How to identify water from thickener aqueous solutions by touch.

Water detection is one of the most crucial psychological processes for many animals. However, nobody knows the perception mechanism of water through our tactile sense. In the present study, we found that a characteristic frictional stimulus with large acceleration is one of the cues to differentiate water from water contaminated with thickener. When subjects applied small amounts of water to a glass plate, strong stick-slip phenomena with a friction force of 0.46 ± 0.30 N and a vertical force of 0.57 ± 0.36 N were observed at the skin surface, as shown in previous studies. Surprisingly, periodic shears with acceleration seven times greater than gravitational acceleration occurred during the application process. Finite-element analyses predicted that these strong stimuli could activate tactile receptors: Meissner's corpuscle and Pacinians. When such stimuli were applied to the fingertips by an ultrasonic vibrator, a water-like tactile texture was perceived by some subjects, even though no liquid was present between the fingertip and the vibrator surface. These findings could potentially be applied in the following areas: materials science, information technology, medical treatment and entertainment.

How do we recognize biological materials by touch?

Biological materials are often used as industrial sources; however, the features of their tactile texture have not been examined. Here, we show that the features of biological materials are warm, silky, and non-slippery sensations, which are governed by thermal conductivity, surface energy, and surface roughness. Interestingly, surface roughness is the most important factor in determining these three tactile sensations. This finding is valuable for developing virtual reality systems and humanoids as well as for understanding the cognitive mechanism of biological materials.