Dual-scale Roughness Research Articles

Fabrication of polymer-based water-repellent surfaces of complex shapes by physical transfer of nanostructures

This work reports large-area fabrication of polymer [polydimethylsiloxane (PDMS), polycaprolactone (PCL) and polytetrafluoroethylene (PTFE)]-based superhydrophobic surfaces decorated with Cu(OH)2 nanowires. The process flow involves fabrication of nanostructured copper hydroxide surface (Cu(OH)2_NSS) by etching and coating the Cu(OH)2_NSS with a hydrophobic silane layer. This is followed by transferring the nanowires on to the polymer surface. The fabricated surfaces [which are Cu(OH)2 nanowires on PDMS: PDMS_Cu(OH)2, Cu(OH)2 nanowires on PCL: PCL_Cu(OH)2 and Cu(OH)2 nanowires on PTFE: PTFE_Cu(OH)2] exhibited dual scale roughness imparting very good water repelling behavior with a static contact angle greater than 150° and contact angle hysteresis of less than 10°. In addition to water repelling behavior, the fabricated surfaces exhibited bloodphobic behavior making them suitable for specific healthcare applications. Using the same technique, fabrication of superhydrophobic surface with complex 3D shape was also demonstrated. The same methodology was further adopted to fabricate a large-area (22 cm × 22 cm) superhydrophobic surface by replacing silane with stearic acid, which is a non-fluorinated surface modifying agent. This helped in reducing the cost and time of fabrication which is important when it comes to large-scale manufacturing.

One-step process for dual-scale ratchets with enhanced mobility of Leidenfrost droplets

HypothesisDroplet depositing onto hot surfaces above the so-called Leidenfrost temperature will float on a cushion of its own vapor. The vapor flow below the drop could be rectified by asymmetric surface textures, resulting the self-propelled droplet motion. Asymmetric structures like ratchets are used to rectify Leidenfrost droplet movement. Hence, it is possible to enhance the droplet mobility using surfaces with combined asymmetric macro/micro-structures. ExperimentsContinuous scale-like microcraters stacked end-to-end were fabricated on steel surfaces by wire electrical discharge machining (WEDM). The crater orientation always vectored towards the machining direction (MD), which oriented the droplet motion. Further, by integrating micro and macro-ratchets, dual-scale ratchets were constructed by one-step process using WEDM. The travelling velocities of Leidenfrost droplets on dual-scale and traditional single-scale ratchets were compared and the enhanced mechanism on dual-scale ratchets was analyzed. FindingsOne-step process was developed to fabricate transport platforms for Leidenfrost droplets, that continuous scale-like microcraters formed simultaneously on the macroratchets. The highest droplet travelling velocity was achieved compared to previous research. Further study shows that the enhanced drop mobility is attributed to the dual-scale roughness which endows a larger propelling force. This finding presents a high-efficiency method to fabricate transport platforms for Leidenfrost droplets.

Unprecedented scaling/fouling resistance of omniphobic polyvinylidene fluoride membrane with silica nanoparticle coated micropillars in direct contact membrane distillation

Recent development of omniphobic membranes shows promise in scaling/fouling mitigation in membrane distillation (MD), however, the fundamental understanding is still under dispute. In this paper, we report a novel omniphobic micropillared membrane coated by silica nanoparticles (SiNPs) (SiNPs-MP-PVDF) with dual-scale roughness prepared by a micromolding phase separation (μPS) and electrostatic attraction. This membrane was used as a model for analysis of scaling behavior by calcium sulfate (CaSO4) scaling and fouling behavior by protein casein in comparison with commercial (C-PVDF) and micropillared (MP-PVDF) membranes. Unprecedented scaling/fouling resistance to CaSO4 and casein was observed in direct contact membrane distillation (DCMD) for SiNPs-MP-PVDF membrane. Similar scaling and fouling occurred for commercial PVDF and micropillared PVDF membranes. The observation corresponds well to the wetting state of all membranes as SiNPs-MP-PVDF shows suspended wetting, but MP-PVDF shows pinned wetting. From a hydrodynamic view, the suspended wetting attributes a slippery surface which reduces the direct contact of foulants to solid membrane part and leads to significantly reduced fouling and scaling. However, a pinned (or metastable) wetting state leads to a stagnant interfacial layer that is prone to severe fouling and scaling. This work highlights that both scaling and fouling resistance are indeed of suspended wetting state and slippage origin.

