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Spin-coated polymer composite hydrophobic surfaces with self-cleaning performance
Materials Research Express
Abstract
Hydrophobic surfaces can be developed with low surface energy materials as per the Wenzel state. To intensify the adherence of the coating on the substrates, polymer has been employed in the coating. The Polymer-silica coating has been prepared with the help of PMMA (Poly methyl methacrylate), PS (Polystyrene) and MTMS (Methyltrimethoxy silane) in the study. Composite of PMMA/MTMS and PS/MTMS were coated uniformly on the substrates by spin-coating technique at a speed of 1500 and 1300 rpm. The functional groups have been identified with Fourier Transform Infrared Ray Spectroscopy. Property of low surface energy (ϒ), for hydrophobic surface can be evolved, with Hamaker's constant (A H ), Vander Waals interaction between the particles of opposite surface charge which form permanent dipoles. The contact angle for PMMA/MTMS is 112° and PS/MTMS is 119° is measured. Further, the optical and structural properties were designated by UV-Visible spectroscopy and FESEM. The particles of dust on the coated surface rolls off smoothly upon contact with the water droplet ensuring the self-cleaning nature.
... To date, many different techniques have been developed for the hydrophobization of metal surfaces, which are based on reducing the surface energy by chemically changing the composition of the surface layer or by applying a hydrophobic agent to the surface. Among the most widespread methods of applying a hydrophobic agent to the surface are methods of adsorption from solutions or vapors [9][10][11], dip coating [12], spin coating [13], and others. However, such methods do not create surfaces with superhydrophobic properties, which are the most promising for use in practice. ...
... Coatings 2023,13, 2058 ...
One of the most common current processing methods in various scientific studies is the modification of surfaces of various structural materials via laser radiation (laser ablation technique). The laser texturizing of metal surfaces is one of the promising applications for the creation of hydrophobic surfaces with a high water contact angle, increased corrosion resistance, and other properties. This paper reports the results of experimental studies to determine the effect of ultrasonic surface cleaning after laser texturizing on the degree of wetting and corrosion resistance of AISI 316L steel. The results show that ultrasonic cleaning leads to the removal of micro-/nano-sized particles formed on the surface following the laser texturizing of roughness. This effect, in turn, helps us to obtain higher values for the water contact angle and to increase the corrosion resistance.
... Superhydrophobic surfaces with water contact angles >150 • have been shown to exhibit the self-cleaning effect and are studied for a wide range of applications [4]. Various strategies for constructing superhydrophobic surfaces on metal substrates have been investigated in the past decades [5,6]. Techniques for surface modification include chemical etching [7], laser patterning methods [8,9], wet etching [10], Chemical Vapor Deposition (CVD) [11], micro-arc oxidation (MAO) [12], and surface coating of intrinsically low energy materials, such as PDMS, fluoropolymers, and fluorinated silanes [13][14][15][16]. ...
Current methods for the protection of metal surfaces utilize harsh chemical processes, such as organic paint or electro-plating, which are not environment-friendly and require extensive waste treatments. In this study, a two-step approach consisting of electrochemical assisted deposition (EAD) of an aqueous silane solution and a dip coating of a low surface energy silane for obtaining a superhydrophobic self-cleaning surface for the enhanced protection of copper substrate is presented. A porous and hierarchical micro-nanostructured silica basecoat (sol-gel) was first formed by EAD of a methyltriethoxysilane (MTES) precursor solution on a copper substrate. Then, a superhydrophobic top-coat (E-MTES/PFOTS) was prepared with 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (PFOTS) for low surface energy. The superhydrophobic coating exhibited anti-stain properties against milk, cola, and oil, with contact angles of 151°, 151.5°, and 129°, respectively. The EAD deposition potential and duration were effective in controlling the microscopic morphology, surface roughness, and coating thickness. The E-MTES/PFOTS coatings exhibited chemical stability against acids, bases, and abrasion resistance by sandpaper. The proposed 2-layer coating system exhibited strong chemical bonding at the two interfaces and provided a brush-like surface morphology with long-lasting superhydrophobicity. The developed method would provide an environment-friendly and expedient process for uniform protective coatings on complex surfaces.
