title: Wettability gradient on the elytra in the aquatic beetle Cybister chinensis and its role in angular position of the beetle at water-air interface.
abstract: The surface of the elytra in some species of aquatic beetles displays relatively low contact angles (CAs), even showing hydrophilic properties. In this study, we report on an observation that both sexes of Cybister chinensis beetle fresh elytral surface do not exhibit uniform CA, but rather a wettability gradient along the longitudinal axis in posterior direction. The wettability is very different between females and males due to the presence (female) or absence (male) of channels on the elytral surface. When a small drop of water touches the elytra surface, it tends to slide towards the anterior having a lower CA on the elytra. This gradient presumably supports a breathing-associated behavior of beetles in which they cause the tip of their abdomen to protrude into the surface of the water in order to collect an air bubble for oxygen uptake and, when floating on the surface, to keep the body inclined at a small angle to the water's surface with their heads immersed.
  
  
  
  Hydrophobicity on surfaces is a fundamental property which has attracted great interest across all scientific disciplines, here we have demonstrated that the gradually changing chemistry of the elytral surface facilitates the tilted beetle posture on the water's surface. The mechanism of water interacting with the elytra demonstrated the most energetically favorable posture in the diving beetles. Surfaces with directional wetting properties that promote droplet drainage are of significant practical importance in many fields. The anisotropic topography and wetting properties of the elytra may inspire microfluidic devices for medical and robotic applications.
authors.affiliation: Key Laboratory of the Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beichen West Road 1-5, Chaoyang District, Beijing 1000101, China; Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, D-24118 Kiel, Germany. Electronic address: sunmx@ioz.ac.cn.
authors.forename: Mingxia
authors.initials: M
authors.lastname: Sun
authors.affiliation: Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Xueyuan Road 37, Haidian District, Beijing 100191, China. Electronic address: chen1027yuan@hotmail.com.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Xueyuan Road 37, Haidian District, Beijing 100191, China. Electronic address: zhengym@buaa.edu.cn.
authors.forename: Yongmei
authors.initials: Y
authors.lastname: Zheng
authors.affiliation: Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Haidian District, Beijing 100190, China. Electronic address: zhenmm@iccas.ac.cn.
authors.forename: Mingming
authors.initials: M
authors.lastname: Zhen
authors.affiliation: Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Haidian District, Beijing 100190, China. Electronic address: shucy@iccas.ac.cn.
authors.forename: Chunying
authors.initials: C
authors.lastname: Shu
authors.affiliation: Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, China. Electronic address: zddai@nuaa.edu.cn.
authors.forename: Zhendong
authors.initials: Z
authors.lastname: Dai
authors.affiliation: Key Laboratory of the Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beichen West Road 1-5, Chaoyang District, Beijing 1000101, China. Electronic address: liangap@ioz.ac.cn.
authors.forename: Aiping
authors.initials: A
authors.lastname: Liang
authors.affiliation: Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, D-24118 Kiel, Germany. Electronic address: sgorb@zoologie.uni-kiel.de.
authors.forename: Stanislav N
authors.initials: SN
authors.lastname: Gorb
mesh_terms: D000388:Air; D000825:Animal Structures / Q000502:physiology* / Q000648:ultrastructure; D000818:Animals; D059001:Aquatic Organisms / Q000502:physiology*; D001696:Biomechanical Phenomena; D001517:Coleoptera / Q000502:physiology*; D005260:Female; D008297:Male; D061848:Optical Imaging; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*; D017155:Wettability
chemical_list: D014867:Water
keywords: Coleoptera; Cuticle; Dytiscidae; Elytra; Surface chemistry; Surface microstructure; Wetting