C. Majidi, R.E. Groff, Y. Maeno, B. Schubert, S. Baek, B. Bush, R. Maboudian, N. Gravish, M. Wilkinson, K. Autumn, and R.S. Fearing.
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Using
design principles inspired by the nanoscopic hairs on the gecko, UC
Berkeley researchers and colleagues have created a novel microfiber
array which has very high friction but is not ``sticky''. Usual high
friction materials, such as soft rubbers or polymers, are tacky, and
would be uncomfortable on shoe soles (think gum stuck to the bottom of
one's shoe). The high friction micro fiber array works by having tens
of millions of contacts per square centimeter which approximate the
intimate contact soft rubber has with a surface. The microfibers are
made from a rigid plastic which is 100 to 1000 times harder than
rubber, and can resist high temperatures without softening. The
microfiber array has friction which is 10 to 30 times greater than the
friction of the starting plastic.
This novel material could potentially replace soft rubber
on surfaces which need high friction like shoes, tires, or sport gloves.
Supported by National Science Foundation and
Defense Advanced Research Projects Agency
Conceptual drawing showing how microfibers engage with a surface. A compressive load brings more hairs into contact, increasing true contact area. Friction increases with true area of contact. High Friction Friction is the
force that resists sliding between two surfaces. High friction
materials can prevent sliding under high loads or steep inclines. Such
materials are typically soft and can achieve intimate contact with an opposing
surface. A typical high friction material is rubber, which is used
in a variety of applications such as shoes and tires.
For more information, please visit NFJ.
The adhesive system
of gecko lizards also demonstrates high friction. Unlike other high
friction materials, the gecko's adhesive is composed of rigid, durable
material. Intimate contact with an opposing surface is achieved through
the bending of millions of compliant micro-sized hairs called setae. Gecko-inspired
High Friction
A synthetic
microstructure similar to the gecko adhesive was made by casting plastic into
a porous mold. This procedure yields an array of vertically aligned
polymer fibers that are each less than a micron in diameter and
20 microns high (about one fifth the thickness of a sheet of paper).
Point of Contact
Prof. Ronald S. Fearing, UC Berkeley; 510-642-9193;
ronf @ eecs . berkeley . edu
Quarter held in place on glass slide by friction force from gecko-inspired
fiber array alone. No adhesive is used to keep quarter from sliding.
Scanning electron microscope picture of synthetic
fibers which are 20 micrometers tall
and 600 nanometers in diameter.
(Microfibers are about 100 times smaller than a human hair.)
There are approximately 40 million fibers per square centimeter
(250 million fibers per square inch).
Scanning electron microscope picture of setae from Anolis lizard.
(Picture copyright Kellar Autumn).
Anolis seta are approximately the same size as the synthetic
fiber array.
Conceptual drawing showing how microfibers engage with a surface. A
compressive load brings more hairs into contact, increasing true contact area.
Friction increases with true area of contact.
The setae on small animals feet conform to follow the surface so a high proportion of the area can have an attractive adhesion. By conforming to the surface there is never a high pressure exerted on a high point that would push the foot away from the surface and counter the adhesion.ItDoesn'tMatter said:
Yes, it is peculiar since the dry surfaces do not immediately cold weld to each other. Maybe there is enough surface oxidation to prevent a weld forming.Andy SV said:
The setae on small animals feet conform to follow the surface so a high proportion of the area can have an attractive adhesion. By conforming to the surface there is never a high pressure exerted on a high point that would push the foot away from the surface and counter the adhesion.The friction coefficient is I believe a different phenomenon. With friction, only the total force is important because some patches will carry higher contact force while others patches will have none. Friction is one sided. It does not allow you to walk across the ceiling, or even to climb a vertical wall.Yes, it is peculiar since the dry surfaces do not immediately cold weld to each other. Maybe there is enough surface oxidation to prevent a weld forming.The crystal structure in the two samples is extremely unlikely to be aligned on the sample interface so the contact surface must be the average of all the diagonal contact “dislocations”, hence the high coefficient without a weld forming.The thing that at first glance surprised me was indium. It has the highest static coefficient against itself in the list, yet it is used as the surface layer on thin shell engine bearings. I believe that is because it is highly resistant to acids and runs only against cast iron, steel or chrome that is very well lubricated with oil.The list also demonstrates another reason why copper sheet makes such a good head gasket for old tractors with cast iron blocks and heads.
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