Diverse Wettability Research Articles

3D Printed Bioinspired Hierarchical Surface Structure with Tunable Wettability

Abstract Nature has examples of impressive surfaces and interfaces with diverse wettability stemming from superhydrophilicity to superhydrophobicity. The multiscale surface structures found in biological systems generally have high geometric complexity, which makes it challenging to replicate their characteristics, especially using traditional fabrication techniques. It is even more challenging to fabricate such complex microstructures with tunable wettability. In this paper, we propose a method to tune the wettability of a micro-scale surface by changing the geometrical parameters of embedded micro-structures in the surface. By taking inspiration from an insect (springtails), we designed micropillar arrays with different roughness by adjusting geometric parameters such as reentrant angle, pitch distance, and the number of spikes and pillars. This study shows that, by changing geometrical parameters in microscale, the apparent contact angle, and hence the surface wettability can be calibrated. The microscale pillars were fabricated using a precise micro direct light processing (uDLP) three-dimensional (3D) printer. Different printing parameters were studied to optimize the geometric parameters to fabricate 3D hierarchical structures with high accuracy and resolution. By controlling the size of different features of the pillar, pillar number, and layout of the mushroom-shaped micropillars, the wettability of the surface is possible to be tuned from a highly nonwetting liquid/material combination to highly wetting material. Such wettability tuning capability expands the design space for many biomedical and thermofluidic applications.

Self‐Adaptive Surface with Tunable Superamphiphobicity Fabricated by Femtosecond Laser Induced Backward Transfer

AbstractIn recent years, a rapid advance in fabricating smart surfaces with tunable wettability have been witnessed. However, switchable superamphiphobic surface remains a great challenge today. Here, glass surfaces with tunable wetting performance between amphipathicity and amphiphobicity, “lotus effect” and “petal effect,” isotropic and anisotropic amphiphobicity are obtained by femtosecond‐laser‐induced backward transfer (LIBT) method. By printing simple patterns, polytetrafluoroethylene (PTFE) is spatial‐selectively transferred with composite micro/nano‐structures formed on the glass surfaces, ensuring a strong repellence for water and oil. Besides, the surface topography and chemistry can be finely regulated in a single‐step process without damage to the glass, which provides the feasibility of controlling the surface wettability. Herein, applications such as high precision mixing reactor, droplet volume screening micro‐device, auto‐filtering droplet channel are designed and demonstrated. Compared to other methods, the LIBT method featuring rapid, large‐area, maskless prototyping is proved to be a promising method for programmable manipulations of functional surfaces with diverse wettability.

Controlled release fertilizers coated by alkylamine-poly (tannic acid) building block with tunable wettability via spraying co-deposition

The wettability of the coating used in controlled release fertilizers (CRFs) is a crucial parameter for the release behavior of nutrients. However, how to control the wettability of coating to tune nutrient release is a big challenge. Herein, the alkylamine-poly (tannic acid) (APTA) building blocks coatings with controlled wettability were prepared as coating materials for CRFs. Alkylamine (octadecylamine (ODA), hexadecylamine (HDA), dodecylamine (DDA), and hexylamine (HEA)) were applied for the modification of poly (tannic acid) (PTA) coating on the silicon wafer and urea surface. The wettability of coatings on silicon wafers were studied and nutrient release of APTA coated fertilizers were characterized in water and soil. The nitrogen nutrient and water vapor permeability coefficient of coatings were characterisized and the mathematical model for predicting the release of nutrients in soil. The alkyl chain length of alkylamine is demonstrated to play a crucial part in controlling the wettability. The rate of nutrient release increases in the order of ODA-PTA < HDA-PTA < DDA-PTA < HEA-PTA coated fertilizer. We envision that APTA building blocks with diverse wettability on urea surface could control the release rate of the nutritional content of fertilizer, which has potential application in the development of controlled-release fertilizers.

Gyroscopic rotation of boiling droplets

The self-propelled motion of boiling droplets has attracted strong interest, and major discoveries are concentrated in the film boiling regime, e.g., translational motion of Leidenfrost drops on ratchets, Leidenfrost wheels. However, little attention was paid to the boiling regimes below the Leidenfrost point. Here, we focus on those boiling regimes and discover a gyroscopic rotation phenomenon of boiling droplets that is ubiquitous on various types of surfaces with diverse wettability and microstructures. The occurrence of gyroscopic rotation can be attributed to the viscous stress from vapor/bubble flows in the gaps of surface microstructures, verified by the results that for the experimental surfaces, the rougher the surface structures and the larger the solid–liquid contact area, the more probable it is to generate gyroscopic rotations. A theoretical model is established to investigate the effect of substrate temperature (boiling regime) on the spinning rate of boiling droplets, and the results further approve the proposed mechanism of gyroscopic rotation. The outcomes of this work help to deepen the understanding of droplet boiling and the corresponding dynamics on surfaces with microstructures.