Initiated chemical vapor deposition of poly(hexafluorobutyl acrylate) thin films for superhydrophobic surface modification of nanostructured textile surfaces

This study demonstrates the synthesis of poly(hexafluorobutyl acrylate) (PHFBA) thin films on textile substrates using initiated chemical vapor deposition (iCVD) method. Owing to its short perfluoroalkyl functional group, PHFBA is a suitable low-surface-energy finish that can be used for hydrophobic functionalization of textile surfaces. During iCVD of PHFBA, the use of initiator helped to achieve deposition rates up to 83 nm/min, which is nearly twice that achieved by PECVD. FTIR and XPS analyses of as-deposited films showed very high retention of carbonyl and perfluoroalkyl functionalities. Polyester and cotton fabrics were made superhydrophobic after conformal coating of PHFBA by iCVD. The decoration of fabric surfaces by SiO2 nanoparticles prior to the iCVD coating helped to create composite structures having dual-scale roughness. The final SiO2-treated and iCVD-PHFBA-coated textile fabrics showed very high water contact angles (> 165°).

Micro-nano textured superhydrophobic 5083 aluminum alloy as a barrier against marine corrosion and sulfate-reducing bacteria adhesion

Abstract Corrosion attack and undesired bio-adhesion are thorny issues threatening and restricting the service performance and security of the widely used structural 5083 aluminum alloys (AA5083) materials in maritime fields. In this paper, we employed a facile ammonia etching approach followed by 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES) modification to fabricate micro-nano textured superhydrophobic AA5083 surface as an effective layer for corrosion inhibition and bio-adhesion suppression. The dual-scale surface morphology and roughness were revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) were employed to depict the chemical components of the micro-nano textured AA5083 surface. X-ray photoelectron spectroscopy (XPS) was employed to confirm the presence of functional groups of fluorinated carbon with low surface energy. The self-cleaning ability and corrosion resistance of the fabricated surfaces were investigated, suggesting an excellent water-proofing and anti-corrosion performance. Moreover, sulfate-reducing bacteria (SRB) was chosen as a typical bacteria to evaluate and demonstrate the anti-adhesion property of the as-fabricated superhydrophobic AA5083 surface. The present study therefore demonstrates that achieving superhydrophobicity on AA5083 substrate is expected to possess quite potential applications in marine corrosion and bio-fouling fields.

Control of Polydimethylsiloxane Surface Hydrophobicity by Plasma Polymerized Hexamethyldisilazane Deposition

The properties of a polydimethylsiloxane (PDMS) surface were modified by a one-step deposition of plasma polymerized hexamethyldisilazane (pp-HMDS) by the arc discharge method. Scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectroscopy analytical techniques were employed for morphological, structural, and chemical characterization of the pp-HMDS modified PDMS surface. The changes in PDMS substrate wetting properties were evaluated by means of contact angle measurements. The unmodified PDMS surface is hydrophobic with a contact angle of 122°, while, after pp-HMDS film deposition, a dual-scale roughness PDMS surface with contact angle values as high as 170° was obtained. It was found that the value of the contact angle depends on the plasma processing time. Chemically, the pp-HMDS presents methyl moieties, rendering it hydrophobic and making it an attractive material for creating a superhydrophobic surface, and eliminating the need for complex chemical routes. The presented approach may open up new avenues in design and fabrication of superhydrophobic and flexible organosilicon materials with a self-cleaning function.

Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial Surface with Dual-Scale Roughness.

