Superhydrophobic Micro Research Articles

Mechanical-robust and polymer-based superhydrophobic coating toward self-cleaning and anti-corrosion

The corrosion of reinforced concrete materials severely threatens cross-sea bridges, buildings and marine ships. The unique air layer on the superhydrophobic coating surface can effectively minimize the reinforced concrete corrosion, which not only extends the long-term usability of concrete but also reduces economic losses and environmental pollution. It is necessary to design a superhydrophobic coating on substrate surfaces to accommodate the requirement for corrosion prevention due to their anti-corrosion and self-cleaning properties. Despite gradually developing superhydrophobic coatings to meet diverse needs, they still encounter limitations in engineering preparation and applications. For example, the rough micro-/nano structure of the superhydrophobic surface is easily damaged by mechanical impact or physicochemical damage, which could affect the antiseptic properties. Moreover, designing and preparing all polymer-based superhydrophobic micro-/nano structures using amorphous elastomer materials is vital for enduringly industrial applications, yet it remains a challenge. Here, we design and fabricate a mechanical-robust superhydrophobic coating composed of amorphous polyurethane (PU) and epoxy resin modified with amino-terminated polysiloxane (APT-PDMS) via a scalable atomization spraying method. A quartz sands impact abrasion test evaluates the mechanical durability of the PU superhydrophobic coating (PU-SC). The superficial PU particles can form elastic micro-protuberances, absorbing impact and preventing fracture through elastic deformation during friction. Furthermore, the as-prepared PU-SC can also resist some chemical mechanical damage, including acids/bases/salts solution corrosion, UV radiation, knife scraping treatment, 3M tape-stripping, water flow impact, outside sunlight shining treatment and temperature treatment. Notably, the robust PU-SC can be efficiently produced on a large scale and applied to prevent electrochemical corrosion in a simulated seawater environment. The corrosion protection efficiency (PE) can reach 98.1%. We believe that the design and development of a mechanical-robust and polymer-based superhydrophobic coating has the prospect of actual application on marine corrosion.

A Fluorine-Free Superhydrophobic Micro–Nano Structured SiO2/TiN Coating for Anti-icing and Photothermal Deicing

A Fluorine-Free Superhydrophobic Micro–Nano Structured SiO₂/TiN Coating for Anti-icing and Photothermal Deicing

Stability of the flow over superhydrophobic micro roughnesses: The influence of the interface

Superhydrophobic surfaces are known for their drag reduction properties. However, the interface between the lubricant and the overlying flow may easily become unstable, leading to the depletion of the superhydrophobic layer and to a consequent drag increase. In this paper, we investigate the modal and non-modal instability of the flow over longitudinal trapezoidal superhydrophobic riblets, including, for the first time, the gas/liquid interface dynamics in the stability analyses. A two-dimensional stability problem, obtained with a domain transform technique and interface modelling using a linearised Young–Laplace equation, is coupled with the n−periodic stability framework introduced by Schmid et al. (2017). The latter technique, using a Bloch wave formalism, allows the computation of the stability of an array of n riblet units of given periodicity at a reasonable numerical cost. For small periodicities, the most unstable mode is a fundamental instability stemming from the three-dimensionalisation of a Tollmien–Schlichting wave. Conversely, in the case of large riblet periodicities, a subharmonic mode linked to capillarity effects becomes the most unstable. Nonmodal transient growth analysis shows that the superhydrophobic riblets have a weak effect on the overall growth. However, riblets having small periodicities induce a slight stabilisation of the flow, while large ones induce an increase of the energy growth on detuned perturbations. The resulting energy growth mechanism induces interface deformations encompassing more than one subunit.

Tuning the Topography of Non‐Wetting Surfaces to Reduce Short‐Term Microbial Contamination Within Hospitals

AbstractMicrobial contamination of hospital surfaces is a major contributor to infectious disease transmission. This work demonstrates that superhydrophobic (Cassie‐Baxter) micro post topographies can significantly reduce cell attachment compared to flat controls. For ordered micro post arrays (post diameters 0.3 to 150 µm), the attachment of four pathogens (Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, and Candida albicans) from discrete contaminant droplets upon short‐term contact (15 s to 30 min) are assessed. There is a 3‐4‐log decrease in microbial attachment when reducing the micro posts diameters from 150 to 0.3 µm for all strains, with large posts (>20 µm) exhibiting similar attachment rates to flat controls. The critical, maximum feature size to prevent attachment can be tuned depending on the ratio of the cell size to post diameter. Two potential mechanisms are discussed for this size effect. First, application of the random sequential adsorption model shows that this relative post/cell size effect may be due to a reduced probability of attachment, which is theorized to be the dominant mechanism. Alternatively, a physical model is suggested for bacterial cell “pull‐off” due to surface tension forces during droplet dewetting. This work may be important for the design of non‐wetting antimicrobial surfaces within healthcare environments.

