Electroadhesion between a flat touchscreen and the human finger with randomly self-affine fractal surface
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Electroadhesion between a flat touchscreen and the human finger with randomly self-affine fractal surface M. FESHANJERDI1,2,* 1
Department of Physics, University of Tehran, Tehran 14395-547, Iran
2
Department of Physics, Alzahra University, Tehran 1993891167, Iran
Received: 22 April 2019 / Revised: 09 August 2019 / Accepted: 12 December 2019
© The author(s) 2019 Abstract: In this study, the effects of finger roughness on the electrostatic potential, electrostatic field, and average effective squeezing pressure between a human finger and a touchscreen are calculated by the perturbation method. This theory is an extension of an earlier work by Persson. It is found that an additional potential ( 2) will appear between the solids when the roughness effect is considered in calculating the perturbation potential. This additional potential is still proportional to the distance u from the bottom surface. Therefore, the effect of the roughness increases the effective potential between the two solids. As a result, the average electrostatic field and average effective squeezing pressure increase. Using the increased effective squeezing pressure, we obtain the contact area, average surface separation, and friction between a human finger and the surface of a touchscreen. The effect of the roughness of the finger skin on the increased average effective squeezing pressure (electroadhesion) increases the contact area and reduces the average surface separation between the finger skin and touchscreen. Therefore, the finger-touchscreen friction increases. The surface topography for the forefinger skin is also measured by atomic force microscopy to obtain more realistic results. The auto spectral density function for the forefinger skin surface is calculated as well. Keywords: friction; touchscreen; perturbation method
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Introduction
The first touchscreen was designed in 1960, and touchscreens are now ubiquitous in daily life. The use of touchscreens is not limited to smartphones and tablets; they are used in many portable computers, smart watches, game consoles, and ATMs, among others. Capacitive touchscreens are currently the most popular type of touchscreens due to their multi-touch technology. A capacitive touchscreen panel consists of a glass substrate coated with a conductive layer such as indium tin oxide (ITO), and an insulating layer such as silicon dioxide (SiO2). When an alternating voltage is applied to the conductive layer of a capacitive touchscreen, touching its surface generates an attractive
force between the finger (as the human body is an electrical conductor) and the touchscreen surface. This force is caused by the induction of charges with opposite signs on the insulating layer of the glass substrate and the finger. This electrostatic attraction force is called “electroadhesion” and increases the fingertouchscreen contact area and the sliding friction force [1–4]. Jonhnsen and Rahbek observed the attraction force between human skin and a charged
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