Why Does Wet Hair Break So Easily?
― Water Unhooks the "Joints" Inside Your Hair
When dry, a single hair is a surprisingly strong thread that can hold about half an apple. But when it gets wet, it stretches the same amount under half the force, and the scales on its surface lift up. The culprit: water temporarily unhooks the hundreds of millions of tiny "joints" inside the hair.
You step out of the bath and run a brush through your wet hair. It catches halfway, you give it a tug, and several broken hairs are left in the brush.
Hairdressers often tell you, "Don't go to bed with wet hair," and "Brush it only once it's dry." But water is just water. It doesn't dissolve hair.
And yet the bed-head you wake up with is easily fixed by wetting it. Hair becoming weaker when wet, and hair becoming easy to reshape when wet: these two facts actually come from the same mechanism.
There are only two reasons wet hair is weaker
Inside a hair, long, thin protein chains are tied to one another by countless weak joints. When water gets in, these come undone, and the hair stretches the same amount under about half the force it needed when dry.
The surface of a hair is covered with thin, scale-like layers that overlap like roof tiles. When the hair gets wet and swells, these scales lift up, and rubbing against a brush or a pillow wears them off more easily.
Let's take them one at a time. Both start with water making the hair swell.
Reason 1: Water unhooks the "joints" between chains
The main material of hair is a protein called keratin. Keratin chains are twisted into a spring-like helix. Two kinds of joints hold this helix in shape and link it to the neighbouring chains.
One is a strong joint, in which two sulphur atoms are firmly bonded together. The other is a weak joint, with a hydrogen atom as the go-between. Each weak joint is feeble on its own, but there are vastly more of them, and they account for much of the stiffness of dry hair.
Hair is said to soak up water amounting to about 30% of its own weight when immersed, and to swell in thickness by more than 10%. The water molecules that get in steal the partners of the weak joints. The bonds between chains loosen, and the hair begins to stretch slowly under a weak pull.
Let's check with Figure 1. The top row is dry hair and the bottom row is wet hair, both pulled to the right with the same force. As you change the force with the slider, you can see that only the wet hair moves quickly toward the dotted line on the right.
To the left of the dotted line, the hair returns to its original length when you let go. Past the line, the helix inside starts to unravel, and a small extra pull stretches it a lot. Wet hair reaches this threshold at about half the force that dry hair needs.
When you pull on a tangled brush, the force is concentrated on a few hairs. With wet hair, more of those hairs get stretched a long way. The force needed to actually snap a hair is also said to drop by 10 to 20% when it is wet.
Reason 2: The scales on swollen hair lift up
The outside of a hair is made of thin scales called the cuticle, layered many times from root to tip. The scales act as armour, protecting the soft core inside.
As on the left of Figure 2, the scales of dry hair lie flat. But when the hair gets wet and the inside swells, the edges of the scales lift, as on the right. The lifted edges snag on brush bristles and pillow fabric, and chip away a little with every rub.
Hair that has bent while you sleep has had its weak joints re-fastened in the "bent" shape. Wetting it undoes those joints, and as it dries they re-fasten in whatever shape it is in at that moment. Because the shape at the moment of drying is what stays, styling with a hair dryer sets your hairstyle. Perm solutions, on the other hand, cut and re-form even the strong joints, so the shape stays even after washing.
Summary
Wet hair breaks easily because water unhooks the weak joints inside it, letting it stretch a long way under about half the force. On top of that, the scales on the swollen surface lift, and rubbing wears them off more easily. The gentlest way to treat wet hair is to rub it as little as possible, work out tangles from the ends with a wide-toothed comb, and dry it early.
Water doesn't dissolve hair.
It just makes the "hands" holding the hair together let go for a while.
You can read about how hair gets replaced in "Why Does Hair Shedding Increase in Autumn?", and about another material whose properties change when stretched in "Why Does a Rubber Band Get Warm When You Stretch It?".
- Prepare two fairly long, naturally shed hairs. Leave one as it is and soak the other in a glass of water for about 5 minutes.
- Pinch both ends of a hair between the fingers of both hands and slowly pull with about the same force. The wet hair should feel as if it stretches more stretchily.
- Wind the wet hair around a pen and let it dry, then unwind it. If the curled shape stays, that is evidence the weak joints have re-fastened.
Results vary with the thickness and condition of the hair. Pulling hard can cut your fingers, so try it slowly, as if handling a fine thread.
For those who want to know more ― terms, formulas and links to textbooksShows which level each topic is at, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school physics, chemistry and biology
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (mechanics of materials, polymer physics, dermatology)
- ResearchNot taught even at university as settled knowledge; being studied by researchers right now
Junior highTerms: this phenomenon has names
- Keratin: The protein that makes up hair, nails and the surface of skin. Inside a hair, helix-shaped chains are bundled together.
- Hydrogen bond: A weak link with a hydrogen atom as the go-between. These are the "weak joints" in the main text. Water molecules form them too, so they can steal partners.
- Disulfide bond: A strong link between sulphur atoms. These are the "strong joints" in the main text, and water cannot undo them.
- Cuticle: The thin, scale-like layer covering the surface of a hair.
Junior highHigh schoolChecking with formulas: how many grams push one hair past its "recoverable range"?
