Aug 26 2016
This is Anand Jagota, Professor of Chemical and Biomolecular Engineering and Director of Bioengineering, Lehigh University. (Credit: Douglas Benedict/Academic Image)
A gecko’s ability to run across smooth walls and hang upside down from improbable surfaces has fascinated scientists for centuries, including Aristotle who made his observation of it in his History of Animals.
It wasn't until around 15 years ago, researchers were able to precisely link the gecko's adhesive powers to the nanoscale threads found on the toes of the gecko. Only then the practical potential of biomimicry at microscopic range began to captivate the minds of researchers.
Recently, a team led by Lehigh University partnered with Michelin Corporation and the National Science Foundation to create materials with surface architectures that have the potential to enhance the reliability and safety of tires.
The NSF's Grant Opportunities for Academic Liaison with Industry (GOALI) program is designed to facilitate firms to "kick the tires," in a manner of speaking, on academic research that probably have major impact upon their industry as well as the society.
Anand Jagota, professor of chemical and biomolecular engineering and director of Lehigh's bioengineering program, is a renowned researcher in biomechanics, biomaterials, and nanobiotechnology for about 30 years; prior to joining Lehigh, he was involved in similar research works with duPont Corporation. During his career, he has developed a keen focus in using biomimetics to optimize the mechanical properties and adhesive of rubbery materials.
Recently, Jagota and his team published a paper in Scientific Reports, a journal of the Nature Publishing Group, which highlights their work developing new bio-inspired film-terminated structures with unique friction features that is likely to have encouraging industrial implications for tires, among other things.
Strongly Modulated Friction of a Film-Terminated Ridge-Channel Structure
Natural contacting surfaces have remarkable surface mechanical properties, which has led to the development of bioinspired surface structures using rubbery materials with strongly enhanced adhesion and static friction. However, sliding friction of structured rubbery surfaces is almost always significantly lower than that of a flat control, often due to significant loss of contact. Here we show that a film-terminated ridge-channel structure can strongly enhance sliding friction. We show that with properly chosen materials and geometrical parameters the near surface structure undergoes mechanical instabilities along with complex folding and sliding of internal interfaces, which is responsible for the enhancement of sliding friction. Because this structure shows no enhancement of adhesion under normal indentation by a sphere, it breaks the connection between energy loss during normal and shear loading. This makes it potentially interesting in many applications, for instance in tires, where one wishes to minimize rolling resistance (normal loading) while maximizing sliding friction (shear loading). (Video Credit: Lehigh University and Anand Jagota)
The paper, "Strongly Modulated Friction of a Film-Terminated Ridge-Channel Structure," was co-written by Jagota and lead author Zhenping He along with Ying Bai, Chung-Yuen Hui of Cornell University and Benjamin Levrard, a researcher at Michelin Corporation.
Michelin was drawn towards Jagota and Hui's biomimetic research when initial results were revealed at a conference in France a few years ago, and now the partnership is going strong.
For tires, there is a typical performance challenge among traction, fuel efficiency, and tire life. Optimizing the quality of one tends to degrade the other.
Nature's Designs, at Work on the Highways
High quality tires minimize rolling resistance, which improves fuel efficiency, while maximizing the sliding friction that basically helps to brake quickly. To help increase this sliding friction, tires currently employ millimeter-scale structures to grip the road and channel water. We are working to create structures at the microscale that will enhance friction and adhesion control.
Anand Jagota, Professor of Chemical and Biomolecular Engineering, Lehigh University
Jagota and his team wanted to work with smooth pad surfaces and looked at the feet of frogs or grasshoppers instead of the hairy fibrils found on gecko toes.
Prior to the latest study, the team had created a thin film containing a range of minute pillars on top of a substrate.
"We placed these pillars or posts in an hexagonal array and covered them with a thin coating that allowed them to make solid contact with rough surfaces and strongly enhances static friction," says Jagota. "Dragging the film in any direction provided the same friction. But tires don't require the same properties in all directions, so we went to an array of parallel ridges. We believed this would provide greater resistance to sideways movement across the film - and greater sliding friction."
They were correct, but the scale of the results amazed the team. The parallel ridges formed a surface where the "good" lateral sliding friction was boosted considerably.
"This was the most unexpected thing: in the ridge-channel geometry, the film improved sliding friction dramatically, by a factor of three or four," Jagota says.
The team developed a film using rubber-like material in the experiment. It had rows of parallel ridges that were uniformly spaced and covered with a thin topcoat. The film was laid flat and then a glass ball was pressed into the film and dragged through it in a perpendicular direction to the ridges.
According to the team, high contortion of the ridges increased sliding friction. The ridges stretched and rode up on each other under the pressure of the sphere, which then formed wide areas of surface and internal contact. This internal sliding allowed surplus energy to be discharged. Furthermore, elastic energy that was absorbed during the contortion was then discharged when the ridges sprung back to their original form.
The results show potential. Enhanced sliding friction could improve the grip of the tire, as forward energy is discharged from the surface of the tire to disperse as sound waves and harmless heat. With no corresponding increase in adhesion noticed, rolling resistance should not be significantly increased.
The NSF's GOALI grant will offer financial support for the ongoing efforts of the team over the next three years.
This has been a very fruitful collaboration already. We are still in the early stages, but the collaborative support from Michelin and the NSF is making it possible for us to put nature's designs to work on the highways.
Anand Jagota, Professor of Chemical and Biomolecular Engineering, Lehigh University