Structure Of Lotus Leaf Research Articles

Preparation of a superhydrophobic film with UV imprinting technology

Superhydrophobic surfaces with an advancing water contact angle (CA) larger than 150 and a water sliding angle (SA) less than 10 are found on lotus leaves and butterfly wings. These surfaces are interesting because of their various applications for water-repellence and anti-sticking properties. The lotus leaf is especially well known for its selfcleaning ability. Rolling water droplets wash off contaminants and dust due to low surface adhesion. According to previous studies, the surface dewetting is governed by the surface energy of coating material and roughness of the surface. Poly(dimethylsiloxane) (PDMS) and fluorocarbon based materials have been used to artificially modify a surface due to their low surface free energies. Nanotubes, nanofibers, nanorods, and porous structures have been introduced to create complex nano structures. Theoretically, two distinct models (Wenzel and Cassie models) were proposed to explain the effect of roughness on a hydrophobic surface. Because roughness impacts contact angles, they introduced the surface roughness factor (r), which is defined as the ratio between the actual and projected surface areas. Wenzel derived a theoretical relationship from the Young equation, which correlates the ‘apparent’ or measured contact angle on a rough surface with a flat substrate (Young’s model). His equation shows that an increased r can enhance surface wettability, allowing water to penerate the surface. Cassie and Baxter, on the other hand, propose that an r above a critical level leads to a great decrease in CA hysteresis, therefore increasing the receding angle. In the Cassie model, water droplets partially sit on surface air pockets and can easily roll off. In the previous study, we showed a simple method to imitate the hierarchical lotus leaf structure using sol-gel technology. Briefly, we used a micro lens array (MLA) pattern to create ordered, micron-sized, large hills and porous silica aerogel to create disordered nano structure, and then chemically modified the surface with a PDMS coating solution. Nano-imprint lithography (NIL) is a promising technique to fabricate patterned structures with high precision and throughput in the micro/nanometer scale region. Additionally, NIL is inexpensive and does not require a complicated apparatus. In conventional NIL technology, hard molds, such as silicon, dielectric material (e.g., silicon dioxide or silicon nitride), and metal material (e.g., nickel) were used. Recently, flexible mold technology was introduced, replacing hard molds, making it easier to release the mold from the polymer surface, providing better conformal contact with the substrate to be patterned, and reducing the pressure needed during imprinting. In a previous study, Kim et al. introduced a simple NIL method to create a superhydrophobic surface with a flexible mold made from anodic aluminium oxides (AAOs). Furthermore, Lee et al. produced well-defined, large, nanostructured polymeric and metallic surfaces with nanoembossing or nanofibers and controlled aspect ratios by employing AAOs or textured Al surfaces as a replication master. In this study, we present a superhydrophobic flexible film preparation with ultraviolet (UV) NIL technology using a flexible master substrate with ordered micronsized patterns and disordered nano structure. The advantage of this study is that it removes the surface treatment step that is required in conventional imprinting. The surface treatment allows for easy demolding due to the low surface energy of the superhydrophobic and modifiedPDMS coating surface.

Stable polytetrafluoroethylene superhydrophobic surface with lotus-leaf structure

A stable polytetrafluoroethylene superhydrophobic surface is prepared with filter paper which is first used as a template. Scanning electron microscope image shows a lotus-leaf like structure appears on the polytetrafluoroethylene surface. Altering the sintering temperature, the microstructure of the as-prepared surface also varied. After treating 12 h in acid, alkali or organic solvents, the as-prepared surface still retains superhydrophobicity and shows excellent stability.

Ultra-hydrophobicity Through Stochastic Surface Roughness

The self-cleaning properties of ultra-hydrophobic surfaces are of great interest for technological applications. Specific surface roughness, essential for the dewetting effect, is not necessarily based on deterministic lotus leaf structures. Ultra-hydrophobicity also occurs on a large variety of stochastic, e.g., self-organized, roughness structures, which offers prospects for cost-effective manufacturing techniques (e.g., thin film deposition). This work addresses the development of roughness design algorithms, which deliver application-focused structural parameters, required for efficient and targeted manufacturing processes. Our approach is based on the assessment of wetting-relevant surface structures by a specific roughness analysis using power spectral density functions. The resulting quantitative roughness criterion, a 'wetting parameter', enables the prediction of ultra-hydrophobicity for design purpose as well as for an efficient control and adoption of the manufacturing process. The reliability of this method is demonstrated for a variety of surfaces for engineering and optical applications. For optical applications, the roughness design takes into account both wetting properties and optical quality (light scattering). Optical coatings manufactured according to the design specifications yielded low-scattering, ultra-hydrophobic surfaces.

The preparation of lotus-like super-hydrophobic copper surfaces by electroplating

The preparation of super-hydrophobic metal surfaces had to combine rough structures and the surface modifications with low surface energy materials. Although there were different views on it, no experiment was done to prove it yet. In this paper, the structure of natural lotus leaf was observed and the lotus-like surfaces on hydrophilic copper substrates were fabricated via electroplating in large current density. These surfaces were super-hydrophobic without any chemical modification. The hydrophobic mechanism was analyzed. It is believed that this was a meta-stable state in Cassie model. As long as the surface structure was suitable, hydrophobic surface, even super-hydrophobic surface, could be got on hydrophilic substrate.

Replication of butterfly wing and natural lotus leaf structures by nanoimprint on silica sol–gel films

An original and low cost method for the fabrication of patterned surfaces bioinspired from butterfly wings and lotus leaves is presented. Silica-based sol–gel films are thermally imprinted from elastomeric molds to produce stable structures with superhydrophobicity values as high as 160° water contact angle. The biomimetic surfaces are demonstrated to be tuned from superhydrophobic to superhydrophilic by annealing between 200 °C and 500 °C.

