Superhydrophobic Microchannels Research Articles

Mixed Convection Flow of a Viscous Dissipative and Heat Generating/Absorbing Fluid Through a Slit Microchannel

This paper is aimed at investigating the consequences of viscous dissipation and super-hydrophobicity on a magnetized mixed convection flow of an electrically conducting fluid across an up facing microchannel influenced by a transverse magnetic field. The plates were alternatively heated and incorporated with heat source/sink effect. The modeled governing equations have been obtained using a semi-analytical (regular perturbation) method. Various line graphs have been plotted to demonstrate the behavior of key parameter dictating the flow. It was found out that the thermal gradient and fluid velocity are significantly enhanced for mounting values of mixed convection Gre, Brickman number Br, Darcy porous number K and heat source S parameters in the superhydrophobic microchannel for constant pressure gradient Г. On the other hand, the velocity deteriorates for increasing levels of magnetic field M and heat sink S factors. Further, the comparison of this present analysis with previously published literature for limiting cases when Г=S=0 and Gre=1 showcased an excellent concurrence, thereby confirming the accuracy and validity of this present investigation. The findings from this research will find relevance in engineering, technological and industrial processes such as for solar collectors, nuclear reactors cooled during emergency shutdown, nuclear power plants, gas turbines and the various propulsion devices for aircraft, missiles, satellites and space vehicles.

Electrokinetically controlled mixed convective heat flow in a slit microchannel

AbstractThis study performs a time‐dependent analysis of mixed convection of an incompressible fluid and heat sink/source factor in an upstanding slit superhydrophobic (SHO) microchannel in the involvement of temperature jump and electroosmotic flow conditions. The internal wall of one of the sides in the microchannel is intentionally modified to demonstrate SHO slip and temperature jump conditions. A transverse magnetic effect is introduced in the path of the flow. The steady‐state solutions of the modeled problem have been analytically derived for temperature, velocity, pressure gradient, sheer stress, and heat transfer rate. The derived results are expounded thoroughly with the use of several plots. It is deduced that the elevating mixed convection (Gre), heat source/sink (Qs), Debye–Hückel (K), and nonlinear parameters (N) are observed to increase the fluid flow as time rises, and these effects are all higher when the velocity slip and temperature jump impacts are present. Further, the application of heat‐generating parameters is viewed to encourage the fluid temperature in the microchannel. Finally, the comparison between the current investigation with the previously published findings demonstrates a very good consistency for the limiting cases.

The hydrothermal performance of non-Newtonian fluids in superhydrophobic microchannels

The hydrothermal performance of non-Newtonian fluids in superhydrophobic microchannels

Analytical Investigation of Arrhenius Kinetics with Heat Source/Sink Impacts along a Heated Superhydrophobic Microchannel

Exothermic Arrhenius-controlled chemical reactions are widely employed for heating applications, such as combustion in heating systems and fuel-burning motors. These effects are critical for manufacturing high thermal systems required for high thermal performances. Therefore, this paper presents the analytical investigation into the Arrhenius-controlled heat-generating/absorbing fluid of a hydromagnetic flow along a heated upstanding plate in a microchannel. One wall had a superhydrophobic surface and temperature jump conditions, but the other was unaltered. The regular perturbation approach investigated the nonlinear, coupled governing equations. With the help of graphical plots, the impacts of crucial and relevant parameters embedded in the flow are described. This investigation concludes that chemically reacting parameters' impact significantly elevates the micro-channel's thermal and hydromagnetic flow. However, applying the heat generation parameter increases fluid velocity, whereas the heat absorption parameter produces the contrary effect. The outcomes of this study can be relevant to the field of biomedical sciences and devices for improving heat transfer efficiency, chemical synthesis, enhancing the performances of micro-electro-mechanical systems (MEMS) and mini-devices, heating and energy generation, designing efficient energy conversion processes, and so on.

