433 SUSTAINABLE HAIR
and form a split end. However, recent experimental findings are refuting this idea. Figure
10 shows a high magnification, scanning electron microscopy (SEM) image of a broken
hair fiber (acquired from the aforementioned repeated grooming experiments) that exhibits
both fibrillation and splitting.
Fibrillation and splitting are commonly seen when examining broken fibers that result
from these experiments (although not necessarily together) however, Figure 11 suggests
that fibrillation occurs prior to breakage rather than in the presumed reverse scenario. This
image was obtained from surveying the topography of the fiber in the immediate vicinity
of the break. In doing so, it was similarly common to find longitudinal cracks in the fibers,
as shown in Figure 12. Both of these occurrences are presumed to result from extreme
bending stimuli that fibers can encounter during hair grooming. It appears logical that a
spit end is obtained if such a crack initiates or terminates at a fiber tip.
Fiber tips, such as that in Figure 10, cannot be technically “repaired.” While “split end
repair” is an attractive consumer proposition, it is not possible to even conceptualize a
means of restoration. As per the previous discourses, the alternative is to temporarily
hide the occurrence such that the consumer does not notice. To this end, it is sometimes
Figure 10. Split end and fibrillation in a broken hair fiber.
Figure 11. Fibrillation prior to breakage.

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.