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Localized Fretting-Vibrotactile Sensations for Large-Area Displays Quang Van Duong, Vinh Phu Nguyen, Fabrice Domingues Dos Santos, and Seung Tae Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09691 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Applied Materials & Interfaces

Manuscript for ACS Applied Materials & Interfaces

June 03, 2019

Localized Fretting-Vibrotactile Sensations for LargeArea Displays

Quang Van Duong†, Vinh Phu Nguyen†, Fabrice Domingues Dos Santos‡ and Seung Tae Choi*,† †School

of Mechanical Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-Gu, Seoul 06974, Republic of Korea

‡ARKEMA-Piezotech,

Arkema-CRRA, Rue Henri Moissan, 69493 Pierre-Benite Cedex, France

Keywords: large-area displays, localized vibrotactile sensation, fretting phenomena, multiple tactile feedback, relaxor ferroelectric polymers

Abstract Tactile perception in large-area displays is currently attracting substantial research attention, since in conjunction with visible and auditory sensations, it provides more immersive and realistic interactions with displayed contents. Here, a new vibrotactile display based on the fretting phenomenon is first developed to provide localized tactile feedback on a large-area display. Normal pressure by a human fingertip activates a locally concentrated electric field in a relaxor ferroelectric polymer (RFP) film under the contact area, which produces a localized electrostrictive strain. The synergetic interplay among the localized electric field, electrostrictive deformation of the RFP film, and contact area dramatically amplifies acoustic vibrations near the contact edge of a human fingertip. A blend of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and poly(vinylidene fluoride-trifluoroethylene) (55:45) copolymer is proposed for the RFP film to provide an enhanced actuation performance even at elevated temperatures. The fretting-vibrotactile mechanism *Corresponding

author. Tel: +82-2-820-5275. Email: [email protected] (Seung Tae CHOI) ACS Paragon Plus Environment

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Localized Fretting-Vibrotactile Sensations for Large-Area Displays

has several interesting properties like tactile feedback on a stationary fingertip, pressure-responsive simple onoff mechanism, multitouch interaction, excellent transparency, and easy integration with capacitive or resistive touch sensors and friction-based haptic-feedback mechanisms. An array of RFP film vibrators can provide addressable content-related multiple tactile feedback on large-area displays by modulating the frequency, amplitude, and profile of the driving voltage signals.

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ACS Applied Materials & Interfaces Quang Van Duong, Vinh Phu Nguyen, Fabrice Domingues Dos Santos and Seung Tae Choi

1. Introduction Haptics has recently been receiving much attention in studies on the multimodal interaction between human and virtual or augmented environments for teleoperation,1-3 rehabilitation,4-6 games,7, 8 training simulators,9-11 automobiles,12-14 and electronic devices.15, 16 Together with visual and aural sensations, haptic sensation can provide more intuitive and realistic interactions with displayed contents such as object shape, texture discrimination, and object manipulation.17, 18 Providing an aural sensation with haptic and visual sensations is relatively easy because synchronization is enough to realize multimodality, and an audio system can usually be spatially separated from haptic and display devices.17 However, integrating haptic devices with high-quality display devices usually involves technological challenges because of spatial interference (e.g., haptic feedback on top of the displayed objects) and optical hindrance. Unlike vision and hearing, tactile perceptions can only be acquired by direct physical contact between the human body and an object through seven sensory receptors (pain, cold, warm, and four mechanoreceptors) spread throughout the skin.19-21 Nevertheless, the desire for multimodal sensations has given an impetus for the development of haptic feedback actuators in large-area display surfaces to which conventional gross-mode actuators such as the eccentric motor22, 23 piezoelectric actuator,24, 25 and linear resonator26, 27 cannot be efficiently adapted. Therefore, several new mechanisms have been developed such as ultrasonic vibration,28, 29 electrovibration,30-35 electrotactile,36, 37 and smart materials38, 39 for large-area displays. The ultrasonic vibration method28, 40 provides tactile sensations by using surface acoustic waves or by squeezing the layer of trapped air in the gap between the user’s fingertip and the vibrating plate to reduce the kinetic friction. The electrovibration display is another friction-based haptic display that utilizes periodic electrical signals on the surface to create the electrovibration effect.30-35 Electrotactile displays directly stimulate electric currents through the skin surface to mechanoreceptors under the skin, thus providing tactile sensations.37, 41, 42 Tactile displays with smart materials are based on various mechanisms of different materials and structures, for example, a thermoresponsive hydrogel-based actuator38 and a flexible tactile actuator with a monolithic pyramidal microstructured dielectric elastomer layer.43 However, each of these methods has its own technological challenges for practical usage on touchscreens. For example, friction-based haptic methods like electrovibration and ultrasonic vibration require the movement of a fingertip and cannot provide any haptic feedback on a stationary fingertip. Moreover, the electrovibration method, which is more suitable for large-area displays among