... cm −1 . 19 The presence of methyl group with condensation in hydrolysed MTMS are detected by bending vibration and C-H stretching of Si-CH 3 peak of 2962.86 cm −1 . The Si-(CH 3 ) 2 stretching vibration of PDMS is addressed by the assimilation tops at 801 and 865 cm −1 . ...
Among all the astonishing equipment found in power transmission and distribution networks, the insulator plays a vital role by providing mechanical support and electrical protection to power system. Despite of all these noteworthy facts, the breakdown of insulators owing to surface contaminants appears to be particularly fascinating in today's scientific world. Researchers provide a plethora of methods to eradicate this problem. From all method, superhydrophobic coating provide better solution for its resistance to moisture, wetness, dust and ice. This unique property of superhydrophobic coatings requires further investigation. Hence, the suggested technique achieves this purpose by preparing the eco-friendly superhydrophobic solution of PDMS (Polydimethylsiloxane) with MTMS (Methyl tri methoxy silane) composites. In this study superhydrophobic coating were prepared by using sol-gel method and spray coating technique Analysis like FTIR, surface morphology and coating thermal study were included. self-cleaning ability of superhydrophobic coating on insulator surface with contact angle (CA) was measured from 165° to 170° and sliding angle 5° to 10° confirming superhydrophobic property. Finally, in an intentionally contaminated conditions based on solid layer method of IEC60507 standards and IEC 60587 standards insulation resistance are tested using a megger instrument.
The flexible and superhydrophobic properties of silica aerogels are extremely important material for thermal insulation and oil spill cleanup applications for their long-term use. Flexible silica aerogels were synthesized by using a two-step sol-gel process with precursors, methyltrimethoxysilane (MTMS) followed by supercritical drying. Silica aerogels were prepared at different molar ratio of methanol to MTMS (M). It was observed that the silica aerogels prepared at M
Here we report, an easy, straightforward and novel way to prepare raspberry-like superhydrophobic silica coatings for self-cleaning applications. The hydrophobic silica particles were obtained by simple condensation of fluoroalkoxysilane (17FTMS) in ethanol at room temperature. These silica particles were embedded into the sol-gel processed silica matrix and deposited on glass plates. On this coating surface, water drops exhibited a contact angle of 152° and rolls off the surface at sliding angle of 10°. This extremely low sliding angle was employed to self-clean the superhydrophobic coating, where dirt particles accumulated on the surface of superhydrophobic coating was efficiently cleaned by quickly sliding water drops. The stability of the microstructure as well as the wetting properties of the coating surface was investigated by scratch resistance and water stream impact test. The superhydrophobic coatings endured against the scratch of applied force of~150 mN. Such one pot synthesis of raspberry-like superhydrophobic silica coatings may open new avenue in the sublime field of superhydrophobic research.
A simple, versatile method has been developed to fabricate the transparent superhydrophobic surface via granuliform silica aerogels. The effect of ageing on the wettability, microstructure morphology and chemical structure of the dried gels has been investigated. Silica aerogel (dried alcogel with ageing) has a 3D porous network exhibiting the high surface area and pore volume. In comparison, large aggregates of silica nanoparticles exist within the backbone of the silica xerogel (dried alcogel without ageing). Both the aerogel and xerogel exhibit analogous chemical composition with abundant of methyl groups on the surface. The rough surface due to the high porosity and low surface energy provided by the methyl groups of aerogel contribute to the superhydrophobicity. Meanwhile, glass slides coated with aerogel film is highly transparent because the roughness created by the aerogel film is limited.
This review article exemplifies the importance of self-cleaning materials and coatings. Self-cleaning coatings are becoming an integral part of our daily life because of their utility in various applications such as windows, solar panels, cements, and paints. In this review, various categories of materials for the fabrication of hydrophilic, hydrophobic, oleophobic, amphiphobic and multifunctional coatings and their synthesis routes have been discussed. Furthermore, different natural organisms exhibiting superhydrophobic behaviour have been analysed. This review also covers the fundamentals of self-cleaning attributes such as water contact angle, surface energy, and contact angle hysteresis.
