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.

Keywords: electroadhesion; haptics; multiscale contact mechanics; skin friction; touchscreens.

Fig. 1.

(A) Physical processes and related length scales leading to tactile sensing during contact between finger and touchscreen under electroadhesion. (B) Schematic of the layered structure of the generic human skin with indication of the relevant biological clues and nerve receptors. (C) The surface roughness PSD as a function of the wavenumber (log–log scale) calculated from the skin surface topography reported in refs. and . The PSD has the rms roughness amplitude $22\hspace{0.17em}\mathrm{\mu }\mathrm{m}$ and the rms slope 0.91. The linear region for $q>4\times 10^5\hspace{0.17em}{\mathrm{m}}^{-1}$ corresponds to a Hurst exponent $H=0.86$. The 3D surface roughness corresponds to a realization of the PSD. (D, Left) Cross-section image of a capacitive touchscreen (model SCT-3250, 3M) obtained by field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus). (Right) The same image is reported with an improved contrast to highlight the different layers of the cross-section: an electric insulator layer (${\mathrm{S}\mathrm{i}\mathrm{O}}_{\mathrm{2}}$) with a thickness of $\approx 1\hspace{0.17em}\mathrm{\mu }\mathrm{m}$, an electric conducting layer ($\mathrm{I}\mathrm{T}\mathrm{O}$) with a thickness of $\approx 250\hspace{0.17em}\mathrm{n}\mathrm{m}$, and glass substrate.

Fig. 2.

Skin–touchscreen mean field model properties and validation against the predictions of BEM simulations. (A) Adopted microcontact model: an elastic solid with surface roughness above a rigid solid with a flat surface (1) and the PSD used for the comparison of the models (2). An electric voltage difference $V$ occurs between the two conducting solids. (B) BEM-predicted roughness upon contact, with a magnified view of the surface and representation of the contact domain. The rough contact is simulated with 16 divisions at the small roughness wavelength. (C) BEM-predicted skin microcontact map, with magnified view of the map and indication of the contact domain. The black and red contour lines show the electroadhesive iso-stress curves around the true contact areas ($h_{rms}$ is the rms surface roughness). (D) Comparison between the normalized contact area as a function of the contact pressure, at different values of applied voltage difference across the interface.

Fig. 3.

Experimental and theoretical results. (A) Schematic of the experimental setup for measuring the force responses of the index finger in the normal and tangential directions, when subjected to a relative sliding motion with respect to the touchscreen. (B) Measured friction force $F_{\mathrm{f}}$ as a function of time. The green colored response was obtained by applying an oscillating electric potential to the touchscreen $\varphi =V_0\cos \left({\omega }_{0}t\right)$ [where $V_0=200\hspace{0.17em}\mathrm{V}$ and $f_0={\omega }_0/\left(2\pi \right)=125\hspace{0.17em}\mathrm{H}\mathrm{z}$]. (C) The friction coefficient $\mu$ obtained from the theory (solid lines) and the experiments (markers) as a function of the applied normal force $F_{\mathrm{N}}$. The amplitude of the voltage applied to the touchscreen is varied between $0\hspace{0.17em}\mathrm{V}$ (red) and $200\hspace{0.17em}\mathrm{V}$ (black), with an increment of $50\hspace{0.17em}\mathrm{V}$, at $125\hspace{0.17em}\mathrm{H}\mathrm{z}$. The relative sliding speed in the experiments was $50\hspace{0.17em}\mathrm{m}\mathrm{m}/\mathrm{s}$. In the calculations, we used $E_{\mathrm{S}\mathrm{C}}=40\hspace{0.17em}\mathrm{M}\mathrm{P}\mathrm{a}$, $d=200\hspace{0.17em}\mathrm{\mu }\mathrm{m}$, $E_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}=10\hspace{0.17em}\mathrm{k}\mathrm{P}\mathrm{a}$, and $h_0=0.2\hspace{0.17em}\mathrm{\mu }\mathrm{m}$. (D) The normalized contact area $A/A_0$ as a function of the applied voltage $V$ and the thickness of the effective insulating layer $h_0=d_1/{ϵ}_1+d_2/{ϵ}_2$. The applied pressure is $p_0=10\hspace{0.17em}\mathrm{k}\mathrm{P}\mathrm{a}$.

Fig. 4.

(Solid lines) The calculated dependency of the contact area on the frequency $f=\omega /\left(2\pi \right)$ of the oscillating electric potential $\varphi =V_0\cos \left(\omega t\right)$. The $A_{\mathrm{o}\mathrm{n}}/A_{\mathrm{o}\mathrm{ff}}$ is the ratio of the real contact area with electroadhesion to that without electroadhesion. The diamond symbols are the measured data for the ratio of friction coefficients with and without electroadhesion, ${\mu }_{\mathrm{o}\mathrm{n}}/{\mu }_{\mathrm{o}\mathrm{ff}}$. See the first section in Discussion for details.