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friction-based methods, suffers a decrease in tactile sensation for human fingertip if the fingertip is not directly grounded through a ground electrode. Moisture, e.g., sweat, can also produce adverse effects like reducing the electric field on the fingertip and the friction force on the display, hence reducing the electrovibration effect. The transparent relaxor ferroelectric polymer film vibrator have been developed to provide the tactile feedback on touchscreen.39 However, in the previous research, the tactile feedback is based on the gross vibration of the film vibrator, which cannot provide localized tactile feedbacks for large-area touchscreen. In this research, a new fretting-vibrotactile display is developed, in which transparent flexible relaxor ferroelectric polymer (RFP) film actuators provide localized multiple vibrotactile sensations via the fretting phenomenon, resolving almost all the challenging aspects mentioned above. The RFP films are attractive due to their mechanical compliance and flexible structure, compatibility with soft human skin, and high optical transparency. Furthermore, they are active under an applied electrical voltage and therefore require simple electronic controls. Two well-known RFPs are poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] and poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFECTFE)], in which the third monomer CFE or CTFE introduced in normal ferroelectric crystalline phases acts as a defect so that all trans conformations can be effectively converted into T3GT3G' or TGTG' conformations that cause destabilization of the normal ferroelectric phase and formation of the non-polar phase at room temperature.44-50 Under an external electric field, RFPs produce giant electrostrictive strain by various mechanisms such as the phase transformation of non-polar to polar crystalline structures, reorientation of nano polar regions (all-trans conformations), and Maxwell stress due to electric Coulombic attractions between opposite charges on two electrodes.47 The RFP film can produce about 6.4% maximum strain under an electric field of 150 V/µm at a low operating frequency (about 1 Hz), and this strain may gradually decrease as the operating frequency increases. However, only the electrostrictive strain across the thickness of the RFP film may not be enough for haptic feedback, i.e., it can be lower than the mechanoreceptors’ threshold amplitude. Using a unimorph structure for bending deformation rather than a direct thickness-mode operation can amplify the displacement of the RFP film with a limited driving voltage. The fretting-vibrotactile display is composed of a flexible touch layer with a top electrode and RFP film, and a bottom electrode on a substrate, separated by an air gap and dot spacers as shown in Figure 1a. With the fretting 4 ACS Paragon Plus Environment

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vibration mechanism, the normal pressure from a human fingertip mechanically activates a locally concentrated electric field under the contact area between the RFP film and bottom electrode. The synergetic interplay among the localized electric field, electrostrictive deformation of the RFP film, and contact area dramatically amplifies the acoustic vibration for localized haptic feedback with no time delay. The fretting vibrotactile display has numerous advantages. First, it provides localized haptic feedback near the contact area of a fingertip, and thus, multiple haptic feedback onto multiple fingertips are possible. Therefore, when combined with an interactive display and a touch input, it can provide addressable and content-related haptic feedback. The fretting vibration has a simple on-off mechanism, i.e., it is activated when there is contact between the RFP film and the bottom electrode by fingertip pressure only. Since the RFP itself is highly transparent, high optical transparency of the device can be achieved by using transparent materials for the top and bottom electrodes such as indium tin oxide (ITO) silver nanowires (AgNWs) in this study or hybrid graphene-metal nanowires51 for better stretchability, which is an important characteristic of flexible electronics application. The structure and mechanism shown in Figure 1a are simple and easy to integrate with fingerprint,52 capacitive or resistive touch sensors without physical interference. Another important characteristic of the fretting-vibrotactile display is good operation under static touch and pressure, which cannot be realized with a friction-based haptic mechanism (e.g., electrovibration). The pressure-responsive vibration and the flexible touch layer provide additional dimensions of feeling, e.g., the depth of touch and click feeling. The combination of fretting vibration and electrovibration methods is promising for creating a complete haptic feedback for basic fingertip gestures like clicking, pressing, and sliding. Furthermore, since the RFP and electrode materials are processed in solution, it is easy to fabricate a continuous fretting vibrotactile device in a large scale. In other words, the fretting vibrotactile display can be scalable to any size and shape for a wide range of applications. Finally, the electrostrictive strain in RFPs dramatically decreases with increasing temperature and this hinders commercial applications of RFPs for electromechanical devices. This should be considered for the durability of devices. Blending RF P(VDF-TrFE-CFE) or P(VDF-TrFE-CTFE) with a small amount (typically less than 30%) of poly(methyl methacrylate) (PMMA),47 poly(vinylidene fluoride-chlorotrifluoroethylene) [P(VDF-CTFE)],5355

PVDF,56, 57 or P(VDF-TrFE)58, 59 has been reported to enhance some electromechanical properties such as the

polarization response, dielectric constant, elastic modulus (i.e., elastic energy density), and breakdown strength.



