434 JOURNAL OF COSMETIC SCIENCE
suggested that treatments might “glue” such splits together in the short term. However, it
is difficult to conceptualize how this might occur without adhering adjected fibers to each
other and creating an undesirable mess.
BENDING AND TORSION
Another commonly heard consumer hair attribute is “softness,” which, thinking literally,
a scientist may consider to be the opposite of stiffness. As previously mentioned, the
contribution of the cuticle is believed to have a far more meaningful impact,26,27 but
measuring the bending stiffness of individual fibers is complicated by their elliptical
cross-section. That is, bending forces are dependent on fiber diameter but this dimension
varies according to the cross-section. That is, a given fiber will have its highest bending
stiffness about its major axis, its lowest about its minor axis, and intermediate values
at all positions in-between. To this end, it is likely easier to assess the overall cuticle
contributions to fiber bending and twisting properties by performing torsional (twisting)
stiffness measurements.28 Historically, these experiments have been performed using a
torsional pendulum where fibers can twist and untwist while measuring the periodicity
and damping of the motion.
As with the extensional properties, hair’s bending and torsional mechanical properties can
be altered by insults but are also highly dependent on the water content. As outlined,
increasing water content solvates secondary, strength-supporting electrostatic bonds, and
lowers mechanical properties. Figure 13 shows extensional modulus (i.e., stiffness) results
for virgin and bleached hair as a function of the relative humidity of the surroundings
(torsional results mirror this behavior). It is seen that the damaged hair is stiffer under low
and medium humidity, but the opposite is found at elevated humidity and when wet.
It seems likely that such alterations in the stiffness of hair fibers should somehow manifest in
terms of consumer perception, perhaps in terms of tactile and/or manageability properties.
Yet, in consumer language, it appears that the term “softness” essentially equates to tactile
properties, where, once again, conventional conditioning treatments significantly boost
consumer perception of this term, while having no effect on fundamental mechanical
properties.
Figure 12. Longitudinal cracking of the cuticle.

435 SUSTAINABLE HAIR
WATER CONTENT OF HAIR
As previously mentioned, many hair properties are highly dependent on its water content.
Outside of purposely wetting the hair by immersion or spraying, the technical water content
is dictated by the relative humidity of the surrounding atmosphere.29,30 This relationship is
depicted by the hair-water adsorption isotherm show in Figure 14.
That is, the water content of hair is not fixed, but instead is a dynamically changing
variable that will change as the wearer moves from indoors to outdoors, room to room,
and to encounters with differing climatic conditions. When the relative humidity rises, so
does the technical water content of hair. Furthermore, as already noted, many properties
vary notably with water content and therefore are also at the mercy of climatic conditions.
A major consumer concern involves hair “drying out,” but technical evidence shows that
extreme conditions are needed to meaningfully change the shape of this isotherm. To this
end, this author’s experience in testing hair from many panelists that reported having “dry,
damaged hair,” all produced unmodified isotherms. Similarly, while many products claim
to “hydrate” or “moisturize,” they also generally have no effect on the isotherm.
Illuminating work by Davis &Stofel31,32 involved using the relative humidity to fix the
water content of hair, and then tresses were presented to panelists for evaluation. That is,
hair was equilibrated at either 15% or 80% relative humidity, which, from the isotherm
in Figure 14, is seen producing approximately 5% and 17% water content, respectively.
Despite having less than a third of the technical water content, the hair equilibrated at 15%
relative humidity was overwhelmingly ranked a feeling the “most moisturized.” This sample
was similarly ranked as being “smoother,” “less tangled,” and “less damaged” by sizable
margins. In short, despite consumer protestations, the tactile properties of hair suffer when
Figure 13. Youngs modulus of hair as a function of the relative humidity.