In this work, we report a large-area fabrication of a flexible superhydrophobic bactericidal surface decorated with copper hydroxide nanowires. This involves a simple two-step method which involves growth followed by transfer of the nanowires onto the polydimethylsiloxane (PDMS) surface by mechanical peeling. Additional roughness in PDMS is obtained through incomplete wetting of the nanoscale gaps which leads to dual-scale roughness and superhydrophobicity with a contact angle of 169° and hysteresis of less than 2°. The simplicity of the process makes it low-cost and easily scalable. The process allows fabrication of nonplanar 3D surfaces. The surface shows blood repellence and antibacterial activity against Escherichia coli with more than 5 log reductions in bacterial colony. The surface also shows hemocompatible behavior, making it suitable for healthcare applications. The fabricated surface is found to be extremely robust against stretching, twisting, sandpaper abrasion, solid weight impact, and tape peel test. The surface is found to withstand human weight multiple times without losing its hydrophobicity, making it suitable for several practical scenarios in healthcare and household applications.

Inkjet Printing Enabled Controllable Paper Superhydrophobization and Its Applications.

Papers' intrinsic interconnected porous structures and hydrophilic properties usually results in difficulty and complexity in partial functionalization and regulation processes because the capillary effect may lead to the fast diffusion of modifiers from one side to the other. Here, we report a simple and innovative inkjet printing approach that led to precise hydrophobic functionalization controllable in both planar and steric dimensions. Fabrication of Janus superwetting papers and superwettable patterned papers with high precision was achieved by computer-controlled inkjet printing. Elaborate controls of ink quantity enabled superhydrophobic functionalization on one side of the paper substrate, with the opposite side superhydrophilic. Static water contact angles up to 154° were obtained on the inkjet-printed side of the paper, thanks to an appropriate combination of surface chemistry with dual-scale surface roughness. Furthermore, paper-based microfluidics were fabricated and the resolution of which were estimated to be ca. 600 μm. Meanwhile, a paper-based analytical device for colorimetric sensing of Ni(II) was designed and demonstrated based on superwettable patterned papers by inkjet printing. The inkjet printing approach reported here represents a key step forward in fabricating Janus materials and complicate patterns for practical applications.

Superhydrophobic porous surface fabricated via phase separation between polyhedral oligomeric silsesquioxane-based block copolymer and polyethylene glycol

A superhydrophobic porous film was fabricated by a simple phase separation process between a polyhedral oligomeric silsesquioxane (POSS) based diblock polymer (P) and polyethylene glycol (PEG) in selective solvents. The POSS-based hybrid diblock polymer was synthesized using a two-step atom transfer radical polymerization method. The polymer, along with PEG, was dissolved in mixed solvents of tetrahydrofuran (THF) and ethanol (AC), and then dip-coated onto the glass slides to form a coating film. After PEG was washed away using water, the polymer film with nano-micrometer dual-scale roughness on the slides was formed due to the phase separation. Then, a superhydrophobic porous film was obtained after surface modification with low surface energy material. The effects of POSS content, P/PEG ratio, and AC/THF ratio were investigated. The results showed that the water contact angle increased with the PEG content within a certain range and the surface roughness was enhanced after adding AC. When the POSS content was 25.3% (P2), the ratio of P2/PEG was 5:5 and AC/THF was 3:7, the film after modification had a static water contact angle of 155.1° and a sliding angle of 6.5°, demonstrating superhydrophobicity.

Design of water-repellant coating using dual scale size of hybrid silica nanoparticles on polymer surface

The dual-scale size of the silica nanoparticles is commonly aimed at producing dual-scale roughness, also called hierarchical roughness (Lotus effect). In this study, we describe a method to build a stable water-repellant coating with controlled roughness. Hybrid silica nanoparticles are self-assembled over a polymeric surface by alternating consecutive layers. Each one uses homogenously distributed silica nanoparticles of a particular size. The effect of the nanoparticle size of the first layer on the final roughness of the coating is studied. The first layer enables to adjust the distance between the silica nanoparticles of the upper layer, leading to a tuneable and controlled final roughness. An optimal size nanoparticle has been found for higher water-repellency. Furthermore, the stability of the coating on polymeric surface (Polycarbonate substrate) is ensured by photopolymerization of hybridized silica nanoparticles using Vinyl functional groups.

