Micropillar-structured Surface Research Articles

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.

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.

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.

Multiscale numerical research on nucleate boiling enhancement from microscale to nanoscale

Nucleate boiling enhancement from surfaces in energy industries is of fundamental significance for improving energy conversion and thermal management efficiency. This study investigates the microscale and nanoscale boiling processes on the hybrid wettability micropillar structured surface (MPS) to continuously enhance nucleate boiling. The microscale and nanoscale boiling processes and underlying enhancement mechanisms are studied by Lattice Boltzmann Method (LBM) and Molecular Dynamics (MD) simulations, respectively. It is revealed that MPS designed with a hydrophilic bottom surface and a hydrophobic top surface decorated with nanopillars can improve nucleate boiling throughout the microscale and nanoscale boiling process. The microscopic boiling process of the hybrid wettability MPS shows that the synergistic effect of wettability and surface structure is attributed to nucleation boiling enhancement due to higher bubble nucleation density and higher bubble departure frequency. The nanoscale boiling process on hydrophobic nanopillar structured surfaces (NPS) shows further nucleate boiling enhancement. The NPS with a nanopillar height of 1.6268 nm is in the Wenzel state with higher heat flux and lower thermal resistance, resulting in faster boiling onset and shorter separation time, while the hydrophobic NPSs in the Cassie state leads to a deterioration in the nucleate boiling.

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.

Wetting behaviors and mechanism of micro droplets on hydrophilic micropillar-structured surfaces

Controlling morphology of liquids on solid surfaces plays an important role in both fundamental research and practical applications. We found that, the shape of the liquid after contact between a droplet and a surface could be tuned by designing the hydrophilic micropillar arrays forming this surface. Such a wetting behavior is a result of imbibition in the micropillars, leading to specific final spreading distance and pattern of the droplet. During the spreading, the morphology of the droplet is controlled by both the arrangement and shape of the micropillars. The experimental results indicated that the surface energy barriers caused by the micropillar edges was a key factor accounting for the anisotropic spreading of the liquid. Moreover, the shape of the meniscus at the front location of the spreading liquid was rebuilt based on computational simulation, which agreed well with the experimental data. Interesting, the time-dependence of the displacement of the liquid transport at the edges was found to be different from that of a wicking film. Theoretical models were accordingly proposed to reveal the shape formation of liquids on such micropillar-structured surfaces.

Contact line-based model for the Cassie-Wenzel transition of a sessile droplet on the hydrophobic micropillar-structured surfaces

The hydrophobic stability to prevent Cassie-Wenzel (C-W) transition is an important property of superhydrophobic surfaces, which is mainly controlled by the micro/nano structure of surfaces. Based on the contact lines (CLs) around and inside the contact region, we analyzed the applied forces on a static sessile droplet deposited on the hydrophobic micropillar-structured surface. A simplified double-radius fitting method was derived to outline the contour of the droplet, and a force-balance model was gained to describe the critical conditions of the C-W transition. Compared with the classical force-balance models, the theoretical predictions from the proposed model agree much better with the experimental results. A reliable estimation of the critical conditions for the C-W transition during evaporation can be readily formed by integrating the depinning mechanism of the receding CL on the micro-patterned surfaces into our model, which obviously cannot be obtained by the classical force-balance models. The effects of gravity and surface tension in the proposed equilibrium model for the C-W transition reach a compromise. The introduction of surface tension acting on the apparent CL in our model will help to provide appropriate geometric parameters for microstructures on the superhydrophobic surfaces to achieve high hydrophobic stability.

Bubble dynamics and heat transfer performance on micro-pillars structured surfaces with various pillars heights

Micro-pillars structured surfaces have been found to enhance nucleate boiling significantly, as compared with smooth surfaces. In this study, a VOF-based (volume of fluid) numerical model is developed to investigate the single-bubble dynamics on micro-pillars structured surfaces in nucleate boiling, aiming to understand the effect of pillar height (h) on boiling enhancement. The model is solving by the commercial CFD (computational fluid dynamics) solver Fluent. The results show that increasing h modifies remarkably the bubble morphology. An interesting phenomenon is observed for larger h that the bubble bottom undergoes expansion, shrinkage, and re-expansion in gaps between micro-pillars in the growth stage, so that a mushroom-like bubble is generated, with a small root and a large head. The modified bubble morphology promotes the bubble departure, leading to a shorter departure time. Moreover, increasing h also enhances both evaporation rate in the microlayer and evaporation rate on the liquid-vapor interface, and thereby increasing bubble departure diameter. Thus, the shorter departure time and larger departure diameter are responsible for the enhanced boiling heat transfer on micro-pillar structured surfaces with larger h.

