Micropillar Arrays Research Articles

In Situ Activation of Superhydrophobic Surfaces with Triple Icephobicity at Low Temperatures.

Superhydrophobic surfaces have been widely studied due to their potential applications in aerospace fields. However, superhydrophobic surfaces with excellent water-repellent, anti-icing, and icephobic performances at low temperatures have rarely been reported. Herein, superhydrophobic surfaces with heating capability were prepared by etching square micropillar arrays on the surface of multiwalled carbon nanotube (MWCNT)/poly(dimethylsiloxane) (PDMS) films. The fabricated superhydrophobic surface has triple icephobicity, which can be activated even at low temperatures. The triple icephobicity is triggered by an applied voltage to achieve excellent water-repellent and icephobic capabilities, even at -40 °C. Additionally, theoretical calculations reveal that a droplet on a superhydrophobic surface loses heat at a rate of 8.91 × 10-5 J/s, which is 2 orders of magnitude slower than a flat surface (2.15 × 10-3 J/s). Also, at -40 °C, the mechanical interlocking force formed between the superhydrophobic surface and ice can be released by the heating property of the superhydrophobic surface. This low-energy, multifunctional superhydrophobic surface opens up new possibilities for bionic smart multifunctional materials in icephobic applications.

Hierarchical Co-Pi Clusters/Fe2O3 Nanorods/FTO Micropillars 3D Branched Photoanode for High-Performance Photoelectrochemical Water Splitting.

In this study, an efficient hierarchical Co–Pi cluster/Fe2O3 nanorod/fluorine-doped tin oxide (FTO) micropillar three-dimensional (3D) branched photoanode was designed for enhanced photoelectrochemical performance. A periodic array of FTO micropillars, which acts as a highly conductive “host” framework for uniform light scattering and provides an extremely enlarged active area, was fabricated by direct printing and mist-chemical vapor deposition (CVD). Fe2O3 nanorods that act as light absorber “guest” materials and Co–Pi clusters that give rise to random light scattering were synthesized via a hydrothermal reaction and photoassisted electrodeposition, respectively. The hierarchical 3D branched photoanode exhibited enhanced light absorption efficiency because of multiple light scattering, which was a combination of uniform light scattering from the periodic FTO micropillars and random light scattering from the Fe2O3 nanorods. Additionally, the large surface area of the 3D FTO micropillar, together with the surface area provided by the one-dimensional Fe2O3 nanorods, contributed to a remarkable increase in the specific area of the photoanode. Because of these enhancements and further improvements facilitated by decoration with a Co–Pi catalyst that enhanced water oxidation, the 3D branched Fe2O3 photoanode achieved a photocurrent density of 1.51 mA cm−2 at 1.23 VRHE, which was 5.2 times higher than that generated by the non-decorated flat Fe2O3 photoanode.

Numerical Study on a Bio-Inspired Micropillar Array Electrode in a Microfluidic Device.

The micropillar array electrode (µAE) has been widely applied in microchip-based electrochemical detection systems due to a large current response. However, it was found that amplifying the current through further adjusting geometrical parameters is generally hindered by the shielding effect. To solve this problem, a bio-inspired micropillar array electrode (bµAE) based on the microfluidic device has been proposed in this study. The inspiration is drawn from the structure of leatherback sea turtles’ mouths. By deforming a μAE to rearrange the micropillars on bilateral sides of the microchannel, the contact area between micropillars and analytes increases, and thus the current is substantially improved. A numerical simulation was then used to characterize the electrochemical performance of bµAEs. The effects of geometrical and hydrodynamic parameters on the current of bµAEs were investigated. Moreover, a prototypical microchip integrated with bµAE was fabricated for detailed electrochemical measurement. The chronoamperometry measurements were conducted to verify the theoretical performance of bµAEs, and the results suggest that the experimental data are in good agreement with those of the simulation model. This work presents a novel bµAE with great potential for highly sensitive electrochemical detection and provides a new perspective on the efficient configuration of the µAE.