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However, not much attention has been given to the strain response of blended RFPs. Furthermore, the strain response of blended RFPs at elevated temperatures have rarely been studied despite being an essential behavior to be explored for commercial applications. In this study, the electrostrictive strain of neat P(VDF-TrFE-CFE) terpolymer and its blends with P(VDF-TrFE) copolymer is measured with unimorph actuators at room temperature and elevated temperatures to determine the best combination of various RFP blends. In summary, RF P(VDF-TrFE-CFE) is blended with a small amount of P(VDF-TrFE) (55:45) copolymer for enhanced actuation performance at room temperature as well as elevated temperatures, and the optimized RFP blend is utilized for vibrotactile actuators. A fretting-vibrotactile touchscreen is also demonstrated in this study, in which transparent flexible RFP film actuators provide pressure-responsive localized multiple vibrotactile sensations via the fretting phenomenon as shown in Figure 1a. With the fretting vibration mechanism, the normal pressure from a human fingertip mechanically activates a locally concentrated electric field under the contact area between the RFP film and bottom electrode. The synergetic interplay among the localized electric field, electrostrictive deformation of the RFP film, and contact area dramatically amplifies the acoustic vibration. This new mechanism is characterized thoroughly by finite element analysis (FEA) and studying optical microscope images of a working device. The fretting vibrotactile touchscreen is composed of all highly transparent components and has a high overall transparency of 83 %. When combined with an interactive display, it can provide addressable content-related multiple tactile feedback touchscreens and displays of any size.

2. Materials and Methods 2.1. Materials Preparation The relaxor ferroelectric terpolymer P(VDF-TrFE-CFE) comprised of VDF, TrFE, and CFE monomers with mole fractions of 59.8, 32.2, and 8 wt%, respectively, and the normal ferroelectric copolymer P(VDF-TrFE) comprised of VDF and TrFE with mole fractions of 55 and 45 wt%, respectively, were synthesized at Piezotech S.A. in France. Ten grams of P(VDF-TrFE-CFE) terpolymer was dissolved in 90 g of methyl isobutyl ketone (MIBK), and 10 g of P(VDF-TrFE) copolymer was dissolved in 90 g of methyl ethyl ketone (MEK). Then, the terpolymer solution and the copolymer solution were blended together.


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2.2. Fabrication of Tactile-Feedback Touchscreen RFP films with a thickness of 5 µm were fabricated with the blended RFP solution using the adhesionmediated film transfer (AMFT) technique.60 AgNWs were used for the upper electrode and fabricated on the capacitive touch sensors formed on the cover PET film (Figure S1). Then, the RFP film was laminated on the AgNWs upper electrode. Meanwhile, an ITO bottom electrode was deposited on a glass substrate. Then, the dotspacer material (DOTITE XB-3146, Fujikura, Japan) was dispensed as a periodic array of dots on the ITO/glass substrate and cured with ultraviolet light. The cover/touch sensors/AgNWs/RFP film and dot spacers/ITO/glass substrate were assembled together to form the tactile feedback touchscreen (Figure S1a). Figure S1b shows the 3D schematics of the 10-in tactile feedback touchscreen assembly. 2.3. Measurement of Fretting Vibration The acoustic vibrations of the circular RFP film vibrator and 10-in tactile feedback touchscreen (Figure 1a) developed in this study were measured with a laser displacement sensor (LDS) (LT-9010M, Keyence) (also refer to Section 3.4). The circular RFP film vibrator or tactile feedback touchscreen was placed on a vibration isolation table, and static pressure was applied on the device by a polydimethylsiloxane (PDMS) bump with a radius of 3.0 mm. A sinusoidal-waveform voltage with frequency of 200 Hz and amplitude of 200 V generated by an arbitrary waveform generator (Agilent, Model: 33210A) and high-voltage amplifier (MATSUSADA, Model: HEOPT-2B20-02) was applied to the fretting vibrotactile touchscreen, and its out-of-plane vibration was measured by the LDS placed on xyz-manual stages. The measured vibration and applied voltage signals were recorded with an oscilloscope. 2.4. Characterization In order to evaluate the electromechanical properties of the RFPs, the unimorph actuator in Figure S2 was fabricated as follows. The solutions of the various blended RFPs are prepared, and their films with a thickness of 3.6 µm were fabricated according to the AMFT technique.60 The RFP film was laminated on an AgNWs (100 nm) bottom electrode on a PET film (130 µm). A gold electrode with a thickness of 15 nm was then sputtered on top of the RFP film. One end of the unimorph actuator was clamped on a fixed support; the width and length of the unimorph actuator are 5 and 45 mm, respectively. A sinusoidal voltage with an amplitude of 400 V and



