Micropillar Surfaces Research Articles

Adsorption and Morphology Analysis of Bovine Serum Albumin on a Micropillar-Enhanced Quartz Crystal Microbalance.

The adsorption of bovine serum albumin (BSA), a widely used blood plasma protein, onto poly(methyl methacrylate) (PMMA) surface is a fundamental phenomenon attracting increasing interests in molecular biology, cell culture, immunology, diagnostics, and vaccinology. The nanostructured PMMA surfaces have shown a considerable effect on the BSA adsorption process. However, the effect of microstructures (e.g., micropillars) on BSA adsorption has seldom been studied. This research reports on the development of an acoustic resonance based method to explore the adsorption of BSA proteins on PMMA micropillars in terms of surface coverage, apparent binding constants, and pH-induced morphology variation. A theoretical model is developed to understand the frequency changes of QCM induced by BSA adsorption by taking into consideration the effects of the hydrodynamic force and an equivalent BSA/liquid layer formed on the micropillar surface. In addition, it was found that the resonance of micropillars with a quartz crystal microbalance (QCM) substrate significantly influenced BSA adsorption on micropillar surfaces.

Dropwise condensation on subcooled micropillar surfaces with 3D lattice Boltzmann method

In this investigation, an alternative geometric formula is proposed to address the force between fluid nodes and fluid ghost nodes, with the aid of which the contact angles can be varied in the range of 48.3°∼131.9°. This formula is applied to the three-dimensional double-distributed thermal lattice Boltzmann method being proved to be accurate and reliable by single droplet condensation. The effects brought by varying micropillar size on the kinetic properties of condensed droplets, including nucleation, growth, coalescence and jumping, are investigated in detail. The results show that the droplet wetting state tends to be the suspended Cassie state as the width and spacing of the micropillars are decreased, and the condensed droplets can merge and jump off the micropillar surface. In the meantime, the average droplet number increases, the average diameter and the diameter of dominant droplets decrease, thus reducing the condensate coverage. When the micropillar spacing is small, increasing the micropillar height results in the condensed droplet state being changed from Wenzel to Cassie state, and the percentage of small droplets also increases. Instead, when the micropillar spacing is large, by increasing micropillar height, droplets can nucleate in the middle of micropillars, and the percentage of large droplets is improved due to increased heat transfer area. In this study, the surface self-cleaning capability is strongest with the combination of dimensionless pillar height 0.4, spacing 0.1 and width 0.1, which reduces the condensate coverage by 66 % compared to its plain competitor.

Effect of the Surface Peak-Valley Features on Droplet Impact Dynamics under Leidenfrost Temperature.

This study explores the kinetic behavior of droplets impacting microtextured surfaces under a Leidenfrost temperature, employing high-speed photography and picosecond laser micromachining techniques. The investigation focuses on two types of microtextured surfaces with totally different surface peak-valley features: a negatively skewed surface with micropit arrays (Ssk < 0) and a positively skewed surface with micropillar arrays (Ssk > 0). The results indicate that both microtextured surfaces contribute to a higher Leidenfrost temperature compared with the original smooth surface, which is consistent with previous studies. However, it is worth noting that the Leidenfrost points of the micropit and micropillar surfaces showed opposite trends with the microtexture area occupancy. Specifically, the Leidenfrost temperature on micropit surfaces increases with greater micropit area occupancy, while it decreases on micropillar surfaces under similar conditions, which is mainly attributed to the differential impact of area occupancy on droplet heat transfer efficiency. When the microtexture area occupancy is 50%, it is worth noting that the micropit and micropillar surfaces have nearly same roughness (Sa), but the Leidenfrost temperature was notably higher on the micropit surface with negative skewness (Ssk < 0), which was related to differences in vapor flow dynamics. We further find that the Weber number (We) significantly influences the Leidenfrost point, with the droplet impact wall behavior going through the states of film bounce back, ejecting tiny droplets and bounce back, and ultimately droplet breakup as the We increases. The dynamic Leidenfrost point was found to be generally higher than the static point and increases with the We. Finally, we compare the cooling efficiency of these surfaces, and it is found that the micropit surfaces with a negative skewness exhibit superior heat dissipation performance under the same conditions, which proved that the negatively skewed surface may have great potential in high-density heat dissipation technology.

