Sonovortex: Aerial Haptic Layer Rendering by Aerodynamic

Vortex and Focused Ultrasound

Satoshi Hashizume∗ Amy Koike Takayuki Hoshi Yoichi OchiaiUniversity of Tsukuba

Abstract

In this paper, a method of rendering aerial hapticsthat uses an aerodynamic vortex and focused ultra-sound is presented. Significant research has been con-ducted on haptic applications based on multiple phe-nomena such as magnetic and electric fields, focusedultrasound, and laser plasma. By combining multi-ple physical quantities; the resolution, distance, andmagnitude of force are enhanced. To combine multi-ple tactile technologies, basic experiments on resolu-tion and discrimination threshold are required. Sep-arate user studies were conducted using aerodynamicand ultrasonic haptics. Moreover, the perception oftheir superposition, in addition to their resolution,was tested. Although these fields cause no direct in-terference, the system enables the simultaneous per-ception of the tactile feedback of both stimuli. Theresults of this study are expected to contribute to ex-panding the expression of aerial haptic displays basedon several principles.

1 Introduction

Aerial haptic feedback is a popular topic in researchfields such as real-world-oriented interaction, aug-mented reality (AR), and virtual reality (VR). Sev-eral methods have therefore been proposed to realizeaerial haptic feedback which include phenomena suchas magnetic forces, ultrasound, and air vortices.

An aerial haptic display has several advantages.First, it projects a force over a distance without phys-

∗hashizume@digitalnature.slis.tsukuba.ac.jp

ical contact or wearable devices. Second, it has ahigh programmability. In particular, it can be setand rearranged at an arbitrary position in a three-dimensional (3D) space, as it does not require physi-cal actuators.

In this study, new aerial interactions were evalu-ated. The aim of the study was the development ofa new aerial haptics system to express a wide rangeof feedback. The proposed system (Figure 1) com-bines aerodynamic and acoustic fields. The aerody-namic vortex [1] from the aerodynamic field and thefocused ultrasound [2] from the acoustic field wereused to develop the device.

The tactile sensations of single and multiple fieldswere then compared. Through a user study, it wasfound that the aerodynamic vortex and focused ul-trasound do not influence each other. By combiningdifferent types of forces, the proposed system can dis-play various textures. Based on the reports in theliterature, this is an early study that combines multi-field physical quantities to render haptic textures.

2 Related Work

2.1 Aerial Haptics Feedback

Several methods have been proposed for aerial hap-tic feedback without physical contact or wearabledevices. The technologies employed without wear-able devices are based on aerodynamic vortices, fo-cused ultrasound, laser induced plasma, and mag-netic forces. These technologies have been appliedto touch panels [3] and VR [4] systems .

Ultrasonic technology, which uses ultrasound, can

1

arX

iv:1

911.

0298

5v2

[cs

.HC

] 9

Dec

201

9

Figure 1: Sonovortex device.

provide tactile sensations in mid-air without the needfor a user actuator [5] [2]. The position of the focalpoint can be changed using a phased array transduceras it represents tactile behavior. The rendering ofvolumetric haptic shapes can also be achieve usingfocused ultrasound [6]. MidAir [3] reflects a virtualimage in the air and provides tactile feedback usingan ultrasonic speaker based on the virtual image andfinger location. HaptoClone [7] enables real-time in-teraction with floating volumetric images using hap-tic feedback. This method is insufficient, given thatthe focused ultrasound force is very weak and onlylimited focal points can be generated.

In [8], an air jet was used to produce contactlesshaptic feedback with a low accuracy. In [4], virtualobjects were represented by air jets from an array ofnozzles. Vortex rings [1] have also been used as anon-contact haptic feedback system, and air vorticeshave been used to produce impact in midair [9] [10].These previous approaches mainly used vortex ringsto add multi-modal sensation to a conventional dis-play. Ueoka et al.[11] evaluated the manner in whichpeople perceive haptic stimuli generated by air vortexrings and how the stimuli affect their emotional stateswhen stressed. However, both these methods have alow fidelity due to the non-focusing stimulating area.