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frequency of 0.1 Hz was applied between the two electrodes. Figure S2 describes the experimental setup. The actuator deflection was measured at a distance L of 30 mm from the support by LDS (LT-9010, Keyence). The unimorph actuator was placed in a thermal chamber, and its temperature was precisely controlled to measure the strain response at an elevated temperature. 2.5. Dynamic Finite Element Analysis In the fretting vibrotactile touchscreen, the normal pressure by a human fingertip mechanically activates an electric field in the RFP film locally concentrated under the contact area. Thus, the RFP film under the concentrated electric field produces a large electrostrictive strain. However, multiphysics finite element analysis (FEA) on the electrostrictive (or piezoelectric) deformation with a variable contact area cannot be easily conducted with commercial FEA software. Therefore, the thermomechanical–electromechanical analogy was utilized in the finite element (FE) simulation, where the anisotropic thermal expansion under a temperature change mimics the linearized electrostrictive deformation under an applied electric field, because thermal expansion can be easily incorporated into a contact problem with a variable contact area. Therefore, FEA was carried out on the dynamic thermomechanical contact with the commercial software ABAQUS.61 An axisymmetric FE model of the circular RFP film vibrator composed of a PET cover, RFP film, glass substrate, and fingertip (including bone and tissue parts) was constructed to evaluate the vibration amplitude and profile, as shown in Figure S3.

3. Results and Discussion 3.1. Fretting Phenomenon of a Relaxor Ferroelectric Polymer Film Vibrotactile sensations can be transmitted to multiple fingertips with a pressure-responsive fretting vibrotactile touchscreen (Figure 1a) consisting of three parts: a flexible touch layer, dot spacers, and a substrate. The flexible touch layer comprises a scratch-resistant polyethylene terephthalate (PET) cover, an array of capacitive touch sensors, a transparent top electrode made of silver nanowires (AgNWs), and an RFP film. A transparent bottom indium tin oxide (ITO) electrode is placed on a substrate made of either glass or hard plastic


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for the display panel. A periodic array of dot spacers with a height of 93 µm is employed to maintain a uniform gap between the RFP film and bottom electrode. The principle of vibrotactile operation is schematically illustrated in Figure 1a. An alternating electric potential is applied between the top and bottom electrodes. When a fingertip applies enough pressure to bend the flexible touch layer and cause the RFP film to touch the bottom electrode, a localized high electric field is generated inside the RFP film immediately above the contact area. This localized electric field generates an electrostrictive strain (negative strain) across the thickness of the RFP film and a positive in-plane strain due to Poisson’s effect (Figure 1b). For example, with an applied voltage maximum of 200 V and an RFP film thickness of 5 µm, the local electric field around the contact area will be about 40 V/µm, which can induce about 2% electrostrictive thickness strain and about 1% in-plane strain.47 Figure 1c and Movie S1 show the deformation profile of the flexible touch layer and fingertip via the fretting vibration phenomenon. Because the localized electric field rises during the ramp-up period of the applied voltage, the RFP film in the contact region contracts in the thickness direction and expands in the in-plane direction. This deformation together with the applied force by a fingertip may induce a local fish-bellied bending of the flexible touch layer and lift-up of the peripheral area of the contact region, resulting in the reduction of the contact area and a large upward fluctuation of the flexible touch layer. On the other hand, during the ramp-down period of the applied voltage, the RFP film expands in thickness and contracts in the in-plane direction. This will release the local bending of the flexible touch layer and lift-up of the peripheral area of the contact region, making the flexible touch layer return to its original shape and thus increasing the contact area. Although the decrease in the thickness of the RFP film is in order of 100 nm, it can cause a dramatic change in the contact radius (up to about 700 µm) since the gap between the RFP film and bottom electrode is very much narrow compared to the in-plane dimension. Therefore, under an alternating electric potential, the synergetic interplay among the localized electric field, electrostrictive strain of the RFP film, and contact area dramatically amplify the deflection amplitude of the flexible touch layer, especially near the contact edge of the fingertip, as shown in Figure 1c. Figure 1d shows the change in contact area between the bottom electrode and RFP film underneath a fingertip measured by a high-speed camera (also refer to Figure S4 and Movie S2). This is known as the fretting vibration phenomenon.62 Thus, a human’s fingertips can easily perceive the haptic sensations from the vibrating flexible touch layer.