Tuning the light emission of a Si micropillar quantum dot light-emitting device array with the strain coupling effect

Using an optical signal to realize sensing of a strain signal is a promising application for tactile sensors. However, most research is now focused on piezophototronic LED arrays, which are difficult to incorporate into the Si-based semiconductor industry. Due to the poor photoelectric performance of Si-based devices caused by the indirect band gap of Si, it has always been challenging to construct high density light-emitting devices with Si. Here, a Si-based quantum dot light-emitting device (QLED) array composed of p-Si micropillars is designed and fabricated, and the mechanism for modulation of the strain coupling effect in Si on the electroluminescence performance of Si-based QLEDs is studied. The introduction of QDs easily provides efficient and adjustable light emission and meets the requirements of different practical applications. The emission intensity of the QLED depends on the injected current density, and the transportation processes of the carriers can be modulated by the strain coupling effect. The combination of Si-based photonic devices with pressure sensing may have a significant impact on the fields of electronic skin and human‒machine interfaces. More importantly, this technology is fully compatible with the dominant Si-based semiconductor industry. Therefore, it shows promise in realizing the integration of large-scale Si-based photonic devices and expanding their application fields.

Well-ordered patterns of nanoparticles induced by an alternately-arranged binary micropillar array

Well-ordered patterns of nanoparticles induced by an alternately-arranged binary micropillar array

Development of a novel microfluidic biosensing platform integrating micropillar array electrode and acoustic microstreaming techniques

Quantifying biomarkers at the early stage of the disease is challenging due to the low abundance of biomarkers in the sample and the lack of sensitive techniques. This article reports the development of a novel microfluidic electrochemical biosensing platform to address this challenge. The electrochemical sensing is achieved by utilizing a micropillar array electrode (μAE) coated with 3D bimetallic Pt–Pd nanotrees to enhance the sensitivity. A bubble-based acoustic microstreaming technique is integrated with the device to increase the contact of analyte molecules with the surface of electrodes to further enhance the electrochemical performance. The current density of Pt–Pd NTs/μAE with acoustic microstreaming is nearly 22 times that of the bare planar electrode in potassium ferrocyanide solution. The developed biosensor has demonstrated excellent sensing performance. For hydrogen peroxide detection, both the Pt–Pd NTs/μAE and acoustic microstreaming contribute to the sensitivity enhancement. The current density of the Pt–Pd NTs/μAE is approximatively 28 times that of the bare μAE. With acoustic microstreaming, this enhancement is further increased by nearly 1.6 times. The platform has a linear detection range of 5–1000 μM with a LOD of 1.8 μM toward hydrogen peroxide detection, while for sarcosine detection, the linear range is between 5 and 100 μM and LOD is 2.2 μM, respectively. Furthermore, the sarcosine biosensing shows a high sensitivity of 667 μA mM−1∙cm−2. Such a sensing platform has the potential as a portable device for high sensitivity detection of biomarkers.

Self-supported AlFeNiCoMo high-entropy alloy with micropillar array structure for efficient oxygen evolution reaction

The development of low-cost and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial for the application of hydrogen energy. In this work, a self-supported AlFeCoNiMo high entropy alloy with a uniformly distributed micropillar array structure on the surface is prepared by melt spinning followed by dealloying. The catalyst exhibits a low overpotential of 228, 265, and 318 mV to drive the current densities of 10, 100, and 1000 mA/cm2, a small Tafel slope of 51 mV/dec, and excellent long-term stability for 30 h in 1.0M KOH electrolyte for OER. The high catalytic activity is attributed to the unique micropillar array with rich active sites and the synergistic effect between multiple metallic elements. The good stability of the catalyst results from the steady micropillar array structure and the formation of oxyhydroxides on the surface during long-term catalysis. This work provides a facile and scalable method for the preparation of self-supported, low-cost, and high-performance OER electrocatalysts.