The tactile presentation of a magnetic field involvesboth direct and indirect presentation. For direct tac-tile feedback, a magnet can be placed on the fin-ger [12] . The authors in [13], presented magneticbased sensing in addition to haptic feedback. Zhang

et al. [14] and Berkelman et al. [15] rendered a 3Dmodel in mid-air using an electromagnet array. Indirect tactile presentation, powerful tactile feedbackcan be achieved without touching the screen.

Light is employed to provide sensation on thehands when the user is experiencing thermal radi-ation [16]. Nanosecond lasers applied to the skininduce a tactile sensation [17] . To date, radio-frequency and superconducting forces have not beenapplied to aerial haptic feedback.

2.2 Cross-Field Haptics

This study combines multiple haptics technologies,thus overcoming their individual drawbacks and im-proving the interaction width.

The wUbi-Pen [18] is a pencil-type tactile interfacethat consist of a vibrator, linear vibrator, speaker,and pin array. It provides functions such as feed-back drag, drop, and movement. Minamizawa etal. [19] developed a device based on tactile presenta-tion, which that combines one-point kinesthetic feed-back and multipoint tactile feedback. The accuracyof the feedback was then improved by combining hap-tics technologies. Impacto [20] was designed to ren-der the haptic sensation of hitting and being hit inthe VR environment. They combined tactile stimu-lation with electrical muscle stimulation. Cross-FieldAerial Haptics [21] involves the drawing of a tactileinterface in the air by combining ultrasonic wavesand laser plasmas. Hashizume et al. [22] developeda touch type haptic device that combines magneticand electrostatic fields. Their report also includes adescription of their implementation method.

Cross-field haptics is not a widely studied field.Aerodynamic vortex and focused ultrasound are si-multaneously utilized. Using both fields helps elimi-nate their drawbacks and it is intended to provide awider tactile presentation. The aerodynamic vortexprovides tactile sensations over large distances andforces. The focused ultrasound delivers distinguish-able high-resolution tactile sensations.

2

Air Cannon

Focused Ultrasound

D

Figure 2: Air cannon and focused ultrasound visual-ized using dry ice and smoke.

3 Implementation

3.1 Aerodynamic Haptics

An air vortex (Figure 3, right) is a ring of air thattypically has a toroidal shape and can travel at highspeeds over large distances. Vortex rings (Figure 2,upper) can be formed by pushing air using a pistonthrough a circular aperture, or hole. The quality ofthe formed vortex is dependents on the volume of airpushed, the velocity of the piston, and the diameterof the aperture.

Gharib et al. [1] defined the stroke ratio RStroke

as a ratio of the length of the theoretical cylindricalslug of air pushed out of the nozzle LS to aperture ofdiameter D:

RStroke =LS

D

The stroke ratio characterizes the stability of the vor-tex as it exits the aperture, and it is used to definethe formation number. A typical value for the forma-tion number is within the range of 3.6-4.5 for severalvarious vortex systems.

According to [23][24], LS can then be expressed as

LS =4VSπD2

where VS is the slug volume.

Thus, for stable vortices, the following is true:

4VSπD3

5 4.5 (1)

An air cannon based on AIREAL, was developed inthis study [9]. Five 2-inch 15W Whisper subwooferswere used as actuators. Sodhi et al. [9] determinedthe total volume of air displaced by all five speakersand the aperture diameter using the previous equa-tions:

VS = 33, 670 mm3, D = 2.1 cm

3.2 Ultrasound Haptics

Ultrasonic haptics are based on acoustic radiationpressure, which exerts a force on the surface of theskin (Figure 2, lower). Ultrasonic haptics can be ap-plied to the skin for a long time-period; however, theyare relatively weak (10-20 mN). The sensation is sim-ilar to that of a laminar air flow within a narrow area.

