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Under certain circumstances, the measured slip resistance of wet walkways has been found to be greater than readings obtained from the same surfaces when dry, a seemingly anomalous result. This phenomenon, sometimes termed sticktion, is most often seen when using a leather test foot. Sticktion has been ascribed to surface-tension adhesion. Surface-tension adhesion is examined with a focus on its production in wet-walkway slip-resistance testing. Past research has indicated that surface hardness is a determinant of frictional resistance. A hypothesis is proposed that, when using leather test feet on wet surfaces, a large portion of the increase in readings over dry values is attributable to water-absorption softening of the leather, and constitutes real slip resistance. Reinterpretation of wet-leather test results as an indication of the presence of sticktion is suggested. Water must physicochemically ‘wet’ both the test foot and test surface for surface-tension adhesion to develop. Tests reveal that such wetting takes place on a number of test-foot and shoe-sole materials of interest. However, for surface-tension adhesion to be of significant magnitude in tribometry, a number of critical conditions must be present. The likelihood of significant surface-tension adhesion occurring when measuring slip resistance is assessed.
slip resistance, sticktion, surface-tension adhesion, hardness, roughness
Engineer, National Forensic Engineers, Inc., Kenmore, WA