An optical microscope photograph shows scales on the wing of an adult painted lady butterfly.
Courtesy of the researchers
“Using a special microscopy technique that peers through openings made in the chrysalis itself, the team continuously photographed individual scales as they grew out of the wing membrane during key periods in the butterfly's development. These images revealed for the first time how the initially smooth surface of the scales wrinkles and forms tiny parallel waves, like the ridges in corduroy. The ripple-like structures eventually grow into finer, more patterned ridges that enable the many functions of the adult wing scales.”
The wavy surface is likely the result of 'buckling', a mechanical process that causes a material to bend inwards when subjected to a compressive force or confined in a confined space. In this case, using a theoretical model that describes a common mechanism for buckling, they found that actin bundles – long fibres that run under the growing membrane and support the scales as they shape – anchor the membrane in place, like ropes around an inflating hot-air balloon.
“Buckling is an instability that, as an engineer, you typically don't want to happen,” said Matthias Collé, an associate professor of mechanical engineering and co-author of the study, “yet in this situation, organisms are using buckling to initiate the growth of these complex, functional structures.”
The research team is working to visualize further stages of butterfly wing development that could provide inspiration for advanced functional materials in the future.
“These materials exhibit optical, thermal, chemical and mechanical properties that are suitable for textiles, building surfaces, vehicles and any surface that must exhibit properties that depend on micro- or nanoscale structures,” Cole says.
“We want to learn from nature not just how these materials work, but also how they form,” says MIT postdoc and co-author Anthony McDougall (SM '15, PhD '22). “For example, if we want to make a wrinkled surface that's useful for a variety of applications, this technique gives us two knobs that allow us to very easily tune how the surface wrinkles: we can change the spacing of where the material is pinned down, or we can change the amount of material that grows between the pinned pieces. And it turns out that butterflies use both of these strategies.”
