22 MOMENTUM • VIRGINIA TECH MECHANICAL ENGINEERING
ability to “ send out ” the optical signals to its potential receivers .
At the same time , mechanically , the presence of micropillars increases the stiffness , strength , and mechanical robustness of the structure by preventing the formation of shear bands , improving the damage resistance of the outer layer , and localizing damage to the exoskeleton .
After gaining an understanding of the basic mechanisms for optical coloration and mechanical reinforcement , Li and his team studied how the arrangement and size of an exoskeleton ’ s micropillars impact both factors .
They found that a balance had to be struck : If there were many micropillars , the mechanical strength would be improved , Li explained . However , this would degrade the structural color , because the area percentage of the horizontal multilayer would be reduced .
The final objective was to determine which property , optical or mechanical , is more optimized when evolution “ designs ” the microstructure . To answer this question , the team examined the microstructure of flower beetles from the same species group , but with different colors .
Optical function won the day . They found that the size and distribution of the micropillars in beetles of differing colors were indeed optimized for achieving the most efficient light redistribution . The improvement of mechanical properties , particularly the stiffness , appeared not to be optimized , since the microstructure was not entirely covered with the stiffer and stronger micropillars . This result indicated that optical performance took priority over mechanical performance during the evolution of this peculiar multilayer , micropillar structure .
“ This work presents a remarkable example of how nature achieves multifunctionality with unique microstructural designs ,” said Li . “ We believe the material strategies revealed in this work can be used in designing photonic coating materials with robust mechanical performance . Our interdisciplinary approach based on materials , optics , mechanics , and biology also offers an important avenue to understanding the evolution at a materials level .”