Dimpled metamaterial in hourglass pattern. Credit: Harvard John A. Paulson School of Engineering and Applied Sciences
Imagine a road cyclist or downhill skier whose clothing adapts to their wind speed, allowing them to shave time just by pulling or stretching the fabric.
Such cutting-edge textiles are within reach, thanks to researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). Led by SEAS mechanical engineering graduate student David Farrell, a study published in Advanced Materials describes a new type of textile that uses dimpling to adjust its aerodynamic properties while worn on the body. The research has the potential to change not only high-speed sports, but also industries like aerospace, maritime, and civil engineering.
The research is a collaboration between the labs of Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics, and Conor J. Walsh, the Paul A. Maeder Professor of Engineering and Applied Sciences.
On-demand golf ball dimples
Farrell, whose research interests lie at the intersection of fluid dynamics and artificially engineered materials, or metamaterials, led to the creation of a unique textile that forms dimples on its surface when stretched, even when tightly fitted around a person's body. The fabrics utilize the same aerodynamic principles as a golf ball, whose dimpled surface causes a ball to fly farther by using turbulence to reduce drag. Because the fabric is soft and elastic, it can move and stretch to change the size and shape of the dimples on demand.
Adjusting dimple sizes can make the fabric perform better in certain wind speeds by reducing drag by up to 20%, according to the researchers' experiments using a wind tunnel.
"By performing 3,000 simulations, we were able to explore thousands of dimpling patterns," Farrell said. "We were able to tune how big the dimple is, as well as its form. When we put these patterns back in the wind tunnel, we find that certain patterns and dimples are optimized for specific wind-speed regions."
Farrell and team used a laser cutter and heat press to create a dual-toned fabric made of a stiffer black woven material, similar to a backpack strap, and a gray, softer knit that's flexible and comfortable. Using a two-step manufacturing process, they cut patterns into the woven fabric and sealed it together with the knit layer to form a textile composite. Experimenting with multiple flat samples patterned in lattices like squares and hexagons, they systematically explored how different tessellations affect the mechanical response of each textile material.
Lattice pattern
The textile composite's on-demand dimpling is the result of a lattice pattern that Bertoldi and others have previously explored for its unusual properties. Stretch a traditional textile onto the body, and it will smooth out and tighten. "Our textile composite breaks that rule," Farrell explained. "The unique lattice pattern allows the textile to expand around the arm rather than clamp down.
"We're using this unique property that [Bertoldi] and others have explored for the last 10 years in metamaterials, and we're putting it into wearables in a way that no one's really seen before," Farrell said.
Provided by Harvard John A. Paulson School of Engineering and Applied Sciences
