Superhydrophilic Surface Research Articles

Cycad-leaf-like crystalline-amorphous heterostructures for efficient urea oxidation-assisted water splitting

Developing efficient bifunctional catalysts for urea oxidation reaction (UOR)/hydrogen evolution reaction (HER) is important for energy-saving hydrogen production. Herein, a catalyst with crystalline-amorphous heterostructure supported by NiCo alloy on nickel foam (NiCoO-MoOx/NC) is reported for the first time. Through simple molybdenum salt etching, 2D NiCo alloy nanosheets are transformed into a unique 3D cycad-leaf-like structure with a super-hydrophilic surface. Simultaneously, the synergistic effect between crystalline NiCoO and amorphous MoOx improves the UOR and HER activity, merely requiring 1.28 V and −45 mV potentials to reach ±10 mA cm−2, respectively. Particularly, the UOR kinetics of NiCoO-MoOx/NC is enhanced significantly compared to that of NiCoO/NC. The electronic structure of NiCoO is modified by MoOx, enabling the rapid generation of NiOOH and CoOOH active species, which would accelerate the synergistic electrocatalytic oxidation of urea molecules. This work inspires the design of highly active and stable bifunctional catalysts for urea assisted H2 production.

Visible light-induced photocatalytic and antibacterial adhesion properties of superhydrophilic TiO2 nanoparticles

Bacterial infections triggered by patient or healthcare worker contact with surfaces are a major cause of medically acquired infections. By controlling the kinetics of tetrabutyl titanate hydrolysis and condensation during the sol–gel process, it is possible to regulate the content of Ti3+ and oxygen vacancies (OVs) in TiO2, and adjust the associated visible light-induced photocatalytic performance and anti-bacterial adhesion properties. The results have shown that the Ti3+ content in TiO2 was 9.87% at the calcination temperature of the reaction system was 300 °C and pH was 1.0, corresponding to optimal photocatalytic and hydrophilic properties. The formation of a hydrated layer on the superhydrophilic surface provided resistance to bacterial adhesion, preventing cross-contamination on high-touch surfaces. The excellent photocatalytic self-cleaning performance and anti-bacterial adhesion properties can be attributed to synergistic effects associated with the high specific surface area of TiO2 nanoparticles, the mesoporous structure, and the presence of Ti3+ and OVs. The formation of superhydrophilic self-cleaning surfaces under visible light can serve as the basis for the development of a new class of anti-bacterial adhesion materials.

Facile fabrication of polymeric-inorganic hybrid coated membrane with super-hydrophilic and underwater super-oleophobic properties for highly efficient crude oil-in-water emulsion separation

The oily wastewater is a huge wastewater stream that can potentially serve as an enormous source of clean water for water reuse applications such as groundwater recharge or irrigation. Given the huge potential of cleaning oily wastewater, we have developed an amino-functionalized nickel oxide nanoparticles coated inorganic-polymeric hybrid membrane with superhydrophilic and underwater superoleophobic properties for excellent crude oil-in-water (O/W) emulsion separation potential using a cross-flow filtration system. The use of ceramic alumina (Al2O3) support offers increased chemical, thermal, and mechanical stability while nickel oxide (NiO) decoration equips the membrane with special surface wettability to avoid membrane fouling. Moreover, the amino-functionalized NiO-coated polyamide network (F-NiO/PA) offers an active layer dense enough to separate the oil from water when the surfactant-stabilized crude oil-in-water emulsion was used as feed during separation experiments. Hence fabricated F-NiO/PA@Alumina membrane was studied by using different membrane characterization techniques to explore the features and understand the structure of F-NiO/PA@Alumina membrane. The F-NiO/PA@Alumina membrane possessed a superhydrophilic (water contact angle; θW, A = 0°) and underwater superoleophobic (oil contact angle; θO, W = 161°) surface wettability which is an ideal feature required for O/W emulsion separation. The F-NiO/PA@Alumina membrane demonstrated a high O/W emulsion separation efficiency of >99 % even with crude oil concentrations as high as 200 ppm using a cross-flow filtration system. The stability test of the separation performance of the F-NiO/PA@Alumina membrane also revealed a separation performance of >99 % for the duration of the test using a cross-flow filtration system.

