Hydrophobic Microstructures Research Articles

Single Condensation Droplet Self-Ejection from Divergent Structures with Uniform Wettability.

Coalescence-induced condensation droplet jumping has been extensively studied for anti-icing, condensation heat transfer, water harvesting, and self-cleaning. Another phenomenon that is gaining attention for potential enhancements is the self-ejection of individual droplets. However, the mechanism underlying this process remains elusive due to cases in which the abrupt detachment of an interface establishes an initial Laplace pressure difference. In this study, we investigate the self-ejection of individual droplets from uniformly hydrophobic microstructures with divergent geometries. We design, fabricate, and test arrays of truncated, nanostructured, and hydrophobic microcones arranged in a square pattern. High-speed microscopy reveals the dynamics of a single condensation droplet between four cones: after cycles of growth and stopped self-propulsion, the suspended droplet self-ejects without abrupt detachments. Through analytical modeling of the droplet in a conical pore as an approximation, we describe the slow isopressure growth phases and the rapid transients driven by surface energy release once a dynamic configuration is reached. Microcones with uniform wettability, in addition to being easier to fabricate, have the potential to enable the self-ejection of all nucleated droplets with a designed size, promising significant improvements in the aforementioned applications and others.

Improving water removal efficiency in a PEM fuel cell: Microstructured surfaces for controlling instability-driven pinching

Proton Exchange Membrane Fuel Cell (PEMFC) is a clean, sustainable energy generation device, and its large-scale usage is becoming popular due to green and secure energy demand worldwide. The performance, efficiency, and lifespan of PEMFC largely depend on the water removal and management within the cell. Under the influence of the cross-air flow, the generated water filaments deform, and as the filament radius lowers, the curvature and capillary pressure increase, ejecting fluid out of the neck at increasing velocities. The moment the filament radius vanishes, the governing equations reach the point of singularity, and the filament breaks. We propose an optimum micro-patterned surface design for efficient water removal from PEMFC. We perform a numerical study of water generation on the surface followed by breakup under shear flow within confinement. We further theoretically identify the breakup behavior with characterization, recognizing the influence of the microstructures toward an efficient design. The hydrophobic microstructures are observed to decrease the dominance of viscous force over inertia and capillary force. This leads to a greater propensity of end-pinching or truncation of the generated droplet at the neck, which reduces the production of undesired satellite droplets that would have otherwise caused flooding of the chamber. In this work, we show that a proper combination of substrate structure and jet velocity-induced shear can mitigate the generation of satellite droplets and reduce the breakup time, significantly increasing the water removal efficiency of the PEMFC.

Experimental Study on Condensation Heat Transfer Performance of Hydrophilic/Hydrophobic Microstructured

In order to study the condensation and heat transfer characteristics of similar microstructure surfaces, two similar microstructure surfaces, cylindrical and circular, were fabricated by femtosecond laser technology on a 0.5 mm silicon wafer. The cylindrical surface is superhydrophobic when the contact angle is more than 150°, and the circular surface is hydrophilic when the contact angle is less than 90°. The difference in condensation heat transfer characteristics between superhydrophobic and hydrophilic microstructures was analyzed, and a visual condensation experimental platform was built. Experimental research showed that: At the same flow rate, the heat transfer coefficient of the superhydrophobic surface and the hydrophilic surface decreases significantly with the increase of the surface subcooling degree, but the heat transfer coefficient of the cylindrical surface is still much larger than that of the circular surface. In addition, the heat transfer performance of the hydrophobic microstructure surface is better than that of the hydrophilic surface at medium and high-speed cooling water flow rates. Although the surface microstructures are similar in shape, the heat transfer performance of cylindrical microstructures is much better than that of circular microstructures under the same conditions, and the heat flux of cylindrical microstructures is 2.2 times that of circular microstructures.

Dynamic Wetting of Ionic Liquid Drops on Hydrophobic Microstructures.

