# FlowHaptics: Mid-Air Haptic Representation of Liquid Flow
## 2 Related Works  
Although there is existing previous research on noncontact approaches to recreating tactile sensations such as the laser-induced evocation of tactile feedback [7] or using acoustic waves [5] and airflow [8; 9; 10; 11] to deliver tactile feedback to the skin, there is not much research exploring the haptic representation of fluid flow. [PERSON] et al. [12] conducted a sensory evaluation of the frictional force generated in the direction parallel to the skin surface when the skin touches water. As a result, it was shown that the frictional force generated in the direction parallel to the skin surface was characteristic of the feeling of water. In this research, we used this finding to not only produce the force in the vertical direction, but also the force parallel to the finger on both sides of the finger.  
[PERSON] et al. [13] proposed the general concept of the haptic representation of the scalar, vector, and tensor fields. [PERSON] et al. [14] proposed a haptic feedback system with the fluid flow field generated by simulation.Forces can be generated with haptic devices, such as through a stylus, e.g., the PHANToM [15], which can generate forces of multiple degrees of freedom on the stylus tip. This successfully reproduces the reaction force of the fluid in real time, but the presentation of the reaction force is transmitted to the user through a graspable device. Furthermore, since the reaction force is presented only at one point of the device, it is difficult to experience fine reaction forces. We believe that users can directly experience the reaction force through the skin and that the reactive force should be presented in a distributed manner in order to have a better immersive experience. [PERSON] et al. [16] created a glove-type device that can locally present vibrations induced by 17 motors to the fingers, palms, and back of the hand to simulate the perception of the flow of the wind. They performed a test on the ability to discriminate the wind direction by changing the amplitude and frequency of the vibration of each motor. The wind direction could be distinguished if there was a difference of more than eight degrees. [PERSON] et al. [17] demonstrated an ultrasound rendering of a hand interacting with fluids. The system simulated an air-like flow and produced a pressure distribution that was suitable for an ultrasound haptic display.  
Instead of just producing tactile feedback on the skin, this research is focused on reproducing the sensation of rapidly falling water on the fingers without the user grasping or wearing a device. The sensation of flowing water is defined as a complex feeling composed of the sensation of force, which is the magnitude of the water's force on the skin, and the tactile sensation, which is the skin friction that occurs when flowing water passes over the fingers. In this paper, the pressure sensation of falling water hitting the fingers was produced using multiple air jets.  
Figure 1: Our prototype _FlowHaptics_, which produces midair haptic feedback to replicate the sensation of flowing water. Although the feeling of flowing water is composed of multiple sensations, such as viscosity and temperature, we focused on pressure, including the drag over the fingers and skin friction.