Experimental research on condensation flow and heat transfer characteristics of immiscible binary mixed vapors on different wettability wall surfaces

In the process of biomass gasification syngas waste heat recovery, water and organic compounds in the syngas will condense at the same time, resulting in immiscible condensates adhering to the wall, which can reduce the heat exchanger heat transfer efficiency and working life. Changing the wettability of the wall surface can affect the condensate flow state on the wall surface, thus affecting the condensation heat transfer performance. Water and organic cyclohexane were used as immiscible working fluids in this paper, the condensation flow and heat transfer characteristics of the mixed vapors on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces were experimentally investigated. The results showed that cyclohexane in immiscible condensates existed as a liquid film on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces, while the water existed in droplets on the hydrophilic and super-hydrophobic wall surfaces, and there were two forms of channels and droplets on the super-hydrophilic wall surface. The condensation heat transfer coefficient on the super-hydrophobic wall surface was higher than that on the hydrophilic wall surface and the super-hydrophilic wall surface, with a maximum of 34% higher than hydrophilic and 45% higher than super-hydrophilic, which was related to the smaller droplet departure diameter on the super-hydrophobic surface. In addition, the super-hydrophilic wall surface had better heat transfer performance than the hydrophilic wall surface only when the immiscible condensate was a "film-channel" flow pattern. The results can provide guidance for enhancing the condensation heat transfer of the water and organic compounds binary mixed vapors.

Porous Polymer Microneedles with Superhydrophilic Surface for Rapid Fluid Transport

Porous Polymer Microneedles with Superhydrophilic Surface for Rapid Fluid Transport

Characterization of modified titanium surfaces by anodisation and immersion tests

This paper explores the features of both nanoporous and compact TiO2 films formed by titanium anodisation in two experimental conditions, given that they have completely different morphologies and properties than those found by our previous studies. After anodisation, samples have been subjected for 20 days to immersion tests in different media (H2O, H3PO4, and KOH). Surface morphology, phase composition and wettability of anodised films were investigated using FESEM, FTIR, Raman spectroscopy, contact angle measurements and XPS, and the hydrophilicity of modified surfaces was investigated by immersion tests. Nanoporous films exhibited hydrophobic surfaces, but contact angle values gradually decreased after immersing films in H2O, H3PO4 and KOH media, respectively. However, compact films produced superhydrophilic surfaces, both before and after immersion tests, with the exception of immersing the film in a H3PO4 medium due to film removal by acid attack. As for compact films, an unusual morphology revealed by the presence of cone-shaped particles might be responsible for the adsorption of –OH groups arranged so as to favour anatase phase formation.

Multifunctional integrated pattern for enhancing fog harvesting water unidirectional transport in a heterogeneous pattern

Solid surfaces with improved wettability as well as geometric structures can enhance capture and droplet removal, thereby improving fog harvesting. We fabricated Al wires by combining superhydrophilic (SHL), superhydrophobic (SHB), and oil-infused SHB (SHBO) surfaces into a pattern whose fog-harvesting efficiency could be measured. The SHL-SHBO-SHL pattern showed the highest promise of water droplet capture and mobility on a solid surface with 42% efficiency compared to the 34% efficiency of Bare. In order to identify the optimal efficiency features, two boundary conditions (boundary I: from SHL to SHBO and boundary II: from SHBO to SHL) were introduced, and the impact of the hydrophilic area was examined. Boundary I boosts capture efficiency whereas boundary II increases drain efficiency. Understanding the forces operating at the wettability gradient surface, as well as incorporating the area ratio of SHL and SHBO via wettability combinations, are key to designing effective fog harvesting systems.

Capacitive sensing of frost growth dynamics on aluminum surfaces with different wettabilities