Multifunctional coatings fabricated from Chinese hemp–derived superhydrophobic micro–nanocellulose

Ecologically feasible strategies for constructing superhydrophobic surfaces offer versatile applications in waterproofing, self–cleaning, selective absorption, and corrosion protection. Herein, we prepared low–surface–energy branched–chain–enriched micronanorod (F@SiO2@MNC) by hydrolyzing silane coupling agent and modifying fluoropolymer using micro–nanocellulose extracted from waste straw (Chinese hemp). These rods were sprayed and adhered to various substrates precoated with a binder, resulting in superhydrophobic surfaces. F@SiO2@MNC addition allowed for the formation of stable spherical liquid droplets when in contact with different types of aqueous liquids. Furthermore, these surfaces demonstrated excellent self–cleaning, robustness, abrasion resistance, UV resistance, cycling stability, and other multifunctionalities. They significantly enhanced the mechanical properties of filter paper, effectively separated oil water mixtures, and improved the corrosion resistance of metals. Our proposed strategy represents a novel approach for developing multifunctional coatings assembled from micronanocellulose.

Experimental investigation on hydrophobic/superhydrophobic micro patterns: New manufacture method and performance

Hydrophobic/superhydrophobic micro patterns have broad application prospect due to their excellent water repellent ability. In order to manufacture hydrophobic micro patterns on metallic surface, a new flexible and high-efficient method called oscillation-loaded dynamic micro embossing (DME) was developed. In this work, the manufacture process and hydrophobic performance of micro patterns were experimentally investigated. Firstly, the oscillation-loaded DME process parameters designing method was provided to fabricate micro patterns with target dimensions. Then, pure copper workpiece with micro parallel channels and cubes array with different feature width were obtained through DME experiments. The experimental results of the geometrical morphology showed that dimensions of micro patterns were close to the theoretical value, demonstrating the effectiveness of the process parameters designing method. In addition, it was revealed that the roughness of the micro pattern top surface was enhanced significantly by increasing grain size and reducing punch width. Furthermore, the hydrophobic performance of the manufactured parallel channels and cubes array were investigated through water contact angle ( WCA ) test. It was demonstrated that the WCA of pure copper workpiece was improved significantly by the fabricated micro parallel channels and cubes array. Moreover, it was found that the WCA can be improved by reducing the feature width for both the parallel channels and cubes array. Lastly, superhydrophobic micro patterns with WCA up to 161.4° were obtained by utilizing the new oscillation-loaded DME process and plasma surface treatment.

Fabrication of sepiolite-based super-hydrophobic stainless steel mesh for enhanced stability and high efficiency oil-water separation

As one of natural nano-clay material, sepiolite is easy to modify to obtain low surface energy, which has great potential in the field of superhydrophobicity application. A novel superhydrophobic micro nanofibers (CSEP/OTS) were prepared by grafting long alkyl chains onto organic activated sepiolite fibers (CSEP) by using octadecyltrichlorosilane (OTS) and tetraethylorthosilicate (TEOS). Then, the superhydrophobic sepiolite-based mesh (SSM-SHSEP) was prepared by spraying a mixed suspension of CSEP/OTS and epoxy (EP). Owing to the excellent chemical and physical resistance of sepiolite, epoxy and stainless steel mesh, the prepared mesh exhibits superior robustness and high stability. As the amount of CSEP/OTS increased, the mesh turned from hydrophobic (pristine mesh, WCA = 133.9°) to superhydrophobic (when EP: CSEP/OTS = 1:1.5, WCA = 153.6°). The screened mesh can be an effective tool for separating different oil-water mixtures (reached up to 97.3%). Furthermore, the coated mesh exhibited remarkable reusability (approximately 50 cycles)，excellent corrosion resistance (pH from 1 to 13), and distinguished durability (20 cycles of sandpaper abrasion test). All these outstanding performances of superhydrophobic sepiolite-based mesh confirmed that it can be widely used in oil-water separation and thereby showed great potential in large-scale industrial applications.

Facile fabrication of micro-/nanostructured, superhydrophobic membranes with adjustable porosity by 3D printing.