How micropatterns affect the anti-icing performance of superhydrophobic surfaces

• Four dynamic-freezing modes are identified on superhydrophobic surfaces (SHSs). • The capillary evacuation contributes to an excellent anti-icing performance. • The coupling effects of nucleation and wetting dynamics are analyzed in detail. • A dimensionless number is put forward to optimally design anti-icing SHSs. Superhydrophobic surfaces (SHSs) have shown significant potential in the anti-icing field over the past decade. In this work, we conduct impact experiments of supercooled drops on SHSs with various micropillar arrays and show how micropatterns contribute to efficient anti-icing performances. Here, we found four dynamic-freezing modes: partial penetration, penetration with evacuation, penetration without evacuation, and internal rupture, and obtained the phase diagrams of these modes in terms of the structural parameters. The hierarchical SHSs, where penetration with evacuation and internal rupture can occur, present more excellent anti-icing performances than single-tier SHSs. The capillary evacuation is proved to play an important role in the reduction of contact time and contact area. A quantitative interpretation of the influence of the capillary evacuation on the contact time is obtained, which demonstrates the potential anti-icing ability of hierarchical SHSs. Coupling the effect of nucleation and wetting dynamics on hierarchical SHSs with capillary evacuation, we put forward a dimensionless number, combined the structural parameters and Weber number We , which is important for optimally designed anti-icing micropatterns of SHSs.

Multiplexed electrospraying of water in cone-jet mode using a UV-embossed pyramidal micronozzle film

The electrospraying of water in the cone-jet mode is difficult in practical applications owing to its low throughput and the electrical discharge caused by the high surface tension of water. A film with multiple dielectric micronozzles is essential for multiplexed electrospraying of water in cone-jet mode without electrical discharge. Thus, a pyramidal micronozzle film with five nozzles was fabricated using the UV-embossing process. The pyramidal micronozzle film consisted of pyramidal micronozzles, a micropillar array, and an in-plane extractor, which were proposed to minimize wetting and concentrate the electric field to the water meniscus at the tip of the pyramidal micronozzle. The electrospraying of water using a single pyramidal micronozzle was visualized by a high-speed camera at a flow rate of 0.15–0.50 ml/h with voltages of 0.0–2.3 kV, −1.6 kV, and −4.0 kV at the water, guide ring, and collector, respectively. Three distinct modes, the dripping, spindle, and cone-jet modes, were observed and distinguished according to the motion of the water meniscus at the nozzle tip. The steady Taylor cone and jet were observed in a voltage range of 1.3–2.0 kV in water, particularly in cone-jet mode. Multiplexed electrospraying of water in cone-jet mode at a flow rate of 1.5 ml/h was performed using a pyramidal micronozzle film, demonstrating the potential for a high-throughput electrospraying system.

Quasicrystalline Arrays and Moiré Patterns in Nematic Liquid Crystals for Soft Photonics

AbstractThe search for new strategies for large‐scale, self‐assembled arrays of soft objects is key for many applications in photonics and bottom‐up manufacturing. This work shows how liquid crystal topological defects can be assembled in controlled, aperiodic arrays. In particular, the focus is on two typical examples: quasicrystals and moiré patterns. Thanks to a combination of topographical cues, specifically a micropillar array and electrical switching, defects can be assembled in a quasicrystal structure, as seen from polarized optical microscopy and from diffraction patterns. In this setting, the liquid crystal defects assemble in multiple patterns that can be switched by tuning the applied electric field and retain the quasicrystalline symmetry. Using topographic cues, it is also possible to induce moiré patterns of defects, characterized by a long wavelength superimposed on the periodic structures over a short scale. Even when the defect density increases and the short‐scale periodicity is lost, the long‐scale one remains. This work shows how versatile the combination of topographic confinement and electro‐optic effect is, giving access to patterns that are otherwise difficult to realize.

Optically tunable metal-dielectric diffractive structures

Hybrid metal-polymer micropillar arrays are elaborated by solvent-assisted embossing techniques and subsequent gold deposition. The polymer is poly(methyl methacrylate) (PMMA) grafted with Disperse Red 1 (DR1) azobenzene derivatives. This photochromic material exhibits spectacular photomechanical properties. Under off-normal $p$-polarized illumination in the absorption band of the DR1, the metal-polymer hybrid micropillars bend in a direction determined by the light polarization. The deformation remains stable when the light excitation is turned off. The optically driven modification of the pillar shape allows us to tune the optical properties of the pillar arrays. Very large intensity changes are measured in the diffraction spectra at zero order (specular reflection), first order, and second order, with almost on and off switching of the diffraction efficiency. The photoinduced bending of the micropillars can be fully reversed by switching the linear light polarization to the orthogonal $s$-polarization state. This allows us to restore the initial optical properties of the pillar array. The polarization-controlled reversibility of the pillar deformation suggests a complex photoinduced deformation mechanism involving stress and stress release assisted by the change in the viscoelastic properties of the polymer under illumination in the absorption band of the DR1 chromophore.

