What materials have high friction?

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. 

High Friction from a Stiff Polymer using Micro-Fiber Arrays, Physical Review Letters, 18 August 2006.

vol. 97, no. 076103

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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.

Point of Contact
Prof. Ronald S. Fearing, UC Berkeley; 510-642-9193; ronf @ eecs . berkeley . edu

Supported by National Science Foundation and Defense Advanced Research Projects Agency

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.

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. 

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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). 

ItDoesn'tMatter said:

For example, I know synthetic setae are very resistant to slipping on surfaces.

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.

Andy SV said:

That's interesting I did not know that about self stick of metals

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.

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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