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3.2. Dynamic Finite Element Analysis and Measurement of Fretting Vibration The vibration amplification by the fretting phenomenon in the fretting vibrotactile touchscreen was examined through axisymmetric dynamic finite element analysis (FEA). Figure 2a shows the vibration profile and amplitude of the flexible cover layer and the changes in the contact radii ( rC1 : between the bottom electrode and RFP film, and rC 2 : between the flexible cover layer and fingertip), when the applied force is increased linearly from 0 mN to 100 mN in 0.5 s (starting from the time 0.1 s), and the sinusoidal voltage

V (t )  V0 (1  sin 2t ) 2 with V0 = 200 V and  = 200 Hz is applied through the top and bottom electrodes. When the applied force is smaller than 10 mN, there is no contact yet between the bottom electrode and RFP film ( rC 2  0 ). Then there is a small transitional region around 10-14 mN of the applied force, in which the RFP film comes into contact with the bottom electrode and retreats completely, alternating in one cycle of applied voltage. After this, the results show an enhanced fretting vibration with increasing applied force, i.e., both the contact radius and vibration amplitude increase. This dependence of the fretting vibration on the applied force is a beneficial effect for haptic feedback, since the sensation on the user’s fingertip is enhanced with firm touch. The contour plots of the typical vertical displacement during one cycle of applied voltage (Figure 2b) clearly show how the fingertip vibrates together with the flexible touch layer. To examine the vibration profile and vibration amplitude along the radial distance of the flexible cover in more detail, a static force of F = 31.8 mN was applied by a human’s fingertip so that the initial static contact radii was 1.95 mm between the fingertip and cover and 0.66 mm between the RFP film and bottom electrode. Figure 2c plots the vertical displacement profile and vibration amplitude as a function of the radial distance from the contact center. From FEA, the vibration amplitude at r (radial distance) ≤ 1 mm was about 0.1 µm representing an electrostrictive strain of 2% in the RFP of thickness 5 µm. As the radial distance increases the vibration amplitude resembles an inverse parabolic curve with a maximum of 2.41 µm at r = 4.2 mm before tapering down. This vibration amplitude is sufficiently large to be perceived by a human’s fingertip. The region with a high vibration amplitude is located near the contact edge of the fingertip, therefore the fretting vibration can effectively stimulate a fingertip with localized vibration. 10 ACS Paragon Plus Environment

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The vibration amplitude was also measured with a laser displacement sensor (LDS) (refer to the Experimental Section) in which a polydimethylsiloxane (PDMS) bump with a radius of 3.0 mm was used to apply static pressure on the flexible touch layer. PDMS was used because it has mechanical properties analogous to the human fingertip. Figure 2c plots the measured vibration amplitude of the flexible cover layer, which also shows a maximum near the contact area and decreases from 1.6 µm to 0 µm when the radial distance increases from 3 mm to 15 mm. The experimental results for the vibration amplitude slightly differ from the FEA results. This may be mainly due to the difference between the PDMS bump used in the experiment and fingertip modeled in the FEA as well as the idealized assumptions used for the FEA (refer to the Experimental Section for complete details). Note that slow and fine tremors can naturally occur with an ordinary person which can change the applied force and thus cause amplitude changes in the fretting vibration. Figure 2d demonstrates the effect of a hand tremor in FEA, which is represented by a sinusoidal force of F  63.6{1  sin[4 (t  0.1)]} mN from t = 0.1 s. The contact radii ( rC1 and rC 2 ) change as a function of time, as does the vibration profile at

r = 4.6 mm.

Generally, a higher pressure produces a larger vibration amplitude, contact area, and radial position of the maximum vibration amplitude. When the applied force is less than about 14 mN (0.17 s < t < 0.28 s), the fretting vibration completely disappears. When the applied force reaches its maximum (F = 127.2 mN) at t = 0.475 s, the vibration amplitude also reaches its maximum. Therefore, a slow and fine hand tremor can cause different tactile sensations by modulating the vibration amplitude.