The Effect of Surface Roughness on Immiscible Displacement Using Pore Scale Simulation

Pore scale immiscible displacement is crucial in oil industry. The surface roughness of throat is an important factor affecting water–oil interface movement. In this paper, the Navier–Stokes (N-S) equation coupled with the phase-field method is adopted to analyze the oil–water flow and interface movement in single channel, considering different surface roughness, diverse wettability and various capillary numbers. The simulation results show that the resistance increases significantly due to the surface roughness. The velocity of interface movement in rough channels is slower than that in smooth channels. The existence of asperities strengthens the interface deformation and promotes the formation of fingering phenomenon. The water flooding process presents different flow patterns in the rough channel with diverse wettability, and the influence of wall roughness on oil–water interface movement is different under various wettability conditions. There is an approximate exponential relationship between the ratio of interface length to channel length and capillary number. When the capillary number exceeds 0.03574, the phenomenon of viscous fingering is obvious, and the influence of capillary number is amplified by roughness.

Sessile Microdrop Coalescence on Partial Wetting Surfaces: Effects of Surface Wettability and Stiffness.

We experimentally investigated the coalescence of two sessile microdrops on rigid surfaces with diverse wettability (macroscopic apparent water contact angles of θapp ≈ 13-110°) and on hydrophobic surfaces (θapp ≈ 110-124°) with very different stiffness properties (Young's moduli of E ≈ 1.1 MPa to 130 GPa). We show that the coalescence contains two fast regimes, in which a liquid meniscus bridging the parent droplets rapidly grows, forming a hemi-ellipsoidal droplet, and a slow regime, in which the merged hemi-ellipsoidal droplet relaxes to the equilibrium hemispherical cap. Whereas the fast bridging regimes last less than 2 ms and are almost independent of surface wettability and stiffness, the relaxation regime, which was only observed on sufficiently hydrophobic and rigid surfaces with low wetting hysteresis, continues for a few tens to several hundreds of milliseconds depending on surface properties. We further demonstrate that the slow droplet relaxation can be described neither by the bulk hydrodynamics nor by a microscopic model concerning liquid evaporation near the droplet edge, but by the molecular kinetic theory for the motion of the three-phase contact line.

Fabrication of Wettability Mesh with Quasi-Rectangular-Restraining Capacity to Water.

A water droplet placed on a surface is usually round owing to surface tension. Restraining a droplet to a rectangle shape has been rarely reported. Herein, we fabricated three meshes with diverse wettability including ordinary mesh, superhydropilic mesh, and quasi-rectangular-restraining mesh. The profiles of water droplets on these three meshes were entirely different from the top view, especially for the quasi-rectangular-restraining mesh, which enables the water droplet on it to achieve the rectangular shape. The surface morphologies and chemical compositions of the meshes were characterized by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Moreover, the influences of processing parameters of the quasi-rectangular-restraining mesh on the quasi-rectangular quality of the water droplet on it were investigated to obtain the relatively optimum processing parameters. The dynamic properties of water droplets on the three meshes were compared, and forces acting on the water droplets during the spreading and shrinking processes on the three meshes were qualitatively analyzed. Additionally, we studied the influences of falling height and water volume on the quasi-rectangular quality of the water droplet on the quasi-rectangular-restraining mesh. Water droplets on the quasi-rectangular-restraining mesh demonstrated good stability under vibration and the droplet could maintain the quasi-rectangular quality on the quasi-rectangular-restraining mesh for about 7 days, revealing a good durability. Further, the large-scaled fabrication of the quasi-rectangular-restraining mesh was realized.