Superhydrophobic Microchannel Heat Exchanger for Electric Vehicle Heat Pump Performance Enhancement

Battery-powered electric vehicles (EVs) have emerged as an environmentally friendly and efficient alternative to traditional internal combustion engine vehicles, while their single-charge driving distances under cold conditions are significantly limited due to the high energy consumption of their heating systems. Heat pumps can provide an effective heating solution for EVs, but their coefficient of performance (COP) is hampered by heat transfer deterioration due to frost accumulation. This study proposes a solution to this issue by introducing a microchannel heat exchanger (MHE) with superhydrophobic surface treatment (SHST) as a heat pump evaporator. A computational fluid dynamics MHE model and a dynamic heat pump model are developed and rigorously validated to examine the detrimental impact of frost accumulation on heat transfer, airflow resistance, and heat pump performance. When the frost layer thickness is 0.8 mm at a given air-side velocity of 1.0 m/s, the air-side heat transfer coefficient can be reduced by about 75%, and the air-side pressure drop sharply increases by 28.4 times. As frost thickness increases from 0 to 0.8 mm, the heating capacity drops from 3.97 to 1.82 kW, and the system COP declines from 3.17 to 2.30. Experimental results show that the frost thickness of the MHE with SHST reaches approximately 0.4 mm after 30 min, compared to that of 0.8 mm of the MHE without SHST, illustrating the defrosting capability of the superhydrophobic coating. The study concludes by comparing the performance of various heating methods in EVs to highlight the advantages of SHST technology. As compared to traditional heat pumps, the heating power consumption of the proposed system is reduced by 48.7% due to the defrosting effect of the SHST. Moreover, the single-charge driving distance is extended to 327.27 km, an improvement of 8.99% over the heat pump without SHST.

Determining The Role of Thermal Radiation on Hydromagnetic Flow in A Vertical Porous Superhydrophobic Microchannel

The focus of this article is to scrutinize the performances of thermal radiation and superhydrophobicity on a free convection of an electrically conducting fluid across an upstanding microchannel affected by a transverse magnetic field. The plates were alternatively heated and incorporated with suction/injection effect. The approximate expressions of the formulated differential equations have been calculated. It is interesting to report that the heat gradient and fluid motion are significantly propelled for mounting values of thermal radiation parameter in both case I: super-hydrophobic surface (SHS) is heated and case II: no slip surface (NSS) is heated. However, this effect brought down the fluid temperature at the NSS, which is attributable to the direction of the heat flow. On the other hand, the velocity deteriorates for mounting values of magnetic field factor by the Lorentz force attracting the molecules, consequently leading to the fluid deterioration.

Analysis of mixed convection flow on Arrhenius-controlled heat generating/absorbing fluid in a super-hydrophobic microchannel: A semi-analytical approach

The theoretical analysis of steady mixed convection with Arrhenius-controlled heat flow for a viscous and an electrically-conducting fluid traveling across an isothermally heated vertical parallel plate in a slit micro-channel is investigated in this study. One wall had super-hydrophobic slip and a temperature spike, while the other did not. A semi-analytical technique (perturbation series) was employed to analyze the nonlinear and coupled leading equations. The results were carefully scrutinized, and the effects of the relevant and pertinent parameters were illustrated using various plots. It is concluded from this work that the actions of mixed convection and chemically reacting factors are noticed to substantially strengthen the fluid movement in the micro-channel for a constant pressure gradient. Additionally, the function of heat source parameter is depicted to raise the fluid flow while heat absorption parameter yields the reverse phenomenon. In the fields of engineering and medicine, it is essential to understand these fluids' characteristics. Owing to the lubrication of micro-channel boundary surfaces where conductivity and viscosity interact with thermos-physical behavior, the results of the present investigation can significantly enhance the functioning of micro-electro-mechanical systems (MEMS) and micro-devices relying on micro-fabrication processes.

