26% narrower than those produced with AWJM, mainly due to the wider jet that resulted from the entrained air in AWJM, again at the same particle velocity. The erosion rate in HASJM was found to be about 27% lower mainly because of a smaller width of micro-channels in HASJM.

Influence of pulsed Nd3+ : YAG laser beam profile and wavelength on microscribing of copper and aluminum thin films

Evenly spaced conductive grids of copper and aluminum thin films on polyimide substrate are used for parabolic reflector antennas aboard telecommunications satellites. Laser microscribing of thin films using a flat-top and Gaussian laser beam profile is analyzed with 95% overlapping of the diameter of the laser spot. Laser scribing is performed using the Q-switched Nd3+: YAG (355, 532 nm) laser. The influence of laser irradiation and beam shape on the scribed microchannel width, depth, and surface characteristics is experimentally analyzed using a noncontact optical profilometer and scanning electron microscope (SEM). Laser scribing using a flat-top profile produced near rectangular microchannels in copper thin films. Using the Gaussian profile, the probability of melting is greater than vaporization as observed using SEM images; this melt pool plays a prominent role in resolidification at the edges. The depth of the scribe channel is observed to be 20% higher for the 532-nm wavelength compared to the 355-nm wavelength. The effect of different environments such as air, water, and vacuum on the channel depth and quality is reported. The response of aluminum and copper thin films for high fluences is also studied. Thermal modeling of the laser-material interaction has been attempted by assuming the plasma electron temperature as the laser ablation temperature for modeling the recession rate and depth for a single laser pulse. Model results agree with experimental data showing greater depth for 532 nm compared to 355 nm.

Two step silicon microfluidics for capillary valve applications

In this publication, we report on a robust and scalable process flow for fabrication of valves, compatible with other microfluidic components using silicon microfabrication techniques. The valves require an abrupt change in both the depth and width of a rectangular microchannel for proper functioning. We report on the process that allow for a very sharp interface between the two areas of the channel. A non-sharp interface will results in leaky valves. We show the variation in silicon etch depth as function of feature size. For small features (3–10)μm there is roughly a 50 percent increase in etch depth compared to 40μm features, while features above 40 μm show minimal differences in depth. For the formation of valves, this provides important design rules allowing one to make structures with precise dimensions. Initial characterization, demonstrating the functionality of the fabricated valves is also be presented.

Microchannel Wetting for Controllable Patterning and Alignment of Silver Nanowire with High Resolution.

Patterning and alignment of conductive nanowires are essential for good electrical isolation and high conductivity in various applications. Herein a facile bottom-up, additive technique is developed to pattern and align silver nanowires (AgNWs) by manipulating wetting of dispersions in microchannels. By forming hydrophobic/hydrophilic micropatterns down to 8 μm with fluoropolymer (Cytop) and SiO2, the aqueous AgNW dispersions with the optimized surface tension and viscosity self-assemble into microdroplets and then dry to form anisotropic AgNW networks. The alignment degree characterized by the full width at half-maximum (FWHM) can be well-controlled from 39.8° to 84.1° by changing the width of microchannels. A mechanism is proposed and validated by statistical analysis on AgNW alignment, and a static model is proposed to guide the patterning of general NWs. The alignment reduced well the electrical resistivity of AgNW networks by a factor of 5 because of the formation of efficient percolation path for carrier conduction.

Fine-tuning width and aspect ratio of an improved microchannel heat sink for energy-efficient thermal management

Computational experiments were performed to study the influence of microchannel width and aspect ratio on performance of a four-compartment, microchannel heat sink with dedicated coolant inlet and outlet for each compartment. Wide range of coolant velocity (0.25–2m/s) & aspect ratio (2–10) and a reasonable range of microchannel width (100–200μm) have been simulated. The appropriate microchannel width and aspect ratio were found to be 200μm and 6, as evidenced from lowest total thermal resistance and lowest non-uniformity of substrate temperature at comparable power consumption among different microchannel widths and aspect ratios simulated. A new parameter ‘ω’ has been proposed for representation of substrate temperature gradient considering maximum, average and minimum substrate temperatures. Correlations have been developed for prediction of dimensionless pumping power, Nusselt number and dimensionless total thermal resistance. Hence, the four compartment heat sink with 200μm wide channels of aspect ratio 6 can be used for energy-efficient thermal management.

A simplified approach to hotspot alleviation in microprocessors

Hotspots in microprocessors arise due to non-uniform utilization of the underlying integrated circuits during chip operation. Conventional liquid cooling using microchannels leads to undercooling of the hotspot areas and overcooling of the background area of the chip resulting in excessive temperature gradients across the chip which adversely affects the chip performance and reliability. This problem becomes even more acute in multi-core processors where most of the processing power is concentrated in specific regions of the chip called as cores. We present a one-dimensional, semi-empirical approach for quick design of a microchannel heat sink for targeted, energy efficient liquid cooling of hotspots in microprocessors. Our approach enables targeted manipulation of the local cooling capacity in the microchannel heat sink, which in turn minimizes the chip temperature gradient. The method is formulated to design a heat sink for an arbitrary chip power map and hence can be readily utilized for different chip architectures. It involves optimization of microchannel widths and flow rate distribution for various zones of the power map under the operational constraints of maximum pressure drop limit for the heat sink. Additionally, it ensures that the coolant flows uninterrupted through its entire travel length consisting of microchannels of varying widths. The resulting design estimate significantly reduces the effort involved in designing hotspot-targeted heat sinks.