Friction at the Tip of the Finger
Earlier last month, the Journal of Experimental Biology published a study about fingerprints and that there is new doubt that fingerprints increase friction, which supposedly facilitates the grip by our hands. Now, they are saying that fingerprints actually reduce the amount of friction between the skin and other surfaces. This is probably contrary to everyone’s intuition.
According to BBC News:
Dr Roland Ennos [from the University of Manchester in England] designed a machine which enabled him to measure the amount of friction generated by a fingerprint when it was in contact with the acrylic glass.
The machine was then strapped to the index finger of one of his students.
Dr Ennos expected the amount of friction to increase in proportion to the strength at which the acrylic glass was pushed against the finger.
This would have supported the theory that the fingerprint was helping to improve grip by ramping up friction levels.
However, the results showed that friction levels increased by a much smaller amount than had been anticipated.
Let’s take a close look at this. Firstly, what is friction and how does it work? Friction can be described as a force that resists movement between two objects that are touching. For a “normal” solid, friction is calculated by multiplying a constant called the coefficient of friction by the amount of force that’s being applied to keep the two surfaces touching. The coefficient of friction is a dimensionless constant that takes into account the two surfaces. The higher this coefficient, the greater the friction force. Using everyday experience, for example, the coefficient of friction between leather shoes on ice is much smaller than between sneakers on the concrete sidewalk. (There are tables we can use to looks these coefficients if we really wanted to. And it’s important to note that these coefficients are generally higher at higher temperatures for the same pair of surfaces.)
See if you can recall the following from high school physics:
Normal Friction
For normal solids, the friction force is strictly dependent on the force that’s pushing the two surfaces together. For instance, the friction force we encounter when we try to push a big aluminum cube on smooth wooden surface depends on how heavy that box is. Interestingly, and intuitively, a smaller cube with the same weight will have the same friction force. This is important to keep in mind!
What makes our fingers so interesting and this new research so surprising is that our fingerprints don’t act like normal solids. According to LiveScience, “the finger was not behaving like a normal solid; it was behaving like rubber. With rubber, friction is proportional to the contact area between two surfaces, not how hard they press together.” This makes things very interesting to consider. The harder we push or the tighter we grip something with our fingers, we are just increasing the surface area of contact. (In fact, because of the ridges on our fingers, there is naturally less contact area than if our fingers were perfectly smooth. So when press hard on something, we are increasing the contact area.) The fact that we increase the normal force (or the squeezing force) with our fingers isn’t directly related to the friction force as it is with normal solids (see above).
If fingerprints don’t help with grip and actually make it worse, why do we have them? ABC News summaries the possibilities:
A French team of researchers working with a mechanical hand loaded with tactile sensors found that fingerprint-like ridges improved the hand’s tactile sensitivity.
Another possibility is that fingerprints help wick water off of our hands, improving grip on wet surfaces, Ennos says. Or alternatively, they might work in coordination with soft finger and foot pads to help skin fit more snugly to abrasive surfaces, reducing shear stress. “We very rarely get blisters on the soles of our feet or our fingertips,” he [Ennos] says.
(Image from Wikipedia.)
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