I’m not the sharpest tool in the shed when it comes to physics. I find some of the terminology and concepts difficult to understand.
So, if you’re like me, this article will help explain the physics of friction blister formation.
In particular, pressure and friction.
Firstly, What Causes Blisters?
The answer is repetitive shear distortions, also referred to as shear deformation.
- STEP 1: Press the tip of your right index finger firmly on the back of your left hand.
- STEP 2: Wobble it back and forth but keep it stuck to the same bit of skin. Notice the “give” in the soft tissues – how your skin stretches back and forth? This is shear deformation.
This is what causes friction blisters.
Blisters are caused by repetitive shear deformation. This simplified diagram shows the relevance of friction force and bone movement.
What Is Friction Force (F)?
Friction is a force. It’s the force that resists the movement of one suface across another.
It’s not a movement – it’s not rubbing. Well it is (double definition), but it’s not the definition you should be thinking about.
Here’s The Physics
f = μN
f = friction force (the parallel force resisting the movement of one suface across another at an interface)
μ = coefficient of friction (the slipperiness of a pair of surfaces at an interface)
N = normal force (the perpendicular force pressing the two surfaces together – pressure is the normal force per unit area)
Here’s The Relevance Of Pressure (N)
- STEP 3: Wobble back and forth again but this time press softly with your finger tip. Notice how there is less shear deformation. Low pressure allows your fingertip to slide across the skin before the shear distortion becomes excessive.
- STEP 4: Now press really hard and wobble back and forth. A high compression force sees your fingertip remain stuck to your hand for longer which causes larger shear distortions.
The higher the normal force (pressure / compressive force / force of contact), the longer your fingertip stays in stationary contact with the back of your hand, resulting in larger shear distortions.
But there’s more.
The Coefficient of Friction (μ)
- STEP 5: Put a drop of oil or dollop of Vaseline on the back of your hand (reduce friction) and wobble your fingertip back and forth again – with really firm pressure. I’m doing it now and I can press like crazy, and yet only a tiny bit of shear deformation results in the soft tissues. You have to do this to believe it.
The more slippery it is, the sooner there is slippage between your fingertip and back of your hand, resulting in smaller shear distortions. The opposite is also true – the more sticky it is, the longer the stationary contact between fingertip and hand, resulting in larger shear distortions.
Take Home Message
You can implement pressure reduction with donut pads made from moleskin or thicker felt, cushioned insoles and toe protectors, or with specific orthotic prescriptions and footwear feature. However, blister prevention is not just about reducing pressure. Don’t forget about the coefficient of friction – how slippery the two materials are. We often concentrate exclusively on pressure reduction. We shouldn’t. Ask yourself how you can make things more slippery by reducing the coefficient of friction.