Roles of Micropillar Topography and Surface Energy on Cancer Cell Dynamics

Microstructured surfaces are renowned for their unique properties, such as waterproofing and low adhesion, making them highly applicable in the biomedical field. These surfaces play a crucial role in influencing cell response by mimicking the native microenvironment of biological tissues. In this study, we engineered a series of biomimetic micropatterned surfaces using polydimethylsiloxane (PDMS) to explore their effects on primary breast cancer cell lines, contrasting these effects with those observed on conventional flat surfaces. The surface topography was varied to direct cells’ attachment, growth, and morphology. Our findings elucidate that surface-free energy is not merely a background factor but plays a decisive role in cell dynamics, strongly correlating with the spreading behaviour of breast cancer cells. Notably, on micropillar surfaces with high surface-free energy, an increase in the population of cancer cells was observed. Conversely, surfaces characterised by lower surface-free energies noted a reduction in cell viability. Moreover, the structural parameters, such as the gaps and diameters of the pillars, were found to critically influence cellular dispersion and adherence, underscoring the importance of the microstructures’ topography in biomedical applications. These insights pave the way for designing advanced microstructured surfaces tailored to specific cellular responses, opening new avenues for targeted cancer therapies and tissue engineering.

Two-Phase Particle Image Velocimetry Visualization of Rewetting Flow on the Micropillar Interfacial Surface.

Boiling heat transfer has a high thermal efficiency by latent heat absorption, which makes it an attractive process for cooling electronic device chips. Critical heat flux (CHF), the maximum heat flux, is a crucial factor determining the operating range of the boiling applications. The CHF can be enhanced by improving the fluid supply to the boiling surface. Herein, micropillar interfacial surfaces have been proposed to increase the CHF by increasing the rewetting flow, which determines the fluid-supply capacity near the bubble contact line. A state-of-art two-phase particle image velocimetry (two-phase PIV) technique is introduced for rewetting flow measurement on micropillar structures (MPSs) to analyze the CHF-enhancement mechanism. The two-phase PIV visualization setup offers high spatial (∼120 μm) and temporal (∼2000 Hz) resolutions for measuring rewetting flow during bubble growth. The MPS samples exhibit enhanced CHF and rewetting flows compared to those on a plain surface. The roughest case, D04G10 sample, had a CHF of 164 W/cm2, 1.84 times higher than that of the plain surface. The D04G10 sample also recorded the highest rewetting velocity of 0.311 m/s, 4.7 times higher than that of the plain surface. The comparison between the rewetting flow and wicking performance shows that wicking-induced flow accounted for a substantial part (∼17%) of the rewetting flow and contributed significantly to the CHF enhancement owing to large rewetting flow by delaying vapor-film formation. Based on these findings, a new CHF model suggested by introducing the rewetting parameter shows a high CHF prediction accuracy of 94%.

Superbiphilic hierarchical aluminum surfaces for exceptional pool boiling performance

The advancement in high-power electronic devices coupled with the need to ensure reliable and efficient heat dissipation of two-phase cooling systems underscores the urgent necessity for breakthroughs in enhancing boiling performance and especially critical heat flux (CHF) to minimize the risk of system failure. In this field, surfaces with tailored wettability have already demonstrated their potential to enhance boiling heat transfer intensity, while surfaces featuring wickable structures like micropillar arrays have shown significant improvements in CHF. In this study, we investigate the use of aluminium micropillar surfaces with tailored wettability to simultaneously enhance nucleate boiling heat transfer performance and specifically increase of the CHF. We fabricated the micropillar surfaces using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while the wettability of selected surfaces was further tailored by application of a fluoroalkyl phosphonic acid and an additional laser texturing step. Three micropillar patterns were tested under pool boiling conditions using saturated twice-distilled water at atmospheric pressure. Importantly, our results revealed that the bottom part of the boiling interface (i.e., the superhydrophilic area) ensured increased liquid supply, while the top parts (i.e., the superhydrophobic area) tend to serve as nucleation sites. When combined, these two effects allowed us to simultaneously improve the CHF and the heat transfer coefficient, resulting in enhancements of up to 113% (2343 kW m−2) and 450% (205 kW m−2 K−1), respectively, compared to the benchmark untreated surface. This research provides a practical and reliable approach to enhancing heat transfer by fabricating hierarchical surfaces, offering potential applications in ultrahigh heat flux thermal technologies.