Frost formation is a critical issue in many industries including air conditioning and refrigeration, aviation, transportation, and cryogenics. Frost buildup on heat exchanger surfaces reduces their efficiency by acting as a thermal resistance and by reducing or blocking the airflow. Defrosting is therefore necessary to maintain system efficiency. However, defrost initiation and cycle duration are critical parameters that govern system performance. Excessively early or late defrosting can result in energy loss. Frost detection is thus essential to carry out defrosting efficiently. To know when to initiate defrost, both the density and thickness of the frost must be known as both affect the frost thermal resistance. However, state of the art approaches are not sensitive to both frost density and thickness. Here, a capacitive sensing method is proposed that is sensitive to both the density and thickness of the frost. The method can achieve precise frost detection, offering insights into the ideal time to initiate defrosting by being sensitive to factors that affect frost specific thermal resistance. Experiments are carried out on plain (hydrophilic), superhydrophobic, and superhydrophilic aluminum surfaces at three different surface temperatures (−6 °C, −10 °C, −16 °C) to investigate the sensing performance. The capacitance measured increases with time as frost forms and grows. This change in capacitance, along with mass and thickness data, was used as a tool to characterize frost growth. The results indicate that decreasing wettability (superhydrophilic to plain (hydrophilic) to superhydrophobic) results in a decreasing trend in mass accumulation, frost density, and change in capacitance and an increasing trend in frost thicknesses and thermal resistance. It was also demonstrated that increasing surface temperature results in decreasing changes in capacitance (measurement sensitivity), mass accumulation, frost thicknesses, and thermal resistance, with an increasing trend in frost density. A strong correlation between frost mass and change in capacitance was observed regardless of surface temperature or wettability. This rendered implications for the best time of defrosting initiation. The proposed approach offers high accuracy, sensitivity, and convenience as a non-invasive method to study dynamic frost growth with insights into more optimal defrosting cycles, contributing to the development of more efficient energy systems.

Janus Membranes with Asymmetric Superwettability for High-Performance and Long-Term On-Demand Oil/Water Emulsion Separation.

Current single-function superwettable materials are typically designed for either oil removal or water removal and are constrained by oil density, limiting their widespread applications. Janus membranes with opposite wettability on their two surfaces have recently emerged and present attractive opportunities for on-demand oil/water emulsion separation. Here, a combination strategy is introduced to prepare a Janus membrane with asymmetric superwettability for switchable oil/water emulsion separation. A mussel-inspired asymmetric interface introduction cooperating with the sequence-confined surface modification not only brings about an asymmetric superwettability Janus interface but also guarantees an outstanding stable interface and remarkable chemical stability surfaces. Specifically, the superhydrophilic surface with underwater superoleophobicity can separate surfactant-stabilized oil-in-water emulsions. Conversely, other surface displays opposite superhydrophobicity and superoleophilicity to treat surfactant-stabilized water-in-oil emulsions. Significantly, this superwettable Janus membrane presents superior long-term on-demand oil/water emulsion separation without obvious flux decline and high recovery ability because of its superwettability and superior stability. Furthermore, the asymmetric superwettability enhances the interfacial floatability at air-water interfaces, enabling the design of advanced interfacial materials. The as-prepared superwettable Janus membrane has established a cooperated separation system, overcoming the monotony of conventional superwettable membranes and expanding the application of these specialized membranes to oily wastewater treatment.

Hierarchical FeOxHy@Ni3B hybrid for efficient alkaline oxygen evolution at high current density

Electrocatalysts with high activity and long-term durability are vital toward large-scale hydrogen production from electrocatalytic water splitting. Here, the self-supported electrode (FeOxHy@Ni3B/NF) with hierarchical heterostructure was simply prepared by using Ni3B chunks grown on nickel foam as substrate to in situ form vertical FeOxHy nanosheets. Such hybrid shows efficient oxygen evolution reaction activity with overpotentials as low as 267 and 249 mV at 100 mA cm−2 in 1 M KOH solution and 30 wt% KOH solution, respectively. Meanwhile, it also exhibits excellent catalytic stability, sustaining catalysis at 500 mA cm−2 in 1 M KOH solution for 200 h, and even for 200 h at 1000 mA cm−2 in 30 wt% KOH solution. Further experimental results reveal that the FeOxHy@Ni3B/NF is endowed with superhydrophilic and superaerophobic surface properties, which not only provide more mass transport channels, as well as facilitated the diffusion of reaction intermediates and gas bubbles. Also, it holds faster reaction kinetics, more accessible active sites and accelerated electron transfer rates due to strong synergistic interactions at the heterogeneous interface.