Anomalously enhanced subcooled flow boiling in superhydrophobic graphene-nanoplatelets-coated microchannels

In boiling heat transfer, superhydrophobic surface is deemed to reduce the critical heat flux (CHF) due to the immediate formation of an insulating vapor film that inhibits effective heat transfer despite its smaller energy barrier for bubble nucleation than that of a superhydrophilic surface. Here, a subcooled flow boiling investigation is carried out in a microchannel heat sink at a low mass flux condition using the superhydrophilic and superhydrophobic graphene nanoplatelets (GNPs) composite coatings. Interestingly, the flow boiling performance of the superhydrophobic GNPs-coated microchannel significantly transcends that of its superhydrophilic counterpart. By benchmarking with the performance of an uncoated microchannel, the CHF and heat transfer coefficient of the superhydrophobic GNPs-coated microchannel are enhanced up to 134% and 135%, respectively. This anomaly is ascribed to the Cassie Baxter-to-Wenzel transition on the superhydrophobic GNPs-coated surface, which is infiltrated with liquid, restricting the nucleating bubbles from expanding into a vapor film and sustaining an efficient boiling. The initial Wenzel state of the superhydrophobic GNPs surface is primarily ascribed to the rapid intercalation of water molecules via the unique graphene nanostructure. This study sheds new light on the unique wetting behavior of graphene-nanostructured surfaces in the context of microscale flow boiling heat transfer.

Nonlinear free convective with longitudinal slits in the presence of super-hydrophobic and non-hydrophobic microchannels in a suspension of nanoparticles: Multi-Linear Regression Analysis

Super hydrophobic microchannels have extremely high water repellency, and such a property is reliant on a set of factors, namely the surface construction and the surface energy. The surface energy can be depressed either by exhausting a chemical film that is coated or stuck to the surface or by adjusting the existing surface interaction or by producing micro- and nanotextures. Further, due to the latter reason of inducing super hydrophobic properties, a study of manufacturing processes to produce surface textures becomes critical to the understanding of the area. The latest generative manufacturing techniques, such as 3-D (three-dimensional) printing, laser machining, and so on are new-generation processes that can be used to write over surfaces at different resolutions. Viewing into this, the natural convection of conductive nanofluids in vertical slot micro-channels with super-hydrophobic slips and temperature jumps, the non-linear Boussinesq approximation approach provides an accurate solution. One inner surface of the microchannel surface has been physically modified to exhibit temperature jumps and super-hydrophobic velocity slips. The transverse magnetic field indicates the direction of flow. In two different physical situations, it is possible to get a single answer. Type I involves heating the super-hydrophobic surface without heating the non-slip surface, and Type II involves heating the non-slip surface without heating the super-hydrophobic surface. Velocity slip reduces the Type I Nusselt number and increases the Type II Nusselt number in both situations. The temperature jump factor has the opposite effect. In contrast to the change in linear density with temperature, the non-linear density change with temperature causes an increase in flow and velocity. It is observed that more slip coefficient corresponds to less Nusselt number and more slip velocity, especially at larger magnetic parameter values to increase the heat transfer rate from the microchannel walls. It is recommended to use Al2O3-water nanofluid the nanofluid composed of nanoparticles with higher thermal conductivity. Applications of the subject include anti-wetting micro technology, electrolytic cell maintenance, micro-hydraulic devices, and micro-electronic heat dissipation.

Self-Driving Underwater "Aerofluidics".

Here, the concept of "aerofluidics," in which a system uses microchannels to transport and manipulate trace gases at the microscopic scale to build a highly versatile integrated system based on gas-gas or gas-liquid microinteractions is proposed. A kind of underwater aerofluidic architecture is designed using superhydrophobic surface microgrooves written by a femtosecond laser. In the aqueous medium, a hollow microchannel is formed between the superhydrophobic microgrooves and the water environment, which allows gas to flow freely underwater for aerofluidic devices. Driven by Laplace pressure, gas can be self-transported along various complex patterned paths, curved surfaces, and even across different aerofluidic devices, with an ultralong transportation distance of more than 1 m. The width of the superhydrophobic microchannels of the designed aerofluidic devices is only ≈42.1µm, enabling the aerofluidic system to achieve accurate gas transportation and control. With the advantages of flexible self-driving gas transportation and ultralong transportation distance, the underwater aerofluidic devices can realize a series of gas control functions, such as gas merging, gas aggregation, gas splitting, gas arrays, gas-gas microreactions, and gas-liquid microreactions. It is believed that underwater aerofluidic technology can have significant applications in gas-involved microanalysis, microdetection, biomedical engineering, sensors, and environmental protection.