Ionic liquids (ILs)─salts in a liquid state─play a crucial role in various applications, such as green solvents for chemical synthesis and catalysis, lubricants, especially for micro- and nanoelectromechanical systems, and electrolytes in solar cells. These applications critically rely on unique or tunable bulk properties of ionic liquids, such as viscosity, density, and surface tension. Furthermore, their interactions with different solid surfaces of various roughness and structures may uphold other promising applications, such as combustion, cooling, and coating. However, only a few systematic studies of IL wetting and interactions with solid surfaces exist. Here, we experimentally and theoretically investigate the dynamic wetting and contact angles (CA) of water and three kinds of ionic liquid droplets on hydrophobic microstructures of surface roughness (r = 2.61) and packing fraction (ϕ = 0.47) formed by micropillars arranged in a periodic pattern. The results show that, except for water, higher-viscosity ionic liquids have greater advancing and receding contact angles with increasing contact line velocity. Water drops initially form a gas-trapping, CB wetting state, whereas all three ionic liquid drops are in a Wenzel wetting state, where liquids penetrate and completely wet the microstructures. We find that an existing model comparing the global surface energies between a CB and a Wenzel state agrees well with the observed wetting states. In addition, a molecular dynamic model well predicts the experimental data and is used to explain the observed dynamic wetting for the ILs and superhydrophobic substrate. Our results further show that energy dissipation occurs more significantly in the three-phase contact line region than in the liquid bulk.

Enhancing the properties of starch-fiber foaming material by adjusting fiber length: The synergistic effect of macro-micro stress conduction

Starch-fiber based foaming materials are a promising alternative to petroleum-based plastics. The fiber length, as an important factor affecting the process of making foamed materials, was studied in this paper to investigate its influence on slurry rheology and the foaming process, and the mechanism of its influence on mechanical properties was analyzed from the perspective of macro-micro synergistic stress transfer. Different lengths [10 mm (L 10 ), 5 mm (L 5 ), 1 mm (L 1 ), 0.5 mm (L 0.5 ), 0.1 mm (L 0.1 )] of fibers were used to prepare different composites. The results showed that the increase in fiber length leads to higher slurry viscosity, a more uniform cell structure, and stronger mechanical properties. The L 10 composite achieved a tensile strength of 3.25 MPa, which was more than 4 times of the L 0.1 composite. Interestingly, the most hydrophobic microstructure was found on the surface of the L 0.5 composite, on which the water contact angle reached 130°. Finally, the effect of fiber length on the degradation rate of composites was explored. In summary, this study provides a new idea for the performance analysis of starch-fiber based foaming materials and is helpful in the selection of fiber length in their manufacturing process. • Starch-fiber composites were made using different lengths (0.1-10 mm) of sisal fibers. • The effect of macro-micro stress conduction on properties were investigated. • Composites with longer fibers can achieve better macro-micro stress conduction. • Longer fibers can build more stable supports, leading to better cell structures.

Droplet Impact on Asymmetric Hydrophobic Microstructures.

Textured hydrophobic surfaces that repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing. In many occasions, surface textures are oriented to direct rebounding droplets. Surface macrostructures (>100 μm) have often been explored to induce directional rebound. However, the influence of impact speed and detailed surface geometry on rebound is vaguely understood, particularly for small microstructures. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars. These insights improve our understanding of droplet impact on hydrophobic microstructures and may be useful for designing structured surfaces for controlling droplet mobility.

Surface-modified Zn0.5Ti0.5NbO4 particles filled polytetrafluoroethylene composite with extremely low dielectric loss and stable temperature dependence