Pool boiling characteristics on the micro-pillar structured surface with nanostructure and heterogeneous wettability

The nucleate boiling phenomenon is significantly affected by the wettability of the surface because it is greatly influenced by the motion of the solid–liquid–gas triplet formed by the behavior of bubbles on the heated surface. Hydrophilic surfaces have a high critical heat flux, and hydrophobic surfaces have low onset of nucleate boiling points and high heat transfer coefficients. To design an optimal boiling surface with the advantages of both hydrophilic and hydrophobic surfaces, in this study, parametric experiments were performed according to the spacing of the micropillar, the presence or absence of nanostructures, and the presence or absence of hydrophobic treatment on the surface where the nanostructure was hierarchically modified to the micropillar array structure in which the upper surface of the pillar was hydrophobic. Using the micro-pillar fabrication process and the heterogeneous hydrophobic surface fabrication process, a micro-pillar structure with hydrophobic treatment on top of the pillar was fabricated. Pool boiling experiments were conducted on uniform hydrophilic surfaces, uniform hydrophobic surfaces, hydrophobic patterns on hydrophilic surfaces, micro-pillar surfaces, micro-pillar with nanostructure surfaces, micro-pillar surfaces with heterogeneous wettability, and micro-pillar with nanostructure surfaces with heterogeneous wettability. Based on the experimental data, we analyzed the change in boiling heat transfer coefficients and critical heat flux according to the surface characteristics. Based on experimental results, it was confirmed that the optimal boiling surface should be a surface with heterogeneous wettability, microstructure, and nanostructure.

Aluminum Micropillar Surfaces with Hierarchical Micro- and Nanoscale Features for Enhancement of Boiling Heat Transfer Coefficient and Critical Heat Flux.

The rapid progress of electronic devices has necessitated efficient heat dissipation within boiling cooling systems, underscoring the need for improvements in boiling heat transfer coefficient (HTC) and critical heat flux (CHF). While different approaches for micropillar fabrication on copper or silicon substrates have been developed and have shown significant boiling performance improvements, such enhancement approaches on aluminum surfaces are not broadly investigated, despite their industrial applicability. This study introduces a scalable approach to engineering hierarchical micro-nano structures on aluminum surfaces, aiming to simultaneously increase HTC and CHF. One set of samples was produced using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while another set underwent an additional laser texturing step. Three distinct micropillar patterns were tested under saturated pool boiling conditions using water at atmospheric pressure. Our findings reveal that microcavities created atop pillars successfully facilitate nucleation and micropillars representing nucleation site areas on a microscale, leading to an enhanced HTC up to 242 kW m-2 K-1. At the same time, the combination of the surrounding hydrophilic porous area enables increased wicking and pillar patterning, defining the vapor-liquid pathways on a macroscale, which leads to an increase in CHF of up to 2609 kW m-2.

Exploring flow boiling characteristics on surfaces with various micro-pillars using the lattice Boltzmann method