A Hierarchically Porous Carbon Fabric for Highly Sensitive Electrochemical Sensors

The hierarchically porous carbon fabrics with controlled conductivity and hydrophilicity have been fabricated by dual templating method of soft templates nested on hard templates. A non‐woven fabric coated with a solution of F127/resol has been carbonized for the synthesis of both macro‐porous structures of 10–15 µm in diameter having meso‐porous carbon structures of 4–6 nm, respectively. After carbonization treatment, not only conductivity is significantly improved, the hierarchically porous carbon also shows superhydrophilicity or water‐absorbing nature due to mild hydrophilic material and its dual scale roughness. The porous carbon becomes conductive with resistivity widely tuned from 5.4 × 103 Ωm to 3.1 × 10−3 Ωm by controlling the carbonization temperature. As the increased wettability for organic liquids could lead organic molecules deep into carbonized fabrics, the sensitivity of hierarchically porous carbon fabrics benefits the detection for methanol(CH3OH) or hydrogen peroxide (H2O2). This new design concept of hierarchically porous structures having the multi‐functionality of high wettability and conductivity can be highly effective for electroanalytical sensors.

Facile, robust and large-scale fabrication method of mechanically durable superhydrophobic PDMS/aerogel coating on fibrous substrates

A simple method was developed for the large-scale fabrication of mechanically durable superhydrophobic surfaces with roughness on the microscale. The method is applicable for a large number of fibrous substrates with different pore sizes. With this method, it is possible to prepare polymeric fibrous substrates with a water droplet (5 μL) roll-off angle of less than 5°. The method developed is based on dip-coating of fibrous substrates in polydimethylsiloxane (PDMS)/toluene solution, followed by the electrostatic spray of aerogel microparticles on the samples. Depending on the PDMS solution concentration, it is possible to obtain a superhydrophobic surface with dual-scale roughness on the microscale. In this article, preparation and characterization of surfaces with electrostatic powder spray on the common fibrous cellulose substrates are provided. Fibrous surfaces obtained were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and static water contact angle measurement. Effects of PDMS/toluene solution concentration on the surface morphology and wettability behavior of surfaces were investigated. We demonstrate the mechanical durability of the surface using high-pressure air flow.

Tribological behavior and wettability of spray-coated superhydrophobic coatings on aluminum

Superhydrophobic surfaces that mimic surfaces found in nature, such as the lotus leaf, are an attractive research topic in various fields of study because of their numerous applications. More recent studies have focused on superhydrophobic surfaces that reduce or completely stop the accretion of ice and snow on power lines and aircraft that operate in cold regions. The superhydrophobic phenomena is usually achieved by creating a dual-scale roughness that is composed of micro- and nano-scale structures that trap air in-between themselves and reduce the surface energy of the textured surface. The objective of this study was to assess the tribological behavior of micro/nano particle based superhydrophobic coating mixtures composed of PTFE, composite PTFE/PEEK, diatomaceous earth (DE), and composite PTFE/ZnO that can be potential candidates for anti-wetting and anti-icing applications for transportation systems. A contact profilometer was used to measure and characterize the average roughness and thickness of coatings. Coating wettability was assessed by measuring the tangent-line contact angle of static water drops on coated surfaces. Friction and cyclic abrasive wear tests were conducted via ball-on-flat tribometer using a spherical tungsten probe at room temperature. Scanning electron microscopy was used to characterize the physical and chemical properties of the coatings and identify the wear mechanisms. The results showed that all coatings except ZnO/PTFE exhibited superhydrophobicity. Abrasive wear mechanisms were the dominant modes for the coatings. PTFE and ZnO/PTFE coatings displayed good wear resistance, superior to that of the DE and PTFE/PEEK coatings.

