Wednesday, March 7, 2012

How Fine Hair is Different

Everybody’s hair is different. It’s really the combination of chemical and physical properties of, and environmental influences on your hair that make it unique, no matter what your heritage or what you do to your hair. Most of us have a mix of hairs on our heads – different diameters, different textures, some variation in color from follicle to follicle. But if you can get over aesthetics - hair color and the visual stuff, there’s some fun physics/engineering in there. I have mostly fine hair and those of us who do know it is not the same to work with as hair which is not fine, so I dug in to try to point out the obvious.
©Science-y Hair Blog 2013
String cheese separates into sections - a
visual aid for hair diameter.
Fine hair has a smaller diameter than average (somewhere between less than 50-70 microns in diameter, depending on the source you consult – I feel it’s closer to the lower end). This doesn’t tell us much unless you’re accustomed to measuring things in microns (micrometers). So I’m falling back on a cheese analogy. Think of fine hair as a few “strings” from a piece of string cheese. Medium or “average” hair could be half the piece of string cheese and coarse hair or larger-diameter would be the entire piece of string cheese. If you want to imagine fine hair being less stiff than average or coarser hair, go right ahead because that could be an advantage (fine hair can feel silky-soft and weightless) or a disadvantage (it can be too soft to easily hold a style, easily “dented” by bobby pins and ponytail holders).

Diameter influences how easily you can de-form (deform, hyphenated so you’d read the meaning) fiber. You could easily bend or reshape a few strings of string cheese. Half the piece (representing “average diameter” hair) would tend to hold it’s shape and deform less readily and the entire piece would be thick and dense enough to resist deformation even more. This character of individual hairs has a lot to do with how those hairs behave as a mass. Fine hair, easily deformed by outside forces, is more susceptible to the effects of gravity, the weight of other hairs and of water, oils and conditioners.
©Science-y Hair Blog 2013
Because fine hair is easily deformed (collapses under weight) it tends to hang with less body or volume than coarser hair. Even wavy or curly hair, which tends to have more body than straight hair because of the waves, will look less full if the hair is fine because the hair has more difficulty maintaining its wave pattern if there is weight on it from the sides or the weight of length.©Science-y Hair Blog 2013

The exception is hair with a very strong wave pattern (tight curls) in which the strength of the waving in the fibers helps them resist collapse.

More Physics and a little Geometry
All hair is covered by several layers of cuticle. If you mentally wrap these layers of cuticle around a few strings of string cheese versus an entire piece of string cheese, you can see that there is a lot less cheese inside the “fine hair” version than there is in the “coarser hair” version. That means there is more surface area (cuticle) relative to mass (cheese or hair cortex) in fine hair.
©Science-y Hair Blog 2013
Because fine hair has more surface area compared to mass or inner volume than medium or wider hair, things like oils and conditioners that either coat the hair shafts or bond to them (cationic conditioners and polymers) have more surface to coat per mass of hair. This is why oils and conditioners can weigh fine hair down more readily – first, fine hair is more easily deformed and second, it has more “surface” per mass of hair for things to coat or stick to.

In the graphic above, you see a fine hair (left) and a medium to wide hair (right). The blue dots surrounding the hair represent “units” of a conditioner or oil – or cuticle. This is a numberless unit, but it worked for the scale-less image – so just humor me. The fine hair is surrounded by 14 units of conditioner and the wider hair has 21 units of conditioner (or cuticle). The dots running across the hair indicate diameter. Here’s the geometry – the area inside the hairs (calculated by using pi!) is 12.57 for the fine hair and a whopping 38.47 for the wider hair. Now for a ratio, because this is where it makes sense; the fine hair has a ratio of conditioner or cuticle to “area inside of hair” of 1.1 to 1 (slightly more blue than brown) and the wider hair has a ratio of 0.55 to 1 (more brown than blue). Because hair cortex is where the "stiffness" comes from, fine hair is not as stiff or well-supported. And because cuticle is where products tend to be, you can see from the graphic how fine hair can suffer from an overly-large dose of anything that makes this already-soft hair even softer. It's more difficult to overwhelm the wider hair with products because it's internal structure, the cortex shown in brown, is larger and provides more support - hairs which are thicker are less deformable.

Solutions for Volume and Style©Science-y Hair Blog 2013
One way to work around the “weight” problem in fine hair is to have layers in your hair. This is the engineering part. If you want to reduce the load on parts of your hair so it is not collapsing under its own weight, then you must remove the weight. Layers do this for fine hair. In wavy or curly hair, the trick is to have those layers “long” so they can still execute their twisting shape. Where the weight from hairs above collapses the wave pattern, the hair can be cut shorter to reduce it’s weight on the areas beneath. Then both layers benefit from the reduction in weight. Think about it – the top layers of your hair, being cut a little short in layering, lose the weight of length because a hair growing from the top of your head down to, perhaps, your shoulders, is much longer than a shoulder-length hair growing from your hairline. So this hair is lighter. And the hair beneath benefits from not being mashed by the hair from above. That is why wavy hair and curly hair look good layered and look bulky when worn all-one-length. A haircut is truly a feat of engineering as well as a bit of artistry.©Science-y Hair Blog 2013

Chemical and Physical Behavior of Human Hair 
Robbins, 1994. 3rd Ed. Springer-Verlag, New York

Yin N., Kissinger RH., Tolgyesi WS., Cottington EM., 1977. 
Journal of the society of Cosmetic Chemistry 28, 139-150

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