Mesostructure of Ordered Corneal Nano-nipple Arrays: The Role of 5–7 Coordination Defects

Abstract

Corneal nano-nipple structures consisting of hexagonally arranged protrusions with diameters around 200 nm have long been known for their antireflection capability and have served as biological blueprint for solar cell, optical lens and other surface designs. However, little is known about the global arrangement of these nipples on the ommatidial surface and their growth during the eye development. This study provides new insights based on the analysis of nano-nipple arrangements on the mesoscale across entire ommatidia, which has never been done before. The most important feature in the nipple structures are topological 5- and 7-fold coordination defects, which align to form dislocations and interconnected networks of grain boundaries that divide the ommatidia into crystalline domains in different orientations. Furthermore, the domain size distribution might be log-normal, and the domains demonstrate no preference in crystal orientation. Both observations suggest that the nipple growth process may be similar to the nucleation and growth mechanisms during the formation of other crystal structures. Our results are also consistent with the most recently proposed Turing-type reaction-diffusion process. In fact, we were able to produce the key structural characteristics of the nipple arrangements using Turing analysis from the nucleation to the final structure development.

A Comparative Study of Crystallography and Defect Structure of Corneal Nipple Array in Daphnis nerii Moth and Papilio polytes Butterfly Eye

“…The grain size and cumulative size distribution for both moths(Male/Female) are shown in Figure S3 in the Supporting Information . Our analysis on grain size distribution of the male Daphnis nerii eye compares well with the Mourning Cloak butterfly eye, 36 while the average grain size of the female Daphnis nerii eye is larger.…”

“…A similar large-angle misorientation was observed in the case of the Mourning Cloak butterfly eye. 36 Some of the (7-5) defects are rotated by 12–15° with respect to each other. These defects combine themselves to form low-angle grain boundaries.…”

“…A similar defect structure is also reported in different butterfly species. 36 , 37 These defects form the demarcation between two adjacent grains of the perfect hexagonal arrangement of the corneal nipples. Even though the defects are formed by the combined 5- and 7-defects, the concentrations of isolated defect sites are very low.…”

Optical Modelling and Phylogenetic Analysis Provide Clues to the Likely Function of Corneal Nipple Arrays in Butterflies and Moths

“…Indeed, eyeshine is visible in the moth Phalaenoides tristifica Hübner (Lepidoptera: Noctuidae), despite it being both diurnal (when reflectance would not be problem) and having 250 nm-high nipples, characteristic of nocturnal moths [57]. In addition, reduced light reflectance is not restricted to eyes with regular nipple structures but is also observed in irregular arrangements [10].…”

“…The surface of the lepidopteran eye consists of hexagonal facets, the outer surface of which may be covered with tiny protuberances termed corneal nipples [7,8,9]. These nipples are generally arranged in closely packed lattices where each nipple has six neighbours, but each facet has defective arrays where nipples have either five or seven neighbours known as 5–7 co-ordination defects [10]. These 5–7 co-ordination defects are aligned in rows forming boundaries between domains with perfect structures.…”