Exploring the Dynamics of Viscous Dissipative Fluid past a Super-hydrophobic Microchannel in the Coexistence of Mixed Convection and Porous Medium

This article investigates the theoretical treatment of mixed convection heat enhancement flow for an electrically-conducting and viscous dissipative fluid traveling vertically through a thermodynamic system where the parallel plates are constantly heated in a slit micro-channel due to mixed convection with porous material. One surface had super-hydrophobic slip and a temperature jump, whereas the other did not. The perturbation technique (semi-analytical method) was employed to solve the nonlinear and coupled leading equations. The results were carefully scrutinized, and the effects of the relevant controlling parameters are shown using different plots. It is concluded from this analysis that the fluid temperature and velocity was found to increase as the viscous dissipation term is increased. Similarly, the function of Darcy porous number is to significantly strengthen the fluid velocity, and these effects are stronger at the heated super-hydrophobic surface, whereas mounting level of magnetic field is seen to drastically weaken the fluid motion in the microchannel. Setting Br and A to zero respectively, Gr/Re=1, and Da to 1000, so that the term 1/Da becomes insignificant, Jha and Gwandu (2017)'s work is retrieved, verifying the accuracy of the current analysis. Further, the outcomes of this research can have possible applications in the lubrication industry and biomedical sciences and has proved very useful to designers in increasing the performance of mechanical systems when viscous dissipation is involved, as well as heat transfer in micro-channels, as it is in combustion.

Investigating the Flow Characteristics of Superhydrophobic U-Shaped Microchannels

Hydrophobicity has been widely reported for its superior behavior in drag reduction, self-cleaning, and anti-corrosion in many areas. Especially in engineering design, the research of the unique property of the slip flow with complex flow patterns is essential for practical applications. In this paper, the flow characteristics of a superhydrophobic U-shaped microchannel are systematically investigated, as the curved part is a fundamental component in microfluids. A slip model is established based on theoretical and experimental solutions. Various types of U-shaped microchannels, radii of curvature, and contact angles are studied with a wide range of Reynolds numbers from 0 to 300. We propose a velocity distribution to examine the non-uniformity of slip velocity on the cross-section. This imbalance is improved with an increase in the apparent contact angle and flow rate, and a decrease in the radius of curvature. The secondary flow and vortices generated by the centrifugal force are enhanced, and their positions are changed due to the slippery boundary. The results show a considerable drag reduction from 10% to 40% with different contact angles. The variation of curvature does not have a decisive impact on the final performance when the surface wettability maintains a steady state. Our research elucidates the physical principle of the slip model in curved channels, showing extensive applications of hydrophobicity in the design of complex microchips and the optimization strategy of heat transfer systems.

Stable and drag-reducing superhydrophobic silica glass microchannel prepared by femtosecond laser processing: Design, fabrication, and properties

The superhydrophobic silica glass microchannel with flow drag reduction performance has broad application prospects. In response to this demand, this paper proposes an integrated design and preparation method of structure and function for drag reduction microchannels on the silica glass surface. The design method of geometric parameters of stable superhydrophobic silica glass surface structures with a periodic micropillar array is studied based on the drag reduction mechanism of superhydrophobic surfaces. Combined with the plasma deposition process, the drag-reducing superhydrophobic silica glass surface is successfully prepared by femtosecond laser technology. The prepared drag-reducing superhydrophobic silica glass surface shows good composite wetting state stability under different test conditions. The drag-reducing superhydrophobic functional structure with a micropillar array is prepared at the bottom of the silica glass microchannels by femtosecond laser direct writing technology. Moreover, the effective integration of the silica glass microchannel structure and the drag-reducing functional structure is achieved. The preparation and flow performance test of microfluidic devices demonstrates that drag-reduction microchannels of silica glass can significantly reduce microfluidic flow resistance. The proposed method effectively improves the functional characteristics of ultrafast laser processing surface microchannels in silica glass and has broad application prospects in fluid flow and control.

