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Generation of Hierarchically Ordered Structures on a Polymer Film by Electrohydrodynamic Structure Formation Hongmiao Tian, Jinyou Shao,* Hong Hu, Li Wang, and Yucheng Ding Micro- and Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, 28 Xianning Road, Xi’an, 710049, P. R. China ABSTRACT: The extensive applications of hierarchical structures in optoelectronics, micro/nanofluidics, energy conservation, etc., have led to the development of a variety of approaches for their fabrication, which can be categorized as bottom-up or top-down strategies. Current bottom-up and top-down strategies bear a complementary relationship to each other due to their processing characteristics, i.e., the advantages of one method correspond to the disadvantages of the other, and vice versa. Here we propose a novel method based on electrohydrodynamic structure formation, aimed at combining the main advantages of the two strategies. The method allows the fabrication of a hierarchically ordered structure with well-defined geometry and high mechanical durability on a polymer film, through a simple and low-cost process also suitable for mass-production. In this approach, upon application of an electric field between a template and a substrate sandwiching an air gap and a polymer film, the polymer is pulled toward the template and further flows into the template cavities, resulting in a hierarchical structure with primary and secondary patterns determined by electrohydrodynamic instability and by the template features, respectively. In this work, the fabrication of a hierarchical structure by electrohydrodynamic structure formation is studied using numerical simulations and experimental tests. The proposed method is then employed for the onestep fabrication of a hierarchical structure exhibiting a gradual transition in the periodicity of the primary structure using a slant template and a flat polymer film, which presents an excellent performance on controllable wettability. KEYWORDS: hierarchical structures, electrohydrodynamic structure formation, micro/nano-structures, polymer structure, controllable wettability
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INTRODUCTION Hierarchically ordered structures have attracted increased attention inspired by bio-organisms unique properties to allow them to survive in a diversity of conditions, which offer inspiring marvel to material properties on providing the capability of the water striders floating on the water becuase of the microscopic hairs on their legs,1 the geckos climbing up walls originating from the hair-like setae covering their feet,2,3 the self-cleaning properties of the lotus plant because of the multilevel structures on the leaves,4,5 and so on. Research on the hierarchical micro/nanostructures in bionics provides the development of applications in electronics/optoelectronics,6,7 photonics,8,9 micro/nanofluidic devices,10,11 energy conservation, and storage,12,13 and other fields, due to the unique ability of these structures to enhance specific functionalities. In practice, hierarchical polymer structures with nanoscale features superimposed to microscale ones have been studied in the field of dry adhesives, with the purpose to transport a very large, thin substrate (e.g., glass panels of size ∼3350 × 3950 mm and thickness
