| Abstract: | A large subset of haptic surfaces employs electro-adhesion to modulate both adhesion and friction at a sliding finger interface. The current theory of electro-adhesion assumes that the applied electric field pulls the skin into stronger contact, increasing friction by enlarging the real contact area. Yet, the high electric field in the interface area damages a dielectric layer of human finger skin and it is unknown how the environmental humidity and temperature at the contact affect interfacial mechanics, which is one of interaction factors for human haptics. This dissertation presents study results of the effects of humidity, electrical voltage, and temperature on the adhesion and the friction between the single nanoscale asperity and the human finger corneocyte using atomic force microscopy (AFM). Measurements of adhesion and friction forces as functions of humidity, electrical voltage, and temperature using the AFM tip applying the critical normal load of 5 nN were performed. The analytical models for the relation of humidity, voltage, temperature, and interface mechanism variables are developed and used to analyze the experimental data to explain those external impacts to nanoscale adhesion, nanoscale friction, and dissipation energy at the interface. The results show that the adhesive force due to the electrowetting effect at high humidity accounts for 35% of the total adhesion, but it is less than 8% contribution to the total friction, implying that the electrofriction can be enhanced by optimizing surface topography to promote the formation and rupture of liquid meniscus bridge between the corneocyte and the AFM tip. For the temperature effect, the energy dissipation from the interface due to high temperature causes decreases in adhesion and friction. These reduced forces are the result of the reduction of capillary force energy of the water meniscus bridge even though surface energy is improved by a larger contact area due to the reduced young modulus of the corneocyte. This research result suggests that design of haptic devices for enhancing the interaction between the user⁰́₉s skin and the touchscreen should consider maintaining the water or liquid bridge formation in the skin-surface interface, and minimizing the energy dissipation when the contact temperature is higher. Another study of sliding velocity dependence of the friction of the chemical surface using lateral force microscopy (LFM) is also reported in this dissertation. It is found that the chemical reaction of the Azide group on a PFPA-graphene substrate to the oxygen occurs when the AFM tip sliding with various velocities cuts through atomic bonds producing low friction force. This finding indicates that any graphene-based surface of the haptics can experience friction reduction when it is contacted with sliding probes. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/197426 |
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