## 2 Related Works  
Recent literature on haptic devices to render sensations of weight and directional forces varies includes a software-based haptic interface, shape-changing device, wearable haptic device, and airflow-based haptic interface. [PERSON] et al. [19] proposed a software-based method that used visual manipulation of digital content to modify perceptions of weight. Interestingly, they found that the levels of presence, immersion, and enjoyment, as rated by the users, could be improved significantly with perceivable tracking offsets. Unlike the software-based approach, [PERSON] et al. [17] proposed a shape-changing haptic device. They proposed Transcalibur, which changes the input device's center of mass to generate different haptic feedback corresponding to specific VR objects held by users. Transcalibur drives weights on a 2D planar area to alter the mass property of the device. For example, the weight shift varies when the user holds a sword or a shield in the VR content to provide shape perception. [PERSON] and [PERSON] [20] proposed Shifty, a rod-shaped dynamic passive haptic proxy device that changes its center of mass. It also changes its perceived thickness and length based on the VR item held by the user. A study was completed wherein the perception of weight was controlled using only visual information. Unlike the shape-changing devices that impart sensations of weight, researchers also investigated the use of wearable haptic devices to simulate physical contact in VR. [PERSON] and [PERSON] [18] proposed a \"jamming suit\", which conveyed a sensation of freezing to the human body. The suit jams joints of the user's body, such as the elbows and knees, to provide the  
Figure 1: DFF device in use.  
physical sensations of being attacked by ice ghouls (VR creatures). [PERSON] et al. [16] developed Dexmo, a state-of-the-art wearable mechanical exoskeleton enabling motion capture and force feedback, which was shown to improve the quality of hand motion control in VR applications. [PERSON] et al. [13] developed a haptic 3-dimensional (3D) shaped output using handheld controllers to enable users to feel shapes, surfaces, forces, and textures based on linear actuation. They demonstrated two devices that render the surface, force feedback, shape, and texture of the VR contents through the fingertips. Since the controller only uses the bottom of the fingertip for haptic information rendering, the device cannot provide complete haptic information [13]. [PERSON] et al. [21] introduced Grabity, a wearable haptic interface which simulates sensations of weight and grasping in VR. Grabity is composed of a kinesthetic pad to simulate grasping sensations for various sized VR objects. Furthermore, it stretches users' skin to simulate sensations of various weights. However, unlike the aforementioned studies that focus on simulating physical contacts in VR through restriction and vibrotactile feedback, there are researchers who have used air flow to generate real force feedback, enabling users to interact in VR environments without handheld devices. [PERSON] et al. [14] generated haptic feedback using a mobile air jet (HAIR), which generated air jets to stimulate the skin. HAIR generated air jets using a robotic arm that tracked and moved underneath the user's hand. [PERSON] and [PERSON] [22] created a device containing a grid of air nozzles. When a virtual 3D object was placed on the device, air flow generated force feedback corresponding to the location of the user's hand and the VR object. [PERSON] and [PERSON] [15] developed VacuumTouch, a force feedback interface based on air suction. VacuumTouch was connected to a touch screen to create pressure when the user touched the digital input interface. The airflow-based haptic interface provides real force feedback, but requires extra installations. Unlike previous literature on haptic devices, this study focusses on rendering actual force feedback by creating force concentrations to render both directional and weight feedback in VR.