The organoclay/poly(methyl methacrylate) (PMMA) nanocomposites were prepared by in situ photopolymerization method using two solvents, ethanol and acetonitrile. The influences of organoclay loading, solvent nature and length of attached surfactant (C8 or C16) on thermal and mechanical properties were studied by thermogravimetric analysis and dynamic mechanical analysis. Alkylammonium surfactants with C8 and C16 chain lengths were evaluated as clay modifiers. All the nanocomposites prepared in acetonitrile exhibited improvement in their thermal stability, mainly due to the interaction between the clay and the polymer which is maximized by the exfoliated clay structure. In the case of PMMA and nanocomposites synthesized in ethanol, the thermal stability of polymer and nanocomposites remained practically the same once the clay structure is predominantly of the intercalated type. In comparison with pure PMMA, glass transition temperature and storage modulus of polymer are notably increased by the presence of clay. It was found that the chain length of surfactant attached to the SWy-1 clay affects the Tg values. Glass transition temperatures of nanocomposites SWy-1-C16/PMMA were significantly higher than the values obtained for nanocomposites SWy-1-C8/PMMA. This can be attributed to the modifying agent C16, which has a greater hydrophobic chain length. The organic tail can provide a better dispersion of the MMA monomer in the organoclay, resulting in a nanocomposite with predominant exfoliated structure. Another significant factor to be considered was the effect of solvent used in the nanocomposite preparation. Considering nanocomposites with the same chain length (C8 or C16), Tg values obtained for nanocomposites prepared with ethanol is higher than those observed for those prepared in acetonitrile. This was attributed to the influence of the average molecular weight; once the nanocomposites prepared in ethanol exhibited higher polymeric chains.
Films of poly (methyl methacrylate) (PMMA) were prepared by the addition of photoinitiator to the polymer. The influence of five organic photoinitiators on thermal stability of poly(methyl methacrylate) was studied by thermogravimetric analysis. Next, the PMMA films doped with these photoinitiators were UV irradiated and investigated in terms of changes in their thermal stability. It was found that the photoinitiators had accelerated thermal degradation of non-irradiated PMMA films due to the action of free radicals coming from the additives’ thermolysis. For UV-irradiated specimens, the effect of photoinitiator on PMMA thermal stability depended on the chemical structure of organic compound modifying the polymer. In general, thermal stability of irradiated samples was higher in the presence of additives. Thermal destruction of modified PMMA can be explained by the formation of resonance structures in aromatic photoinitiators and consumption of energy in dissipation processes.
Eu3+ ions-doped cubic mesoporous silica thin films with a thickness of about 205 nm were prepared on silicon and glass substrates using triblock copolymer as a structure-directing agent using sol-gel spin-coating and calcination processes. X-ray diffraction and transmission electron microscopy analysis show that the mesoporous silica thin films have a highly ordered body-centered cubic mesoporous structure. High Eu3+ ion loading and high temperature calcination do not destroy the ordered cubic mesoporous structure of the mesoporous silica thin films. Photoluminescence spectra show two characteristic emission peaks corresponding to the transitions of5D0-7F1 and 5D0-7F2 of Eu3+ ions located in low symmetry sites in mesoporous silica thin films. With the Eu/Si molar ratio increasing to 3.41%, the luminescence intensity of the Eu3+ ions-doped mesoporous silica thin films increases linearly with increasing Eu3+ concentration.
Liquid-solid (LS) and vapor-liquid-solid (VLS) interfaces are important to the fundamental understanding of how surface chemistry impacts industrial processes and applications. Superhydrophobic surfaces, from structural hierarchies, were fabricated by coating flat smooth surfaces with hollow glass microspheres. These surfaces are referred to as structural hierarchical modified microsphere surfaces (SHiMMs). Two phase LS and three-phase VLS interfaces of water droplets on SHiMMs, with an apparent static contact angle (aSCA) of ~160° were probed at microscale using environmental scanning electron microscopy (ESEM) and high resolution optical microscopy (OM). Both ESEM and OM confirmed the presence of air pockets in 3 μm-150 μm range at the VLS triple phase of the droplet peripheral contact line. The wetting characteristics of the LS interface in the interior of the water droplet was probed using energy dispersive spectroscopy (EDS) which corroborated well with the VLS triple-phase observations, confirming the presence of both the microscale air pockets and fractional complete wetting of the SHiMMs. The superhydrophobic water droplets on the SHiMMs also exhibited relatively high adhesion to the SHiMMs – a tilt angle of 10°-40° was needed for detaching the droplets off the surfaces. Semi-quantitative three-phase contact line analysis and experimental data indicated high water aSCA and large adhesion on the microscale roughened SHiMMs is attributed to pinning of the probe liquid both at the triple VLS and interior LS interfaces. The control over microroughness and surface chemistry of the SHiMMs will allow tuning of both the static and dynamic liquid-surface interactions.