Influence of hydrophobic area fraction of a wettability-patterned evaporator in an ultrathin vapor chamber

Integrating wettability-patterned surfaces into a multiscale micro/nanostructured evaporator has demonstrated promising thermal performance enhancement of an ultrathin vapor chamber (UTVC). However, the mechanism behind it still remains unknown. To investigate the synergetic effect of wettability-patterned micro/nanostructures on the thermal performance of the UTVC, the hydrophobic area fraction of the wettability-patterned surface in this study varies from 6.25% to 66.9%. The wettability-patterned surface is fabricated on the substrate of the evaporator, and a micro/nanostructured mesh functions as a multiscale wick structure intimately pressed on the wettability-patterned surface to form the evaporator. The total thickness of UTVCs under the investigation is less than 250 µm including the multiscale evaporator, the condenser, and the supporting micropillar array. The thermal performance of UTVCs with different hydrophobic area fractions is evaluated under the optimum charge ratio. To understand the mechanism of the enhancement of wettability-patterned surfaces, a visualization experiment was conducted utilizing a wettability-patterned glass substrate. It is found that the wettability-patterned surface can increase the length of the triple contact line, which enhances the evaporation process. Specifically, the optimum hydrophobic area fraction was found at around 34%. The UTVC with the wettability-patterned surface achieves a maximum 13,820 W/(m·K) in-plane effective thermal conductivity, which is a 162% enhancement compared to the one with a hydrophilic surface. Furthermore, it also provides a 29.1% maximum reduction of vertical thermal resistance of UTVCs. An evaporation model of the wettability-patterned surface integrated with the wire mesh is proposed based on its elongation effect on the triple contact line. The optimum hydrophobic area fraction predicted by the model is 29.9%, which is very close to the experimental results.

Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the “Salvinia Effect,” this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Flows of healthy and hardened RBC suspensions through a micropillar array.

Red blood cell (RBC) deformability is an important haemorheological factor; it is impaired in many pathologies leading to microvascular complications. Several microfluidic platforms have been utilized to examine the role of deformability in RBC flows but their geometries tend to be simplified. In the present study, we extend our previous work on healthy RBC flows in micropillar arrays [1] to probe the effect of impaired RBC deformability on the velocity and haematocrit distributions in microscale RBC flows. Healthy and artificially hardened RBC suspensions at 25% haematocrit were perfused through the micropillar array at various flow rates and imaged. RBC velocities were determined by Particle Image Velocimetry (PIV) and haematocrit distributions were inferred from the image intensity distributions. The pillars divide the flow into two distinct RBC streams separated by a cell-depleted region along the centreline and in the rear/front stagnation points. RBC deformability was not found to significantly affect the velocity distributions; the shape of the velocity profiles in the interstitial space remained the same for healthy and hardened RBCs. Time-averaged and spatiotemporal intensity distributions, however, reveal differences in the dynamics and local distributions of healthy and hardened cells; hardened cells appear to enter the cell-depleted regions more frequently and their interstitial distributions are more uniform. The study highlights the importance of local RBC distributions and the impact of RBC deformability on cell transport in complex microscale flows.

Ultrarapid and ultrasensitive detection of SARS-CoV-2 antibodies in COVID-19 patients via a 3D-printed nanomaterial-based biosensing platform.

Rapid detection of antibodies during infection and after vaccination is critical for the control of infectious outbreaks, understanding immune response, and evaluating vaccine efficacy. In this manuscript, we evaluate a simple ultrarapid test for SARS‐CoV‐2 antibodies in COVID‐19 patients, which gives quantitative results (i.e., antibody concentration) in 10–12 s using a previously reported nanomaterial‐based three‐dimensional (3D)‐printed biosensing platform. This platform consists of a micropillar array electrode fabricated via 3D printing of aerosolized gold nanoparticles and coated with nanoflakes of graphene and specific SARS‐CoV‐2 antigens, including spike S1, S1 receptor‐binding domain (RBD) and nucleocapsid (N). The sensor works on the principle of electrochemical transduction, where the change of sensor impedance is realized by the interactions between the viral proteins attached to the sensor electrode surface and the antibodies. The three sensors were used to test samples from 17 COVID‐19 patients and 3 patients without COVID‐19. Unlike other serological tests, the 3D sensors quantitatively detected antibodies at a concentration as low as picomole within 10–12 s in human plasma samples. We found that the studied COVID‐19 patients had higher concentrations of antibodies to spike proteins (RBD and S1) than to the N protein. These results demonstrate the enormous potential of the rapid antibody test platform for understanding patients' immunity, disease epidemiology and vaccine efficacy, and facilitating the control and prevention of infectious epidemics.

