  Rendering tactile effects on a touch screen via electrovibration has many
potential applications. However, our knowledge on tactile perception of change
in friction and the underlying contact mechanics are both very limited. In this
study, we investigate the tactile perception and the contact mechanics for a
step change in friction under electrovibration during a relative sliding
between finger and the surface of a capacitive touchscreen. First, we conduct
magnitude estimation experiments to investigate the role of normal force and
sliding velocity on the perceived tactile intensity for a step increase and
decrease in friction, called as rising friction (RF) and falling friction (FF).
To investigate the contact mechanics involved in RF and FF, we then measure the
frictional force, the apparent contact area, and the strains acting on the
fingerpad during sliding at a constant velocity under three different normal
loads using a custom-made experimental set-up. The results show that the
participants perceived RF stronger than FF, and both the normal force and
sliding velocity significantly influenced their perception. These results are
supported by our mechanical measurements; the relative change in friction, the
apparent contact area, and the strain in the sliding direction were all higher
for RF than those for FF, especially for low normal forces. Taken together, our
results suggest that different contact mechanics take place during RF and FF
due to the viscoelastic behavior of fingerpad skin, and those differences
influence our tactile perception of a step change in friction.
