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Researchers have figured out why human hair can wear down a razor's edge, even though hair is 50 times softer than steel. Medioimages/Photodisc/Getty ImagesYou’ve likely stood over the bathroom sink or in the shower, wondering: How does your hair manage to dull a razor blade made of steel? Hair is famously strong for its thickness, but it’s a stainless-steel razor blade! That should definitely be tougher than a single strand of hair, right?
Scientists have asked themselves the same question, and the answer is now clear: heterogeneity.
The Winner by a Hair
Hair is much softer than steel. In fact, it’s about 50 times softer than the stainless steel found in razor blades. Despite this, the razor blades we use for shaving dull fairly quickly. To understand why, a team of scientists from MIT's materials science and engineering department conducted a study, with their findings published in the journal Science on August 6, 2020.
Gianluca Roscioli, a researcher, decided to shave his facial hair with disposable razors, bringing the blades into the lab for closer inspection using a scanning electron microscope. The results showed that, instead of the metal edges simply rounding or wearing down, they were actually chipping and cracking.
In response, he developed a mechanized shaving device in the lab to run more precise experiments using hair from both himself and his colleagues. The entire apparatus was small enough to fit inside the electron microscope, making it quite the convenient setup.
Chip Off the Old Razor Blade
Roscioli and his colleagues discovered that the formation of chips on the blade’s edge was more likely when the hair was able to bend before the blade cut it. To further investigate, they created computer simulations with multiple variables: different hair types, varying cutting angles, directions of force applied, and blade materials.
The researchers identified three key conditions under which chips occurred:
- When the blade approached the hair at an angle
- When the blade had a varied composition
- When the hair met the blade at a vulnerable point
"Our simulations clarify how heterogeneity in a material can increase stress on it, causing a crack to form, even when the stress is from a soft material like hair," explains C. Cem Tasan, the Thomas B. King associate professor of Metallurgy at MIT and a co-author of the study.
"Heterogeneous" refers to the blade material being imperfectly uniform, containing microscopic flaws that make it more prone to chipping when it comes into contact with hair. And where one chip appears, more will follow, leading to a dull blade.
The team of researchers is currently focused on developing a more consistent and uniform material, one that will result in blades that are sharper and have greater durability over time.
When discussing the strength of human hair, we often refer to its tensile strength, which measures how far it can be stretched before breaking. This is quantified in megapascals (MPa). Human hair has a tensile strength of around 200 MPa, while structural steel comes in at approximately 400 MPa. Spider webs, however, boast a much higher strength at 1,200 MPa. Among the strongest known materials, carbon nanotubes reach an impressive 62,000 MPa.