The time delay ∆tij for the (i, j)-th transducer isgiven by:

∆tij =l00 − lij

c(2)

where l00 and lij are the distances from the focalpoint to the (0, 0)-th reference and (i, j)-th transduc-ers, respectively; and c is the speed of sound in theair. The focal point can be moved by recalculationand the setting of the time delays for the next coor-dinates.

Haptic images are generated by an acoustic phasedarray system(Figure 3, middle). Haptic image Hi isthe summation of the time series of the focal points:

Hi =∑

fp(x, y, z) × p× t (3)

where fp is the ultrasonic focal points, p is the acous-tic pressure, and t is the time duration.

3

Air cannon

Phased array

Displayed areaPhased array Air cannon

40kHz transducers

Whisper subwoofer

Nozzle

Figure 3: Aerodynamic and ultrasound system.

3.3 Latency

The amount of time required to produce a tactilepresentation using an aerodynamic vortex is differ-ent from that required using the focused ultrasound.The focused ultrasound advances in the air at thespeed of sound. Therefore, the focused ultrasoundcan achieve a near simultaneous tactile presentationwith the generation of signals. The speed and delayof the aerodynamic vortex are greater than those ofultrasound. According to [10], the vortex speed isgiven by

vs =LS

tcone

vvortex =vs2

where tcone is the time required for the displaced airto move through the aperture. The average speedof the aerodynamic vortex device used in this studywas 7.2m/s [9]. A delay of 30mms was applied to thefocused ultrasound to ensure that the aerodynamicvortex and focused ultrasound reach the user simul-taneously.

4 EXPERIMENTS AND RE-SULTS

In this section, a discussion on the user experimentsfor the evaluation proposed haptic system is pre-sented.

4.1 Experiment of generated force

The magnitude of the force generated by an ultra-sonic wave and air cannon were considered. The pre-cision electronic balance was set vertically and placed15 cm from the ultrasonic device and air cannon (Fig-ure 4). The mass displayed on the precision electronicbalance was then converted to force. The air cannonwas output at 30 Hz, and the power supply voltagewas varied from 5 to 17.5 V in increments of 2.5 V.In addition the change in the magnitude of the gen-erated force was examined. The ultrasonic wave wasoutput at a modulation frequency of 50 Hz, and theoutput intensity was changed from 0 to 600 in incre-ments of 100. Moreover, the change in the magnitudeof the generated force was investigated. To convertthe output intensity to the output force, a conversioncould be carried out using sin2(πp/1248) [2]. Theultrasonic focal length was set as 15 cm.

Figure 5 present the results, in which both the ul-trasonic wave and air cannon increase the force gen-erated in proportion to the output intensity.

4.2 Experiment of double-pointthreshold

The user study was conducted to investigate spatialresolution. The double-point threshold[25] for acous-tic radiation pressure induced by focused ultrasoundand air vortex pressure was evaluated. Five peopleparticipated in the user study (20.2 years old on av-

4

Air cannon

(a) Setup of air cannon in experiment of generated force.

15cm

Electronicbalance

15cm

Electronicbalance

Ultrasonicarray

(b) Setup of ultrasonic array in experiment of generated force.

Figure 4: Setup of experiment on generated force

erage, with one female and four males). The par-ticipants were isolated from visual information usingblindfolds. Participants then placed their hands ona table positioned 15 cm away from the haptic de-vice(Figure 7 right). The platform could be movedwith an accuracy of 0.1 mm, as participants were toldnot to move their hands. The output force was setwith reference to Figure 5.

The method of limits was used to measure thedouble-point threshold. First, the standard stimuluswas applied to the palm of the hand of the partic-ipant. The standard stimulus is a 10 s ultrasoundwave and five air vortex cycles. After administeringthe standard stimulus, the platform was moved andthe comparative stimulus was administered. Afteradministering the comparative stimulus, each partic-ipant gave one of the following answers: ”(1) the twopoints are divided,” (2) ” the two points not are di-vided,” or (3) ”I do not know.” The distance betweenthe standard and the comparative stimuli was ap-proximated until the participant answered ”the twopoints are not divided” or ”I do not know” (descend-ing series). The distance between the standard andcomparative stimuli was then increased until the par-ticipant answered with ”the two points are divided”(ascending series). The test for the descending andascending series were carried out twice.

