Electro-mechanical contact interactions between human finger and touchscreen under electroadhesion

Electroadhesion is a promising technology with potential applications in robotics, automation, space missions, textiles, tactile displays, and some other fields where efficient and versatile adhesion is required. However, a comprehensive understanding of the physics behind it is lacking due to the limited development of theoretical models and insufficient experimental data to validate them. In this thesis, we have developed an electro-mechanical model to estimate the magnitude of electrostatic forces between human finger and touchscreen under electroadhesion. We also measured the friction forces between the finger and touchscreen to infer the magnitude of electrostatic forces experimentally. The model is in good agreement with the experimental data and showed that the change in magnitude of the electrostatic force is mainly due to the leakage of charge from the Stratum Corneum layer of the skin to the touchscreen at frequencies lower than 250 Hz and electrical properties of the Stratum Corneum at frequencies higher than 250 Hz. In addition, we proposed a new and systematic approach based on electrical impedance measurements, where skin and touchscreen impedances are measured and subtracted from the total impedance to obtain the remaining impedance in order to estimate the electrostatic forces between the finger and the touchscreen. This approach also marks the first instance of experimental estimation of the average air gap thickness between human finger and voltage-induced capacitive touchscreen. Moreover, the effect of electrode polarization impedance on electroadhesion was investigated. Precise measurements of electrical impedances confirmed that electrode polarization impedance exists in parallel with the impedance of the air gap, particularly at low frequencies, giving rise to the commonly observed charge leakage phenomenon in electroadhesion. We also investigated tactile perception by electroadhesion for DC and AC voltage signals applied to the touchscreen using ten participants with varying finger moisture levels. Our study showed that the voltage detection threshold for an AC signal was significantly lower than that of the corresponding DC signal and we explained this discrepancy by charge leakage at lower frequencies again. We have also observed that the participants with a moist finger had significantly higher threshold levels than the rest of the participants, which is supported by our electrical impedance measurements. Finally, we aimed to investigate the effect of touchscreen's top coating layer on our tactile sensing with and without electroadhesion, and within the time frame of this thesis, we have focused on the latter only. Hence, we first performed psychophysical experiments to quantify human tactile discrimination ability of touchscreen surfaces coated with different materials, followed by multiple physical measurements. The results showed that coating material has a strong influence on our tactile perception and human finger is capable of detecting differences in surface chemistry due to, possibly, molecular interactions. In conclusion, the findings of this thesis provide new insights into the physics of finger-touchscreen interactions under electroadhesion and have implications for the design of robotic systems and haptic interfaces utilizing this technology.