Duckweeds mimicked self-clean Janus solar evaporator

Solar water evaporation has been considered as a promising method to solve the global water scarcity due to its low energy consumption, environmental friendly, as well as high water evaporation efficiency. However, salt crystallization on evaporation surface drastically decrease the evaporation rate of seawater. In this work, we propose a novel bionic solar evaporation system based on 3D printing technology inspired by natural duckweeds. Water can be easily transported to the evaporation surface through regularly arranged “S” shaped microchannels. Superhydrophilic surface enables the water spread quickly on evaporation surface and eventually fall back to bulk water, realizing the self-clean capability. The evaporation rate of our solar evaporation system can reach up to 1.40 kg m−2 h−1 with an evaporation efficiency of 95.7% under 1 sun irradiation. In addition, the evaporator demonstrates amazing desalination and purification capabilities, which can effectively remove metal ions as well as organic impurities with a purification rate up to 99.9%. Moreover, long-term desalination performance of our evaporation system has been well achieved, demonstrating the effective and stable performance of our bionic evaporation system for seawater desalination.

Experimental Investigation on Pool Boiling Heat Transfer Performance of Superhydrophilic, Hydrophilic and Hydrophobic Surface

Experimental Investigation on Pool Boiling Heat Transfer Performance of Superhydrophilic, Hydrophilic and Hydrophobic Surface

Fog harvesting efficiency analysis of biomimicking surfaces: A review

With a rapidly increasing population, pollution levels and industrial water and energy demand are on a constant rise in the past century. Populations residing in arid and semi-arid regions have been victims of water shortage issues since decades. Efficient water harvesting techniques are becoming highly significant today for ensuring the availability of essential resources throughout the globe. Inspired from nature where certain creatures have attractive surfaces which can collect water from the fog or mist even in no rainfall conditions, researchers have developed high-efficient novel water harvesting materials that could effectively address water scarcity issues. Through an in-depth analysis of the wettability and water droplet transportation of these creatures in nature, novel bionic surfaces that combine desirable natures and properties have been developed through innovative fabrication and coupling techniques. This could achieve 2.5 to 5.3 times more efficient water harvesting than conventional superhydrophobic and superhydrophilic surfaces. The efficiency value increased by 250% to 600% by merging different morphologies like Desert beetle, cactus and Janus, compared to their individual uniform morphology surfaces. The review also presents current challenges and scope for future development in water harvesting materials.

Superwettable surfaces and factors impacting microbial adherence in microbiologically-influenced corrosion: a review.

Microbiologically-influenced corrosion (MIC) is a common operational hazard to many industrial processes. The focus of this review lies on microbial corrosion in the maritime industry. Microbial metal attachment and colonization are the critical steps in MIC initiation. We have outlined the crucial factors influencing corrosion caused by microorganism sulfate-reducing bacteria (SRB), where its adherence on the metal surface leads to Direct Electron Transfer (DET)-MIC. This review thus aims to summarize the recent progress and the lacunae in mitigation of MIC. We further highlight the susceptibility of stainless steel grades to SRB pitting corrosion and have included recent developments in understanding the quorum sensing mechanisms in SRB, which governs the proliferation process of the microbial community. There is a paucity of literature on the utilization of anti-quorum sensing molecules against SRB, indicating that the area of study is in its nascent stage of development. Furthermore, microbial adherence to metal is significantly impacted by surface chemistry and topography. Thus, we have reviewed the application of super wettable surfaces such as superhydrophobic, superhydrophilic, and slippery liquid-infused porous surfaces as "anti-corrosion coatings" in preventing adhesion of SRB, providing a potential avenue for the development of practical and feasible solutions in the prevention of MIC. The emerging field of super wettable surfaces holds significant potential for advancing efficient and practical MIC prevention techniques.

The biofilm removal effect and osteogenic potential on the titanium surface by electrolytic cleaning: An invitro comparison of electrolytic parameters and five techniques.