Designing Assembly of Meshes Having Diverse Wettability for Reducing Liquid Ejection at Terminal Velocity Droplet Impact

In this paper, we have studied the effect of surface wettability and geometrical parameters on the velocity of the ejecting jet during droplet impacts on meshes. This paper presents detailed experiments and modeling which helps in explaining a previous demonstration, where an assembly of meshes with different wettabilities was used to significantly reduce the amount of liquid leakage during impact of droplets travelling at terminal velocity. Drop impact on meshes at various Weber numbers (We) was captured using high-speed imaging. At higher We, the velocity of the ejected liquid jet (Vi) was found to be higher than the velocity of the impacting droplet (Vd) for most of the superhydrophobic meshes. In contrary, for all values of We the velocity of the ejected liquid through superhydrophilic meshes was lower than the droplet velocity for all meshes. While jet width for superhydrophobic mesh was close to the pore opening dimension, in case of superhydrophilic mesh merging of the ejected jets meant that the width was close to the impacting droplet diameter. A combination of superhydrophilic and superhydrophobic mesh was used to significantly reduce liquid ejection for droplet impact at terminal velocity (~8 m/s). Less than 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> % volume ejection was observed for the designed combination. This configuration can be used to fabricate rain repellent air-transparent surfaces.

Diverse wettability of superoleophilicity and superoleophobicity for oil spill cleanup and recycling

The frequent marine oil spill accidents in nowadays has aroused great attention all over the world. Lots of superwetting absorption materials with 2D or 3D structures are fabricated to efficiently remove oil spill, but the desorption of the absorbed oil is more difficult. Oil contaminants adhere on the surface will decline the performance and reusability of the absorption materials. Discarding or burning them will bring secondary pollution. Faced with these problems, we prepared an oil extractor integrated by a superhydrophobic-superoleophilic stainless steels wire mesh and a 3D porous cellulose sponge with superhydrophilicity-superoleophilicity in air and superoleophobicity in water. The oil extractor can in situ collect oil spill no matter on the water or under the water. More importantly, it is good at underwater oil desorption, which ensure the recovery and reuse of oil, and meanwhile avoid the materials being fouled by oils. More than 85.5% of crude oil could be successfully discharged only in 2min when the oil-contaminated sponge was placed in water. The findings in this work not only put forward a new strategy for collection and reuse of various “oils”, but also offer a facile method to achieve a combination of two kinds of materials with diverse wettability.

Development of Dual-Phobic Surfaces: Superamphiphobicity in Air and Oleophobicity Underwater

In the present work, we describe a simple method to fabricate dual-phobic fluorosilane-coated polydimethylsiloxane/camphor-soot/polydimethylsiloxane (FPCP) composite surfaces. The surface morphology and silane treatment provide the needed texture on the FPCP composite surface to demonstrate superamphiphobicity in the air and oleophobicity in the underwater environment. High-resolution field emission-scanning electron microscopy (FESEM) imaging of the FPCP composite surface illustrates the top surface with an array of hollow cylindrical pillars. The dimensions of surface texture are measured, and the relationship between the wetting states of liquid and textures surface in the air as well as in an underwater environment is studied. Also, X-ray photoelectron spectroscopy (XPS) analysis demonstrates different changes of plain polydimethylsiloxane (PDMS), PDMS/camphor-soot/PDMS (PCP), and fluorosilane-coated PCP (FPCP) composite surfaces that are responsible for diverse wettability properties. We compared the...

Stepwise anodic electrodeposition of nanoporous NiOOH/Ni(OH)2 with controllable wettability and its applications

A nanostructured surface with diverse wettability which has the properties of superhydrophilicity, superhydrophobicity and distinguished wetting contrast is important to several key technologies. To meet these requirements, a facile and cost-effective approach via the electrochemical fabrication of a nano-sponge-like nickel oxyhydroxide/nickel hydroxide film on conductive glass followed by the application of external stimuli or chemical treatments is presented. The external stimuli of an environmental chamber and UV/ozone can dynamically and selectively control the aspect of wettability between hydrophobic and superhydrophilic in a short period. In addition, the aid of a chemical treatment with self-assembled non-fluorinated silane on annealed nickel hydroxide film can result in superhydrophobicity without degradation of optical transparency. The effectiveness of these achieved functions is demonstrated using transparent 2D microfluidic channels and direct selective electroplating.

