• contact angle;
  • friction coefficient;
  • human skin;
  • wettability

Introduction: The objective of this study was to investigate whether hydrophilic/hydrophobic balance (Hi/Ho) of the skin surface strongly modifies the friction coefficient (μ).

  The Hi/Ho balance is determined using the relationship between the critical surface tension γc (Zisman's principle: which delimits the wetting capacity) and the surface tension of water γH2O (water: reference element of Hi/Ho balance).

Method: Critical surface tension γc was determined (according to Zisman's principle) through the measurement of advancing contact angle θ 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 μ.

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 μ. For example, Teflon® with high hydrophobia has a low μ. Friction coefficient increased when hydrophobia of sliding and slider surfaces decreased.

Conclusion: 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.