To determine the optimal current and time of electrolytic cleaning (EC), compare its biofilm removal effect with generic treatments and evaluate the influence of EC to surface characteristics and osteogenic potential of SLA titanium (Ti) discs. The six-species biofilm-covered Ti discs were placed as cathodes in physiologic saline and subjected to various current and time treatments. The residual biofilms were evaluated to determine the optimal parameters. The contaminated Ti discs were randomized and treated by rotating Ti brush; ultrasonic-scaling with metal tips; ultrasonic-scaling with PEEK tips; air-polishing and EC. The residual biofilms were compared using a lipopolysaccharide kit (LPS), scanning electron microscope (SEM), confocal laser scanning microscopy and colony-forming unit counting. Non-contaminated Ti discs were treated and characterized. The bone marrow mesenchymal stem cells (BMSCs) were cultured on treated non-contaminated Ti discs. The adhesion, proliferation, alkaline phosphatase (ALP) activity and osteocalcin level of BMSCs were assessed. The parameters at 0.6A5min were considered optimal. For LPS and SEM, EC promoted a significantly greater biofilm removal than the other groups. There were no changes in the Ti discs' colour, topography, roughness and chemical elements after EC, and the electrolysis-treated Ti discs obtained a super-hydrophilic surface. EC positively impacted the proliferation and ALP activity of BMSCs, surpassing the efficacy of alternative treatments. EC achieves a near-complete eradication of contaminants on the SLA surface, causes no surface damage with improved hydrophilicity, and promotes the early osteogenic response of BMSCs, which makes it a promising treatment for peri-implantitis.

Synthesis of Hierarchical Superhydrophilic/Superhydrophobic Nanostructured Surfaces for Oil/Water Separation

AbstractSurface wetting, the phenomenon where a liquid spreads or adheres to a solid surface, plays a crucial role in both natural and technological fields. This study focuses on elucidating the relationship between surface properties and wetting behavior, emphasizing the significance of hierarchical structures. A 3D hierarchical structure is created by controlling shape and size through electroplating and chemical reactions, adjusted by current intensity, ammonium persulfate, and ammonium hydroxide concentrations. This modification is achieved by modifying the surface's chemical properties. This control directly impacted the surface wetting properties, providing a means to regulate wetting behavior by altering surface structure. Through control of surface chemistry, a superhydrophilic surface is able to successfully create with a contact angle of 0° and a superhydrophobic surface with a contact angle of 171.3.

Effect of Surface Wettability on Nanoparticle Deposition during Pool Boiling on Laser-Textured Copper Surfaces

Prior studies have evidenced the potential for enhancing boiling heat transfer through modifications of surface or fluid properties. The deployment of nanofluids in pool boiling systems is challenging due to the deposition of nanoparticles on structured surfaces, which may result in performance deterioration. This study addresses the use of TiO2–water nanofluids (mass concentrations of 0.001 wt.% and 0.1 wt.%) in pool boiling heat transfer and concurrent mitigation of nanoparticle deposition on superhydrophobic laser-textured copper surfaces. Samples, modified through nanosecond laser texturing, were subjected to boiling in an as-prepared superhydrophilic (SHPI) state and in a superhydrophobic state (SHPO) following hydrophobization with a self-assembled monolayer of fluorinated silane. The boiling performance assessment involved five consecutive boiling curve runs under saturated conditions at atmospheric pressure. Results on superhydrophilic surfaces reveal that the use of nanofluids always led to a deterioration of the heat transfer coefficient (up to 90%) compared to pure water due to high nanoparticle deposition. The latter was largely mitigated on superhydrophobic surfaces, yet their performance was still inferior to that of the same surface in water. On the other hand, CHF values of 1209 kW m−2 and 1462 kW m−2 were recorded at 0.1 wt.% concentration on both superhydrophobic and superhydrophilic surfaces, respectively, representing a slight enhancement of 16% and 27% compared to the results obtained on their counterparts investigated in water.

