The Science Of Friction... Skin vs Rubber

Friction between fingers and rock

Having dug into the science of friction that applies to rubber (on rock) there is still the question of how friction works for the skin on our fingers. The rubber on the shoes are only half of the equation, we slip just as much in our gripping of the rock as we do in our step.

There is not much industrial use for this information and the funding of research and studies into the subject is equally absent. I will however offer an abstraction based on the science of rubber friction.

Comparing skin to rubber

The qualities of rubber that are relevant for the different components of the friction between rubber and rock is the rubber surface:

• the atoms on the contact surface that come into contact with the rock
• the plasticity of rubber making the surface bend into shape by the shape of the rock surface
• the ability of the rubber to compress
• the fact that the surface is rough
• the fact that it gets worn by the contact with the rock.

Now... as far as I can tell... skin, or more precisely skin on the finger with all its underlying layers of tissue, display very similar characteristics:

• It has a rough surface
• It adapts in shape, filling gaps and small divots etc in the rock when loaded
• On a microscopic level it also compresses when loaded
• It definitely gets worn when we climb.

Back to the science

Looking back at how this friction breaks down for rubber on rock:

FT = FA + FHS + FHb + FC

where:

FT = The total frictional slip resistance
FA = Friction from adhesion
FHS = Friction from surface deformation (microhysteresis)
FHb = Friction from surface bulk deformation (macrohysteresis)
FC = Friction from rubber wear

F_A - Adhesion

Adhesion is the force between two surfaces that exists on the molecular level, between molecules and atoms of the two surfaces. This is undeniably a factor for skin on rock, the atoms are bound to come into contact and thus to be bonds between atoms of the different materials.

F_HS - Microhysteresis

Microscopic asperities in the surface of the finger interlocks with microscopic asperities in the rock surface (or climbing hold surface). This is also likely to be a factor for skin as the skin surface is not a smooth surface.

F_Hb - Macrohysteresis

The tissue/skin on the finger will flex somewhat around the protuberances of the rock, causing a larger contact area between the two surfaces. The added adhesion this creates has to be a significant part of this friction contribution just as it is for rubber on rock.

F_C - Wear

The skin on your fingers gets worn, tearing off microscopic pieces of skin from your fingers takes force. This force contributes to the total friction force experienced.

Conclusions

The similarities between rubber and the skin on your fingers are undeniable. This makes for a plausible comparison of the mechanics of friction for rubber on rock to the mechanics of friction for skin on rock.

We also instinctively know that there is a lot of differences, shoes stick a lot better than fingers. This however does not mean that the mechanics are not similar, but it points to the fact that rubber at least have a much stronger adhesion (to rock) than skin does.

There are also factors that apply to skin that don't apply to rubber, our skin breathes and perspires. This causes a film of moisture between the surfaces that influences the mechanics. The effect of moisture on rubber friction is well documented and studied and it is likely that this applies similarly to skin on rock.

However well this comparison gets or how detailed the different factors are broken down, the overall conclusion is that as a climber you have to address friction in the same way for your fingers as you do for your feet (shoes). You may not want to prolong contact time as perspiration may become a problem, but otherwise it all applies and you have to be vigilant when it comes to observing and addressing The Normal Force.

How well do you find this reasoning to match your experience with skin friction?

May The Normal Force Be With You!