Hydrophobic phase columns (also known as RP columns) have a very wide range of utilizations in HPLC. Besides RP-HPLC, they are also used in ion-pair chromatography (IP), hydrophobic interaction chromatography (HIC), and nonaqueous reversed-phase chromatography (NARP). The main commercial types of stationary phases used in RP-HPLC are included in this section and in Appendix 2 . Basic physical and chemical properties of hydrophobic stationary phases and columns are essential in their selection. End-capping and silanol activity, wettability, and stability towards pH and salts are very important issues in selecting among various RP stationary phases that are commercially available. Phase ratio as a fundamental parameter of RP-RPLC columns is explained and estimated for a number of reversed-phase columns including conventional C18, polar-embedded, and polar-endcapped. Some examples of column selections are discussed such as those used for the separation of small molecules with a hydrophobic moiety, or for the separation of proteins and peptides.
Vapor-phase covalently bound siloxane thin films of various functionalities on a variety of surfaces, including glass, aluminum, and polyester, were demonstrated in a one-step process. Siloxane films a few nanometers thick were deposited on surfaces from the vapor phase using different inert polydimethylsiloxanes (PDMSs) without the use of solvent, initiator, or a crosslinking agent. Functional PDMSs fluoro-polysiloxane (F-PDMS) and amino-polysiloxane (A-PDMS) were also coated on glass substrates, providing functional surfaces for patterned array of microlenses (MLs) for near-field imaging applications. We call our siloxane film deposition method SOLVED which refers to Siloxane-bound Layers through Vapor-Enhanced Deposition. Our ellipsometry data on the PDMS SOLVED surfaces indicated the film thickness was 6.5 ± 0.3 nm. The siloxane-surface binding was a thermally activated reaction with an apparent activation energy of ∼11 kcal mol⁻¹. Our optical spectroscopic X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and mass-spectrometry analysis data provided strong evidence that the SOLVED films retained the chemical nature of the polymers used for the coatings. The redox diffusion coefficients of Fe(CN)64-/3- species on SOLVED films using cyclic voltammetry (CV) studies were comparable with that of highly packed C18-silane self-assembled monolayers (SAMs) on ITO surfaces-implying high quality pinhole-defect-free SOLVED coatings on the surfaces. Further, both the CV and water contact angle (CA) measurements indicated that the photostability of SOLVED and octadecyltrimethoxysilane (OTMS) SAM films on the ITO substrates was comparable. Unlike bulk PDMS, our ultra-thin SOLVED films did not exhibit any significant changes in the water CA over two weeks. A reproducible and stable hydrophobicity with tunable water CAs between 30° and 97° was obtained through plasma treatment. By selectively removing the SOLVED films thru oxygen plasma treatment, we demonstrated the fabrication of self-assembled aqueous ZnCl2 ML array. Near-field imaging using MLs yielded much higher spatial resolution than can be obtained using diffraction-limited wide-field imaging.
Large majority of superhydrophobic surfaces have very limited mechanical wear robustness and long-term durability. This problem has restricted their utilization in commercial or industrial applications and resulted in extensive research efforts on improving resistance against various types of wear damage. In this review, advances and developments since 2011 in this field will be covered. As such, we summarize progress on fabrication, design and understanding of mechanically durable superhydrophobic surfaces. This includes an overview of recently published diagnostic techniques for probing and demonstrating tribo-mechanical durability against wear and abrasion as well as other effects such as solid/liquid spray or jet impact and underwater resistance. The review is organized in terms of various types of mechanical wear ranging from substrate adhesion, tangential surface abrasion, and dynamic impact to ultrasonic processing underwater. In each of these categories, we highlight the most successful approaches to produce robust surfaces that can maintain their non-wetting state after the wear or abrasive action. Finally, various recommendations for improvement of mechanical wear durability and its quantitative evaluation are discussed along with potential future directions towards more systematic testing methods which will also be acceptable for industry.
