Contact mechanics between the human finger and a touchscreen under electroadhesion

Abstract

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.

Electroadhesion Technologies for Robotics: A Comprehensive Review

“…Two methods of exploiting EA in haptic feedback for touchscreen applications have been developed: electrovibration [111], [112], which uses ac voltages with varying amplitudes, frequencies, and waveforms; and electroadhesion [113], [114], which uses dc voltages. Tactile sensations are produced by modulating the friction between users and touchscreens in both methods.…”

Electroadhesion for soft adhesive pads and robotics: theory and numerical results

“…where p 0 is the (external) applied pressure. Intuitively, one expect this approach to be accurate when the interaction force between the surfaces is long-range, and a similar approach has been used for the attraction resulting from capillary bridges [23,24] and also in an earlier study of electroadhesion [13][14][15]. Using (16) and (17) we have:…”

A Macro Model for Electroadhesive Contact of a Soft Finger With a Touchscreen

“…(6) and (15), it follows that u z (a) = −σ 0 /k. The substitution of (12) and (14) into Eq. (9) yields…”

“…Electroadhesion between two nominally flat surfaces having different electric potentials has been modeled in a number of publications [10,13]. The effect of surface roughness was recently investigated in [14], where, in particular, it is assumed that the effective loading pressure is represented as the sum p+p a , where p a is the electric attraction stress and p = F N /A is the applied pressure. However, the latter assumption (that is p = const) is expected to be accurate at the micro scale, whereas at the macro scale the applied pressure is supposed to vary across the apparent contact area.…”