3.3. Enhanced Actuation with Blended Relaxor Ferroelectric Polymers For the RFP for the fretting vibrotactile touchscreen, the normal ferroelectric P(VDF-TrFE) copolymer was blended into the RF P(VDF-TrFE-CFE) terpolymer to provide enhanced actuation performance at room temperature and elevated temperature. With the P(VDF-TrFE-CFE)/P(VDF-TrFE) blends with a low copolymer content (less than 20%), both components form separate crystalline phases but the normal ferroelectric copolymer is converted into a relaxor due to interfacial coupling, therefore the blends entirely exhibit relaxor behavior.59 Note that the Curie temperature TC of the P(VDF-TrFE) copolymer is about 65 C at a composition



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of VDF 50% and gradually increases with the VDF composition to a maximum of above 130 C at a composition of VDF 80%.63 Therefore, 10% or 20% P(VDF-TrFE) copolymer with two compositions (VDF:TrFE = 75:25 and 55:45) designated as Co75 and Co55, respectively, were blended with the P(VDF-TrFE-CFE) terpolymer (VDF:TrFE:CFE = 71.2:25:3.8). Unimorph cantilever actuators were fabricated with the blended RFPs, of which the displacement hysteresis was measured with the LDS as shown in Figure 3a, b (also refer to Figure S2). Interestingly, compared to the neat terpolymer, 10% and 20% blends with Co55 increased the displacement response by 5.9% and 16.5%, respectively, while 10% and 20% blends with Co75 decreased the displacement response by 2.5% and 17.2%, respectively. At an elevated temperature of 60 C, the displacement of the neat terpolymer was 25% less than that at 25 C, and the hysteresis loop was wider at 60 C than at 25 C, presumably due to high dielectric and mechanical losses at high temperature. The 20% Co55 blend also shows a maximum displacement at 60 C comparable to that of the neat terpolymer at 25 C. The conflicting performance of Co55 and Co75 blends can be explained based on the strain mechanisms of the RFP. Note that the high electrostrictive strain in RFPs may originate from various mechanisms such as the phase transformation from non-polar to polar, reorientation of nano-polar regions (all-trans conformations), and Maxwell stress due to electric Coulombic attractions between opposite charges on two electrodes.47 Co75 has a higher crystallinity, a larger amount of  phases, and stronger ferroelectric interactions than Co55 (refer to Figure S5), and the Curie temperatures of Co75 and Co55 are about 120 C and 75 C, respectively.64-66 Therefore, Co75 is more resistant to conversion into a relaxor than Co55,59,

67

and Co55 may contain more paraelectric (non-polar) phases than Co75 at room

temperature and at elevated temperature (60 C), which would be beneficial to RF behavior. 2 The electrostrictive strain ( 11 ) (refer to Equation S1), strain energy density ( U e  0.5Y311 ), and film stress

(  f  Y311 ) in the RFP film can be obtained from the displacements given in Figure 3b, of which the maximum values are given as box plots in Figure 3c. Here, Y3 represents the Young’s modulus of the RFP film. Unlike the actuator displacement, the electrostrictive strain in the neat terpolymer increases from 2.2% to 3.2% as temperature increases from 25 C to 60 C. However, all of the blended RFPs show reduced strains, and the strain reduction is more severe in the Co75 blends than in the Co55 blends. The film stress (  f  Y311 ), plotted in Figure 3c, may be more suitable than the electrostrictive strain for explaining the trend of the actuator 12 ACS Paragon Plus Environment

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displacement given in Figure 3b, because the bending moment and thus displacement in a unimorph actuator are proportional to the film stress, for which Young’s modulus and electrostrictive strain both play an important role. Even though the strain increases at elevated temperatures, the film stress at 60 C decreases to almost half of that at 25 C. Therefore, the decrease of the actuation performance (displacement) at the elevated temperature is mainly caused by the decrease in the Young’s modulus of the neat P(VDF-TrFE-CFE) film.56 The addition of Co75 decreases the film stress, while the addition of Co55 tends to increase the film stress. Therefore, blending the P(VDF-TrFE) (55:45) copolymer into the neat P(VDF-TrFE-CFE) terpolymer is beneficial for preventing the decrease in the Young’s modulus at an elevated temperature with an acceptable tradeoff for the electrostrictive strain.

