Butterfly wings cannot be very far behind geckos' toes so far as sources of inspiration for biomimicry research goes. Various properties of the wings of lepidopterans have triggered research into banknote forgery prevention, light reflection and solar cells. New research from Ohio State University suggests the delicate membranes may hold clues to dirt-resistance surfaces.

The researchers turned to the Giant Blue Morpho (Morpho didius), a Peruvian butterfly species with beautiful iridescent-blue wings in males. "Their wings are so delicate that getting dirt or moisture on them makes it hard to fly," said Ohio State Professor of mechanical engineering, Bharat Bhushan. "Plus, males and females recognize each other by the color and patterns on their wings, and every species is unique. So they have to keep their wings bright and visible in order to reproduce."

Having studied the butterfly wings, and the (less headline-friendly) leaves of a rice plant (Oriza sativa), under electron microscope, Oriza sativa Bhushan and doctoral student Gregory Bixler made plastic replicas mimicking their textures. They then compared its dirt-repelling properties to plain flat surfaces, and similar plastic replicas of fish scales and shark skin from prior research. The researchers applied a nanoparticle surface to all of their surfaces, inspired by the waxiness and slipperiness of the leaves and sharkskin.

Each surface was then covered with a dusting of silicon carbide powder, elevated to an angle of 45º, and syringed with water for two minutes. By comparing the number of dust particles before and after the syringing, the researchers were able to quantify the dirt resistant properties of each surface.

The researchers report that they were able to clean 85 percent of the dust from the butterfly-mimicking plastic compared to just 70 percent from a flat surface. Part of their explanation for this is that, viewed under the microscope, the butterfly wing is far from smooth. Rather, the surface resembles a "clapboard roof" of shingles that, Bhushan suggests, dirt and moisture "rolls" off.

The butterfly wing's micrometer-scale shingles viewed under electron microscope (Image: Bharat Bhushan and Gregory Bixler, courtesy of Ohio State University)

However, the butterfly-inspired surface performed worse than the sharkskin-inspired surface, from which 98 percent of dust was cleared; and the rice leaf-inspired surface, from which 95 percent washed off.

In a separate test, the surfaces were applied to the inner surface of a pipe, and the resulting drop in water pressure recorded, with the three materials finishing in the same order on the podium, but all ahead of the same old flat surface.

As a result of the tests Bhushan suggests that the rice leaf-inspired surface is particular suited to lining pipelines while a Giant Blue Morpho-like material might be best suited to coating medical equipment where it apparently might prevent the growth of bacteria.

The research paper, Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity, appears in the latest edition of the journal Soft Matters.

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