Numerical Study on Laminar Drag Reduction for Superhydrophobic Surfaces with Continuous V-Shaped Microstructures

Superhydrophobic surfaces have attracted great attention owing to their capacity of reducing fluid resistance. Most of the previous numerical simulations on drag reduction of the superhydrophobic surfaces have concentrated on the rectangular microstructures, whereas few studies have focused on the continuous V-shaped microstructures. Based on the gas–liquid two-phase flow theory and volume-of-field model, combined with the semi-implicit method for pressure-linked equations algorithm, the effects of laminar drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were numerically studied. Three different sizes of superhydrophobic microchannels with continuous V-shapes were simulated according to the experimental data. Results showed that the drag reduction effects of continuous V-shaped microstructures were mainly determined by the width of adjacent microstructures, with the height of the microstructures only having minimal influence. At the same time, the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were compared with those with V-shaped and rectangular microstructures. The results indicated that the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were obviously better than for those with V-shaped microstructures, whereas the superhydrophobic surfaces with rectangular microstructures were more effective in reducing their drag than those with V-shaped microstructures under the condition of the same shear-free air–water ratios. Therefore, in the preparation of superhydrophobic materials, the continuous V-shaped microstructures are recommended; in addition, increasing the microstructure width should be emphasized in the preparation of superhydrophobic materials with continuous V-shaped microstructures.

Longitudinal flow in superhydrophobic channels with partially invaded grooves

Analytical expressions are derived for the longitudinal flow in a superhydrophobic microchannel where flat menisci in the Cassie state have partially invaded the grooves between no-slip blades. Using these solutions, the effective slip lengths are computed and compared with recent analytical results for unbounded shear flow over the same class of surfaces. Expressions for the first-order corrections to these effective slip lengths when the menisci are weakly curved are also derived. A mathematical connection to superhydrophobic channel flows where the flat menisci are still pinned to the tops of the pillars is also made, resulting in novel analytical expressions for those solutions too.

Effects of convergence and superhydrophobicity on the hydrothermal features of the tapered double-layer microchannel

The performance of the tapered double-layer microchannels with the hydrophobic surfaces is studied numerically. Hydrothermal performance is analyzed by different parameters, such as pressure drop, pumping power, thermal resistance, Nu number, etc. The governing equations are solved by the finite volume method (FVM). User-defined functions (UDFs) are used to model temperature-dependent properties of water and hydrophobic and superhydrophobic surfaces. The effects of different parameters, including slip length, Re number, and tapered factors are examined. The results showed that by using the superhydrophobic surfaces, the pumping power decreases due to fluid slips, and the thermal resistance increases due to the temperature jump on the surface. It is found also that at Re = 50, the convergent microchannel with the superhydrophobic wall has a higher Nu number as compared with the direct microchannel at all values of slip length. However, at higher Re numbers, the Nu number of the convergent microchannel with superhydrophobic surfaces is lower than that of the direct microchannel with conventional surfaces. In addition, at Re = 50, the use of superhydrophobic walls decreases the thermal resistance in comparison with the conventional walls. The superhydrophobic surfaces give higher hydrothermal performance factors (η) for Re≤ 150, while they give lower performance factors at Re > 250. For Re ≤ 100, increasing the convergence of superhydrophobic microchannels increases the hydrothermal performance factors.