3.4. Fretting-Vibrotactile Sensation A 10-in fretting vibrotactile touchscreen (Figure 1a) was fabricated according to the processes illustrated in Figure S1, in which the 20% Co55 blend was used in the RFP film. A PDMS bump was used to apply a static force on the 10-in fretting vibrotactile touchscreen as shown in Figure 4a (also refer to Movies S3 and S4). A sinusoidal voltage with a maximum of 200 V was applied and the vibration amplitude was measured using LDS as a function of frequency, as shown in Figure 4b. The vibration amplitude reaches almost 3.5 µm at 1 Hz, and as the operating frequency increases it gradually decreases to about 1 µm at 500 Hz. The vibration amplitude is higher than the threshold amplitude of the Pacinian channels in a fingertip for the frequency range of 60-500 Hz, for which the fretting vibrotactile touchscreen can be operated to provide tactile sensation. Note that a thicker flexible touch layer can enhance the reliability of the device but will reduce the vibration amplitude. The vibration measurements show that increasing its thickness from 230 µm to 600 µm decreases the vibration amplitude from 1.7 µm to 1.1 µm at the applied voltage of 200 V and 200 Hz, which is still acceptable for haptic perception. Figure 4c shows various vibration profiles of the fretting vibrotactile touchscreen corresponding to various forms of applied voltage (e.g., sine, ramp, and square-burst signals). As can be seen from Figure 4c, the vibration profiles are highly dependent on the waveform of the applied voltage. This opens the possibility of introducing discriminable tactile sensations to the user’s fingertips and hence rendering different virtual surface textures by controlling the waveform, amplitude, and frequency of the operating voltage.



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3.5. Multiple Addressable Tactile-Feedback Display Figure 5a and Movie S5 demonstrate the pressure-responsive fretting vibrotactile touchscreen with capacitive touch sensors developed in this study. It is worth noting that instead of capacitive touch sensors, resistive touch sensors with almost the same structure and working principle as the fretting RFP film actuators can also be combined in our device. Based on the fretting vibrotactile touchscreen, Figure 5b shows a proposed schematic diagram for a multiple addressable and content-related tactile feedback touchscreen for large-area displays. The tactile RFP film actuator can be split into many small areas with patterned electrodes. Multi-touch gesture information via the touch sensor array and tactile information of objects in the display panel are delivered to the actuator controller. Therefore, the actuator controller can generate object-dependent tactile signals on specific RFP film actuators where multiple touches take place. If vibration profiles are linked to surface textures, lateral force profiles to 3D tactile features, and then operating voltages to vibration profiles and lateral forces, an algorithm may be developed to simulate the appropriate object shape, texture discrimination, and object manipulation. The pressure-responsive tactile feedback touchscreen presented in this paper works well under static pressure but is not very suitable for sliding motion. The opposite situation occurs for friction-based haptic displays via electrovibration or ultrasonic vibration because they require the movement of the fingertip to provide a tactile sensation. Therefore, if the fretting RFP film actuator is integrated with other friction-based haptic mechanisms such as electrovibration or ultrasonic vibration, it can provide ample tactile stimulation to the user’s fingertips through various interactions (e.g., touch, slide, pinch, and zoom) and render the user’s tactile perception. More specifically, vibration under the static pressure of fingertip will help create the sensation of discriminable textures, and lateral forces under a sliding fingertip can define predominantly 3D tactile features on the surface. The major technical challenges that need to be addressed involve the air gap and spacer dots between the RFP film and bottom electrode. The quality of the displayed image may be reduced by the partial reflection of visible light from the free surfaces of the RFP film and bottom electrode. By following ASTM Standard D1003, total luminous transmittance and haze of the pressure-responsive fretting vibrotactile touchscreen are measured to be 83.5% and 3.51%, respectively. To increase the optical efficiency, several types of antireflective film can be 14 ACS Paragon Plus Environment

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coated on the bottom electrode and RFP film, such as multilayered optical films and inhomogeneous films with a gradual change in the refractive index. Another promising approach for antireflective screen coatings is the moth-eye nanostructure, an excellent antireflective structure found in nature composed of numerous ommatidia lenses.

