“…3 show the friction reduction at a number of velocities in the x and y directions for a given vibration amplitude. It is important to note that, while the friction reduction produced by the ultrasonic longitudinal vibration seems relatively small in comparison to the reduction observed at high velocities, even without vibration, a comparison with classic ultrasonic devices (which use out-ofplane modes) [21] shows that longitudinal modes have the potential to be more energy efficient in terms of the active power required to produce a given friction contrast, and thus a desired sensation.…”