Study on the Effect of Processing Parameters on Residual Stresses of Injection Molded Micro-Pillar Array.

As one of the main methods for fabricating microstructured surfaces, micro-injection molding has the advantages of short cycle time, high production efficiency, and the potential for batch manufacturing. However, non-negligible residual stresses inside the molded part could affect the replication quality, dimensions, and physical properties of the microstructure. Therefore, studying the effects of processing parameters on residual stresses is a necessary prerequisite to ensure the successful fabrication of microstructured parts. In this paper, an injection molding simulation model of micro-pillar arrays was developed using molecular dynamics software, and a series of injection molding experiments were conducted. It was found that increasing the mold temperature and melt temperature can reduce the thermal residual stresses and molecular orientation stresses, and effectively improve the uniformity of residual stress distribution. The increase in the packing pressure can make the shear field of flow more intense and increase the molecular orientation stresses, thus making the residual stresses more severe.

P-029: OCCLUSIONCHIP: A FUNCTIONAL MICROCAPILLARY OCCLUSION ASSAY COMPLEMENTARY TO EKTACYTOMETRY FOR ASSESSING RED CELL HEALTH IN SICKLE CELL DISEASE

Purpose: Red blood cell (RBC) deformability is a biomarker to assess the clinical status and response to therapy in sickle cell disease (SCD). RBC deformability has been measured by ektacytometry for decades, using shear or osmolar stress. However, ektacytometry averages the RBC population, and cannot report small fractions of abnormal RBCs. A single cell-based, functional RBC deformability measurement provides essential information on the presence of unmodified cell populations that can perpetuate disease complications after the implementation of drug or cell-based therapies. There are two goals in this study: to determine the reproducibility of values generated by the standardized OcclusionChip assay and commercially available Oxygen Gradient Ektacytometry (LORRCA); and to determine if the OcclusionChip assay could complement Ektacytometry for detection of abnormal RBCs in small fractions. Materials and methods: OcclusionChip microfluidic assay design includes a gradient of capillary network-inspired micropillar arrays embedded into the microfluidic channel forming microcapillaries from 20 μm down to 4 μm. This mimics the non-uniform, continuously changing capillaries in the capillary bed, retaining RBCs with significantly decreased deformability in the upstream array with coarser openings, and those with modestly decreased deformability in the downstream array with finer openings. Micropillar arrays were coupled with two 60-μm side passageways mimicking capillary bed arteriovenous anastomoses. This helps regulate blood flow; when an upstream portion of the array is obstructed, incoming RBCs can flow into the microfluidic anastomosis and re-enter the array downstream. To test the relative merits of the OcclusionChip and Ektacytometry, we measured the elongation index (EI by Oxygen Gradient Ektacytometry) and occlusion index (OI by OcclusionChip) of blood samples containing gradient concentrations of glutaraldehyde-stiffened RBCs. We also tested clinical blood samples obtained from subjects with SCD using oxygen gradient ektacytometry, which measures the EI at a range of pO2 level (159 mmHg to ~5 mmHg), and the hypoxic OcclusionChip assay, which measures OI at physiologic hypoxic pO2 level (45 mmHg). Results: We tested glutaraldehyde-stiffened RBCs for up to 1% volume fraction; ektacytometry detected no significant change in Elongation Index (EI), while the OcclusionChip showed significant differences in OI (Figure 1). OcclusionChip detected a significant increase in OI in RBCs from an individual with sickle cell trait (SCT) and from a subject with SCD who received allogeneic hematopoietic stem cell transplant (HSCT), as the sample was taken from normoxic (pO2:159 mmHg) to physiologic hypoxic (pO2:45 mmHg) conditions (Figure 2). Oxygen gradient ektacytometry detected no difference in EI for SCT or HSCT. Conclusion: The OcclusionChip assay is more sensitive to alterations in RBC deformability in SCD, particularly when %hemoglobin S (HbS) is small, thus able to complement Ektacytometry for a more accurate assessment of patient clinical status. These results suggest that the single cell-based OcclusionChip enables the detection of HbS-related RBC abnormalities in SCT and SCD, particularly when the HbS level is low. We conclude that the OcclusionChip is complementary to the population-based Ektacytometry assays and provides additional sensitivity and capacity to detect small but clinically significant populations of abnormal RBC.Figure 1. The OcclusionChip enables the detection of small-fraction glutaraldehyde-stiffened RBCs in mixtures with normal RBCs. (A) Elongation Index (EI) by ektacytometry. (B) Occlusion Index (OI) results on the same RBC mixtures.Figure 2. OcclusionChip enables detection of HbS-related RBC abnormalities under hypoxia in SCT and SCD. Shown are (A) Elongation Index (EI) results by oxygen gradient ektacytometry and (B) Occlusion Index (OI) results by the OcclusionChip. The authors do not declare any conflict of interest

