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Norwegian University of Science and Technology (NTNU) 2021

Conical micro-structures for super-repellent surfaces and their effect on droplet impact

Ding, Wenwu

Titre : Conical micro-structures for super-repellent surfaces and their effect on droplet impact

Auteur : Ding, Wenwu

Université de soutenance : Norwegian University of Science and Technology (NTNU)

Grade : Philosophiae Doctor 2021

Résumé partiel
Surface wetting properties control is crucial to various applications, such as anti-wetting, self cleaning and heat transfer processes. Design of surface wetting properties can be achieved by mechanical and chemical methods. The mechanical methods include surface roughness design, while chemical methods involve changing the intrinsic wetting properties of the surface. The intrinsic wetting properties, which can be characterized by the contact angle on a flat surface, can reach a maximum of 120°. The combination of roughness and chemical treatment can distinctly enlarge the contact angle range, including wicking (zero contact angle), partial wetting (finite contact angle) and superhydrophobicity (contact angle larger than 150°). Thus, a lot of works showing various types of structure design with different wettability properties can be found in the literature. Among the various possible micro-structures, conical structures (well-known structures found in the Lotus leaf) are frequently used due to their unique properties. The Lotus leaf has a lot of micro-scale tapered bumps and also nano-scale roughness. Various previous works artificially produce this kind of conical structures and achieve similar wetting properties. However, a lot of these works use non-regular conical like structures, while the number of works using patterned conical structured surfaces is limited. Therefore, it remains unclear how the different conical geometries can affect the wetting properties of the surface. To bridge this gap, we fabricate patterned conical micro-structured surfaces with different cones geometry and topography and study how the static and dynamic wetting properties are affected by the structures. The conical micro-structures are produced on silicon substrates using photo-lithography and plasma etching techniques. By varying the fabrication process recipe, different types of conical structures are fabricated. This thesis presents the study of both static and dynamic wetting properties for various conical structured surfaces. In addition, cylindrical pillar structured surfaces are also used for comparison.

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