This award will allow for a combined experimental and theoretical study of hierarchical surfaces exhibiting the "lotus effect". The main experimental objectives are to prepare hierarchical surfaces that exhibit superhydrophobicity, self-cleaning and drag reduction, and to optimize the durability of these surfaces to identify which fabrication techniques and materials can potentially withstand real world applications. Functional testing will be conducted on the durable surfaces coated on glass windows and solar panels to evaluate their water repellency and durability. To better understand the experimental results, the theoretical basis of how hierarchical roughness governs the wetting regime transitions, contact angle hysteresis, the length scale dependence of the contact angle, and dynamic effects will be investigated through mathematical modeling.

The insights generated from this study will further advance the use of biomimetic surfaces for various applications where both energy efficiency and mechanical reliability are crucial. Since superhydrophobic surfaces can be used in diverse applications such as windows, windshields, solar panels, textiles, ships, micro/nanochannels, among others, eliminating the need for cleaning will reduce energy usage. Since the hydrophobicity of a surface affects the capillary force, new ways of energy conversion, such as the microscale capillary engine, will be further developed. Identifying the processes and materials that improve the mechanical reliability of these surfaces will lead to immediate commercial applications. The nature of this research project will benefit society in general by furthering the interactions in the nanotechnology community. This research project will also enhance interest in science and engineering among high school students though seminars and lab visits.

Project Report

, Self-Cleaning and Drag Reduction Prof. Bharat Bhushan Inspired by Lotus leaf (Nelumbo nucifera), superhydrophobic and self-cleaning surfaces were designed and developed with hierarchical structure, where microscale roughness was created with microparticles or micropatterns and nanoscale roughness was created by deposition of nanoparticles using a spray method. Comparisons were made between use of microparticles and micropatterns in terms of wettability across different pitch values, and similar trends in contact angle (CA) and contact angle hysteresis (CAH) were found. Additionally, transparent superhydrophobic surfaces were created with nanoparticles via dip coating. The static water CA and CAH were measured to characterize superhydrophobicity and potential for self-cleaning effect. Self-cleaning were confirmed through examination of the removal of dust particles by water droplets. The mechanical durability and wear resistance of surfaces were tested using waterfall/water jet techniques, AFM techniques, and conventional ball-on-flat tribometer. Superhydrophobic and self-cleaning surfaces were investigated for antifouling. Nature’s antifouling surface properties included a combination of mechanisms such as surface topography, sloughing, and chemical interactions. Rice leaves and butterfly wings were shown to provide drag reduction and self-cleaning, which combined the shark skin (anisotropic flow leading to low drag) and lotus (superhydrophobic and self-cleaning) effects. Rice leaf and butterfly wing inspired samples were fabricated using photolithography and hot embossing approaches, where experiments were conducted to evaluate their drag reduction and self-cleaning effectiveness. Nanostructured coatings were applied to samples to study the effects of superhydrophobicity and superoleophobicity on replicas. Anti-biofouling properties of the micropatterned surfaces were studied with Escherichia coli (E. coli) microorganisms using various concentrations and incubation times. The anti-inorganic fouling properties were evaluated using simulated dirt particles. Results were discussed to understand the role of sample geometrical dimensions and conceptual models were developed describing the role of surface structures related to low drag, self-cleaning, and antifouling properties. Superomniphobic (superhydrophobic and superoleophobic) surfaces were fabricated with composites of nanoparticles and low surface energy fluorobinders using dip coating and spray coating techniques. The particle-to-binder (P-B) ratio was optimized. The mechanical durability of single layered coatings was improved by dual layered coatings, because dual layered coatings strengthened the adherence of the nanoparticles with the binder as well as with the coated substrate. The mechanical durability was examined under mechanical rubbing action. The anti-smudge properties were examined by wiping an artificially contaminated coating using oil-impregnated microfiber cloth. Superhydrophobic and self-cleaning surfaces combined with mechanical durability are of interest in many commercial and industrial applications, including drag reduction in biosensors, self-cleaning glass, and antifouling surfaces for navigation ships. As an example, in medical arena, hydrophobic polymers with antimicrobial properties are of interest in various technologies including catheters, endotracheal tubes and orthopedic hip implants. In marine arena, more environmentally friendly antifouling and fouling-release coatings are desired. There has been significant interest so far by industries involved in fabrication of consumer goods, aircrafts, mobile devices, oil pipelines and sporting equipment. This research will provide guidance for researchers and engineers in development of surfaces with hydrophobicity, low drag, anti-biofouling, and anti-inorganic fouling characteristics. Aditionally, efficient and effective approaches were reported in this study to fabricate surfaces with different characteristics. The relevant knowledge, skills, technologies and methods of this study can be transferred among universities and other institutions. The scientific and technological developments can be accessible to a wider range of users when they are exploiting the technology to develop new products, processes and applications. The technology has been submitted for IP protection. It is anticipated, that company would license this technology. In this project, 15 peer-reviewed journal papers were published and one is in press; 2 patents were submitted (pending); the scientific achievements were reported in various media. The researchers on the project received extensive training on Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), laser etching, soft lithography, micromolding, photolithography, closed channel drag and wind tunnel drag measurements, contaminant wash testing, hot embossing, clean room training, and various wear test procedures. In addition, the lab has provided the opportunity and actively assisted a STEM school student in conducting open channel drag experiments. Undergraduate students have designed and built highly relevant equipment in the laboratory for capstone design projects. In addition, courses were offered in nanotechnology and tribology. Presentations were given at various conferences attended by students and researchers of various races and ethnic origin. Efforts were constantly planned to attract high school students with ethnic diversity, including summer courses and lab tours. In summary, the outcome of this study is expected to be beneficial to both research community and industry when designing and developing novel surfaces for various applications and solving numerous engineering problems. As our findings from this research are adopted to practical products, the desired properties would be achieved and our society will benefit from this study. Thus it will help to save energy and money in real life.

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Ohio State University
United States
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