Polymer-ceramic composites are widely applied in microwave substrate materials due to the excellent dielectric properties and simple preparation process recently. Polytetrafluoroethylene-based (PTFE) composites filled with Zn0.5Ti0.5NbO4 (ZTN) ceramic particles were fabricated by hot-pressing. The particles were modified by C14H19F13O3Si to enhance the interface compatibility between PTFE and ZTN powders, which was characterized by X-ray photoelectron spectroscopy (XPS) and contact angle. The surface characteristic of particles transformed into hydrophobicity and tight microstructure as well as better dielectric properties were obtained after the surface modification. The microstructure, dielectric, thermal, mechanical properties, and water absorption of the composites concerning ZTN content were investigated. Modified ZTN/PTFE composites with 50 vol% ZTN particles exhibit excellent dielectric properties with a high dielectric constant of 8.3, an extremely low dielectric loss of 0.00055 at 7 GHz, and a stable temperature coefficient of the dielectric constant of −12.2 ppm/°C. All the properties show modified ZTN particles filled PTFE composite is the potential material for microwave substrate application.

High-Performance Unidirectional Manipulation of Microdroplets by Horizontal Vibration on Femtosecond Laser-Induced Slant Microwall Arrays.

The high-performance unidirectional manipulation of microdroplets is crucial for many vital applications including water collection and bioanalysis. Among several actuation methods (e.g., electric, magnetic, light, and thermal actuation), mechanical vibration is pollution-free and biocompatible. However, it suffers from limited droplet movement mode, small volume range (VMax /VMin <3), and low transport velocity (≤11.5mm s-1 ) because the droplet motion is a sliding state caused by the vertical vibration on the asymmetric hydrophobic microstructures. Here, an alternative strategy is proposed-horizontal vibration for multimode (rolling, bouncing/reverse bouncing, converging/diffusing, climbing, 90o turning, and sequential transport), large-volume-range (VMax /VMin ≈ 100), and high-speed (≈22.86mm s-1 ) unidirectional microdroplet manipulation, which is ascribed to the rolling state on superhydrophobic slant microwall arrays (SMWAs) fabricated by the one-step femtosecond laser oblique ablation. The unidirectional transport mechanism relies on the variance of viscous resistance induced by the difference of contact area between the microdroplet and the slant microwalls. Furthermore, a circular, curved, and "L"-shaped SMWA is designed and fabricated for droplet motion with particular paths. Finally, sequential transport of large-volume-range droplets and chemical mixing microreaction of water-based droplets are demonstrated on the SMWA, which demonstrates the great potential in the field of microdroplet manipulation.

How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures.

Surfactants, as amphiphilic molecules, adsorb easily at interfaces and can detrimentally destroy the useful, gas-trapping wetting state (Cassie-Baxter, CB) of a drop on superhydrophobic surfaces. Here, we provide a quantitative understanding of how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different roughness (r) and solid fraction (ϕ). Experimentally, at low surfactant concentrations (C), some drops attain a homogeneous wetting state (Wenzel, W), while others attain the CB state whose large contact angles can be predicted by a thermodynamic model. In contrast, all of our high-C drops attain the Wenzel state. To explain this observed transition, we consider the free energy and find that, theoretically, for our surfaces the W state is always preferred, while the CB state is metastable at low C, consistent with experimental results. Furthermore, we provide a beneficial blueprint for stable CB states for applications exploiting superhydrophobicity.

Pore-Scale Displacement Efficiency during Different Salinity Water Flooding in Hydrophilic and Hydrophobic Microstructures

Previous macroscopic core flooding tests have shown that injecting low-salinity water improves oil recovery in sandstone and carbonate reservoirs through wettability alteration. However, consistent mechanistic clarification of the underlying physicochemical mechanisms involved in oil wettability at the pore-scale level is not fully understood. In this work, a microfluidic approach is used to provide in situ visualization of oil–brine flow to give an indication of the micromechanisms affecting oil sweep efficiency. The potential of enhancing oil recovery by low-salinity flooding at the microscale is also investigated, which would help in predicting a reservoir’s performance before committing to production processes at a large field scale. Two types of crude oils with various acid numbers were used, and hydrophilic and hydrophobic physical microstructures were used to mimic sandstones and carbonates. The results revealed a reduction by 7–10% in the residual oil for the water-wet microstructure when the seaw...

