In general, the friction coefficient of a soft object, which has a low elastic modulus, is greater than that of a hard object. Briefly, friction and hardness are physically correlated. Given this relationship, a question naturally arises: are they perceptually coupled?We hypothesized that the higher an object’s surface friction coefficient, the softer it would feel in a physically consistent manner. To confirm this hypothesis, we conducted two types of psychophysical experiments using skin-like materials made of polyurethane and human cheeks as stimuli. In experiment 1, skin-like objects with the same dimensions and stiffness were coated with powders so that they had different friction coefficients. Participants actively explored and evaluated the softness of these surfaces using their fingers. Their exploratory motions were restricted to either pressing or rubbing. When participants repeatedly pressed the surfaces with no sliding motion, they judged the softness of all the surfaces to be equal. In contrast, when participants rubbed the surfaces, they judged the surfaces with lower friction (i.e., more slippery) as softer than the surfaces with higher friction. In experiment 2, the same results were obtained using human cheeks, one side of which was lubricated to be more frictional. This psychophysical interference between hardness and friction is paradoxical in terms of tribology and contact mechanics. We discuss the potential reasons that led to these results.

Background: Tactile sensation plays a crucial role in object manipulation, communication, and even emotional well‐being. It has been reported that the deformability of skin (also described as skin compliance) that shows a large mechanical response to stimuli is associated with high tactile sensitivity. However, although the compliance of the stratum corneum, the outermost layer of skin, can change daily due to skin care and environmental factors, few studies have quantified the effect of the stratum corneum on tactile sensation.Aims: We investigated the changes in tactile sensitivity resulting from skin hydration and identified corresponding alterations in the compliance of the stratum corneum.Methods: A randomized controlled trial was conducted. Participants were randomly assigned to an intervention group (n = 20) that had a moisturizing cream applied to their cheeks or a control group (n = 19) that had Milli‐Q water applied to their cheeks. Tactile discrimination performance was assessed using psychophysical techniques before and after application. The water content, mechanical response characteristics, and penetration of PEG/PPG‐17/4 dimethyl ether from the cream in the stratum corneum were evaluated to identify hydration effects. Skin deformations occurring during tactile sensation were measured concurrently using a suction device employed for tactile stimulation.Results: Tactile sensitivity was increased in participants who had cream applied to the skin surface, while no significant change was observed in participants who received Milli‐Q water. The improved discrimination of tactile stimulus intensity was directly related to the magnitude of skin displacement. The higher water content of the stratum corneum due to cream application decreased the dynamic modulus of elasticity of the stratum corneum and increased the skin's extensibility in response to tactile stimuli.Conclusions: Hydrating the stratum corneum significantly enhances tactile sensitivity and is accompanied by an increase in skin extensibility, a factor in tactile intensity perception. The compliance of the thin stratum corneum layer plays a crucial role in tactile experiences that involve skin stretching.