The Ohio State University

Strong beautiful hair is universally desired across various cultures. Hence, the ways in which cleaning conditioning, grooming and styling processes affect the strength, feel and manageability of hair is of interest to beauty care science and technology.  Human hair is a complex nanocomposite biological fiber. In this thesis, experiments are performed to better understand mechanical and electrical properties of human hair fibers.

Human hair fibers experience tensile forces during many grooming and styling processes. Hence, characterizing the tensile response of human hair fibers is of considerable interest. The effect of tension on human hair is studied using an atomic force microscope (AFM) and a custom built tensile sample stage. The tensile sample stage allows loading a single hair fiber in tension. In the methodology used here, straining of the hair fiber is stopped intermittently and a marked control area is imaged with the AFM at different strains, upto break point. Morphological changes and deformation that occur with increasing strain are studied. Stress strain data is also simultaneously captured by the sample stage. To understand the effect of damage and treatment on hair fibers virgin, chemically damaged, virgin conditioner treated and chemically damaged conditioner treated, Caucasian hair fibers are studied. To understand differences between damaging processes, mechanically damaged hair is also studied and compared to chemically damaged and virgin hair. Hair from different ethnic types is known to have significantly different properties. In order to understand this difference better, the tensile response of virgin Caucasian, Asian and African hair is investigated. The wet tensile response of hair is particularly important to explain the effect of wet combing and wet styling processes. Also by studying the wet tensile properties, pathways of penetration of water molecules into the hair fiber can be explained. Hence, virgin damaged and treated hair fibers soaked in water are subjected to tension, and a control area is imaged intermittently. While tension experiments are good baseline studies, most grooming processes are not simple tension. In order to better simulate grooming processes, fatigue tests are carried out, where a single hair fiber is subjected to fatigue. The fatigued fiber is then tested and imaged in tension. Mechanisms for all observed results are proposed.

Another property of interest is the surface charge of human hair. Surface charge of hair has a significant effect on manageability, feel, and appearance. For this reason, controlling charge buildup to improve these factors is an important issue in the commercial hair care industry. Previous studies have looked at static charging characteristics of hair on a macroscale. In this study, static charging characteristics of hair are studied on a nanoscale with an AFM. Surface charge is created on hair by rubbing a control area with a voltage biased AFM tip, to which a small voltage bias is applied. The resulting charge distribution is characterized by measuring the surface potential of the control area in situ with Kelvin probe microscopy. The rubbing load is progressively increased, and the effect of this increase on the charge build up is assessed. Virgin, damaged and conditioner treated hair samples are studied, for a better understanding of charge build up and dissipation. Relevant mechanisms are discussed.