4. Conclusion A pressure-responsive vibrotactile sensation mechanism based on the fretting phenomenon of a RFP film was first demonstrated to efficiently generate an amplified acoustic vibration near the contact edge of a human fingertip on a large-area touchscreen. The normal pressure by a human fingertip mechanically activates a locally concentrated electric field under the contact area, which produces a large electrostrictive strain in the RFP film. The synergetic interplay among the localized electric field, electrostrictive strain of the RFP film, and contact area dramatically amplifies the acoustic vibration. Then, a blend of 80% RF P(VDF-TrFE-CFE) (VDF:TrFE:CFE = 71.2:25:3.8) terpolymer and 20% normal ferroelectric P(VDF-TrFE) (VDF:TrFE = 55:45) copolymer is proposed as the RFP material for the actuator to provide an enhanced actuation performance even at elevated temperatures. A high overall transparency of 83 % for the fretting vibrotactile display is achieved by using highly transparent components. The fretting vibrotactile display is scalable to any size and shape and can provide localized pressure-responsive haptic feedback on a stationary fingertip. If the fretting RFP film actuator is integrated with other friction-based haptic mechanisms (e.g., electrovibration or ultrasonic vibration), ample tactile stimulation can be provided to the user’s fingertips through various interactions (e.g., touch, slide, pinch, and zoom). An array of fretting RFP film actuators can provide multiple addressable content-related tactile feedback on large-area displays by modulating the frequency, amplitude, and profile of the driving voltage signals.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:…



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Observation of Fretting Vibration Phenomenon; Finite Element Analysis of the Fretting Vibration Phenomenon; Strain Measurement of the Blended RFPs; Characterization (XRD and FTIR) of Blended RFPs; Fabrication of the Fretting Vibrotactile Touchscreen (PDF) Movie S1 - Fretting vibrotactile sensation principle (GIF) Movie S2 - Observation of fretting mechanism with fingertip (MP4) Movie S3 - Side-view of fretting mechanism with PDMS bump (MP4) Movie S4 - Measurement of vibration profile (MP4) Movie S5 - Demonstration of touchscreen (MP4)

Acknowledgement Q. V. Duong and V. P. Nguyen contributed equally to this work. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017R1A2B4012081) and by the Nano·Material Technology Development Program through the NRF funded by the Ministry of Science, ICT, and Future Planning (NRF-2016M3A7B4910531).

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Figure Captions

Figure 1. a. 10-inch localized fretting-vibrotactile touchscreen and its vibration amplification principle via the fretting phenomenon of the relaxor ferroelectric polymer (RFP) film vibrator. b. Electrostrictive strain mechanisms in P(VDF-TrFE-CFE) RFP blended with normal ferroelectric P(VDF-TrFE) copolymer. c. Deformation profiles of the flexible touch layer obtained from dynamic finite element analysis during oscillation of applied voltage, which show large fluctuation of the contact area and flexible touch layer. d. Optical microscope images of the contact area between the bottom electrode and RFP film captured with a high-speed camera at different voltage levels. Figure 2. a. FEA results on vibration profile at r (radial distance) = 4.6 mm and contact radii between the RFP film and bottom electrode (contact radius 1, rC1) and between the human fingertip and cover film (contact radius 2, rC2) as a function of time, when a force increases from 0 (t = 0.1 s) to 100 mN (t = 0.6 s) through a human fingertip. b. FEA snapshots for deformed shape and displacement field of the RFP film actuator at various values of applied voltage. c. Displacement profiles (FEA result) and vibration amplitudes (experimental and FEA results) of the flexible touch layer along the radial distance of the circular RFP film actuator. d. FEA results on vibration profiles at r (radial distance) = 4.6 mm and contact radii (contact radius 1 and contact radius 2) as a function of time, when a hand tremor with debonding is considered, i.e., a sinusoidal force of F  63.6 1  sin  4 (t  0.1)  mN is applied from t = 0.1 s. Figure 3. a. Experimental set-up of the displacement measurement of the bimorph RFP actuator at elevated temperature. b. Displacement hysteresis loops at room temperature (25 ℃) and elevated temperature (60 ℃) of unimorph actuators made of relaxor ferroelectric P(VDF-TrFE-CFE) polymer blended with normal ferroelectric P(VDF-TrFE) polymer. c. Strain, elastic strain energy density, and film stress in the blended RFP films. Figure 4. a. Experimental set-up for fretting vibration measurement of a 10-inch fretting vibrotactile touchscreen. b. Experimental and FEA results on vibration amplitude as a function of frequency, 22 ACS Paragon Plus Environment

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when the sinusoidal voltage of maximum 200 V is applied, and threshold vibration amplitudes of mechanoreceptors (Pacinian, NP I and NP III) in human’s fingertips.68-70 c. Experimental results on the fretting vibration profiles for applied voltages of sinusoidal, ramp, and square-burst types. Figure 5. a. Demonstration of the 10-inch pressure-responsive fretting vibrotactile touchscreen. b. Schematic diagram of control algorithm of the array of tactile-feedback RFP film actuators for large-area display.



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