Preparation of material surface structure similar to hydrophobic structure of lotus leaf

Nano/micro replication, a technique widely applied in the microelectronics field, was introduced to prepare the hydrophobic bionics microstructure on material surface. Poly(vinyl alcohol) (PVA) and polystyrene (PS) moulds of the mastoid microstructure on lotus leaf surface were prepared respectively by the nano/micro replication technology. And poly(dimethylsiloxane) (PDMS) replicas with the mastoid-like microstructure were prepared from these two kinds of polymer moulds. Scanning electronic microscope (SEM) was employed to investigate the morphology and microstructures on moulds and replicas. Both the static and dynamic contact angles between water droplet and PDMS replicas’ surface were also measured. As a result, similar microstructure can be observed clearly on the surface of PDMS replicas and the static contact angle on PDMS replicas was enhanced dramatically by the existence of these microstructures.

Preparation of super-hydrophobic surface on stainless steel

To mimic the lotus leaf structure, binary microstructures at both micro- and nano-scale were constructed on the stainless steel surface by the eletroless plating. Super-hydrophobicity was achieved with a water contact angle of 150° and the sliding angle of 4° by modifying the textured surface by means of HFTHTMS ((heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane).

PREPARATION OF SUPER-HYDROPHOBIC SURFACES BY USING ELASTOMER TEMPLATES AND UV-CURABLE RESIN

Super-hydrophobic surfaces have attracted great research interest due to their novel performance as self-cleaning,anti-fog,and anti-oxidation.In this work,a new approach,based on elastomeric templates and UV-curable resins,has been developed to prepare super-hydrophobic surfaces.Poly(dimethylsiloxane) (PDMS,Sylgard 184) was purchased from Dow Corning and used to replicate lotus-leaf surfaces.In the replication step,PDMS blocks were prepared through replica molding against lotus-leaf surfaces.PDMS prepolymer was prepared by mixing the elastomer base and curing agent in a ratio of 10∶1. After 20 min,the PDMS prepolymer was poured into molds within fresh lotus-leaf surfaces as masters.Then the prepolymer was cured in 40℃ oven for 4 h.After curing and peeling off,the PDMS blocks with negative lotus-leaf surfaces' structures were obtained.Microscope observation indicates that the PDMS blocks' surfaces were featured with hollow cavities of 7～12 microns in diameter.The PDMS blocks could be used as stamps in the following curing step of ultraviolet (UV)-curable prepolymers,which consist of oligomer,cross-linking agent,reactive monomer,and photo-initiator.In the experiment,isobornyl acrylate,siliconized urethane acrylate,benzoin dimethyl ether and dipentaerythritol pentaacrylate were used as monomer,oligomer,photoinitiator and crosslinking agent,respectively.In a typical process,UV-curable prepolymer was prepared by mixing monomer,oligomer,photoinitiator and crosslinking agent in a ratio of 10∶68∶2∶20. Then the prepolymer was put into a 0.085 MPa vacuum oven to remove air bubble.The prepolymer was spread on substrates (silicon wafer or glass slide),pressed by the PDMS stamp with replicated lotus-leaf surface.Finally,the UV-curable prepolymer covered with the PDMS stamp was exposed to UV-light (500 μW/cm~2).After curing for 15 min,the PDMS stamp was peeled off,and the cured sample was store in 40℃ oven for 5 h before further experiments.Measured by scanning electron microscopy (SEM,Hitachi S-4500),a large amount of papillary microstructures with an average diameter of about 7～10 microns and average height of 10～18 microns could be seen on the surfaces.These papillary microstructures are replicas of the surface patterns of the lotus leaves.The obtained surfaces had water contact angles larger than 150° and small sliding angles,exhibiting fascinating super-hydrophobicity like lotus-leaf surfaces.Obviously,the replicated papillary microstructures played important role in the increase of surface hydrophobicity from about 105° (smooth film surface) to 157°.This result can be explained with the Wenzel theory.Factors influencing the wettability of resulted surfaces,such as content of monomer,cross-linking agent,initiator and light irradiation time,were also studied in detail.Experiments showed that the surface contact angles increase with the content of photoinitiator before 0.7%,and decrease with the content of crosslinking agent when larger than 20%.A monomer content of about 10% and irradiation time longer than 10 min were also preferable for achieving high hydrophobicity.Combining the advantages of soft-lithographic and UV-curing techniques,this approach can be potentially applied in fabrication of large-area super-hydrophobic surfaces.

Artificial lotus leaf structures from assembling carbon nanotubes and their applications in hydrophobic textiles

Artificial lotus leaf structures were fabricated on cotton substrates via the controlled assembly of carbon nanotubes onto the surface of cotton substrates. Both pristine carbon nanotubes (CNTs) and surface modified carbon nanotubes (PBA-g-CNTs) were used as building blocks to biomimic the surface microstructures of lotus leaves at the nanoscale. Cotton fabrics, which otherwise have perfect water absorbabilities, have been endowed with super-hydrophobic properties—water contact angles greater than 150° were measured. The method provides a bionic route to create hydrophobic textiles. Furthermore, considering the novel mechanical and electric properties of carbon nanotubes, these carbon nanotubes coated cotton fabrics will find potential application in sensing, conducting and special textiles.

Stable Superhydrophobic Coatings from Polyelectrolyte Multilayers

The present study demonstrates that the superhydrophobic behavior of the lotus leaf structure can be mimicked by creating a honeycomb-like polyelectrolyte multilayer surface overcoated with silica nanoparticles. Superhydrophobicity was achieved by coating this highly textured multilayer surface with a semifluorinated silane. The surface maintains its superhydrophobic character even after extended immersion in water. The key structural elements needed to create stable, superhydrophobic coatings from polyelectrolyte multilayers are discussed.