Superhydrophobic surface prepared by micro-milling and WEDM on aluminum alloy

To simulate the hydrophobic microstructure of rice leaf surface, high-speed precision micro-milling machine was used to fabricate micro groove array structure on the surface of aluminum alloy. The micro-and nanostructure was constructed on the surface of the grooved convex platform by Wire Cut Electrical Discharge Machining (WEDM). The surface morphology and hydrophobic properties of the aluminum alloy microstructures fabricated by two processing methods were observed respectively, and the hydrophobic mechanism was analyzed. The results show that the contact angle was effectively improved from 49° up to 158.4° in the vertical direction, and 146.7° in the parallel direction. The change of surface wettability from hydrophilic to hydrophobic was realized. By comparison, the micro-and nanostructure fabricated by WEDM had improved the hydrophobic stability of the aluminum alloy surface while enlarging the contact Angle, and the micro-milling groove structure further amplified the contact angle and greatly reduced the contact area of the water droplet, it was also observed that the drop took longer to completely spread on the sample after WEDM.

Salvinia-Effect-Inspired "Sticky" Superhydrophobic Surfaces by Meniscus-Confined Electrodeposition.

Inspired by the Salvinia effect, we report the fabrication and characterization of a novel "sticky" superhydrophobic surface sustaining a Cassie-Baxter wetting state for water droplets with high contact angles but strong solid-liquid retention. Unlike superhydrophobic surfaces mimicking the lotus or petal effect, whose hydrophobicity and droplet retention are typically regulated by hierarchical micro- and nanostructures made of a homogeneous material with the same surface energy, our superhydrophobic surface merely requires singular microstructures covered with a hydrophobic coating but creatively coupled with hydrophilic tips with different surface energy. Hydrophilic tips are selectively formed by meniscus-confined electrodeposition of a metal (e.g., nickel) layer on top of hydrophobic microstructures. During the electrodeposition process, the superhydrophobic surface retains its plastron so that the electrolyte cannot penetrate into the cavity of hydrophobic microstructures, consequently making the electrochemical reaction between solid and electrolyte occur only on the tip. In contrast to typical superhydrophobic surfaces where droplets are highly mobile, the "sticky" superhydrophobic surface allows a water droplet to have strong local pinning and solid-liquid retention on the hydrophilic tips, which is of great significance in many droplet behaviors such as evaporation.

Thermal performance analysis of pool boiling on an enhanced surface modified by the combination of microstructures and wetting properties

Characterized by low temperature difference and high heat flux, pool boiling heat transfer has been widely applied in various engineering technical fields. The aim of this study was to explore the thermal performance of pool boiling heat transfer on an enhanced surface modified by the combination of microstructures and wetting properties. The two-dimensional transient volume of fluid model and numerical simulation were selected to investigate each case which was established. Variation of the vapor bubble behavior, the velocity field of computational domain, the temperature at the liquid/solid interface and heat transfer coefficient (HTC) of each case were analyzed. The heat transfer performances of mixed hydrophilic and hydrophobic microstructures were all superior to those of hydrophilic microstructures only, except the flat. With respect to hydrophilic microstructures, the average growth rates of HTC for mixed hydrophilic and hydrophobic microstructures were 17.59%, 21.59%, 2.74% and 3.81% when the heights of microstructures were 3 μm, 5 7, 7 μm and 9 μm, respectively. Moreover, the average growth rate of HTC for mixed hydrophilic and hydrophobic microstructures decreased by 12.44%, 5.31% and 5.21% with increasing width.