A micropillar array-based microfluidic chip for label-free separation of circulating tumor cells: The best micropillar geometry?

IntroductionThe information derived from the number and characteristics of circulating tumor cells (CTCs), is crucial to ensure appropriate cancer treatment monitoring. Currently, diverse microfluidic platforms have been developed for isolating CTCs from blood, but it remains a challenge to develop a low-cost, practical, and efficient strategy. ObjectivesThis study aimed to isolate CTCs from the blood of cancer patients via introducing a new and efficient micropillar array-based microfluidic chip (MPA-Chip), as well as providing prognostic information and monitoring the treatment efficacy in cancer patients. MethodsWe fabricated a microfluidic chip (MPA-Chip) containing arrays of micropillars with different geometries (lozenge, rectangle, circle, and triangle). We conducted numerical simulations to compare velocity and pressure profiles inside the micropillar arrays. Also, we experimentally evaluated the capture efficiency and purity of the geometries using breast and prostate cancer cell lines as well as a blood sample. Moreover, the device’s performance was validated on 12 patients with breast cancer (BC) in different states. ResultsThe lozenge geometry was selected as the most effective and optimized micropillar design for CTCs isolation, providing high capture efficiency (>85 %), purity (>90 %), and viability (97 %). Furthermore, the lozenge MPA-chip was successfully validated by the detection of CTCs from 12 breast cancer (BC) patients, with non-metastatic (median number of 6 CTCs) and metastatic (median number of 25 CTCs) diseases, showing different prognoses. Also, increasing the chemotherapy period resulted in a decrease in the number of captured CTCs from 23 to 7 for the metastatic patient. The MPA-Chip size was only 0.25 cm2 and the throughput of a single chip was 0.5 ml/h, which can be increased by multiple MPA-Chips in parallel. ConclusionThe lozenge MPA-Chip presented a novel micropillar geometry for on-chip CTC isolation, detection, and staining, and in the future, the possibilities can be extended to the culture of the CTCs.

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.

Anisotropic Wettability of Bioinspired Surface Characterized by Friction Force.

Bioinspired surfaces with special wettabilities attract increasing attention due to their extensive applications in many fields. However, the characterizations of surface wettability by contact angle (CA) and sliding angle (SA) have clear drawbacks. Here, by using an array of triangular micropillars (ATM) prepared by soft lithography, the merits of measuring the friction force of a water droplet on ATM over measurements of CA and SA in characterizing the surface wettability are demonstrated. The CA and SA measurements show ignorable differences in the wettabilities of ATM in opposite directions (1.13%) and that with different periodic parameters under the elongation ranging from 0 to 70%. In contrast, the friction measurement reveals a difference of > 10% in opposite directions. Moreover, the friction force shows a strong dependence on the periodic parameters which is regulated by mechanical stretching. Increasing the elongation from 0 to 50% increases the static and kinetic friction force up to 23.0% and 22.9%, respectively. Moreover, the stick-slip pattern during kinetic friction can reveal the periodic features of the measured surface. The friction force measurement is a sensitive technique that could find applications in the characterization of surface wettabilities.