Contact mechanics between the human finger and a touchscreen under electroadhesion

Significance The technology for generating tactile feedback on a touchscreen via electroadhesion is already available—and straightforward to implement—but the knowledge on human skin contact mechanics is limited. To better understand the contact mechanism between the finger pad and touchscreen under electroadhesion, we investigated the sliding friction as a function of normal force and voltage using (i) a mean field theory based on multiscale contact mechanics, (ii) a full-scale computational contact mechanics study, and (iii) experiments performed on a custom-made tribometer. We show that the real contact area and the electroadhesion force depend strongly on the skin surface roughness and on the nature of the touchscreen coating. Thus, by reducing the effective thickness of the latter, the human tactile sensing can be drastically enhanced. The understanding and control of human skin contact against technological substrates is the key aspect behind the design of several electromechanical devices. Among these, surface haptic displays that modulate the friction between the human finger and touch surface are emerging as user interfaces. One such modulation can be achieved by applying an alternating voltage to the conducting layer of a capacitive touchscreen to control electroadhesion between its surface and the finger pad. However, the nature of the contact interactions between the fingertip and the touchscreen under electroadhesion and the effects of confined material properties, such as layering and inelastic deformation of the stratum corneum, on the friction force are not completely understood yet. Here, we use a mean field theory based on multiscale contact mechanics to investigate the effect of electroadhesion on sliding friction and the dependency of the finger–touchscreen interaction on the applied voltage and other physical parameters. We present experimental results on how the friction between a finger and a touchscreen depends on the electrostatic attraction between them. The proposed model is successfully validated against full-scale (but computationally demanding) contact mechanics simulations and the experimental data. Our study shows that electroadhesion causes an increase in the real contact area at the microscopic level, leading to an increase in the electrovibrating tangential frictional force. We find that it should be possible to further augment the friction force, and thus the human tactile sensing, by using a thinner insulating film on the touchscreen than used in current devices.