Building Dual-Scale Roughness Using Inorganic Pigments for Fabrication of Superhydrophobic Paper

This study reports the fabrication of superhydrophobic paper via a facile two-step dip coating method. Ground calcium carbonate (GCC) and montmorillonite (MTM) were used as inorganic pigment nanoparticles, and cellulose nanofibers were utilized as a binder to study the effect of forming a superhydrophobic surface on filter paper with alkyl ketene dimer (AKD) treatment. The micro GCC-treated paper showed a contact angle of 156.7 ± 2.4° but a relatively large tilt angle (∼15°). Counterintuitively, the nano MTM-treated paper surface was only hydrophobic with large hysteresis even after AKD sizing three times. Although none of the pigments by themselves could form good performance superhydrophobic coating on paper, by combining the two pigments, we built MTM nanostructures on top of the GCC micrometer-structures to form a surface with dual-scale roughness. The contact angle measurements showed that the MTM/GCC treated paper had a contact angle of 162.4 ± 3.3° and a tilt angle of less than 5°.

Bi-stage time evolution of nano-morphology on inductively coupled plasma etched fused silica surface caused by surface morphological transformation

In this work, we experimentally investigate the surface nano-roughness during the inductively coupled plasma etching of fused silica, and discover a novel bi-stage time evolution of surface nano-morphology. At the beginning, the rms roughness, correlation length and nano-mound dimensions increase linearly and rapidly with etching time. At the second stage, the roughening process slows down dramatically. The switch of evolution stage synchronizes with the morphological change from dual-scale roughness comprising long wavelength underlying surface and superimposed nano-mounds to one scale of nano-mounds. A theoretical model based on surface morphological change is proposed. The key idea is that at the beginning, etched surface is dual-scale, and both larger deposition rate of etch inhibitors and better plasma etching resistance at the surface peaks than surface valleys contribute to the roughness development. After surface morphology transforming into one-scale, the difference of plasma resistance between surface peaks and valleys vanishes, thus the roughening process slows down.

Influence of alkaline treatment on surface roughness and wetting properties of hydrophobized silk fabrics

The purpose of this research was to develop silk fabrics with dual-scale surface roughness, hydrophobicity, and self-cleaning properties. The surface roughness of 100% raw silk fabric was controlled by varying the temperature and time of degumming with NaOH solution to adjust the extent of sericin dissolution. Hydrophobization was carried achieved by depositing n-dodecyltrimethoxysilane on the silk fabric. The surface morphology, chemical composition, weight loss, tensile strength, luster, and color change of the fabric were measured, and the surface wettability was evaluated by water contact and shedding angle measurements. It was observed that the sericin on the fiber surface dissolved in the alkaline solution and formed nanoclusters. Consequently, because of the nanoscale roughness induced via the alkaline treatment, the contact angle increased from 0° to 152° after the hydrophobization, and the shedding angle decreased from 90° to 15°, indicating the superhydrophobicity of the resulting fabric. Therefore, alkaline treatment of silk fabric at 70℃ for 10 min was determined as the optimum condition for developing a self-cleaning silk fabric with an outstanding superhydrophobicity. The physical properties, such as tensile strength, luster, and color, were found to be the same even after the treatment.

Influence of micro and nano-scale roughness on hydrophobicity of a plasma-treated woven fabric

A superhydrophobic fabric surface was fabricated by forming a dual roughness structure in combination with lowered surface energy. The contribution of the innate micro-scale roughness resulting from the waviness of filaments and yarns in a woven fabric on hydrophobicity was investigated in comparison with a smooth film surface. Though the micro-scale roughness coming from the multi-filaments of fabric was conducive in enhancing the hydrophobicity of the surface, the micro-scale roughness itself was not enough to create superhydrophobicity. Thus a nano-scale roughness was introduced by an anisotropic etching employing oxygen plasma etching followed by plasma enhanced chemical vapor deposition. As for the nano-scale roughness, however, it was possible to achieve the superhydrophobicity only with nano-scale roughness, but with a very large aspect ratio of nano-pillar structure. In the presence of dual-scale roughness consisting of both micro- and nano-scale structures, the superhydrophobic characteristic was effectively achieved even at a small aspect ratio of nano-pillar. By adjusting the number of filaments in a yarn and by controlling the plasma process time, it was possible to control the dual-scale roughness of a woven fabric and its wettability. An excessive thinning and lengthening of nano-pillars may negatively affect the hydrophobicity by the collapse and aggregation of pillar tips, and an appropriate processing condition is critical to design a durable superhydrophobic surface.