The experiments were conducted when only ultra-sonic waves were applied, when only an air cannonwas used, when an air cannon was used with a con-stant ultrasonic wave, and when an ultrasonic wavewas applied with a constant air vortex. The ultra-sonic waves were generated at modulation frequenciesof 50 Hz and 200 Hz. The focal length of the ultra-sonic device was set as 15 cm, and the output force

was set as 5.73 mN. The participants were stimulatedwith ultrasound for 5 s. The output force of the aircannon was set as 7.67 mN, and the stimulation wasapplied five times. To provide a constant air vortex,the air cannon was implemented at 15 Hz.

Results : The results are shown in Figure 6. In thecase of only ultrasonic waves (Figure 6 (a) and (b)),the double-point threshold was approximately 6 mmregardless of the modulation frequency. The double-point threshold for an air cannon only (Figure 6 (c))was 11 mm. The double-point threshold of an air can-non while an ultrasonic wave was constantly provided(Figure 6 (d) and (e)) was not much different fromthat of an air cannon only. The double-point thresh-old of the ultrasonic wave while an air vortex wasconstantly provided (Figure 6 (f) and (g)) was ap-proximately 3 mm larger that of the ultrasonic only.In the case in which the air cannon was affected, thevariation of the double-point threshold by the partic-ipant was large.

4.3 Experiment of perceptual thresh-old

The user study was conducted to evaluate the percep-tual threshold for acoustic radiation pressure inducedby the focused ultrasound and air the pressure vor-tex. Seven people participated in the user study (19.6years old on average with two females and five males).The participants were isolated from visual informa-tion using blindfolds, and auditory information waseliminated using headphones with white noise (Fig-ure 7, left). Participants placed their hands on a tablepositioned 15 cm away from the haptic device(Figure7 right). The output force was set with reference toFigure 5.

Focused ultrasound : The focused ultrasoundhaptic stimulation was applied to the right palm ofeach participant. Moreover, vibrotactile stimulationmodulated by 200-Hz and 50-Hz rectangular waveswas applied. The output force was set as one of sixvalues (min:0.70 mN, max:10.9 mN) near the thresh-olds. The ultrasound output time was 200ms. Eachforce condition was applied once (i.e., one trial) andthe number of trials was 10 per participant. The or-der of trials was randomized. In each trial, the par-

5

0

2

4

6

8

10

12

0 20 40 60 80 100

Forc

e [m

N]

Ultrasonic output power [%]

0

2

4

6

0 20 40 60 80 100 120

m[ ecroF

Ultrasonic output power [%]

0

2

4

6

8

10

12

14

16

0 5 10 15 20

Forc

e [m

N]

Air cannon output power [V]

(a) Experiment on force of ultrasonic (b) Experiment on force of air cannon

Figure 5: Results of experiment on generated force

Figure 6: Results of double-point threshold:(a)ultrasonic waves(50Hz), (b)ultrasonic waves(200Hz), (c)an air cannon, (d) an air cannon whileultrasonic wave (50Hz) was constantly provided, (e)an air cannon while ultrasonic wave (200Hz) wasconstantly procided, (f) ultrasonic wave(50Hz) whilean air vortex was constantly provided. (g) ultrasonicwave(200Hz) while an air vortex was constantlyprovided.

ticipants were asked whether they perceived stimulion their forefingers.

Aerodynamic vortex : The aerodynamic vortexhaptic stimulation was applied to the right palm ofeach participant. The output force was set to oneof six values (min:0.66 mN, max:13.7 mN) near thethresholds. Each force condition was applied once(i.e., one trial) and the number of trials was 10 per

Blindfold

Head Phone

Air cannonPhased array

Displayed area

Displayed area

Figure 7: Overview of experimental setup

participant. The order of trials was randomized. Ineach trial, the participants were asked whether theyperceived the stimuli on their forefingers.