Universal wetting transition of an evaporating water droplet on hydrophobic micro- and nano-structures

Water-repellent, rough surfaces have a remarkable and beneficial wetting property: when a water droplet comes in contact with a small fraction of the solid, both liquid-solid adhesion and hydrodynamic drag are reduced. As a prominent example from nature, the lotus leaf-comprised of a wax-like material with micro- and nano-scaled roughness-has recently inspired numerous syntheses of superhydrophobic substrates. Due to the diverse applications of superhydrophobicity, much research has been devoted to the fabrication and investigations of hydrophobic micro-structures using established micro-fabrication techniques. However, wetting transitions remain relatively little explored. During evaporation, a water droplet undergoes a wetting transition from a (low-frictional) partial to (adhesive) complete contact with the solid, destroying the superhydrophobicity and the self-cleaning properties of the slippery surface. Here, we experimentally examine the wetting transition of a drying droplet on hydrophobic nano-structures, a previously unexplored regime. In addition, using a theoretical analysis we found a universal criterion of this wetting transition that is characterized by a critical contact angle. Different from previous results showing different critical droplet sizes, our results show a universal, geometrically-dependent, critical contact angle, which agrees well with various data for both hydrophobic micro- and nano-structures.

Fabrication of Anion Exchange Membrane with Excellent Selectivity and Chemical Stability for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) have recently been revitalized as one of the most promising candidates for large scale electrical energy storage due to their significant merits, including the minimization of crossover contamination, excellent electrochemical reversibility and high efficiency. [1] As a key component of VRFBs, the membrane plays the role of preventing the migration of vanadium ions and transporting ions to complete the internal circuit. [2] Hence, the fabrication of novel membranes with the high chemical stability, low vanadium permeability, acceptable ionic conductivity and mechanical properties is one of the critical research objectives for the VRFB research community. [3, 4] In most reported works, the sulfonated polymers, i.e., proton exchange membrane (PEMs), have been widely researched for VRFBs. Although the ionic conductivity is acceptable, yet the energy efficiency and capacity retention are always plagued by the severe migration of vanadium ions. To address this issue, anion exchange membranes (AEMs) have been gradually employed in VRFBs due to the Donnan repulsion effect that can largely decrease the crossover of vanadium ions. But the chemical stability of functional groups is the critical issue for AEMs in the harsh chemical environments (high concentration of supporting electrolyte (H2SO4) and high oxidative reactive species (VO2 +)). Hence, the fabrication of AEMs with excellent chemical stability is highly required. In this regard, we here developed a novel cross-linked quaternary ammonia poly (2, 6-dimethyl-1, 4-phenylene oxide) (xQAPPO). The xQAPPO was prepared in four steps: bromomethylation, partial hydrophysis, quaternization and a final cross-linking process, as illustrated in Figure 1. Contrary to the conventional AEMs, not only the functional groups were grafted away from the polymer backbones, but also the side chain were cross-linked by glutaraldehyde. The cross-linked process can derive the aggregation of the backbones and side-chains and repel the functional groups, which enable to form a well-established hydrophilic /hydrophobic microstructure. Therefore, the polymer backbones could be well protected due to the reduced attack by the oxidative species. Meanwhile, the conductivity of the as-prepared AEMs enable to be readily tailored by the degree of bromo-methylation (x). The bromomethylated PPO with the x ranging from 40% to 80% was synthesized by controlling the ratio between PPO and the brominating agent. Our preliminary result showed that an ionic conductivity of 30.4 mS cm-1 was achieved at room temperature. More importantly, xQAPPO with the x value of 40% possesses a permeability of VO2+ ions of 0.15 × 10-7 cm2 min-1, which is largely lower than that of Nafion 212 (5.25 × 10-7 cm2 min-1). Moreover, the ex situ immersion test by immersing the membrane samples in 0.1 M VO2 + with 5 M H2SO4 was performed to determined the chemical stability of the as-prepared xQAPPO. It was shown that the weight loss of xQAPPO was as low as 6 wt.% for as long as 60 days. Further characterizations, including the water uptake, swelling ratio, mechanical properties, will be presented at the meeting. Meanwhile, the cell performance of single-cell VRFB respectively assembled with xQAPPO will also be presented at the meeting.

