Human beings often explore and perceive the characteristics of objects by touching with their fingers. During this process, the contact pressure and shear stress acting on the skin also modulate the tactile sensation. The external environment is an important factor that influences tactile perception as well as the finger friction characteristics. The purpose of this study was to investigate the effects of fluid environments, such as air, deionized water (DW) and thickened water (TW), on perceived roughness and relevant friction behaviour during finger movement. Two studies were performed to analyse the effect of fluid environment as well as the influence of lubricant viscosity on finger tactile friction behaviour. Participants conducted perception and sliding friction tests with their index finger in air and submerged in DW and TW, respectively. Perception tests were performed using a pairwise comparison, scoring the perceived roughness difference between a reference sample and the test sample. The statistical analysis showed that there was no significant difference in the roughness perception between air and DW, while the sensitivity of perception reduced with increasing lubricant viscosity. An approximate calculation of the film thickness was combined with classical lubrication theory to investigate the relationship between perception and friction. In TW, the thick film formed between the finger and the polytetrafluoroethylene plate changed the contact of the asperities with the skin, thus changing the subjective judgement and friction.
Keywords: finger friction; fluid environment; lubrication theory; tactile perception.