Bubble dynamics and heat transfer characteristics on a micropillar-structured surface with different nucleation site positions

Microstructured surfaces have been extensively employed to enhance nucleate boiling. However, enhancement mechanisms have not been fully understood. In this work, a three-dimensional numerical model is developed to investigate growth and departure dynamics of a single bubble for nucleate boiling on a micropillar-structured surface. In the model, the volume of fluid method is used to capture vapor–liquid interfaces, and evaporations in both vapor–liquid interface and microlayer are taken into account. Moreover, a pressure outlet boundary condition is proposed to reasonably describe the inflow and outflow on side surfaces of the computational domain. The focus of this work is to answer how the location of nucleation site affects bubble dynamics and the resultant heat transfer characteristics. Two typical locations are considered: the center in the micropillar gap and the corner between the micropillar and substrate, referred to as the center nucleation and corner nucleation. The results show that the bubble with the center nucleation exhibits symmetric growth and departure, whereas the symmetry is broken up for the bubble with the corner nucleation. Asymmetric growth and departure induce asymmetric temperature profiles inside/around the bubble, leading to a faster departure and a smaller departure diameter for the bubble with the corner nucleation. Moreover, contributions of microlayer evaporation and vapor–liquid interface evaporation for the two nucleation locations are also discussed.

Nanopattern-Embedded Micropillar Structures for Security Identification.

A novel method was developed for fabricating nanopatterns embedded on micropillar-structured surfaces using nanowelding technology for security identification. Commonly used substrates, that is, polyethylene films, glass wafers, Si wafers, and curved surfaces, were employed and their characteristics were evaluated. Cr was deposited onto the selected substrate to strengthen the adhesion force, and an adhesive layer of ultra-thin metal was deposited on top of the Cr layer. Lastly, nanopatterns were embedded on the substrates by nanowelding. The morphologies, cross sections, and three-dimensional (3D) images of the fabricated nanostructures were evaluated, and their crystalline structures and compositions were analyzed. Using the same method, nanopatterns embedded on micropillar-structured surfaces were fabricated for the first time as security patterns to improve security identification. The fabricated security patterns were characterized in three stages. First, micropillar structures and structural color were simply observed via optical microscopy to achieve a preliminary judgment. The appearance of structural color was due to the nanostructures fabricated on the micropillar surface. Next, the designed nanopatterns on the micropillar-structured surfaces were observed by scanning electron microscopy. Lastly, the changes in the spectral peaks were precisely observed using a spectrometer to achieve an enhanced security pattern. The fabricated security patterns can be suitable for valuable products, such as branded wines, watches, and bags. In addition, the proposed method offers a simple approach for transferring metal nanopatterns to common substrates. Moreover, the fabricated security patterns can have potential applications in semiconductor electrodes, transparent electrodes, and security identification codes.

Lattice Boltzmann simulation of nucleate boiling in micro-pillar structured surface

A two-particle distribution functions lattice Boltzmann method is used to simulate periodic bubble nucleation, growth and departure from a micro-pillar structured surface. Bubble growth process and the effects of micro-pillar character scales on bubble departure period are investigated in this paper. Fluid hydrodynamic and thermodynamic are characterized by the density distribution function and the temperature distribution function respectively. Pseudo-potential model and Peng-Robinson equations of state are selected in our simulations. Constant temperature boundary condition is applied to the bottom solid. It is found that bubble generated in the vicinity of the micro-pillars, but in the same superheat, the smooth surface has no phase change phenomenon. This result indicates that micro-structure can contribute to the nucleation of the fluid phase change. Geometrical dimensions of micro-pillars have obvious effects on the growth of the bubble. Specifically, we investigated the influence of the distance between two pillars and the height of the pillars on the bubble growth. Results show that, to some extent, the distance of two micro-pillars “n” and the height of micro-pillar “h” have obvious effects on the bubble growth. For the height “h”, the smaller of “h”, the bigger of heat flux and the smaller of bubble departure periodic. For the distance “n”, it has no obvious effect on surface heat flux and bubble departure periodic but has an important effect on deciding whether it can form an activated nucleate site. Only proper “n” and “h” can be chosen to ensure an activated nucleation site according to our simulation results.

