Department: NanoEngineering
Faculty Advisor(s): Marc A. Meyers

Primary Student
Name: Yang Yu
Email: yay047@ucsd.edu
Phone: 858-263-6144
Grad Year: 2017

Keratinous fibrous materials, such as wool and hair, have been observed to exhibit viscoelasticity. We show that hair exhibits a yield stress that is strain rate dependent. In order to better understand the strain rate sensitivity and the contributing factors, both human and horse hair were studied before and after the matrix were chemically disrupted. Figs. 1a and b show the stress-strain curves of human and horse hair fibers, respectively. As the strain rate increases, the Young?s modulus remains constant, while the yield stress rises. The post-yield stress in the human hair shows an increase in slope, indicative of the alpha to beta keratin transformation (Fig. 1a). In the horse hair, this increase in slope is not evident.The strain rate sensitivities of both human and horse hair were measured and are equal to 0.11 and 0.05, respectively. In order to further understand these differences in strain rate sensitivity, both human and horse hair were treated with an established method to disrupt the disulphide bonds within the keratin matrix, while leaving the intermediate filaments intact. Results in Figs. 1c and 1d show that human hair shows a considerable decrease in strain rate sensitivity (from 0.11 to 0.05), while that for horse hair is not significantly affected by the treatment. This is partially explained by difference in the percentage of the matrix, since the strain rate sensitivities become close in human and horse hair after the contribution of matrix is excluded. This work was supported by the AFOSR MURI Program (AFOSR-FA9550-15-1-0009).

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