Porous membranes with special wetting properties have attracted great interest due to their various functions and wide applications, including water filtration, selective oil/water separation and oil skimming. Special wetting properties such as superhydrophobicity can be achieved by controlling the surface chemistry as well as the surface topography of a substrate. Three-dimensional (3D) printing is a promising method for the fast and easy generation of various structures. The most common method for 3D printing of superhydrophobic materials is a two-step fabrication process: 3D printing of user-defined topographies, such as surface structures or bulk porosity, followed by a chemical post-processing with low-surface energy chemicals such as fluorinated silanes. Another common method is using a hydrophobic polymer ink to print intricate surface structures. However, the resolution of most common printers is not sufficient to produce nano-/microstructured textures, moreover, the resulting delicate surface micro- or nanostructures are very prone to abrasion. Herein, we report a simple approach for 3D printing of superhydrophobic micro-/nanoporous membranes in a single step, combining the required topography and chemistry. The bulk porosity of this material, which we term “Fluoropor”, makes it insensitive to abrasion. To achieve this, a photocurable fluorinated resin is mixed with a porogen mixture and 3D printed using a stereolithography (SLA) printing process. This way, micro-/nanoporous membranes with superhydrophobic properties with static contact angles of 164° are fabricated. The pore size of the membranes can be adjusted from 30 nm to 300 nm by only changing the porogen ratio in the mixture. We show the applicability of the printed membranes for oil/water separation and the formation of Salvinia layers which are of great interest for drag reduction in maritime transportation and fouling prevention.

Fabrication of Superhydrophobic Micro Post Array on Aluminum Substrates Using Mask Electrochemical Machining

Surfaces with controllable micro structures are significant in fundamental development of superhydrophobicity. However, preparation of superhydrophobic surfaces with array structures on metal substrates is not effective using existing methods. A new method was presented to fabricate super-hydrophobic post arrays on aluminum (Al) substrates using mask electrochemical machining and fluoridation. Electrochemical etching was first applied on Al plates with pre-prepared photoresist arrays to make the post array structures. Surface modification was subsequently applied to reduce the surface energy, followed by interaction with water to realize superhydrophobicity. Simulation and experimental verification were conducted to investigate how machining parameters affect the array structures. Analysis of the water contact angle was implemented to explore the relationship between wettability and micro structures. The results indicate that superhydrophobic surfaces with controllable post structures can be fabricated through this proposed method, producing surfaces with high water static contact angles.

Superhydrophobic Breakdown of Nanostructured Surfaces Characterized in Situ Using ATR-FTIR.

In situ characterization of the underwater stability of superhydrophobic micro- and nanostructured surfaces is important for the development of self-cleaning and antifouling materials. In this work, we demonstrate a novel attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy-based method for large-area wetting characterization of silicon nanopillars. When air is present in between the structures, as is characteristic of the Cassie-Baxter state, the relative intensities of the water bands in the absorption spectrum change because of the wavelength-dependent attenuation of the evanescent wave. This phenomenon enables unambiguous identification of the wetting state and assessment of liquid impalement. Using mixtures of isopropanol and water with different concentrations, the breakdown of superhydrophobic states and the wetting hysteresis effects are systematically studied on uniform arrays of silicon nanopillars. A transition from the Cassie-Baxter to Wenzel state is observed when the isopropanol concentration exceeds 2.8 mol %, corresponding to a critical surface tension of 39 mN/m. Spontaneous dewetting does not occur upon decreasing the isopropanol concentration, and pure water can be obtained in a stable Wenzel state on the originally superhydrophobic substrates. The developed ATR-FTIR method can be promising for real-time monitoring of the wetting kinetics on nanostructured surfaces.

Experimental and theoretical investigations on the flow resistance reduction and slip flow in super-hydrophobic micro tubes

Super-hydrophobic micro tubes with contact angles larger than 150° on inner walls are obtained by solidifying a hydrophobic solution prepared by adding 2% perfluorinated octyl fluorine silane and other additives to modified silicone dilute solution, and diameters of micro tubes are 0.447mm, 0.728mm and 0.873mm, respectively. The changing coefficients of the pressure drops and friction factors in micro tubes due to the super-hydrophobic processing are experimentally measured with Reynolds number ranging from 0 to 2500. A theoretical method is employed to obtain the theoretical correlations of the slip length and velocity in super-hydrophobic micro tubes, so the relationships between the flow resistance reduction and slip flow are quantitatively analyzed. The results illustrate that the changing coefficient of the pressure drop and the friction factor reaches over 60% due to super-hydrophobic surfaces, but the reduction of the pressure drop decreases gradually with the increase of Re and the decrease of tube diameter. The theoretical predictions to changing coefficients of pressure drops and friction factors are consistent with the experimental results. Moreover, the reduction rate of slip length and slip velocity with the increase of pressure and Re is the dominant factor to decide the total resistance reduction in super-hydrophobic micro tubes.