Cross-field : Three types of user studies were con-ducted on the cross-field. First, focused ultrasoundand an aerodynamic vortex tactile stimulus were ap-plied simultaneously. The output force of the air can-non was set as 7.67 mN. The vibrotactile stimulationof ultrasound was set as 200 Hz and 50 Hz. Second,the air vortex perceptual threshold in space with aconstant ultrasound was evaluated. The output forceof the ultrasound was maintained at 9.7 mN. The out-put force of the air vortex was set to one of six val-ues (min:0.66 mN, max:13.7 mN). Third, the focusedultrasound perceptual threshold in space with a con-stant aerodynamic vortex was evaluated. The outputforce of the air vortex was set as 7.67 mN and the aircannon was actuated to 20Hz. The output force of ul-trasound was set as one of six values (min:0.70 mN,

6

Table 1: Results of simultaneous tactile presentationwith a focused ultrasound and aerodynamic vortex

The ultrasound The perceptualvibrotactile stimulation threshold of haptic

[Hz] [%]50 95.2200 100

max:10.9 mN) and the vibrotactile stimulation wasmodulated to 200 Hz and 50 Hz.

Results : The results are presented in Figure 8and Table 1. The perception rate on the verticalaxis indicates the ease of tactile sensation. When theperception threshold was 100%, the examinee couldperceive the tactile sensation.

In the case wherein only the ultrasound was ap-plied, the perception threshold was nearly 100% whenthe output exceeded 4 mN. In the case wherein onlythe air cannon was used, the perception threshold in-creased as the voltage increased. In addition, whenthe output force exceeded 11 mN, the participantscould perceive feel the tactile sensation. When the aircannon was operated with an ultrasonic wave, sens-ing when the output force reached 2 mN was difficult.However, when the output force was higher than 2mN, the results were the same as that wherein onlythe air cannon was used. When ultrasonic waves wereapplied with an air cannon, the perception thresholdwas generally low compared with the case whereinonly ultrasonic waves was applied. In particular,when the modulation frequency was 50 Hz, the per-ception threshold was less than 20 %. When pre-sented simultaneously, participants could recognize aperception threshold of 95 % or greater.

5 Discussion and Conclusion

From the experiments conducted on double-pointthresholds, the double-point threshold was found toincrease the when air cannon and ultrasonic wavewere combined. In addition, from the experimentson perceptual thresholds, the air cannon was easyto perceive in the ultrasonic presentation state; how-ever, the ultrasonic tactile stimulus was difficult to

perceive in the air cannon presentation state. Thisis because the magnitude of the force that the aircannon can apply is larger than the ultrasonic tac-tile stimuli. It is necessary to appropriately adjustthe magnitude of the applied force. However, whenthe tactile stimulus was simultaneously applied, nei-ther tactile sense could be perceived. When usingSonovortex, it is effective to present the air cannontactile sense in the ultrasonic presentation state, orto simultaneously present the air cannon and the ul-trasonic.

In this study, a method was developed that com-bines multiple tactile technologies. This method gen-erates a tactile sensation using an ultrasonic deviceand air cannon. In addition, the ranges of possibleresolutions and thresholds were discussed. Cross-fieldhelps eliminate the drawbacks of each field and pro-vide a wider tactile presentation. An aerodynamicvortex provides tactile sensations over large distancesand with considerable force levels. The focused ultra-sound delivers distinguishable high-resolution tactilesensations.

However, there are still several drawbacks. Bothan air cannon and ultrasonic device generate envi-ronmental noise. In particular, given that the aircannon produces a loud sound, using it in a quietspace such as a hospital or company office is difficult.However, this does not present a problem in noisyspaces such as shops and towns. Moreover, to useSonovortex, an air cannon and phased array must bedeferred. For wearing, Sonovortex is heavy and large.However, Sonovortex is expected to be incorporatedinto the environment and used for digital signage andamusement.

Acknowledgement

We would like to thank University of Tsukuba forsupporting this work. We are also thankful to all themembers of the Digital Nature Group at Universityof Tsukuba for their discussions and feedback.