A novel method to control hydrolytic degradation of nanocomposite biocompatible materials via imparting superhydrophobicity

Acceleration of hydrolytic degradation of biomedical materials is not always desirable. For instance, terpolymers based on l-lactide, glycolide and trimethylene carbonate exhibit very fast hydrolytic degradation due to their amorphous structure, hydrophilicity, and high water absorption capability. Therefore, an attempt was made in the current study to impede the hydrolytic degradation for these materials through imparting superhydrophobicity to their surfaces. The used terpolymer has been shown to have promising potential applications as bio-absorbable surgical sutures and other biomedical materials, and thus, its applicability could be further extended upon impeding its hydrolytic degradation. Moreover, a novel method including combined use of non-solvent and nanoparticles was utilized to achieve superhydrophobicity. Very diverse wettability results were obtained which were attributed to the obtained various morphologies according to scanning electron microscopy results. More importantly, a unique hierarchical morphology was found to be responsible for the observed water repellent behavior. X-ray photoelectron spectroscopy results revealed co-existence of nanosilica particles and terpolymer chains on the surface's top layer. Finally, it was found that the superhydrophobic sample exhibited a significantly impeded hydrolytic degradation as compared with the hydrophilic pure terpolymer which was attributed to the formation of air pockets on the surface's top layer.

Biological effects of sol-gel derived ZrO2 and SiO2/ZrO2 coatings on stainless steel surface--In vitro model using mesenchymal stem cells.

The objective of this study was to determine biocompatibility of zirconia-based coatings obtained by the sol-gel method. Two matrices, ZrO2 and SiO2/ZrO2, were created and applied on stainless steel type 316L with dip-coating technique. The morphology and topography of biomaterials' surface were characterized using energy-dispersive X-ray spectroscopy and atomic force microscopy, while chemical composition was analyzed by Raman spectroscopy. Additionally, wettability and surface free energy were characterized. Biocompatibility of obtained biomaterials was evaluated using an invitro model employing mesenchymal stem cells (MSCs) of adipose and bone marrow origin. Biological analysis included determination of proliferation activity and morphology of MSCs in cultures on synthesized biomaterials. Osteoinductive properties of biomaterials were determined both in non-osteogenic, as well as osteogenic conditions. The results showed that investigated biomaterials exerted different impact on MSCs. Biomaterial with ZrO2 layer was more biocompatible for adipose-derived MSCs, while SiO2/ZrO2 layer promoted proliferation of bone marrow derived MSCs. Moreover, hybrid coating exhibited greater osteoinductive properties than ZrO2 coating, both on cultures with adipose-derived stromal (stem) cells and bone marrow stromal cells. Observed biological effects may result not only from different chemical composition, but also from diverse wettability. The ZrO2 coating was characterized as hydrophobic layer, while SiO2/ZrO2 exhibited hydrophilic properties. The results obtained suggest that behavior of MSCs in response to the biomaterial may vary depending on their origin, therefore we postulate, that screening analysis of implants' biocompatibility, should incorporate model applying both adipose- and bone marrow derived MSCs.

Facile preparation of poly(ethyl α-cyanoacrylate) superhydrophobic and gradient wetting surfaces

Hollow microspheres of poly(ethyl α-cyanoacrylate) were prepared via vapor phase polymerization using micro-waterdroplets as template and initiator. Depending on the ratio of the shell thickness to the radius, the hollow microspheres would crimple to form either microballoons or microcups during drying. These two types of microparticles were used as building blocks to construct surfaces with diverse wettability. The microballoons linked up to form a porous-netlike surface which was rough enough to render the surface superhydrophobic, while the microcups-built surface showed less hydrophobicity. In addition, surfaces consisting of both microparticles with gradual decrease of roughness along the length direction were obtained, which presented gradient wetting property varied from superhydrophobic to hydrophobic. The as-formed superhydrophobic or gradient wetting surfaces may find potential applications in biomedical field because of the biocompatibility of poly(ethyl α-cyanoacrylate).

Hydroxyapatite nanoceramics: Basic physical properties and biointerface modification

A new method of surface energy modification and engineering of the hydroxyapatite (HAp) nanoceramics coatings is presented. It is performed by electron-induced surface energy modification resulting in deep and tunable variation of its wettability state. It is found from electronic traps state spectroscopy studies of the HAp ceramics implemented by various methods such as photoluminescence and surface photovoltage spectroscopy, that the HAp nanoceramics is a wide band gap p-type semiconductor with complex structure of electron/hole bulk and surface localized states. It is shown that a low-energy electron irradiation leads to surface potential modulation and provides tailoring any wettability state in a wide range of contact angles by variation of injected and trapped electron charge. The diverse wettability states engineered on the HAp surface enable selective adhesion of basic biological cells such as proteins, DNA and various bacteria.