Hair seems to take a delight in surprising us. While we think it fragile and that a situation hanging by just a hair seems extremely precarious, in fact hair is incredibly resilient.
While water is essential for the hair's hygiene, beauty and well-being, it is not necessarily its best friend... And what should we think of its relationship with electricity which can, like fear, make it stand on end?

  Strong as a hair

Modification of the structure of keratin in an elongated hair before its breaks.

While it is not obvious when handling a single hair, you only need to try to break a small lock to be convinced: hair is extremely strong.
The organisation of keratin within its cortex allows it to resist a strain of up to about a hundred grams. A lock of 100 hairs can thus withstand a weight of 10 kilograms. As to the average head of hair, it could withstand 12 tons, if the scalp were strong enough!

Before breaking, a hair undergoes changes. For example, by delicately handling a reasonably long hair it can easily be shown that it behaves like a piece of elastic; after extending slightly, it returns to its original length.

The use of the extensiometer which progressively stretches a hair at the rate of 1 cm per minute allows precise study of the modifications hair undergoes before it breaks.
Thus, for lengthening of up to 5%, hair is elastic. This is due to the structure of the keratin molecule. Called keratin a in its natural state, stretching arranges it into keratin b. When the stretching stops it returns to its initial form like a spring. Then the hair enters a condition known as flowing where, almost without effort, it can elongate by 25%: keratin a unwinds as keratin b.
Beyond that, keratin b begins to resist. However, in this phase before breaking, the hair can still be elongated and it often breaks only after its length has actually doubled!

Beyond the elastic phase, the hair has another property: at least for a while, it keeps the shape it has been given. Thus, if a hair is wound around a pen and after several hours the pen is removed, the hair retains its curled shape. This is known as the plasticity of hair. In combination with water and heat, this property allows temporary modification of the hair's shaper by using, for example, the technique of blow-drying.

These properties vary greatly depending on the shape of the hair.
Breaking under a strain of 60 grams after an elongation of 40%, African hair seems to be the most fragile. At the other end of the scale, Asian hair is the strongest, withstanding a weight of 100 grams and an elongation of 55%. For either of these features Caucasian hair occupies an intermediate position.
However, by extrapolating these various measurements to equal shapes and surface areas, scientists can show that hair from these three ethnic groups behaves in an intrinsically comparable fashion and can thus confirm a common structure for all the world's hair.

Another factor influences the properties of hair, namely water.

  A tumultuous relationship : hair and water

Wet hair is heavier than dry hair: this seems obvious and it is. However, this simple observation illustrates an important characteristic of hair: it is permeable.
Despite the close fitting scales of its cuticle and the sebum which naturally coats it, a hair in good condition can absorb more than 30% of its own weight of water. If the hair is alread damaged by other factors, this percentage can reach 45%. Its length can thus increase by 2% and its diameter by 15% to 20%!

In cosmetics, this swelling is used to good effect to make large molecules penetrate into the body of the hair. This is the case for dyes.

But, in general, water is harmful to hair and considerably amplifies the many factors damaging it. It particularly accentuates the negative effects of sunlight, hair's other great enemy. Melanin degradation is encouraged and sun linked decoloration intensifies. Keratin itself is altered, making the hair fragile and easily damaged.
So while we are enjoying the pleasure of letting our hair dry in the sun after bathing in the sea, it is in fact undergoing real torture.

That being said, the hair takes up water even without bathing in the sea, showers or shampooing. In fact, it is permeable to water vapour which is always present in the surrounding air to a greater or lesser extent. This is what makes hair so much more difficult to manage in humid weather.

However, this property does also have an unexpected application. In the hair hygrometer, the variation in hair length linked to the absorption of water vapour is used to measure the degree of relative humidity in the atmosphere. This apparatus consists of a hair attached to a needle which records the variation in the hair's length by moving in front of a graduated dial.

  Electric hair

The contact of hair with certain synthetic garments can go as far as to produce sparks. More often, it is sometimes enough to take off a pullover to see your hair stand up on your head. Similarly, a plastic ruler rubbed on a piece of material then placed near the hair has a tendency to attract it.

These phenomena are associated with the hair's ability to become charged with static electricity. Since keratin is a good insulator, it is mainly friction which gives hair an electric charge: this effect is said to be triboelectric . It increases when the hair cuticle is damaged, encouraging the exchange of electric charges. It decreases on the other hand when the hair is damp: the plastic ruler held close to damp hair has no effect. On the other hand, some types of hair are very difficult to manage in dry weather






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