7

(a) Experiment on perceptual threshold of ultrasonic (b) Experiment on perceptual threshold of air vortex

(c) Experiment on perceptual threshold of air vortex in ultrasonic space

(d) Experiment on perceptual threshold of ultrasonic in air vortex space

0

20

40

60

80

100

0 2 4 6 8 10 12 14

]%[ etar noitpecrep

Ultrasonic output force [mN]

200Hz

50Hz

0

20

40

60

80

100

0 2 4 6 8 10 12 14

]%[ etar noitpecreP

Air cannon output force [mN]

0

20

40

60

80

100

0 2 4 6 8 10 12 14

]%[ etar noitpecreP

Air cannon output force [mN]

50Hz

200Hz

Air vortex only

0

20

40

60

80

100

0 2 4 6 8 10 12 14

]%[ etar noitpecrep

Ultrasonic output force [mN]

50Hz

200Hz

Ultrasonic only (200Hz)

Ultrasonic only (50Hz)

Figure 8: Results of perceptual threshold.

References

[1] A. Weigand and M. Gharib. On the evolutionof laminar vortex rings. Experiments in Fluids,22(6):447–457, 1997.

[2] T. Hoshi, M. Takahashi, T. Iwamoto, andH. Shinoda. Noncontact tactile display basedon radiation pressure of airborne ultrasound.IEEE Transactions on Haptics, 3(3):155–165,July 2010.

[3] Yasuaki Monnai, Keisuke Hasegawa, MasahiroFujiwara, Kazuma Yoshino, Seki Inoue, and Hi-royuki Shinoda. Haptomime: Mid-air haptic in-teraction with a floating virtual screen. In Pro-ceedings of the 27th Annual ACM Symposium onUser Interface Software and Technology, UIST’14, pages 663–667, New York, NY, USA, 2014.ACM.

[4] Yuriko Suzuki and Minoru Kobayashi. Air jetdriven force feedback in virtual reality. IEEEComput. Graph. Appl., 25(1):44–47, January2005.

[5] Tom Carter, Sue Ann Seah, Benjamin Long,Bruce Drinkwater, and Sriram Subramanian.Ultrahaptics: Multi-point mid-air haptic feed-back for touch surfaces. In Proceedings of the26th Annual ACM Symposium on User Inter-face Software and Technology, UIST ’13, pages505–514, New York, NY, USA, 2013. ACM.

[6] Benjamin Long, Sue Ann Seah, Tom Carter,and Sriram Subramanian. Rendering volumetrichaptic shapes in mid-air using ultrasound. ACMTrans. Graph., 33(6):181:1–181:10, November2014.

[7] Yasutoshi Makino, Yoshikazu Furuyama,Seki Inoue, and Hiroyuki Shinoda. Hap-

8

toclone (haptic-optical clone) for mutualtele-environment by real-time 3d image transferwith midair force feedback. In Proceedings ofthe 2016 CHI Conference on Human Factors inComputing Systems, CHI ’16, pages 1980–1990,New York, NY, USA, 2016. ACM.

[8] Yuriko Suzuki, Minoru Kobayashi, and SatoshiIshibashi. Design of force feedback utilizing airpressure toward untethered human interface. InCHI ’02 Extended Abstracts on Human Factorsin Computing Systems, CHI EA ’02, pages 808–809, New York, NY, USA, 2002. ACM.

[9] Rajinder Sodhi, Ivan Poupyrev, Matthew Glis-son, and Ali Israr. Aireal: Interactive tactileexperiences in free air. ACM Trans. Graph.,32(4):134:1–134:10, July 2013.

[10] Sidhant Gupta, Dan Morris, Shwetak N. Patel,and Desney Tan. Airwave: Non-contact hapticfeedback using air vortex rings. In Proceedingsof the 2013 ACM International Joint Conferenceon Pervasive and Ubiquitous Computing, Ubi-Comp ’13, pages 419–428, New York, NY, USA,2013. ACM.

