Swelling Behavior, and the Effect of Fiber Shape on Reactivity

Dimensions, Swelling Behavior, and the Effect of Fiber Shape on Reactivity 

Two of the most commonly measured hair fiber dimensions are its length and its diameter. Assuming that a hair fiber approximates a cylinder, its volume, cross-sectional area, radius, and surface area may be obtained from formulae that describe the volume of a cylinder (y), the area of a circle A), and the surface area of a right cylinder (Su), in terms of its diameter D) and length (L). 

Although human hair fibers vary in cross-sectional shape, from nearly circular to elliptical, normalizing most elastic and other properties to fiber thickness can significantly reduce experimental scatter. Thickness is usually characterized as fiber diameter or cross-sectional area. Corrections to diameter for ellipticity are generally not employed. Hair fiber dimensions are also necessary to calculate fundamental elastic properties, and dimensional changes are often employed to follow the course of chemical reactions with hair. 

For measurement of short lengths (in the millimeter range or less), a microscope may be used, but for longer lengths (several centimeters or longer), a cathetometer is useful. Although fiber diameter may be measured directly with a microscope, or more crudely with micrometer calipers, other excellent methods are available for determining cross-sectional dimensions of human hair. 

Both single-fiber and multiple-fiber methods are available for determining hair fiber cross-sectional dimensions or changes. Single-fiber methods include linear density, microscope (light or electron), vibrascope, micrometer caliper, and laser beam diffraction. For multiple-fiber determinations, a centrifuge may be used. 

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