The field of Tribology is not just limited to metals and their lubrication. The science of Tribology is applicable to our day to day life. One such area where friction, wear and lubrication persists is our own hairs. The hair care product industry is multi-million dollar industry and there are a range of products at disposal to the customers. Research and development work of these companies focuses on characterization of friction and wear properties of hair and their lubrication so that a high quality product such as shampoo, conditioner, hair lotion, hair spray can be developed.
Before entering into hair Tribology, a brief idea about the hair structure and scalp is a must. There are two main parts of the hair: hair root and hair shaft.
Fig. 1. Image of hair structure
The hair root consists of:
- Sebaceous gland: Produces an oily substance called sebum which lubricates the hair and skin
- Dermal papilla: Also known as mother of hair. Contains blood capillaries and provide nutrients for the hair root to grow.
- Arrector pilli muscle: It is a tiny, involuntary muscle which causes goose bumps
- Hair bulb: It is the lowest part of the hair strand
- Follical: Tube like structure which protects the root of hair
The hair shaft consists of:
- Cuticle: it’s the outermost layer of the hair and is responsible for creating the shine and smoothness
- Cortex: It’s the middle layer of the hair and contains melanin pigment responsible for providing elasticity and natural colour.
- Medulla: It’s the innermost layer and is not involved in salon services.
The hair scalp consists of:
- Dermis: It’s the innermost layer of the skin
- Epidermis: It’s the outermost layer of skin
Let’s talk about the three main processes present in hair
- Friction: Processes like combing of hairs via thin spaced comb or plastic comb and environmental conditions like dust, dirt, humidity, sweat also contribute towards friction between hairs.
- Wear: Processes like blow-drying, chemical hair dyes, hair colorants and hair straighteners leads to hair damage or wear of hairs.
- Lubrication: Hair shampoos and conditioners provide cleaning and lubricating functions.
Friction force measurement of a single hair fibers is difficult. Although there are some methods that are widely used:
- The capstan method: This is one of the oldest method and the test setup consists of a cylindrical rod, a weight and a hair fiber. The single hair fiber is passed over the cylindrical rod and the tension required to cause slippage is measured. The friction coefficient of the hair fiber sliding over the cylinder is given by the relation:
where θ is the contact angle of the hair on the cylinder , T is the tension of the hair fiber, and W is the hanging weight. This equation assumes that friction is independent of load.
- Cross fiber test setup: In a much recent study, V. Krasmik, J. Schlattmann carried out hair fiber tests on a linear reciprocating nanotribometer. The friction force was computed by measuring the deflection of a dual beam cantilever with high resolution capacitive sensors.
- Atomic force microscopy (AFM): A relatively new technique for measuring friction coefficient of hair fiber using movement of stylus tip over the hair surface. Apart from this, AFM also helps in determining surface properties of hair fibers.
From a lubrication and hair repair point of view, shampoos and conditioners are of prime importance. A brief review about their composition and function of individual ingredient is given below.
- Cleaning agents: Surfactants act as cleaning agents and removes dirt, scalp sweat, hair oil and other foreign entities that produces friction. These molecules have the ability to be both hydrophobic (water repelling) and hydrophilic (water absorbing). The anionic surfactants provide a lot of the lather and detergency in the shampoo. Other surfactants like Non-ionic surfactants and Amphoteric surfactants are generally used as additives to boost foam, stabilize foam and improve conditioning.
- Conditioning agents: Cationic surfactants are generally used as conditioning agents. They are not used alone and are generally paired with long chain fatty alcohols (replacement of traditionally used natural oils) in order to provide lubrication and softness.
- Functional agents: Shampoos have certain amount of these agents to prevent dandruff or hair fall.
- Preservatives: Preservatives are used in shampoos to kill bacteria, fungi, and yeasts and other germs.
- Perfumes: Perfumes are used in shampoos to impart herbal, fruity or floral fragrance.
- Opacifying agents: Opacifying agents added to make shampoos opaque and pearlescent.
- pH balancing agent: Control the pH value of the shampoo and try to keep its formulations slightly towards acidic side.
Evaluation of performance of shampoo or conditioner
- One of the method generally used to check the performance of a shampoo or a conditioner is the swatch-on-swatch sliding test. Prior to the test a hair swatch is treated with the shampoo or the conditioner whose performance has to be evaluated. Then the treated swatch of hair is made to slide out from two adjacently placed stationary hair swatches. A friction force transducer is used to measure the force required to pull the hair swatch out of the two stationary swatches. The lesser the force required to pull, the higher the lubrication capability of the shampoo or conditioner.
Fig. 2. A hair swatch
The above article provides a brief idea about how Tribology can be used in manufacturing of high quality shampoos and conditioners. Further research and development into this field can provide new breakthroughs which would prove to be beneficial for the associated companies and consumers.
- Bhushan Bharat. “Chapter 1: Introduction—Human Hair, Skin and Hair Care Products.” Biophysics of Human Hair Structural, Nanomechanical, and Nanotribological Studies. Berlin: Springer Berlin, 2013. 1-20 Google books preview only
- Spencer, Nicholas D. “Chapter 4: Aqueous Lubrication in Cosmetics.” Aqueous Lubrication: Natural and Biomimetic Approaches. New Jersey: World Scientific, 2014. 103-44 Google books preview only
- V. Krasmik, J. Schlattmann “Measuring human hair friction with a crossed fibres test setup”, Proceedings of Malaysian International Tribology Conference 2015, pp. 60-61, November 2015 Google books preview only
- Robbins, Clarence R. “Method of Measuring Friction on Hair Fiber.” Chemical and Physical Behavior of Human Hair. Berlin: Springer, 2012. 336-39 Google books preview only
- Blau, Peter J. “Section 3.3.4: Tests on Flexible Surfaces.” Friction Science and Technology from Concepts to Applications. 2nd ed. Boca Raton, FL: CRC, 2009. 65-67 Google books preview only
- Frictional effects in human hair by Anthony M. Schwav and D.C. Knowles, Presented April 9, 1963, Chicago
- Adhesion and Nanotribology of Biofibres by Hiroyasu Mizuno, Doctoral Thesis, KTH Royal Institute of Technology School of Chemical Science and Engineering Surface and Corrosion Science, 2012
- Tribology and hair care by R. David Whitby, Tribology and Lubrication Technology, March 2007, Pg. 64
- Basic Hair Structure and Function, Hair Growth SOS, http://www.hairgrowthsos.com/hair-structure.html
- Science of Hair, Hair and scalp expert (expert advice), http://www.hairandscalpexpert.com/structure_of_the_hair.htm
- Types of surfactants in your shampoo written by Lorna Casse, Hair Momentum, http://hairmomentum.com/types-of-surfactants-in-shampoo/
- Ingredients shampoo, Science toys, http://scitoys.com/ingredients/shampoo.html
- Dandruff deconstructed, What are the best active ingredients for my dandruff?, https://www.dandruffdeconstructed.com/active-ingredients/
- Formulation & evaluation of shampoo PPT, V. Manimaran, Lecturer Department of Pharmaceutics, SRM College of Pharmacy, http://www.srmuniv.ac.in/sites/default/files/files/SHAMPOOS.pdf
- Hair colour experiments/swatch tests (discussion), http://www.salongeek.com/threads/hair-colour-experiments-swatch-tests.226689/
- Chapter 11, Properties of the hair and scalp PPT
- Properties of the hair and scalp PPT
- Images used are “free for commercial use”