[11] Ryoko Ueoka, Mami Yamaguchi, and Yuka Sato.Interactive cheek haptic display with air vor-tex rings for stress modification. In Proceedingsof the 2016 CHI Conference Extended Abstractson Human Factors in Computing Systems, CHIEA ’16, pages 1766–1771, New York, NY, USA,2016. ACM.

[12] Malte Weiss, Chat Wacharamanotham, SimonVoelker, and Jan Borchers. Fingerflux: Near-surface haptic feedback on tabletops. In Pro-ceedings of the 24th Annual ACM Symposium onUser Interface Software and Technology, UIST’11, pages 615–620, New York, NY, USA, 2011.ACM.

[13] Kasun Karunanayaka, Sanath Siriwardana,Chamari Edirisinghe, Ryohei Nakatsu, and Pon-nampalam Gopalakrishnakone. Magnetic FieldBased Near Surface Haptic and Pointing Inter-face, pages 601–609. Springer Berlin Heidelberg,Berlin, Heidelberg, 2013.

[14] Q. Zhang, H. Dong, and A. El Saddik. Magneticfield control for haptic display: System designand simulation. IEEE Access, 4:299–311, 2016.

[15] P. Berkelman, M. Miyasaka, and J. Anderson.Co-located 3d graphic and haptic display us-ing electromagnetic levitation. In 2012 IEEEHaptics Symposium (HAPTICS), pages 77–81,March 2012.

[16] Satoshi Saga. HeatHapt Thermal Radiation-Based Haptic Display, pages 105–107. SpringerJapan, Tokyo, 2015.

[17] Yoichi Ochiai, Kota Kumagai, Takayuki Hoshi,Jun Rekimoto, Satoshi Hasegawa, and YoshioHayasaki. Fairy lights in femtoseconds: Aerialand volumetric graphics rendered by focusedfemtosecond laser combined with computa-tional holographic fields. ACM Trans. Graph.,35(2):17:1–17:14, February 2016.

[18] Ki-Uk Kyung and Jun-Young Lee. wubi-pen:Windows graphical user interface interactingwith haptic feedback stylus. In ACM SIG-GRAPH 2008 New Tech Demos, SIGGRAPH’08, pages 42:1–42:4, New York, NY, USA, 2008.ACM.

[19] K. Minamizawa, D. Prattichizzo, and S. Tachi.Simplified design of haptic display by extendingone-point kinesthetic feedback to multipoint tac-tile feedback. In 2010 IEEE Haptics Symposium,pages 257–260, March 2010.

[20] Pedro Lopes, Alexandra Ion, and Patrick Baud-isch. Impacto: Simulating physical impactby combining tactile stimulation with electri-cal muscle stimulation. In Proceedings of the28th Annual ACM Symposium on User InterfaceSoftware & Technology, UIST ’15, pages11–19, New York, NY, USA, 2015. ACM.

[21] Yoichi Ochiai, Kota Kumagai, Takayuki Hoshi,Satoshi Hasegawa, and Yoshio Hayasaki. Cross-field aerial haptics: Rendering haptic feedbackin air with light and acoustic fields. In Pro-ceedings of the 2016 CHI Conference on Human

9

Factors in Computing Systems, CHI ’16, pages3238–3247, New York, NY, USA, 2016. ACM.

[22] S. Hashizume, K. Takazawa, A. Koike, andY. Ochiai. Cross-field haptics: Multiple direc-tion haptics combined with magnetic and elec-trostatic fields. In 2017 IEEE World HapticsConference (WHC), pages 370–375, June 2017.

[23] K Shariff, , and A Leonard. Vortex rings. AnnualReview of Fluid Mechanics, 24(1):235–279, 1992.

[24] Ari Glezer. The formation of vortex rings.Physics of Fluids, 31(12), 1988.

[25] A Lee Dellon, Susan E Mackinnon, andPage McDonald Crosby. Reliability of two-pointdiscrimination measurements. The Journal ofhand surgery, 12(5):693–696, 1987.

10