Virtual targets on touchscreens (e.g., icons, slide bars, etc.) are notoriously challenging to reach without vision. The performance of the interaction can fortunately be improved by surface haptics, using friction modulation. However, most methods use position-dependent rendering, which forces users to be aware of the target choice. Instead, we propose using tactile feedback dependent on users’ speed, providing a viscous feeling. In this study, we compared three viscous damping conditions: positive damping, negative damping, and variable damping (viscosity was high during slow movements and low during fast movements), against a baseline condition with no tactile feedback. These viscous fields are created by changing net lateral forces based on velocity. Results indicate that, during the initial phase of movement when the finger approaches the target, various viscous feedback has an insignificant impact on targeting trajectories and movement velocity. However, positive damping and variable damping significantly influence behavior during the selection phase by reducing oscillation around the target and completion time. Questionnaire responses suggest user preference for viscous conditions and disapproval of negative viscous forces. This study provides insights into the role of viscous resistance in touchscreen interactions.@en

The sensation of touching virtual texture and shape can be provided to a touchscreen user by varying the friction force. Despite the saliency of the sensation, this modulated frictional force is purely passive and strictly opposes finger movement. Therefore, it is only possible to create forces along the direction of movement and this technology cannot stimulate a static fingertip or provide forces that are orthogonal to the direction of movement. The lack of orthogonal force limits the guidance to a target in an arbitrary direction and there is a need for active lateral forces to give directional cues to the fingertip. Here, we introduce a surface haptic interface that uses ultrasonic traveling waves to create an active lateral force on bare fingertips. The device is built around a ring shape cavity where two degenerate resonant modes around 40 kHz are excited with 90$^{\circ }$ phase shift. The interface provides active forces up to 0.3 N to a static bare finger uniformly over a 140×30 mm$^{2}$ surface. We report the model and design of the acoustic cavity, force measurements, and an application to create a key-click sensation. This work demonstrates a promising method for uniformly producing large lateral forces on a touch surface.

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