Researchers Find Inspiration in Butterfly Wings and Rice Leaves

Humans have long marveled at the water repellency of lotus leaves and the way sharks glide through water, but a new study shows that butterfly wings and rice leaves combine the favorable surface characteristics of both. Inspired by these natural structures, researchers at Ohio State University (Columbus) have created materials that could someday help reduce friction and contamination in medical devices and even enable them to clean themselves.

Bharat Bhushan, director of the university's nanoprobe laboratory for bio- and nanotechnology and biomimetics, and doctoral candidate Gregory D. Bixler set out to develop surfaces to address three common engineering challenges faced by industries, including the medical device sector: fluid drag, biological fouling, and self-cleaning. They began by studying butterfly wings and rice leaves, which solve these problems naturally.

"Nature does everything down to the nanoscale and even the atomic scale, in some cases," remarks Bhushan, an Ohio Eminent Scholar and Howard D. Winbigler professor in the department of mechanical and aerospace engineering. "So we looked to nature to see how it's done."

The researchers observed the surface characteristics of butterfly wings and rice leaves in order to discover what enables them to reduce drag, exhibit low adhesion, and clean themselves. Butterfly wings, they found, feature shinglelike scales and microgrooves arranged like rays, while the surfaces of rice leaves have sinusoidal grooves and micropapillae topped by waxy nanobumps.

In an article published in the journal Soft Matter, Bhushan and Bixler detail how they created nanostructured materials to mimic these characteristics. Creating silicone rubber negative molds from original butterfly wing and rice leaf samples, the researchers filled the molds with urethane polymer to produce positive replicas. Next, they applied a coating of hydrophobized silica nanoparticles to mimic the lotus effect on the rice leaf replicas.

The researchers then conducted experiments to compare the characteristics of the butterfly wing and rice leaf replicas with their original counterparts as well as with original and replica samples of rainbow trout fish scales and mako shark skin. Experiments showed that the butterfly wing replicas exhibited a 79 to 85% ability to self-clean, whereas the coated rice leaf replicas exhibited a 95% self-cleaning rate. Tests also showed that the coated rice leaf replicas exhibited high contact angles, indicating superhydrophobicity, and an adhesion force of just 0.11 N, the lowest among all of the samples. The butterfly wing replicas showed an adhesion force between 0.3 and 0.4 N, which is lower than the replicas of the fish scales and shark skin. The coating applied to the rice leaf replicas was also shown to reduce drag significantly.

Dubbing the combination of low drag, resistance to biofouling, and self-cleaning ability the "rice leaf  and butterfly wing effect," the researchers comment that their findings could inspire designs in medical device and other applications. For example, materials created to take advantage of the rice leaf and butterfly wing effect would be useful in catheters, biosensors, and drug-delivery devices. "Anywhere you have a fluid flow, this work would be of interest," Bhushan notes.

Especially excited about the rice leaf replicas, the scientists plan to research them further, using modeling to inspire the development of new structures. "We've already created a replica," Bhushan adds. "So the next step is to fabricate structures ourselves, as opposed to using the natural object as a master."