Bionic function surface inspired by the preventing moisture property mechanism of <italic>Ephemera pictiventris</italic> mcLachlan compound eyes

Abstract

The fogging phenomenon affects light transmittance on the surface of a transparent substrate, and what’s more, the substrate surface will therefore become wet, which leads to some secondary problems such as corrosion and deterioration etc. There are two ways to solve the antifogging problem. One is hydrophilic and the other is hydrophobic. However, hydrophilic antifogging method cannot change the “wet” nature. The anti-moisture function on Ephemera pictiventris mcLachlan compound eyes surface was studied in this paper, and the compound eyes structure was characterized via Scanning Electron Microscope (SEM) and Laser Scanning Confocal Microscope (LSCM). Then anti-moisture mechanism was explained by Cassie model based on above structure data. Next artificial Ephemera pictiventris mcLachlan compound eyes surfaces were prepared by direct three-beam Laser Interference Lithography (LIL) and Colloidal Lithography (CL) respectively, and further wettability was also discussed. Results show that the compound eyes surface of Ephemera pictiventris mcLachlan exhibits striking hydrophobic anti-moisture property. The compound eyes are composed of hundreds of micron-scale ommatidia closely arranged on ellipsoidal surface. The morphology of each ommatidium is almost uniform and made up of two parts. The upper part is spherical and the lower is frustum of a cone with the upper spherical diameter of about 20 μ m and the overall height of about 11 μ m. There is no more tiny nanostructure on the ommatidia surface. The morphology and dimension of bionic convex array surface that is artificial compound eyes surface are close to the parameters of true compound eyes surface. The water contact angle (WCA) value of this surface can reach 123°, showing a strong hydrophobic property, after etched 2 min by HF with the concentration of 5%. The mechanism analysis results show that the calculated apparent WCA value is 119.2°, in agreement with the experimental value. Also the apparent WCA value increases with the increase of intrinsic WCA value. The smaller the ratio of ommatidia radius, the greater the apparent WCA value. When the ratio is less than or equal to 0.4, the apparent WCA value is up to 150° or more. The static WCA value and water sliding angle (WSA) value of artificial optimized Ephemera pictiventris mcLachlan compound eyes surface with the feature size of 350 nm are 160°±5° and 2°, respectively, showing excellent superhydrophobic and self-cleaning functions. The result indicates that Ephemera pictiventris mcLachlan compound eyes is dry-style antifogging function structure surface. The synthesis of proper size, compact arrangement of ommatidia and arc shape of the compound eyes is one of the origins of hydrophobic antifogging function. Artificial optimized compound eyes surface shows excellent superhydrophobic and self-cleaning functions.