Layered double oxide (LDO) particle containing photoreactive hybrid layers with tunable superhydrophobic and photocatalytic properties

Inorganic/organic hybrid layers have been prepared having superhydrophobic as well as photoreactive properties. The hybrid thin films with micro- and nanosized dual-scale surface roughness consist of ∼25μm layered double oxide (LDO) photocatalyst particles and low surface energy poly(perfluorodecyl acrylate) [p(PFDAc)] fluoropolymer binder material. The application of [p(PFDAc)] resulted in the decrease in the surface free energy of the hydrophilic LDO. The structured surface LDO with ∼12% ZnO phase content were synthesized from layer double hydroxide (LDH) spheres. The determined excitation wavelength and the calculated band gap energy values were 386nm and 3.23eV, respectively. The hybrid thin films were prepared by a simple spray-coating method, which is a low-cost, fast and scalable film-forming technique. The surface roughness and also the wetting properties of the two-component hybrid layers proved to be finely adjustable by the LDO:fluoropolymer ratio. It was found that at 80–90wt% LDO content, the thin films with a surface free energy value of ∼12mJ/m2 displayed superhydrophobic behaviour (Θ>150°) with satisfactory photocatalytic properties. This means special photoreactive surfaces with superhydrophobic properties instead of the conventional superhydropilic photocatalyst layers. According to the benzoic acid photodegradation test experiments of benzoic acid, the hybrid layers with 80–90wt% LDO content photooxidized 22–24% of the initial test molecule concentration (0.17g/L) under UV-A (λmax=365nm) illumination.

Superhydrophobic and self-cleaning surfaces prepared from a commercial silane using a single-step drop-coating method

Abstract A simple and time-saving approach for preparing superhydrophobic and self-cleaning surfaces from a commercial silane, Octadecyltrichlorosilane (ODTS) is reported in this study. A solution of the silane was prepared by dissolving in hexane. Prior to film deposition, the substrates were treated with oxygen plasma etching to remove organic contaminants from the surface followed by immediate immersion in deionized (DI) water to form hydroxyl groups. A few drops of the prepared silane solution were then placed on the treated surface and allowed to dry under ambient conditions. The modified surfaces exhibited superhydrophobic behavior with advancing water contact angles (WCA) in the range 160–170° and the contact angle hysteresis (CAH) less than 10°. Surface characterization with scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed the presence of dual-scale roughness. Dust particles readily adhered to and were washed away with the rolling water droplets placed on the deposited film. Furthermore, the resulting surface was able to maintain these characteristics under the impact of a water jet. It should be mentioned that the demonstrated efficiency of this method makes it very feasible candidate for preparing self-cleaning surfaces for a variety of industrial applications.

Water-resistant surfaces using zinc oxide structured nanorod arrays with switchable wetting property

Abstract This study presents an experimental approach for fabricating super-hydrophobic coatings based on a dual roughness structure composed of zinc oxide nanorod arrays coated with a sputtered zinc oxide nano-layer. The ZnO nanorod arrays were grown by means of a low-temperature electrochemical deposition technique (75 °C) on FTO substrates. The ZnO nanorods show a (002) orientation along the c-axis, and have a hexagonal structure, with an average length of 710 nm, and average width of 156 nm. On the other hand, the crystallite size of the top-coating sputtered ZnO layer is of 30 nm. The as-deposited ZnO nanorod arrays exhibited a hydrophobic behavior, with a surface water contact angle of 108°, whereas the dual-scale roughness ZnO nanorods coated with sputtered ZnO exhibited a super-hydrophobic behavior, with a surface water contact angle of 157° and a high water droplet adhesion. The photo-catalytic activity of the samples was investigated against the degradation of methylene blue under UV-A irradiation (365 nm). The ZnO nanorod arrays showed good photocatalytic activity whereas the superhydrophobic ZnO nanorod arrays top-coated with sputtered ZnO showed minimal activity regarding the degradation of methylene blue. The superhydrophobic films exhibited high sensitivity to UV light, with a UV-induced switching behavior from super-hydrophobic to super-hydrophilic after only 30 min of UV exposure.