Effect of surface coating on defrosting water drainage characteristics of vertical-fin microchannel frosting evaporator

Vertical-fin microchannel heat exchanger has better frosting and defrosting performance owing to its better water drainage along vertical fins. Effect of hydrophilic and superhydrophobic coatings on frosting and defrosting performance is experimentally compared and drainage performance is investigated by dynamic dip testing and simulation. Results show that since water has smaller adhesive force on superhydrophobic fins, defrosting water carries some frost while is separated from the evaporator in form of frost-water mixture. Hence, the superhydrophobic vertical-fin microchannel evaporator has shorter defrosting duration by 9.8%. Both hydrophilic and superhydrophobic samples have satisfactory cyclic repeatability without obvious capacity attenuation. During defrosting, most water can flow down the leading edge extension. The residual water of the superhydrophobic sample is 61 g/m2, which is 19.6% less than that of hydrophilic one.

Underwater gas self-transportation along femtosecond laser-written open superhydrophobic surface microchannels (

Underwater transportation of bubbles and gases has essential applications in manipulating and using gas, but achieving this function at the microscopic level remains a significant challenge. Here, we report a strategy to self-transport gas in water along a laser-induced open superhydrophobic microchannel with a width less than 100 µm. The femtosecond laser can directly write superhydrophobic and underwater superaerophilic microgrooves on the polytetrafluoroethylene (PTFE) surfaces. In water, the single laser-induced microgroove and water medium generate a hollow microchannel. When the microchannel connects two superhydrophobic regions in water, the gas spontaneously travels from the small region to the large area along this hollow microchannel. Gas self-transportation can be extended to laser-drilled microholes through a thin PTFE sheet, which can even achieve anti-buoyancy unidirectional penetration. The gas can overcome the bubble’s buoyance and spontaneously travel downward. The Laplace pressure difference drives the processes of spontaneous gas transportation and unidirectional bubble passage. We believe the property of gas self-transportation in the femtosecond laser-structured open superhydrophobic and underwater superaerophilic microgrooves/microholes has significant potential applications related to manipulating underwater gas.

Evaluation of Thermal Hydraulic Characteristics of a Two Phase Superhydrophobic Microfluidic Device using Volume of Fluid Method

ABSTRACT The slug flow in a superhydrophobic microchannel with a T-junction was studied computationally. The continuous phase passed through the main channel while the dispersed phase 1 1. was introduced through the side channel. The volume of fluid (VOF) was employed to track the interface to study the dynamics of slug flow. First, a mesh independence study was carried out to select the optimum mesh by comparison of CFD results with experimental data. The developed model of microchannel was used to study slug flow heat transfer enhancement for micro cooling of electronic chips. The constant heat flux was applied on the walls of the microchannels and the axial wall temperature profile was noted. Upon quantification of heat transfer augmentation in terms of wall temperature reduction, Nusselt number and heat transfer coefficient enhancement, it was noted that slug flow performed much better vis-à-vis single-phase flows at similar conditions.

Thermal Analysis of Microchannels Heat Sink using Super-hydrophobic Surface

Abstract: Electronics devices are the major part of modern technology and with the rapid growth of miniaturizations of electronic devices, the heat dissipation from these devices have been the objective for researchers. This heat dissipation has to done effectively otherwise this will affect the life of device and will result decrement in efficiency. Increasing the heat transfer rates from electronic devices has long been a quest. Microchannel heat sink is one of the best option for removing heat from the electronics devices due to its compact size which provides high surface area to volume ratio that enables higher heat transfer rates. Microchannels are the flow passages having hydraulic diameter ranges from 10 micrometer (µm) to 200µm. Microchannel heat sink enhances the feasibility of electronics device. Microchannels with hydrophobic surface are a promising candidate for cooling of electronics devices, as hydrophobic surface can be used to create friction free regions with a channel which effectively reduce pumping power, flow pressure drop and frictional factor compared to Microchannel without Hydrophobic surface. This paper deals with the detailed behavior of Microchannel with hydrophobic surface. In this work, rectangular cross section with 0.8 mm (800 micron) hydraulic diameter super hydrophobic microchannel is used. Keywords: Microchannel, Hydrophobic surface, Heat transfer rate, Frictional factor.