Nicole sent these articles to me, and I found them fascinating. It’s kind of a dense read, but it breaks down the physics of combing and its effects on the hair.
The research was done by a C. Robbins, and these are summaries of the full experiments.
Impact loading and hair breakage
During combing of hair, short fiber fragmentation (less than 2.5 cm) and longer segment breaks occur by different pathways. Longer fiber breaks most likely occur principally by impact loading. Impact loading causes hair breakage at lower loads than tensile loading, with essentially no increase in strain versus normal tensile testing, which produces large strain increases. L says~“Impact loading” is applied stress on the strands over time, “Tensile loading” is stress/strain that challenges the fundamental strength of the strand (example, pulling a strand from the root). Strain rates in impact loading are more similar to combing rates than rates of extension in tensile loading, and the looped and crossed hair formations in snags fit impact-load breakage better than simple extension of straight/non-crossed hairs in tensile testing. Extension or impacting hair fibers with flaws or damaged hair sections such as damaged wrapped ends produces short fiber fragmentation, while longer segment breaks may be produced in fibers with natural flaws such as fiber twists, cracks, or badly abraded or chemically weakened hair or even knots.
Pathways of breakage
Hair breakage during combing was evaluated by combing tresses and examining photographs of snags of hair fibers in combs. The resultant hair fiber arrangements suggest that breakage likely involves hair-on-hair interactions, and broken fragment size suggests that breakage occurs primarily at or near the hair-comb interface. Compression forces during combing were also measured, and impact loading of a hair fiber over another hair versus a hair fiber over a comb tooth shows that compression and abrasion are important to breakage during combing and that impact loading of one hair fiber over another during snagging is a probable and important pathway for hair breakage.
Brushing and combing hair
During combing of hair, longer fiber breaks (>2.5 cm) occur principally by impact loading of looped crossover hairs, while short segment breaks (2.5 cm) occur primarily by end wrapping. Brushing provides breakage similarly but with a higher ratio of long-to-short segment breaks, and the ratio of long-to-short segment breaks (L/S) is a good way to follow these two pathways of breakage under different conditions. For example, bleaching hair, a longer comb stroke, increasing fiber curvature, wet combing versus dry combing, and brushing versus combing all provide for an increase in long segment breaks and this ratio, with the largest effect produced by brushing.
The effects of bleaching and conditioning on short and long segment breakage by wet and dry combing of tresses
A recent publication (1), provided evidence for two types of hair breakage during combing, short segment breakage (approximately less than 1.27 cm) and longer segment breakage. We have confirmed these results and refined the separation distance between short and long segment breakage at about 2.54 cm. Furthermore, chemical bleaching increased both short and long segment breakage while a commercial hair conditioner decreased both types of breakage. Whether the hair is chemically bleached or conditioned, for dry combing, short segment breakage increases with increasing comb strokes, that is, short segment breakage increases as combing damages the ends of the hair, however, long segment breakage does not increase with increasing comb strokes. Wet combing provided a decrease in short segment breakage and an increase in long segment breaks, but no increase in breakage with increasing comb strokes. Mechanical combing of tresses shows similar results qualitatively, however the variance was too large and adjustments need to be made to provide for a larger number of broken hairs to bring the mechanical and hand combing results in line. For dry combing, as the comb descends through the hair, hairs above it are made parallel and those beneath are either made parallel or knot by hairs looping around other hairs or hairs looping around comb teeth and other hairs several cm between the comb and the hair tips. As the comb advances through the looped/knotted hairs long breaks occur or as the comb descends near the tips wrapped ends can result. End wrapping by inertia & possibly static charge produces short segment breaks which are more severe if the hair is cut at 90 degrees versus a tapered cut. For wet combing, clumping of hairs by a capillary action produces fewer short segment breaks, by reducing end wrapping: however, crossed hair interactions occur & because of higher friction more severe snags arise higher up in the tress, and lower hair breaking load due to plasticization by water, producing a larger number of long segment breaks. The very best practical way to evaluate hair strength is by counting the actual number of short and long segment breaks and by considering both wet and dry combing.
Interesting stuff.… All the scientists/science students out there, feel free to jump in!