Nanoscale capillarity



As device dimensions shrink into the nanometer range, interfacial forces become increasingly important. At the same time, traditional continuum theories of interfacial forces become inadequate, and fundamentally new phenomena can appear. We seek to determine the limits of traditional theories, identify new interfacial phenomena, and explore processes that may enable new active nanodevices. We study interfacial forces to obtain information about field-induced changes in the charge distribution and forces at solid/liquid and fluid interfaces that are important in electrowetting on dielectric (EWOD). Electrowetting on dielectric (EWOD) is a popular method to move fluid in a microchannel and to vary the focal length of a fluidic lens, and it is being envisioned as a means for oil-water separation. While the phenomenon is relatively well-understood for large (mm) drops, the limits of traditional theories for a fluid drop in a nanoscale channel had not been explored. Moreover, the mechanism at play during electrowetting is not fully understood and the origin of anomalous features, such as contact angle saturation, remains debated. Using the surface force apparatus (SFA) we designed experiments to determine the mechanism driving electrowetting. We have modified the instrument to allow for external potential control of both interacting surfaces and used capillary condensation to generate nanoscale water droplets. Our experiments allowed us to probe contact angle changes within the first tens of nanometers of a drop, and are not limited by possible issues caused by contact angle hysteresis. Using this approach, we unequivocally demonstrated that the real contact angle does not change in electrowetting experiments. Our results show that there is no measurable change in the solid-liquid surface energy in EWOD and that the mechanism at play is electromechanical in nature. One key implication of our work is the demonstration that electrowetting is not a viable mechanism to control surface for micro-/nanoscale devices.



Invariance of the Solid-Liquid Interfacial Energy in Electrowetting Probed via Capillary Condensation (Langmuir)

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