Treating a hair as a thin rod, we calculate from the relation between pull and stretch. First the weight at which it breaks, then the weight that pushes it past the range where it springs back (2% stretch), comparing dry and wet hair. The units are N (newtons) for force and mm for length.
| In symbols | F ＝ E × A × ε (breaking force is F ＝ σ × A, cross-sectional area is A ＝ π × r²) |
| In words | Pulling force ＝ stiffness of the material (Young's modulus) × cross-sectional area × fractional stretch |
| Where it comes from | Hooke's law for springs, "force is proportional to stretch", rewritten in terms of force per unit cross-sectional area (stress) so that it works for rods of different thicknesses |
| Hair diameter (radius r) | said to be about 0.08 mm (radius 0.04 mm) |
| Stress at breaking σ (dry hair) | said to be about 200 N/mm² (a range of roughly 150 to 250 depending on the study) |
| Young's modulus E (dry hair) | said to be about 4000 N/mm² (4 GPa) |
| Young's modulus E (wet hair) | about half the dry value, said to be about 2000 N/mm² |
| Rough limit of the springy range ε | said to be up to about 2% stretch (0.02) |
| Radius squared | 0.04 × 0.04 ＝ 0.0016 mm² |
| Cross-sectional area A | 3.14 × 0.0016 ＝ 0.005024 mm² |
| Breaking force (dry hair) | 200 × 0.005024 ≒ 1.0 N |
| Convert to weight (1 N is about 102 grams) | 1.0 ÷ 9.8 ≒ 0.10 kg |
| Dry hair: E × A | 4000 × 0.005024 ≒ 20.1 N |
| Dry hair: force for 2% stretch | 20.1 × 0.02 ≒ 0.40 N |
| Wet hair: E × A | 2000 × 0.005024 ≒ 10.0 N |
| Wet hair: force for 2% stretch | 10.0 × 0.02 ＝ 0.20 N |
| Convert to weight (wet hair) | 0.20 ÷ 9.8 ≒ 0.020 kg |
Dry hair breaks at about 100 grams and springs back up to about 40 grams. Wet hair passes its recoverable range at only about 20 grams. Twenty grams is the weight of 20 one-yen coins. That is quite a plausible force on a single hair when a brush gets tangled.
High schoolHigh school+Hair stretches in "three regions"
High schoolHooke's law, in which a spring's stretch is proportional to force, holds for hair too, up to about 2% stretch. The quantity describing stiffness in this region is Young's modulus (the longitudinal elastic modulus).
High school+If you keep pulling a hair, the relation between force and stretch is said to divide into three regions: the first, proportional region; the "yield region", where it stretches by some 25 to 30% with hardly any increase in force; and a final region where it stiffens again until it breaks. In the yield region, the helical chains are thought to unravel and shift into a straight form. Water greatly lowers the force needed to enter this region.
UniversityThe stress–strain curve and plasticisation by water
In mechanics of materials, this relation is drawn as a stress–strain curve. In keratin fibres, the yield region is attributed to the "α–β transition", in which helical α (alpha) keratin is stretched and shifts into a β (beta) sheet structure. Water breaks the hydrogen bonds between chains and makes the chains more mobile. In polymer physics this is called "plasticisation", and water is a classic plasticiser for hair. The fact that Young's modulus drops as humidity rises is explained by the same reasoning.
📖 Derivations and further reading: Stress–strain curve (Wikipedia, Japanese) ／ Keratin (Wikipedia, Japanese)
ResearchWhat is still not well understood
- How far water penetrates. Hair contains a mixture of parts that let water through easily and parts that do not, and it is said that the order and depth of water's entry look different depending on the measurement method.
- How damaged hair breaks. With repeated drying and friction, at what stage do tiny cracks begin to spread? The mechanism of fatigue fracture is still under study.
- The role of surface oils. The surface of healthy hair is said to have a thin, water-repelling layer of fat. How much the way and strength of water uptake change once this wears off is still being researched.
In other words, what this article says is also "an explanation of what is known so far." In particular, the numbers for Young's modulus and strength vary greatly with the hair's thickness, ethnicity, degree of damage and how it is measured.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science Field 1, "Action of forces (stretch of a spring)" | Force and stretch are proportional (Figure 1) |
| High school | Chemistry "Hydrogen bonds", Biology "Protein structure" | Weak and strong joints (Figure 2) |
| High school+ | Advanced physics "Young's modulus and stress" | Calculating the force that passes 2% stretch |
| University | Mechanics of materials, polymer physics | Stress–strain curve, α–β transition, plasticisation |
| Research | Fibre science, dermatology | How water enters, breakage of damaged hair |
| ― | Links to daily life | How to brush wet hair, fixing bed-head, styling with a hair dryer |
- C. R. Robbins, Chemical and Physical Behavior of Human Hair, 5th ed., Springer (2012)
- M. Feughelman, Mechanical Properties and Structure of Alpha-Keratin Fibres, UNSW Press (1997)
- Wikipedia, "Hydrogen bond" (水素結合, Japanese)
- Wikipedia, "Keratin" (ケラチン, Japanese)
※This article is a general-audience science explainer. The numbers given are rough guides and estimates to help you understand the mechanism. If you have concerns about your hair or scalp, please consult a dermatologist or a hair-care professional.