title: Influence of the hydrophobic and hydrophilic characteristics of sliding and slider surfaces on friction coefficient: in vivo human skin friction comparison.
abstract: BACKGROUND
  The objective of this study was to investigate whether hydrophilic/hydrophobic balance (Hi/Ho) of the skin surface strongly modifies the friction coefficient (mu). The Hi/Ho balance is determined using the relationship between the critical surface tension gammac (Zisman's principle:which delimits the wetting capacity) and the surface tension of water gammaH2O (water: reference element of Hi/Ho balance).
  
  
  METHODS
  Critical surface tension gammac was determined (according to Zisman's principle) through the measurement of advancing contact angle theta of a series of ethanol/water dilutions. Friction coefficient depends on several parameters: types of probe motions (rotational vs. linear), surface roughness and physicochemical parameters of surfaces in contact). In this study, the wettability parameters for six surfaces (human skin forearm, Teflon, silicone impression material 'Silflo', vinyl polysiloxane impression material 'resin' steel and glass) were measured and their influences were compared to friction coefficient mu.
  
  
  RESULTS
  This study shows that the higher hydrophobia tendency of the surfaces, the lower friction coefficient. The use of three sliding materials (Teflon, steel and glass) of different Hi/Ho balance confirms the importance of these physicochemical parameters in mu. For example, Teflon with high hydrophobia has a low mu. Friction coefficient increased when hydrophobia of sliding and slider surfaces decreased.
  
  
  CONCLUSIONS
  Friction coefficient value depends on the type of slider surface and its physicochemical properties. In vivo,the friction coefficient may quantify the influence of lubrificant/emolients/moisturizers. For example, the friction coefficient of hydrated skin (through the action of moisturizing products) is higher than the friction coefficient of dry skin. The relationship between the friction coefficient and the Hi/Ho balance can be reversed in the presence of water and sebum on forehead, for example.
authors.affiliation: Laboratoire d'ingénierie et de biologie cutanée, Université de Franche Comté, Besançon, France.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Elkhyat
authors.forename: Carol
authors.initials: C
authors.lastname: Courderot-Masuyer
authors.forename: Tijani
authors.initials: T
authors.lastname: Gharbi
authors.forename: Philippe
authors.initials: P
authors.lastname: Humbert
mesh_terms: D000328:Adult; D001834:Body Water / Q000502:physiology*; D003198:Computer Simulation; D005260:Female; D017276:Friction; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008954:Models, Biological; D012212:Rheology / Q000379:methods*; D012879:Skin Physiological Phenomena; D013223:Statistics as Topic; D013500:Surface Tension; D017155:Wettability