In this study, the lattice Boltzmann method is utilized to simulate flow boiling within a microchannel featuring a micro-pillar surface. This investigation aims to explore the impacts of micro-pillar shape and quantity on the flow boiling characteristics across various superheats and Reynolds numbers (Re). A systematic examination is conducted on three types of micro-pillars, five quantities of micro-pillars, four Re values, and 18 superheat levels. The mechanisms contributing to enhanced heat transfer in flow boiling are elucidated through a comprehensive analysis of bubble dynamics, temperature and velocity fields, local and transient heat fluxes, and boiling curves. Moreover, the critical heat fluxes (CHF) of all surfaces are evaluated to identify the superior micro-pillar configurations. The findings revealed that microchannels with micro-pillar surfaces induce more vortices compared to those with smooth surfaces, attributable to the combined effects of bubble dynamics and micro-pillars. Bubble patterns and boiling curves demonstrated the significant impact of micro-pillar geometrical shapes on the boiling regime and heat transfer performance. As flow boiling progressed, an increase in micro-pillar quantity and Re can mitigate the fluctuation and decline rate in transient heat flux, respectively. Among the three types of micro-pillar surfaces, the circular shape exhibited the highest flow boiling performance, followed by the triangular and rectangular shapes. For all surfaces, the CHF increased with Re, and each micro-pillar type displayed an optimal quantity for achieving maximum CHF, with the highest increase reaching 45.2%. These findings are crucial for optimizing microchannel designs to enhance flow boiling heat transfer efficiency.

Enhanced boiling heat transfer on three-dimensional hybrid micropillar array surfaces

Boiling heat transfer is widely used in numerous industrial systems because of its excellent heat dissipation performance. The critical heat flux (CHF) represents the upper limit of efficient heat removal. The most widely applied CHF model is Zuber's hydrodynamic instability model, which posits that the interference between vapor and liquid counterflows results in the CHF. In this paper, a three-dimensional (3D) hybrid micropillar array surface for hindering the liquid–vapor interface is proposed to enhance the CHF and heat transfer coefficient (HTC). The 3D hybrid surface comprises a tall hydrophobic (HB) pillar array surrounded by a short hydrophilic (HP) pillar array. The tall HB pillar array acts as a potential nucleation site for increasing the HTC, and the short HP pillar array acts as a wicking structure for facilitating liquid rewetting. In addition, the height difference between the HB and HP pillar arrays can potentially reduce the interference between liquid and vapor counterflows and increase the CHF. Relative to a plain silica surface, the 3D hybrid surface increases the CHF and HTC by 107% and 360%, respectively. Theoretical modeling suggests that the CHF on the 3D hybrid surface was still limited by pool hydrodynamics.

Fabrication of a three-dimensional micro/nanocarbon structure with sub-10 nm carbon fiber arrays based on the nanoforming and pyrolysis of polyacrylonitrile-based jet fibers

To produce a three-dimensional micro/nanocarbon structure, a manufacturing design technique for sub-10 nm carbon fiber arrays on three-dimensional carbon micropillars has been developed; the method involves initiating electrostatic jetting, forming submicron-to-nanoscale PAN-based fibers, and maximizing the shrinkage from polyacrylonitrile (PAN)-based fibers to carbon fibers. Nanoforming and nanodepositing methods for polyacrylonitrile-based jet fibers as precursors of carbon fibers are proposed for the processing design of electrostatic jet initiation and for the forming design of submicron-to-nanoscale PAN-based fibers by establishing and analyzing mathematical models that include the diameter and tensile stress values of jet fibers and the electric field intensity values on the surfaces of carbon micropillars. In accordance with these methods, an array of jet fibers with diameters of ~80 nm is experimentally formed based on the thinning of the electrospinning fluid on top of a dispensing needle, the poking of drum into an electrospinning droplet, and the controlling of the needle–drum distance. When converting thin PAN-based jet fibers to carbon fibers, a pyrolysis method consisting of the suspension of jet nanofibers between carbon micropillars, the bond between the fibers and the surface of the carbon micropillar, and the control of micropillar spacing, stabilization temperature, and carbonation rate is presented to maximize the shrinkage from PAN-based fibers to carbon fibers and to form sub-10 nm carbon fiber arrays between three-dimensional carbon micropillars. The manufacturing design of a three-dimensional micro/nanocarbon structure can produce thin PAN-based jet nanofibers and nanofiber arrays aligned on micropillar surfaces, obtain shrinkage levels reaching 96% and incorporate sub-10 nm carbon fibers into three-dimensional carbon micropillars; these actions provide new research opportunities for correlated three-dimensional micro/nanocarbon structures that have not previously been technically possible.

