Moving across slippery surfaces

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2011

A study examines whether quick movement across a surface minimizes slipping. 

KEY CONCEPTS
• An individual in motion slips on a low-friction surface because he or she is unable to maintain stable contact with the ground.
• Guinea fowl birds moving over a low-friction surface did not fall if they moved at a speed higher than three meters a second.
• This study has direct relevance to human beings and indicates that moving at faster speeds minimizes slipping because the individual has less contact with the surface.

Important lubrication parameters such as friction have relevance in our day-to-day lives. While those in the lubricant industry are very focused on solving important issues involving friction and wear, they may not be as aware about how much these concepts impact their personal lives.

For example, a previous TLT article discussed how consumers select clothing by how the fabric feels as we pass our fingers through a piece of clothing (1). Research in this area showed that Amontons’ law of friction will not apply to fabrics. Instead, a new parameter known as the Refined Friction Factor was proposed to evaluate common fabrics such as denim and lightweight cotton.

Another real-world situation that we have all faced at some point is how not to fall when moving across a slippery surface. Andrew Clark, assistant professor of biology at the College of Charleston in Charleston, S.C., says, “Slipping occurs on a low friction surface when a person in motion is unable to keep stable contact between his or her limb and the ground.”

Clark indicates that usually the person slipping will fall backward unless something strange happens. The slip is caused because the person is not able to achieve the minimum required coefficient of friction to maintain stability while moving across a surface.

One of the key parameters evaluated in the process of moving across a surface is the contact angle between the limb and the ground. Clark says, “A person taking a large step sticks out his or her foot at a smaller contact angle. This motion does not enable the foot to stick to the ground, leading to slipping because ground reaction forces are insufficient to support the person’s body weight.”

Ground reaction force is based on Sir Issac Newton’s Third Law of Motion, which states that for every action there is an equal or oppositely directed force generated in response. Clark explains, “In the act of standing still on a surface, the person exerts a downward force that is equal to his or her body weight. This force leads the surface to respond with a vertical ground reaction force of equal magnitude. When running or walking, the vertical ground reaction forces increase to support the moving foot as it accelerates and decelerates. Thus, the amount of ground reaction force is dependent upon how well the individual can exert its body weight on the surface.”

Intuitively, it makes more sense for a person to move slowly across a surface to minimize the chances for slipping. But past studies suggest that the opposite occurs.

Clark and his fellow researchers decided to conduct an empirical study to verify if quick movement across a surface will minimize slipping. The results of this experimentation may provide insight to help those individuals who are physically challenged to better able to move across slippery surfaces.

HELMETED GUINEA FOWL
The researchers decided to evaluate the movement of guinea fowl on a low friction, polypropylene surface and on a 150-grit sandpaper, high friction surface. Clark says, “We selected guinea fowl because they are representative of birds that move in a similar fashion to human beings. The guinea fowl also are warm-blooded and have a muscular system similar to human beings.”

Four helmeted birds of comparable weight and age were used in the study. They moved along a 6 meter-by-0.4 meter runway. Their motion was filmed for future study. Figure 2 shows the movement of a guinea fowl down the runway.


Figure 2. Guinea fowl birds were used in a study that determined quick movement across a low friction surface minimized slipping. (Courtesy of Timothy Higham/Clemson University)

The researchers confirmed that guinea fowl moving slowly over a low friction surface have a high probability of slipping if a small contact angle exists between their limbs and the surface. Clark says, “Contact angles exceeding 70 degrees minimized the chances for the guinea fowl to fall.”

A second parameter investigated by the researchers is the minimum distance threshold that will enable guinea fowl to slip. Clark says, “We found that a slip distance less than 10 centimeters is needed to minimize the chances for the guinea fowl to fall. This distance is also applicable to human beings walking over slippery surfaces.”

Most important, the researchers found that guinea fowl moving across the surface at speeds higher than three meters per second never fell even with small contact angles on a low-friction surface. Clark says, “This finding means the birds gain more stability on a slippery surface by moving at a faster speed. The key factor is that movement at a faster speed means that the guinea fowl have less contact with the surface. Furthermore, at higher speeds, the guinea fowl’s center of mass (hips) will successfully pass forward and over its base of support (talons), thus keeping slip distance below 10 centimeters and preventing the body from falling backwards.”

Clark draws an analogy with driving a motorcycle on a road. He explains, “Riders moving at a higher speed can negotiate obstacles more smoothly than at lower speeds.”

One other interesting aspect is that the researchers noted that the guinea fowl use the same approaches to avoid falling as human beings.

Future work will involve trying to determine the required coefficient of friction for a surface to minimize the chances of slipping. Three-dimensional kinematic data will also be generated from the filmed experiments.

Further information can be obtained from a recent article (2) or by contacting Clark at clarkaj@cofc.edu.

REFERENCES
1. Canter, N, (2006), “Determining the Friction on Fabrics,” TLT, 62 (6), pp. 14–16.
2. Clark, A. and Higham, T., (2011), “Slipping, Sliding and Stability: Locomotor Strategies for Overcoming Low-Friction Surfaces,” The Journal of Experimental Biology, 214 (8), pp. 1369–1378.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.