The Ohio State University

WETTING BEHAVIOR AND SURFACE POTENTIAL CHARACTERISTICS OF HUMAN HAIR

Richard A Lodge,
B.S. M.S. Thesis,
Department of Mechanical Engineering, December 2006

ABSTRACT

The ways in which common hair care products, such as conditioner, deposit onto hair and change hair properties are of interest in beauty care science, since these properties are closely tied to product performance. In this thesis, a variety of studies are conducted to advance the current characterization of human hair. Tapping mode atomic force microscopy (AFM) is used to generate surface height maps of hair, which are then compared to maps generated using contact mode AFM. Contact mode has been used in the past and has the potential of damaging small surface features, while tapping mode does not. The frequency spectra of both data sets are also compared. This comparison is made to determine the most appropriate method for imaging compliant biological samples that may be subject to damage from interaction with the AFM tip during contact mode. Additionally, force calibration mode of AFM is used to obtain the local conditioner thickness distribution and adhesive force mapping of various hair surfaces. The conditioner thickness is extracted by measuring the forces on the AFM tip as it approaches, contacts, and pushes through the conditioner layer. Because the interaction of hair with skin is one of the most common in the real world, and clean skin is hydrophobic, a Si₃N₄ tip is coated with Z-TETRAOL to create a hydrophobic tip to measure friction and adhesion. These values are compared with those measured with the more common, hydrophilic tips.

To further the study on the effect of contact angle, the wettability of human hair is studied by using the Wilhelmy balance method to measure dynamic contact angle of hair with water. The method uses a microbalance to measure the force exerted on a single fiber when it is immersed into the wetting liquid of interest. This measured force is related to the wetting force of the liquid on the fiber, and the dynamic contact angle can be calculated. Hair samples were measured dry, and then also allowed to soak in water before being measured to determine if a wet environment affects the wetting properties of the hair surface. Additionally, hairs from subjects of different ethnicities are measured and compared. Further, the mechanisms driving a significant directionality dependence are studied and discussed. The results are also used to explain tribological properties found in previous studies.

The final property of interest in this thesis is the surface potential 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. In this portion of the thesis the surface potential of human hair is measured using the Kelvin Probe method with an atomic force microscope (AFM). In the first part of this study, samples are mounted in conductive silver paint, and a DC voltage is applied through the sample puck. The surface potential is then measured as the applied voltage is changed. In the second part of the study, physical wear and triboelectric charging are investigated with the Kelvin probe technique. Physical wear has been shown to cause surface potential change in conductors and semiconductors, and it is of interest whether or not physical wear alone can cause a surface potential change on hair and other insulating materials. It is known that interaction of hair with dissimilar materials, such as plastic combs, hands, and latex balloons, creates a charge on hair, and determining the mechanisms of this phenomenon is the purpose of this study. A variety of samples are worn with a diamond tip to investigate the effect of physical wear on surface potential. Additionally, hair samples are rubbed with latex to study the effect of triboelectric charging on the microscale. The surface potential is measured both prior to and after rubbing to observe how the surface charge changes.

Caucasian virgin (undamaged), chemically damaged, and mechanically damaged hair samples are studied to determine the effect of damaging treatments on surface charge properties. Samples treated with PDMS silicone conditioner as well as those treated with an amino silicone conditioner are also studied in some cases to determine the effect of conditioner treatment. Finally, hair samples from subjects of varying ethnicities are used to examine any differences in properties due to ethnicity.