Condensation mode transition and droplet jumping on microstructured surface

Condensation enhancement on the micropillar structured surfaces is studied using a three-dimensional lattice Boltzmann model, and the optimal micropillar configuration is assessed. The impacts brought by varying micropillar geometric parameters on heat transfer and droplet dynamics during dropwise-to-filmwise transition are explored. Increasing micropillar width and spacing or decreasing height accelerates nucleation and improves nucleation density in the dropwise condensation stage, while radial contraction of the liquid film is hindered and the heat transfer area is increased in the filmwise condensation stage. Compared with its plain competitor, the maximum number of the isolated droplets is increased by 525%, the nucleation time is advanced by 57.1% and the heat flux is increased by 187.4% on the optimal micropillar surface. The entire condensation process spanning over nucleation, coalescence and jumping of multiple condensate droplets on the micropillar surface is realized using the present three-dimensional lattice Boltzmann model for the first time. The effects of height and spacing of micropillar arrays on jumping height and velocity are analyzed. The results show that the micropillar height poses little effect on the jumping height and velocity when the micropillar spacing is 0.1. When the micropillar spacing is 0.15, the jumping height and velocity first increase but then decrease with the increase of the micropillar height. Finally, the condensation heat transfer mechanism under different subcooling on the hierarchical surface is analyzed in detail.

Numerical Study of Microstructures and Roughness Design Effects on Surface Hydrophilicity through the Lattice Boltzmann Method

Hydrophilicity is one of the most vital characteristics of titanium (Ti) implants. Surface structure design is a powerful and efficient strategy for improving the intrinsic hydrophilic ability of Ti implants. Existing research has focused on experimental exploration, and hence, a reliable numerical model is needed for surface structure design and corresponding hydrophilicity prediction. To address this challenge, we proposed a numerical model to analyze the droplet dynamics on Ti surfaces with specific microstructures designed through the lattice Boltzmann method (LBM). In this work, a Shan-Chen (SC) model was applied in the simulations. We simulated droplets spreading on smooth and micropillar surfaces with various wettability and provided a comprehensive discussion of the edge locations, contact line, droplet height, contact area, surface free energy, and forces to reveal more details and mechanisms. To better tune and control the surface hydrophilicity, we investigated the effects of micropillar geometric sizes (pillar width a, height h, and pitch b) on hydrophilicity via single factor analysis and the response surface method (RSM). The results show that the hydrophilicity initially increases and then decreases with an increasing a, increases with an increasing h, and decreases with an increasing d. In addition, the interaction effects of a-d and h-d are significant. The optimization validation of the RSM also demonstrates the accuracy of our lattice Boltzmann (LB) model with an error of 0.687%. Here, we defined a dimensionless parameter ξ to integrate the geometric parameters and denote the surface roughness. The hydrophilicity of Ti surfaces improves with an increasing surface roughness. In addition, the effect of the microstructure geometry shape was investigated under the same value of surface roughness. Surfaces with micropillars show the best hydrophilicity. Moreover, this study is expected to provide an accurate and reliable LB model for predicting and enhancing the intrinsic hydrophilicity of Ti surfaces via specific microstructure and roughness designs.

Pool boiling on micro-structured surface with lattice Boltzmann method

In this paper, pool boiling on micro-pillar structured surfaces is studied by using a three-dimensional pseudo potential phase-change lattice Boltzmann method (LBM). The joint enhancing effects on bubble nucleation and boiling performance are discussed in detail regarding various surface wettability and pillar geometrical parameters. Results show that on neutral and hydrophobic surfaces, enlarging the spacing of micro pillars delays the nucleation but can reduce the temperature inside vapor film and improve heat conduction. Despite that increasing the pillar height can improve heat flux, it is adverse to nucleation due to the increased cooling effect on the roots of micro pillars. In contrast, on hydrophilic surfaces, the impact of pillar spacing on nucleation is not monotonous and relatively much complicated. The heat flux is enhanced with increasing pillar spacing because of the extended three-phase contact line. And, the nucleation positions differ significantly by varying pillar geometrical parameters on hydrophilic surfaces.

Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition.