Analysis of Carbon Nanoparticle Coatings via Wettability

Wettability, typically estimated through the contact angle, is a fundamental property of surfaces with wide-ranging implications in both daily life and industrial processes. Recent scientific interest has been paid to the surfaces exhibiting extreme wettability: superhydrophobic and superhydrophilic surfaces, characterized by high water repellency and exceptional water wetting, respectively. Both chemical composition and morphology play a role in the determination of the wettability “performance” of a surface. To tune surface-wetting properties, we considered coatings of carbon nanoparticles (CNPs) in this study. They are a new class of nanomaterials synthesized in flames whose chemistry, dimension, and shape depend on combustion conditions. For the first time, we systematically studied the wettability of CNP coatings produced in a controlled rich ethylene/air flame stabilized over a McKenna burner. A selected substrate was intermittently inserted in the flame at 15 mm above the burner to form a thin coating thanks to a thermophoretic-driven deposition mechanism. The chemical-physical quality and the deposed quantity of the CNPs were varied by opportunely combing the substrate flame insertion number (from 1 to 256) and the carbon-to-oxygen ratio, C/O (from 0.67 to 0.87). The wettability of the coatings was evaluated by measuring the contact angle, CA, with the sessile drop method. When the C/O = 0.67, the CNPs were nearly spherical, smaller than 8 nm, and always generated hydrophilic coatings (CA < 35°). At higher C/O ratios, the CNPs reached dimensions of 100 nm, and fractal shape aggregates were formed. In this case, either hydrophilic (CA < 76°) or superhydrophobic (CA ~166°) behavior was observed, depending on the number of carbon nanoparticles deposed, i.e., film thickness. It is known that wettability is susceptible to liquid surface tension, and therefore, tests were conducted with different fluids to establish a correlation between the flame conditions and the nanostructure of the film. This method offers a fast and simple approach to determining mesoscale information for coating roughness and topographical homogeneity/inhomogeneity of their surfaces.

Transparent photothermal films with high optical selectivity for anti/de-icing

A variety of transparent photothermal films with enhanced ultraviolet and near-infrared solar radiation absorption has been investigated to operate in cold environments in recent years. However, the enhancement of solar energy harvesting has to excessively sacrifice the transmittance of visible light in most existing anti-frosting and anti/de-icing systems, which leads to a widespread constraint in the applications requiring high transparency. In this study, a new type of transparent photothermal film, PMMA-iAg, was proposed and optimally designed. The polymethyl methacrylate (PMMA) is doped with indium tin oxide (ITO) nanospheres and silver nanocylinders (AgNCs). Effects of the particle doping concentration ratio and volume fraction fv on the light transfer of the PMMA-iAg film have been systematically studied. An evaluation parameter of “deviation from the ideal spectrum” σT is established to assess the photothermal performance and transparency simultaneously. The results show that the present PMMA-iAg film can achieve a UV and NIR absorbance of 93% and a visible transmittance Tp of 80%. Moreover, the PMMA-iAg films can absorb more high-energy light (UV) instead of letting them transmit. In particular, the effect of ice-drops on the transmittance and haze of the transparent photothermal film was investigated at different projected surface coverage ratios fa and wettability. The results indicate that ice-covered films with both hydrophilic and super-hydrophobic surfaces achieve outstanding transmittance. The ice-covered films with super-hydrophilic surfaces have a remarkably low haze. This work provides guidance for designing efficient transparent photothermal films for the application of anti/de-icing.

Dynamic behavior of droplet impact on micro-nano structures with extreme wetting properties

Extreme wetting surfaces represented by superhydrophilic and superhydrophobic surfaces have important applications in heat transfer engineering such as boiling and condensation. The droplet impact behavior is helpful to deepen the understanding of extreme wetting characteristics and guide practical engineering applications. In this study, superhydrophilic and superhydrophobic micro-nano structures were fabricated by micro embossing and subsequent chemical treatment, and the droplet impact dynamics of microstructures, nanostructures, and micro-nano structures were analyzed using water and alcohol. Alcohol has the lower surface tension and wets the fabricated surfaces more easily than water. Among all fabricated structures, micro-nano multiscale structures exhibit the most extreme droplet impact dynamics. Whether it is water or alcohol droplets, the superhydrophilic micro-nano structures show the best spreading properties, and the maximum spreading factor (the ratio of the droplet wetting diameter to the initial diameter) of water is 134.1 % higher than the flat surface, which is also higher than both the microstructures and the nanostructures. Water droplets are more likely to bounce after impacting the superhydrophobic micro-nano structures than flat and nanostructures. However, due to the pinning effect of the larger-scale microstructures, the droplets impact on micro-nano structures would break and splash while the Weber number exceeds 43.41. The high-temperature impact dynamics show that the superhydrophobic micro-nano structures has excellent high-temperature resistance and can fully bounce within the tested range. The superior bouncing properties indicates the excellent condensation potential of the superhydrophobic micro-nano structures. The superhydrophilic micro-nano structures will be beneficial to enhance boiling heat transfer engineering applications, and the boiling heat transfer of droplets on the superhydrophilic micro-nano structures is the most severe during the boiling temperature range, and the drying time after alcohol impact is ∼ 63.4 % shorter than that of the superhydrophobic micro-nano structures.