Biomimetic fabrication of polymer film with high adhesive superhydrophobicity by duplicating locust wing surface

Locust is a common flying insect. Locust wings were used as biomimetic templates to fabricate multi-functional polymer(polydimethylsiloxane, PDMS) films by soft lithography. The microstructure and wettability of the natural and artificial locust wing surfaces were investigated by means of a scanning electron microscope(SEM) and a video-based contact angle meter. The natural locust wing surface exhibits complicated hierarchical structures and high adhesive superhydrophobicity(contact angle 152°). The prepared polymer film faithfully reproduces the surface microstructures of the bio-template, and displays a good hydrophobicity and high adhesion(contact angle 144°). The complex wettability of the natural and artificial locust wing surfaces ascribes to the cooperative effect of hydrophobic composition and multi-dimensional rough microstructures. This work not only promotes our understanding of the wetting mechanism on bio-surfaces, but offers an inexpensive and effective approach for biomimetic fabrication of multi-functional interfacial materials.

Hydrophobic Mg–Al layered double hydroxide film on aluminum: Fabrication and microbiologically influenced corrosion resistance properties

Mg-Al layered double hydroxide films with an ab-oriented microstructure are fabricated in situ by urea hydrolysis using pure Al as both the substrate and Al source. After surface treatment with sodium oleate, sodium laurate, and sodium stearate, respectively, the surface of the film changes from hydrophilic to hydrophobic, which can be attributed to the aliphatic carboxylate films formed on the film surface. Scanning electron microscopy revealed that the unique microstructure of the precursor film can be inherited. The hydrophobic property of the sodium laurate and sodium stearate treated films can reduce the adsorption and adhesion of sulfate reducing bacteria and retard biofilm formation. The hydrophobic layered double hydroxide films obtained in the present work have excellent microbiologically influenced corrosion resistance activities that are due to the unique hydrophobicity microstructure and may be potentially be applied in microbiologically influenced corrosion resistance coatings. (C) 2015 Elsevier B.V. All rights reserved.

0603 疎水性微細構造のアスペクト比と接触角が気液界面形成に及ぼす影響

0603 疎水性微細構造のアスペクト比と接触角が気液界面形成に及ぼす影響

Surface Tension Flows inside Surfactant-Added Poly(dimethylsiloxane) Microstructures with Velocity-Dependent Contact Angles

Filling of liquid samples is realized in a microfluidic device with applications including analytical systems, biomedical devices, and systems for fundamental research. The filling of a disk-shaped polydimethylsiloxane (PDMS) microchamber by liquid is analyzed with reference to microstructures with inlets and outlets. The microstructures are fabricated using a PDMS molding process with an SU-8 mold. During the filling, the motion of the gas-liquid interface is determined by the competition among inertia, adhesion, and surface tension. A single ramp model with velocity-dependent contact angles is implemented for the accurate calculation of surface tension forces in a three-dimensional volume-of-fluid based model. The effects of the parameters of this functional form are investigated. The influences of non-dimensional parameters, such as the Reynolds number and the Weber number, both determined by the inlet velocity, on the flow characteristics are also examined. An oxygen-plasma-treated PDMS substrate is utilized, and the microstructure is modified to be hydrophilic. Flow experiments are conducted into both hydrophilic and hydrophobic PDMS microstructures. Under a hydrophobic wall condition, numerical simulations with imposed boundary conditions of static and dynamic contact angles can successfully predict the moving of the meniscus compared with experimental measurements. However, for a hydrophilic wall, accurate agreement between numerical and experimental results is obvious as the dynamic contact angles were implemented.

Drag Reduction by Hydrophobic Microstructures

We studied the mechanism of drag reduction due to textured hydrophobic surfaces in Newtonian laminar flow through a rectangular channel. The test wall surfaces were fabricated with different fine groove patterns and groove area ratios, and were then coated with PTFE to produce hydrophobic surfaces. Drag reduction was estimated by pressure loss measurements in a 12 × 12 mm channel. Visualization of the gas-liquid interface was carried out using a 0.5 × 5 mm microchannel to investigate the mechanism of drag reduction. A series of experiments showed that the gas-liquid contact area ratio and the air layer thickness influence the drag reduction, the maximum drag reduction ratio is 3.7 %.