Superhydrophobic surfaces have long been the center of attention of many researchers due to their unique liquid repellency and self-cleaning properties. However, these unique properties rely on the stability of the so-called Cassie state, which is a metastable state with air-filled microstructures. This state tends to transit to the stable Wenzel state, where the inside of the microstructures eventually wets. For potential industrial applications, it is therefore critical to maintain the Cassie state. We investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces by squeezing a water drop between the surface and a transparent superhydrophobic force probe. The probe's transparency allows the use of top-view optics to monitor the area of the drop as it is squeezed against a micropillared surface. The impalement, or Cassie-to-Wenzel transition, is identified as a sharp decrease in force accompanied by an abrupt change in the drop's contact area. We compare the force measured by the sensor with the capillary pressure force calculated from the observed drop shape and find a good agreement between both quantities. We also study the force and pressure at impalement as a function of the pillar's slenderness ratio. Finally, we compare the impalement pressure with three literature predictions and find that our experimental values are consistently lower than the theoretical values. We find that a possible cause of this earlier Cassie-to-Wenzel transition may be the coalescence of the squeezed drop with microdroplets that nucleate around the base of the micropillars.

Spontaneous Charging of Drops on Lubricant-Infused Surfaces.

When a drop of a polar liquid slides over a hydrophobic surface, it acquires a charge. As a result, the surface charges oppositely. For applications such as the generation of electric energy, lubricant-infused surfaces (LIS) may be important because they show a low friction for drops. However, slide electrification on LIS has not been studied yet. Here, slide electrification on lubricant-infused surfaces was studied by measuring the charge generated by series of water drops sliding down inclined surfaces. As LIS, we used PDMS-coated glass with micrometer-thick silicone oil films on top. For PDMS-coated glass without lubricant, the charge for the first drop is highest. Then it decreases and saturates at a steady state charge per drop. With lubricant, the drop charge starts from 0, then it increases and reaches a maximum charge per drop. Afterward, it decreases again before reaching its steady-state value. This dependency is not a unique phenomenon for lubricant-infused PDMS; it also occurs on lubricant-infused micropillar surfaces. We attribute this dependency of charge on drop numbers to a change in surface conductivity and depletion of lubricant. These findings are helpful for understanding the charge process and optimizing solid–liquid nanogenerator devices in applications.

Vapor condensation on micropillar structured surface with lattice Boltzmann method

Pure vapor condensation process and heat transfer on micropillar structured surfaces with various wettabilities are simulated numerically based on an improved three-dimensional phase-change lattice Boltzmann method (LBM). Condensed droplet dynamic behaviors containing nucleation, growth and departure are mainly explored. Two preferable droplet nucleation sites are detected successfully: at the corner between micropillar sides and subcooled substrate, and at the center of substrate. The effects of contact angle, micropillar spacing and height on droplet nucleation time, departure diameter and departure frequency are investigated quantitatively. Results indicate that as the contact angle increases, nucleation of condensate is delayed, while the departure diameter is reduced and departure frequency is increased. The increase in micropillar spacing, height and contact angle changes droplet from Wenzel state to partial wetting state. Moreover, it is also shown that the highest temperature and maximum local heat flux appear at the three-phase contact line. Despite the very complex roles of micropillar structures and wettability on condensation heat transfer, an optimal heat transfer surface with configurations of contact angle of 90°, micropillar spacing of 12 and height of 18 is presented. This study provides a fundamental understanding in regards to vapor condensation on microscale structured surface from numerical view.

