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

Abstract

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.

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

“…Figure 3 shows the measured adhesion force as a function of both the temperature and RH. As was shown in previous work, the adhesion force increases by roughly 50% when increasing the relative RH from 0.05 to 80% at 25 °C and 0 V. 19 However, the adhesion force slightly decreases by roughly 3% when increasing the temperature from 25 to 40 °C (see Supporting Information 1, Figures S1−8, and S1−9). The temperature-dependent surface tension of water is well-known and can be linearly approximated as γ = − 0.1589T[°C] + 75.95 mN/m, where the relative decrease between 25 and 40 °C is ∼3%.…”

“…Therefore, to estimate W 132 , we first measured the adhesion force between the AFM tip and the corneocyte by engaging the AFM tip on the corneocyte surface with 5 nN tip force at 0.05%RH for each temperature level. 19 Then, the adhesion force due to contact between two surfaces (when the water screening layer at the interface between the AFM tip and the corneocyte at 0.05%RH is assumed minimal) is substituted into eq 2, getting the average W 132 values as follows: 62.54, 59.40, 58.72, and 56.41 mN/m for 25, 30, 35, and 40 °C respectively, which is low compared to the previous study result. 32 Although the decreasing work of adhesion cannot explain the observed increase in energy dissipation, a dramatic decrease in the elastic modulus of the skin cell could lead to a significant increase in the contact area between the corneocyte and the tip.…”

“…Previous measurements of an AFM tip and a single human corneocyte under varied humidity and applied electric field showed that the presence of the water meniscus provided minimal increases to sliding friction but had a significant effect on the adhesion to the surface, implying that liquid bridges at the human finger contact would provide a larger effect for microscopically rough surfaces where menisci can form and break during sliding. 19 Yet, the effects of temperature changes on the mechanical energy dissipation due to adhesion, sliding, and separation are still unclear.…”

Replaceable Dielectric Film for Low-Voltage and High-Performance Electrowetting-Based Digital Microfluidics

“…Among them, digital microfluidics (DMF) based on electrowetting-on-dielectric (EWOD-DMF) is an important technology that uses programmable electrode arrays to accurately control droplet generation, transportation, merging, and splitting. The EWOD-DMF technology has been widely applied in chemical sensing, drug delivery, liquid robotics, − and biomedicine fields. , To achieve reliable EWOD-DMF, several major factors should be considered, including actuation voltage, breakdown voltage, contact angle change, contact angle saturation, and contact angle hysteresis.…”

“…The EWOD-DMF technology has been widely applied in chemical sensing, 8 drug delivery, liquid robotics, 9−11 and biomedicine fields. 12,13 To achieve reliable EWOD-DMF, several major factors should be considered, including actuation voltage, breakdown voltage, contact angle change, contact angle saturation, and contact angle hysteresis.…”