Wetting characteristic of bubble on micro-pillar structured surface under a water pool

In this study, the contact angle of air bubbles on a micro-structured surface under a water pool is experimentally investigated. A previous study “Kang and Jacobi, Equilibrium Contact Angles of Liquids on Ideal Rough Surfaces, Langmuir (2011)” adopted the work of adhesion as the additional work of the process of droplet contact on a textured surface, and it explained the reason for failure of the classical theoretical model (Wenzel equation) in real wetting tests. We extended the above concept to bubble interaction with the textured surface. First, the bubble contact angles on the textured surface were modeled based on the concept of the work of adhesion in the free energy calculation. Second, the contact angle of air bubbles was measured on the bottom of test surfaces under a water pool. The test-section surfaces have micro-pillars with a size of 5–40 μm prepared by the microelectromechanical systems (MEMS) technique. The bubble contact angles were compared with both the modeled bubble shape and classical prediction (Wenzel & Cassie–Baxter equations). In addition, detail wetting features which shows wetting transition and critical wetting behavior of bubble were discussed.

Revisiting the Critical Condition for the Cassie-Wenzel Transition on Micropillar-Structured Surfaces.

Biological and engineering applications of superhydrophobic surfaces are limited by the stability of the wetting state determined by the transition from the Cassie-Baxter state to the Wenzel state (C-W transition). In this paper, we performed water droplet squeeze tests to investigate the critical conditions for the C-W transition for solid surfaces with periodic micropillar arrays. The experimental results indicate that the critical transition pressures for the samples with varying micropillar dimensions are all significantly higher than the theoretical predictions. Through independent measurements, we attributed the disparity to the incorrect assessment of the contact angle on the sidewall surfaces of the micropillars. We also showed that the theoretical models are still applicable when the correct contact angle of the sidewall surfaces is adopted. Our work directly validates and improves the theoretical models regarding the C-W transition and suggests a potential route of tuning superhydrophobicity using finer scale surface features.

Profile Characterization and Temperature Effect on the Wettability of Microstructured Surfaces

Wetting is one of the omnipresent phenomena governed via natural laws. Moreover, surface wettability at non-ambient temperature especially at high temperature (30°C to 90°C) is of great importance in many industrial processes. In this study, Si wafers with various structures were fabricated to investigate wettability at different temperatures. Three shapes with micro-pillar structured surfaces were designed and fabricated. Pillar-structured surfaces were fabricated by photolithography and ICP etching. The temperature-dependent wettability of single-phase regime droplets was characterized using contact angle measurements. The wetting behavior of a water droplet was observed.

Frequency analysis of micropillar structured surfaces: A characterization and design tool for surface texturing

Various systematically controlled, ordered and randomized micropillar structures were fabricated onto polypropylene by injection molding. Mold inserts were prepared by a micro-working robot. The advantages of this simple surface texturing application include the rigorous control of the pillar positions and the use of a variety of polymers. The textured surfaces were characterized both in horizontal and vertical directions. Power spectral density (PSD) curves combined with filtered root-mean-square (RMS) values provide structure periodicities and roughness differences between various spatial frequencies, respectively. The developed characterization tool is useful in texture mimicking when substantial periodicities and roughness is obtained and imitated in the input data of the micro-working robot. Hence, injection molded micropillar surfaces can possess similar spatial frequency and roughness features to that of mimicked surfaces.The developed characterization method was also connected to the texture design. The height profiles of structures corresponding to the fabricated surfaces were predicted from the experimental input data of the micro-working robot, and the PSD curves were generated to obtain the connection between experimental and predicted structures. The rigorous correspondence between the experimental and predicted structures and the filtered RMS values indicates that the developed design tool enables the theoretical inspection of desired structures. This can also be used in texture mimicking to expedite experimental work.

Patterning nanowire and micro-nanoparticle array on micropillar-structured surface: Experiment and modeling.

DNA molecules in a solution can be immobilized and stretched into a highly ordered array on a solid surface containing micropillars by molecular combing technique. However, the mechanism of this process is not well understood. In this study, we demonstrated the generation of DNA nanostrand array with linear, zigzag, and fork-zigzag patterns and the microfluidic processes are modeled based on a deforming body-fitted grid approach. The simulation results provide insights for explaining the stretching, immobilizing, and patterning of DNA molecules observed in the experiments.