Electrowetting: A Consideration in Electroadhesion

Effects of Temperature and Humidity on Energy Dissipation between Human Corneocytes and Nanoasperity Sliding Contacts

Human haptic perception relies on the ability of sensory receptors underneath the skin corneocyte layer to sense external load, where adhesion and friction play an essential role in nanoscale solid-solid contact. Energy dissipation present at the surface interface due to the change of separation distance during sliding contact was uncovered, but the energy dissipation of human finger skin cell-nanoprobe contact under humidity and temperature conditions has not been investigated yet. In this paper, the energy dissipation of skin corneocyte-nanoprobe interface under variation of both humidity, 0.05-80%RH, and temperature ranging from 25 to 40 °C is directly measured by atomic force microscopy (AFM). Analytical models of dissipation energy for this nanomaterial interface mechanism are developed, and the results are compared to the measured values. AFM measurements of dissipation energy reveal that the amount of dissipated energy caused by water meniscus stretching monotonically increases with humidity and temperature, resulting in adhesion and friction decreases. The purposed analytical model represents that dissipation energy trend.

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate's hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics.

Closed-Loop Control of Electroadhesion Using Current Regulation

Electroadhesion displays provide controllable friction between the fingertip and screen. However, the change of contact condition causes variability in the produced friction. In this paper, we demonstrate a novel method for closed-loop control using current regulation to improve the precision of the electroadhesion force regardless of contact conditions. The current sensor obtains static current (when the finger is stationary) and dynamic current (when the finger is sliding). The static current is used to estimate the apparent contact area. The estimated contact area modulates the driving voltage along with the dynamic current. To verify the proposed method, we measured electroadhesion forces under open-loop control and closed-loop control. The benefit of using this closed-loop control is shown by comparing the relative static error of open-loop control and closed-loop control. The relative error reductions achieved over 34 % (max 112 %) for four changing contact conditions.

Surface haptic rendering of virtual shapes through change in surface temperature

Compared to relatively mature audio and video human-machine interfaces, providing accurate and immersive touch sensation remains a challenge owing to the substantial mechanical and neurophysical complexity of touch. Touch sensations during relative lateral motion between a skin-screen interface are largely dictated by interfacial friction, so controlling interfacial friction has the potential for realistic mimicry of surface texture, shape, and material composition. In this work, we show a large modulation of finger friction by locally changing surface temperature. Experiments showed that finger friction can be increased by ~50% with a surface temperature increase from 23° to 42°C, which was attributed to the temperature dependence of the viscoelasticity and the moisture level of human skin. Rendering virtual features, including zoning and bump(s), without thermal perception was further demonstrated with surface temperature modulation. This method of modulating finger friction has potential applications in gaming, virtual and augmented reality, and touchscreen human-machine interaction.

General theory of electroadhesion

We present a general theory of electroadhesion assuming layered materials with finite electric conductivity and an air gap resulting from interfacial surface roughness. The theory reduces to the results derived in Persson (2018J. Chem. Phys.148144701) in the appropriate limits. We present numerical results to illustrate the theory.

Nanotexture Shape and Surface Energy Impact on Electroadhesive Human-Machine Interface Performance

With the ubiquity of touch screens and the commercialization of electroadhesion-based surface haptic devices, modeling tools that capture the multiphysical phenomena within the finger-device interface and their interaction are critical to design devices that achieve higher performance and reliability at lower cost. While electroadhesion has successfully demonstrated the capability to change tactile perception through friction modulation, the mechanism of electroadhesion in the finger-device interface is still unclear, partly due to the complex interfacial physics including contact deformation, capillary formation, electric field, and their complicated coupling effects that have not been addressed comprehensively. A multiphysics model is presented here to predict the friction force for finger-surface tactile interactions at the nanoscale. The nanoscopic multiphysical phenomena are coupled to study the impacts of nanotexture and surface energy in the touch interface. With macroscopic friction force measurements as verification, the model is further used to propose textures that have maximum electroadhesion effect and minimum sensitivity to relative humidity and user perspiration rate. This model can guide the performance improvement of future electroadhesion-based surface haptic devices and other touch-based human-machine interfaces.

Evaluation of the Electrowetting Effect on the Interfacial Mechanics between Human Corneocytes and Nanoasperities

A large subset of haptic surfaces employs electroadhesion to modulate both adhesion and friction at a sliding finger interface. The current theory of electroadhesion assumes that the applied electric field pulls the skin into stronger contact, increasing friction by increasing the real contact area, yet it is unknown what role environmental moisture plays in the effect. This paper uses atomic force microscopy (AFM)to determine the effect of humidity on the adhesion and friction between the single nanoscale asperity and individual human finger corneocytes. An analytical model of the total effective load of the AFM tip is developed to explain the humidity-voltage dependence of nanoscale adhesion and friction at contacting asperities. The results show that the electrowetting effect at the interface at high humidity accounts for 35% of the adhesive force but less than 8% of the total friction, implying that the electrowetting effect can be enhanced by optimizing surface topography to promote the formation and rupture of liquid menisci.

Finger Pad Topography beyond Fingerprints: Understanding the Heterogeneity Effect of Finger Topography for Human-Machine Interface Modeling

With the rapid development of haptic devices, there is an increasing demand to understand finger pad topography under different conditions, especially for investigation of the human-machine interface in surface haptic devices. An accurate description of finger pad topography across scales is essential for the study of the interfaces and could be used to predict the real area of contact and friction force, both of which correlate closely with human tactile perception. However, there has been limited work reporting the heterogeneous topography of finger pads across scales. In this work, we propose a detailed heterogeneous finger topography model based on the surface roughness power spectrum. The analysis showed a significant difference between the topography on ridges and valleys of the fingerprint and that the real contact area estimation could be different by a factor of 3. In addition, a spatial-spectral analysis method is developed to effectively compare topography response to different condition changes. This paper provides insights into finger topography for advanced human-machine interaction interfaces.