Contact Mechanics and Friction on Dry and Wet Human Skin
The surface topography of the human wrist skin is studied using an optical method and the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for both dry and wet condition of the skin. For dry skin, plastic yielding becomes important and will determine the area of contact observed at the highest magnification. The measured friction coefficient [M.J. Adams et al., Tribol Lett 26:239, 2007] on both dry and wet skin can be explained assuming that a frictional shear stress σf ≈ 15 MPa acts in the area of real contact during sliding. This frictional shear stress is typical for sliding on polymer surfaces, and for thin (nanometer) confined fluid films. The big increase in the friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as resulting from the increase in the contact area arising from the attraction of capillary bridges. This effect is predicted to operate as long as the water layer is thinner than ∼14 μm, which is in good agreement with the time period (of order 100 s) over which the enhanced friction is observed (it takes about 100 s for ∼14 μm water to evaporate at 50% relative humidity and at room temperature). We calculate the dependency of the sliding friction coefficient on the sliding speed on lubricated surfaces (Stribeck curve). We show that sliding of a sphere and of a cylinder gives very similar results if the radius and load on the sphere and cylinder are appropriately related. When applied to skin the calculated Stribeck curve is in good agreement with experiment, except that the curve is shifted by one velocity-decade to higher velocities than observed experimentally. We explain this by the role of the skin and underlying tissues viscoelasticity on the contact mechanics.
KeywordsContact mechanics Skin friction Water layer
We thank M.J. Adams and S.A. Johnson for useful communications, and for sending us their surface topography data for skin.
- 2.Park, A.C., Baddiel, C.B.: Effect of saturated salt solutions on elastic properties of stratum corneum. J. Soc. Cosmet. Chem. 23, 3, 471–479 (1972)Google Scholar
- 6.Persson, B.N.J., Albohr, O., Tartaglino, U., Volokitin A.I., Tosatti, E.: On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion, J. Phys.: Condens. Matter 17, R1–R62 (2005)Google Scholar
- 7.Kendall, M.A.F., Carter, F.V., Mitchell, T.J., Bellhouse, B.J.: University of Oxfort, UK, research article: Comparison of the transdermal ballistic delivery of micro-particles into the human and porcine skin. http://www.dtic.mil/dtic/tr/fulltext/u2/a410062.pdf
- 8.Persson, B.N.J., Ganser, C., Schmied, F., Teichert, C., Schennach, R., Gilli, E., Hirn, U.: Adhesion of cellulose fibers in paper, subm. to J. Phys.: Condens. MatterGoogle Scholar
- 18.Wangenheim, M., Kröger, M.: Friction phenomena on microscale in technical contacts with rubber. Proceedings 9th ASME Conference on Engineering Systems Design and Analysis 3, ESDA (2008), Haifa, Israel. S., pp. 541–547Google Scholar
- 21.Niewiarowski, P.H., Lopez, S., Ge, L., Hagan, E., Dhinojwala, A.: Sticky gecko feet: the role of temperature and humidity. PLoS ONE 3, e2192 (2008). doi: 10.1371/journal.pone.0002192
- 31.Johnson, K.L.: Contact Mechanics, p. 452. Cambridge University Press, Cambridge (1966)Google Scholar
- 35.Johnson, S.A., Gorman, D.M., Adams, M.J., Briscoe, B.J.: The friction and lubrication of human stratum corneum. In: D. Dowson et al (eds.) Thin films in technology, pp 663–672. Elsevier Science Publishers (1993)Google Scholar
- 41.Geerligs, M.: A literature review of the mechanical behaviour of the stratum corneum, the living epidermis and the subcutaneous fat tissue, Technical Note PR-TN 2006/00450, Koninklijke Philips Electronics N.V. (2006)Google Scholar