Experimental investigation of pool boiling heat transfer on the radial micro-pillar surfaces

An experimental study on the pool boiling heat transfer with heterogeneous micro-pillar surfaces was performed in this study. Six kinds of surfaces, including single-block radial micro-pillar surfaces (MS-1, MS-2 and MS-3) and four-blocks radial micro-pillar surfaces (BS-1, BS-2 and BS-3), were fabricated by the dry etching technique. FC-72 was used as the working medium under four different liquid subcoolings. A high-resolution camera was used to capture the boiling phenomenon. It could be found that the heat transfer performance of these radial micro-pillar surfaces was remarkably enhanced in comparison with that of a smooth surface, which showed an earlier onset of nucleate boiling (ONB) and an improved heat transfer coefficient (HTC). Moreover, the critical heat flux (CHF) was dependent on the heat transfer area enhancement ratio at the high subcooled temperature. BS-1 surface had the highest CHF 1.57–2.05 times more than the smooth surface. For the MS-1 and BS-2 surfaces, their CHF was almost the same due to a similar heat transfer area enhancement ratio, but the HTC of the MS-1 increased by 20% compared with the BS-2 surface. This could be due to different solid fractions φS (36.78% for MS-1 and 25.81% for BS-2). Under the saturation condition, the overall HTC of micro-pillar surfaces first increased to a critical value, and then decreases with the increase of heat flux. In addition, the maximum heat transfer coefficient enhancement ratio (HTCmax,PF/HTCmax,S) was almost constant when ΔTsub > 15 K, which was somewhat significant for subcooled pool boiling.

Enhanced boiling heat transfer by nucleation patterning with self-assembly of reduced graphene oxide coating

Boiling heat transfer is a favorable method for cooling high heat flux devices, and its performance is evaluated using critical heat flux (CHF), which indicates the maximum heat dissipation capacity. CHF occurs when a surface is covered with a vapor film due to bubble coalescence. Here, we propose a new nucleation patterning surface using rGO-coated micropillar-free cavities in order to enhance boiling heat transfer by suppressing bubble coalescence. Nucleation patterned surface is achieved by a sectored self-assembly on surfaces with artificial cavities embedded in micropillar array. The nucleation pattern is designed with spacings of 1.0 and 1.5 mm, with reference to the bubble departure diameter on the rGO-coated micropillar surface. The rGO particles deposited on the bottom of the micropillar-free cavity cause bubble formation in the cavities, and the micropillars around the cavities supply liquid to bubbles through wicking. Moreover, rGO deposition with varying heat flux schemes suggests the capability of constructing toned rGO layers on patterned micropillar surfaces. The results confirmed that high heat transfer performance can be obtained by applying denser bubble nucleation with a bubble generation spacing to bubble departure diameter ratio of 1, under the condition of preventing bubble coalescence. The heat transfer coefficient and critical heat flux were augmented by 340% and 203%, respectively, by preserving flow paths for water imbibition under the floating rGO layer and delaying bubble coalescence.

Micro/nanoscale surface on enhancing the microchannel flow boiling performance: A Lattice Boltzmann simulation

Microchannel Heat Sink (MCHS) has been widely adopted in thermal engineering fields, such as refrigerators, chip cooling, battery packs, etc. To meet the ever-increasing demand for heat dissipation, surface modification methods adopting micro/nanoscale-modified surfaces have received considerable attention. In this paper, an advanced micro/nanoscale surface modification design is proposed based on a Lattice Boltzmann method (LBM) simulation study. Coupled boundary treatments at the inlet/outlet are developed with better numerical stability. The effects of surface wettability and micropillar on MCHS heat transfer performance are analyzed through bubbles' dynamic behaviors, Nusselt number, heat flux, and pressure drop. Design-based suggestions are proposed, and the enhancement mechanisms are explained. Results show that hydrophobic surface is more preferred for temperature-sensitive devices with low superheat requirement (Ja<0.1115), while the hydrophilic surface is more preferred for devices with a large heat dissipation requirement (Ja≥0.1286). Furthermore, the micropillar surfaces with pillar geometric factor Sp of 7 can yield the optimum heat transfer performance under a wide range of superheat conditions. Finally, an advanced design of a biphilic micropillar surface is proposed with superhydrophobic regions located at the top of the pillars, and other regions remain hydrophilic surfaces. An excellent heat transfer enhancement of 105.8% is achieved even compared with a pure hydrophilic micropillar surface. The enhancement is attributed to the superhydrophobic top regions efficiently blocking the bubbles merging process, which leads to more intense bubbles departure. These results provide a valuable guide for MCHS design and unravel the enhancement mechanism of the flow boiling process.