  A noncontact tactile stimulus can be presented by focusing airborne
ultrasound on the human skin. Focused ultrasound has recently been reported to
produce not only vibration but also static pressure sensation on the palm by
modulating the sound pressure distribution at a low frequency. This finding
expands the potential for tactile rendering in ultrasound haptics as static
pressure sensation is perceived with a high spatial resolution. In this study,
we verified that focused ultrasound can render a static pressure sensation
associated with contact with a small convex surface on a finger pad. This
static contact rendering enables noncontact tactile reproduction of a fine
uneven surface using ultrasound. In the experiments, four ultrasound foci were
simultaneously and circularly rotated on a finger pad at 5 Hz. When the orbit
radius was 3 mm, vibration and focal movements were barely perceptible, and the
stimulus was perceived as static pressure. Moreover, under the condition, the
pressure sensation rendered a contact with a small convex surface with a radius
of 2 mm. The perceived intensity of the static contact sensation was equivalent
to a physical contact force of 0.24 N on average, which was 12 times the
radiation force physically applied to the skin.