A method to fabricate wettability gradient surface with raspberry-like composite microspheres was described. The synthesis of composite microspheres was through the ethanol sol–gel processing of TEOS on carboxyl functionalized polystyrene (PS) template particles. The factors influencing the raspberry-like morphology and the deposition efficiency of SiO2 onto templates were discussed, including the surface carboxyl density, TEOS amount and water amount, and the optimal formulation of synthesis was given. The wettability gradient surface was built up by coating the suspension prepared on substrate and putting it in thermal gradient field for heat treatment. The resultant surface demonstrated wettability gradient from hydrophobic to superhydrophilic with the increasing of calcination temperature, which was because the physical topography of surface changed gradually from spike-type into pore-type and the main chemical compositions changed from polystyrene into SiO2.
The contribution group method consists of attributing to each chemical group of a molecule or a macromolecule a contribution to the Hamaker constant and to the surface free energy. From the Hamaker constants and the surface energies of organic liquids, the contributions of several carbonated groups are determined in order to calculate the surface properties of different polymers. The calculated values of their surface free energies are found to be in fairly good agreement with those obtained either from wettability measurements or by surface tension measurements of molten polymers.
Thionin doped silicate gels, initially synthesized with tetraethyl orthosilicate (TEOS), were modified by incorporating a second precursor, methyltrimethoxysilane (MTMS). The optical spectra revealed the formation of a second band in thionin gels containing over 50% MTMS. These new bands for the absorption and fluorescence spectra were found to reside at 560 nm and 665 nm, respectively. According to the behavior of thionin dissolved in aqueous solutions, it was readily deduced that these newly formed peaks were a result of dimers. Furthermore, FTIR spectroscopy revealed that gels synthesized with MTMS possess a significantly lower concentration of SiOH groups than gels prepared from TEOS. Due to the reduction in SiOH concentration, the polarity of the gel was decreased which accounted for the blue shifts in the fluorescence spectrum of thionin doped gels synthesized with raised MTMS contents. Finally, dye aggregation was suggested to be the result of fewer sites provided by the organically modified gels.
a b s t r a c t Herein, we report a simple and low cost method for the fabrication of superhydrophobic coating surface on quartz substrates via sol-gel dip coating method at room temperature. Desired surface chemistry and texture growth for superhydrophobicity developed under double step sol–gel process at room temper-ature. The resultant superhydrophobic surfaces were characterized by Field-emission scanning electron microscopy (FE-SEM), Atomic force microscopy (AFM), water contact angle (WCA) measurement, dif-ferential thermal gravimetric analysis-differential thermal analysis (TGA-DTA) calorimetry and optical spectrometer. Coating shows the ultra high water contact angle about 168 ± 2 • and water sliding angle 3 ± 1 • and superoleophilic with petroleum oils. This approach allows a simple strategy for the fabri-cation process of superhydrophilic–superhydrophobic on same surfaces with high thermal stability of superhydrophobicity up to 560 • C. Thus, durability, special wettability and thermal stability of superhy-drophobicity expand their application fields.
The Lifshitz theory of van der Waals interaction has been employed to calculate the contact angles of diiodomethane, 1-bromonaphthalene, 1-methylnaphthalene, bromobenzene, 1-tert-butylnaphthalene, liquid paraffin, and hexadecane on poly(dimethylsiloxane), poly(4-methyl-1-pentene), polyethylene, natural rubber, and polystyrene surfaces. The theoretical treatment is based on the equation cos θ = [(2APVL/ALVL)(HoLVL/HoPVL)2] − 1, where θ is the contact angle and APVL and ALVL are the non-retarded Hamaker constants for the heterointeraction between polymer and liquid across vacuum and the homo interaction of the liquid across vacuum, respectively. Nonretarded Hamaker constants have been determined from the dielectric properties of the materials and application of the Lifshitz theory. HoLVL and HoPVL are the equilibrium “contact” surface separations associated with the liquid−liquid homointeraction and polymer−liquid heterointeraction across vacuum, respectively. HoLVL values, and the analogous HoPVP values associated with polymer homointeraction, have been estimated from the surface free energies of the dispersive liquids and polymers. Four different approaches, each with a different assumption regarding the heterointeraction between polymer and liquid, have been used to obtain HoPVL values: (i) HoPVL = HoLVL, (ii) HoPVL = HoPVP, (iii) a geometric mean relationship, HoPVL = (HoPVPHoLVL)1/2, and (iv) an arithmetic mean relationship, HoPVL = (HoPVP + HoLVL)/2. Theoretical contact angles obtained with the four approaches have been compared with experimental contact angles. In general, the approaches which employ the combining rules, whether geometric or arithmetic, provide the best agreement between theory and experiment. Previous work that has dealt with the contact angles of n-alkanes on poly(tetrafluoroethylene) has also been re-examined. For the perfluorocarbon−hydrocarbon system none of the approaches are able to reconcile theory and experiment.
