24th International Colloquium Tribology - January 2024 217 Active, Real-Time Friction Control with ElectroAdhesion: Application to Soft Contacts for Augmented Tactile Perception Luigi Portaluri 1* , Luciana Algieri 2 , Massimo De Vittorio 1,2 and Michele Scaraggi 1,2,3** 1 Department of Engineering for Innovation, University of Salento, Lecce, IT 2 Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia, Arnesano (LE), IT 3 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK * luigi.portaluri@iit.it ** michele.scaraggi@unisalento.it 1. Introduction Virtual reality technologies make it possible to interact in an environment recreated entirely by a computer. The sense of sight and hearing are reproduced through visors and headsets respectively, which have developed exponentially in recent years, reproducing reality almost faithfully. Haptic technologies, on the other hand, enable/ modulate the sense of touch experienced by humans when touching an object, adapting in real time the mechanical interaction that occurs between the object itself and the skin tissue. The human body recognizes stimuli from the external world through the presence of receptors in the skin. When pressure is applied to the skin, the deformation stimulates these receptors, generating the sense of touch [1]. One way to achieve the sense of touch is to exploit the phenomenon of electro-adhesion (EA), where an electrical potential applied between two nominally flat conductive surfaces in contact, with an interposed dielectric gap, causes charges of opposite sign to accumulate on the contacting surfaces. This generates an electrostatic attraction (known as Maxwell stress) which, when added to the externally applied normal load, increases the actual contact area (maintaining the external normal load) [2,3,4]. This modulation of contact area determines the change in friction and adhesion (and, to name a few, electrical contact resistance, thermal contact resistance, etc.). In order to provide an augmented sense of touch, most haptic feedback studies have focused on recreating a surface through a rising/ lowering pin array. This movement of the pins was enabled by piezoelectric actuators [5], linear electromagnetic actuators [6], shape memory alloy (SMA) actuators [7], electroactive polymers, or EAPs [8]. But these displays can only be improved by making the actuator system smaller, to increase the pin density of the surface. These displays can only be improved by making the actuation system smaller, to increase the pin density of the surface. By exploiting the phenomenon of electro-adhesion, friction modulation can be obtained by simply varying the voltage applied to the display, without using an actuation system. The objective of this work was to set the basis for the control of the sliding friction occurring in the finger-touchscreen contact, with the purpose of developing a tactile feedback (augmented sense of touch) screen prototype. 2. Materials and method The phenomenon of electro-adhesion (EA) for soft contacts was investigated to mimicking the human finger vs touchscreen interaction. A soft rubber spherical sample was fabricated for mimicking the effective elastic properties of the human finger, whereas electrical conductivite is added by coating the prototyped finger with a compliant conductive layer. The prototyped finger is thus put in contact with a home-made EA-active transparent display, and the contacting pair dynamically actuated in pure squeeze motion thanks to a novel home-made thermally-controlled opto-electromechanical tribometer. The latter allows to quantify the extent to which the applied voltage, thus the EA, is effective into increasing, reversibly, the true contact area between the prototyped finger and the EA-display, thus to provide a modulated active interaction and, finally, sense of touch. The prototyped finger (Figure 1.a). was developed using soft Polydimethylsiloxane (PDMS), hemisphere shaped, covered with a rough layer (mimicking the finger small scale roughness) of a stretchable and conductive polymer Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). EA-active screen (Figure 1.b) was built through a microscope slide covered with a transparent and conductive patterned thin layer of Indium Tin Oxide (ITO), on the top of which a few µm Parylene C optically-transparent dielectric layer was deposited. The prototyped finger is attached at a load cell to evaluate the load present in the contact. The Johnson-Kendall-Roberts (JKR) experimental technique was adopted to quantify the EA-tailored adhesion and contact formation in the contact pair described above. In the typical experiment: 1. the PEDOT covered PDMS dome is moved in contact at constant approach speed of 0.05mm/ s until a squeezing load of 0.1 N is reached; 2. the contact is left under constant penetration for 5 s; 3. the dome is moved out of contact at constant speed of 0.05 mm/ s until the original (starting) position is reached. Cycle 1) to 3) is repeated N times at the same voltage, then a new voltage is set and the cycle 1) to 3) repeated again N times until the total number M of voltage is tested. The adopted voltages are 0, 10, 20, 40 and 80 V. Active, Real-Time Friction Control with ElectroAdhesion: Application to Soft Contacts for Augmented Tactile Perception 218 24th International Colloquium Tribology - January 2024 Figure 1. (a) PDMS hemispherical dome coated with a thin layer of PEDOT: PSS to replicate the surface and geometrical finger properties. (b) Microscope slide coated with 35 nm layer of ITO and a thin film of ParyleneC to replicate a generic transparent touchscreen. (c) Schematic of the prototypes contact stack. (d) Contact area at different voltage with 4x magnification. (e) Schematic of the opto-electro-mechanical tribometer adopted in this study. (f) Pull-off force measured at different voltage. 3. Conclusion Figure 1.d shows how the contact area remarkably increases with increasing applied voltage, and this finally qualitatively validate the adoption of EA as a physical approach to augment the interaction between two solids in a controllable way, i.e., it is expected a higher pull-off force and contact area as the applied voltage increases. Also the pull-off (Figure 1.f) was successfully measured to quantify the strength of the EA process through the rough interaction, mimicking the finger vs touchscreen contact. The greatest effect of electroadhesion occurs at 80V. Applying the other voltages, the work remains constant or increases slowly. References [1] Johnson KO, Yoshioka T, Vega-Bermudez F. Tactile functions of mechanoreceptive afferents innervating the hand. J Clin Neurophysiol. 2000 Nov; 17(6): 539- 58. doi: 10.1097/ 00004691-200011000-00002. PMID: 11151974. [2] Ayyildiz M, Scaraggi M, Sirin O, Basdogan C, Persson BNJ. Contact mechanics between the human finger and a touchscreen under electroadhesion. Proc Natl Acad Sci U S A. 2018 Dec 11; 115(50): 12668-12673. doi: 10.1073/ pnas.1811750115. Epub 2018 Nov 27. PMID: 30482858; PMCID: PMC6294909. [3] Omer Sirin. Mehmet Ayyildiz. Bo Persson. Cagatay Basdogan. Electroadhesion with application to touchscreens. Soft Matter. 10.1039/ C8SM02420K. [4] Persson BNJ. 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