You've probably stood over the bathroom sink or in the shower and wondered: How is it possible that your hair can dull a steel razor blade? Hair is notoriously strong for its thickness, but come on. It's a stainless-steel razor blade! That's got to be stronger than a little old strand of hair, right?
Scientists have wondered the same thing, and now we've got an answer, and that answer is heterogeneity.
The Winner by a Hair
Hair is indeed softer than steel. You might have noticed this yourself. It's about 50 times softer than the stainless steel used in razor blades. And yet the razor blades we use for shaving get dull fairly quickly. A team of scientist at MIT's department of materials science and engineering wanted to find out why, and their results were published Aug. 6, 2020, in the journal Science.
Researcher Gianluca Roscioli shaved his own facial hair with disposable razors and took them into the lab to be examined with a scanning electron microscope. He found that the edges of the metal weren't rounding or being worn down as you might expect. Rather, they were chipping and cracking.
So he created a mechanized shaving apparatus in the lab for more controlled testing using hair from himself and his labmates. The whole machine fit inside the electron microscope. Handy.
Chip Off the Old Razor Blade
What Roscioli and his co-authors on the study found was that chips in the blade's edge were more likely when the hair was able to bend before being cut by the blade. So the team went even further to create computer simulations with more variations: different hair, different cutting angles, direction of force being applied and the materials used in the blade.
They found that the chips appeared under three conditions:
- When the blade approached the hair at an angle
- When the blade was heterogeneous in composition
- When the hair met the blade at a weak point
"Our simulations explain how heterogeneity in a material can increase the stress on that material, so that a crack can grow, even though the stress is imposed by a soft material like hair," says C. Cem Tasan, the Thomas B. King associate professor of Metallurgy at MIT and a researcher on the study.
"Heterogeneous" means the blade's material is not perfectly uniform. There are microscopic imperfections that allow chips to happen when it comes into contact with a hair. And where there's one chip, there will be more chips, resulting in a dull blade.
The researchers are now working on a more homogeneous, or uniform, material for sharper, longer-lasting blades.