a b s t r a c t The superhydrophobic surfaces have drawn lot of interest, in both academic and industries because of optically transparent, adherent and self-cleaning behavior. Surface chemical composition and mor-phology plays an important role in determining the superhydrophobic nature of coating surface. Such concert of non-wettability can be achieved, using surface modifying reagents or co-precursor method in sol–gel process. Attempts have been made to increase the hydrophobicity and optical trans-parency of methyltrimethoxysilane (MTMS) based silica coatings using polymethylmethacrylate (PMMA) instead of formal routes like surface modification using silylating reagents. The optically transparent, superhydrophobic uniform coatings were obtained by simple dip coating method. The molar ratio of MTMS:MeOH:H 2 O was kept constant at 1:5.63:1.58, respectively with 0.5 M NH 4 F as a catalyst and the weight percent of PMMA varied from 1 to 8. The hydrophobicity of silica coatings was analyzed by FTIR and contact angle measurements. These substrates exhibited 91% optical transmittance as compared to glass and water drop contact angle as high as 171 ± 1 • . The effect of humidity on hydrophobic nature of coating has been studied by exposing these films at relative humidity of 90% at constant tempera-ture of 30 • C for a period of 45 days. The micro-structural studies carried out by transmission electron microscopy (TEM).
The experimental results on the synthesis and physicochemical properties of superhydrophobic silica aerogels, with the highest ever obtained contact angle (∼173°), using methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent and ammonium hydroxide (NH4OH) catalyst, are reported. The molar ratios of NH4OH/MTMS (N), H2O/MTMS (H) and MeOH/MTMS (M) were varied from 4.25 × 10−2 to 3.5 × 10−1, 2 to 10 and 1.75 to 17, respectively. It has been found that the gelation time decreases with increase in N and H values and it increases with increase in M values. The bulk density of the aerogels was found to decrease with increase in N, H and M values. It has been observed that the volume shrinkage increases with decrease in N and H values and increases with M values. In the case of catalyst concentration variation, the contact angle (θ) increases slightly from 159° to 167° with increase in N values. On the other hand, in the case of H2O and MeOH variations, the θ first increases from 162° and 160° up to the values of 173° and 167° and then decreases to 160° and 159° with increase in H and M values, respectively. All the MTMS aerogels are opaque to the visible light. The aerogels retain their hydrophobicity up to a temperature of ∼480 ° C. The thermal conductivity of the aerogels was found to be around 0.095 W/m K except for the aerogels with higher bulk density (>0.3 g/cm3, at a lower H value of 2) whose thermal conductivity was around 0.109 W/m K. The aerogels have been characterized by Fourier transform infrared spectra (FTIR), thermogravimetric and differential thermal analyses (TGA-DTA) and scanning electron microscopy (SEM) techniques. The results have been discussed by taking into account the hydrolysis and condensation reactions and SEM observations.
With a view to developing repellent materials combining both low surface energy and rough structure, original semi-fluorinated polythiophenes have been chemically and electrochemically synthesized and characterized by cyclic voltammetry, GPC, and UV-visible measurements. Polymer films have been deposited onto different substrates by drop casting a soluble polymer fraction on glass plate or by electrodeposition on ITO plate. Surface properties and particularly water and oil repellencies have been investigated by goniometry and correlated with the surface morphology and structure observed by SEM. The incorporation of fluorocarbon chains in the rigid polythiophene backbone yields very low surface free energy materials. Moreover, the way of deposition has a huge influence on the quality and performance of the film surface properties. Electroformed polymers, due to rough surfaces, exhibit great super-hydrophobic and lipophobic properties together with exceptional time stability.