Customizable local superhydrophobic micro channels by BNNTs growth

The local superhydrophobic micro channels were fabricated by BNNTs growth with the method of ball milling assisted with B ink annealing. Both the ball milling and annealing processes in the preparation were optimized to obtain high quality and density of BNNT films. The structure of anode plate is designed to synthesize BNNTs selectively under control. Besides, the customizable growth by the calculation of flow model in COMSOL as well as surface treatment were achieved to further reduce the hydro-resistance. This study exhibits the predictability and controllability of BNNTs growth in customizable microstructures.

Camellia japonica-polysiloxane based superhydrophobic hybrid powder for the selective adsorption of metal ions from a mixture of metal ions in artificial sea water

Superhydrophobic hybrid micro–nanocomposite powder was prepared from a mixture of tannin rich (Camellia japonica) leaf powder, polymethylhydroxysiloxane and phenyl substituted silica ormosils. The surface properties, functional groups, and thermal stability of the hybrid material were analysed by various characterization techniques. The hybrid powder exhibited a hierarchical surface morphology, mesoporous structure, higher thermal stability and superhydrophobicity. The superhydrophobic powder was evaluated for its potential use in the adsorption of metal ions from a mixture of metal ions solutions prepared in artificial sea water. The hybrid material adsorbed copper (II) and nickel (II) ions preferably compared to other competitive metal ions in the mixture solution. The metal ions adsorbed by the superhydrophobic materials were also checked using other leaf powder hybrid materials. The results showed that the preferable adsorption of metal ions can vary according to the type of leaf powder used in the hybrid system.

Biologically inspired tunable hydrophilic/hydrophobic surfaces: a copper oxide self-assembly multitier approach

In this study, a fabrication method for biologically inspired superhydrophobic micro- and nano-structured tier surfaces, each made of a self-assembled copper oxide, is presented. The method is controllable and applicable to bulk production when compared to existing high-end fabrication techniques. By modulating wet chemistry, different shapes and scales of tier structures were created. We demonstrated that their wetting behaviors are closely related to morphological information such as pitch, height and shape. To characterize their wetting behaviors, several experiments were designed and executed. In static water contact angle (WCA) measurements, morphological modulation led to wide WCA range (17°–95°). After hydrophobic self-assembly monolayer of 1-dodecanethiol, their WCA was escalated into superhydrophobic regime. In dynamic WCA, the contact angle hysteresis is greatly reduced by hybridizing the micro- and nano-tier (multiple tiers) when compared to utilizing a single tier. Also, the modification of the surface structure influences the rate of evaporation. In an analytical approach, the multiple tiers show a lower surface free energy compared to that of the single tier. By hybridizing different scales and shapes of tiers–such as hemispheric and conic shapes–the multiple tiers can efficiently reduce the surface energy barrier. Eventually, these manipulations lead to a subtle WCA hysteresis during the liquid motion testing. The analytical results are consistent with the dynamic WCA measurements. The multiple tiers also stabilize the Cassie regime and result in an increased hydrophobicity, which is more than when a single tier is employed.

Generation and Characterization of Super-Hydrophobic Micro- and Nano-structured Surfaces

Here we describe the use of a backside exposure method for the creation of high aspect ratio tapered microstructures with nano-porous sidewalls. These sidewalls result from the exposure and curing conditions of the SU8 matrix. The structures show ultra-hydrophobic behavior with water contact angles of more than 160°, without an additional coating of the SU8 polymer. By choosing appropriate exposure conditions, needlelike structures can be created with a high level of porosity, covering their entire surface. The contact angle hysteresis values of such structures lie in the range of 20°. After an additional deposition of smooth and thin metallic gold film and coating it with an alkyl thiol selfassembled monolayer (SAM), we were able to decrease this hysteresis to values of around 10°. By using thin and rough metallic films like wet chemically oxidized titanium oxide and a fluoralkylsilane SAM, the hysteresis values could be reduced to only 4°.

Superhydrophobic Micro- and Nanostructures Based on Polymer Sticking

Superhydrophobic polytetrafluoroethylene (Teflon®, DuPont) sub-micro and nanostructures were fabricated by the dipping method, based on anodization process in oxalic acid. The polymer sticking phenomenon during the replication creates the sub-microstructures on the negative polytetrafluoroethylene nanostructure replica. This process gives a hierarchical structure with nanostructures on sub-microstructures, which looks like the same structures as lotus leaf and enables commercialization. The diameter and the height of the replicated nano pillars were 40 nm and 40 um respectively. The aspect ratio is approximately 1000. The fabricated surface has a semi-permanent superhydrophobicity, the apparent contact angle of the polytetrafluoroethylene sub-micro and nanostructures is about 160 °, and the sliding angle is less than 1°.