Evaporative transport in thin liquid films and electrically actuated droplets

The basis of operation for two-phase devices such as heat pipes and vapor chambers is phase change heat transfer from a meniscus in a wetted wick structure. The heat transport capability of the device largely depends on the characteristics of the meniscus very near the contact line, termed the thin-film region. Only recently have measurement techniques matured to the point where the hydrodynamic and thermal characteristics of thin liquid films can be probed. The present work reports on the use of high-resolution temporal, spatial, and thermographic measurements to describe transport phenomena of liquid films and droplets in fundamentally critical configurations. Non-contact microscale infrared thermography is employed to directly measure the temperature of a V-groove wetted by an evaporating meniscus. A temperature suppression in the vicinity of the contact line is clear evidence of the intensive evaporation occurring in the thin-film region. A 50 μm long meniscus sub-region adjacent to the contact line is estimated from the experiments to account for ∼ 45% of the total meniscus heat transfer. Similarly, an evaporating meniscus wetting a bed of spheres in a carefully instrumented, saturated-ambient chamber is examined. A kinetic-theory based numerical algorithm reveals that up to ∼ 55% of the total meniscus mass transfer is attributable to a meniscus sub-region of \u3c10 μm thickness (sub-region length typically ∼ 30 μm). In both cases, microregions with length scales on the order of tens of μm account for \u3c 5% of the total meniscus surface area, but contribute roughly half of the total heat and mass transport, demonstrating that high-performance thermal systems should be designed to exploit the superior heat transfer rates supported by thin-film evaporation. Droplet-based thermal management techniques offer precise spatial and temporal control, making them ideal for cooling hot spots or other high heat density systems. The manipulation of liquid droplets by electrowetting on superhydrophobic surfaces is a well-studied technique that has recently been applied to thermal management systems. The present work describes a fundamental investigation of the dissipative effects in the electrowetting-induced Cassie-Wenzel transition of droplets on hydrophobic rough surfaces. The droplet shape evolution during the transition is recorded by high-speed imaging (at 10,000 frames per second). A quantitative surface energy-based analysis suggests that contact line friction—the dominant dissipative mechanism for droplets spreading on smooth surfaces—is overshadowed by other viscous and/or hysteresis effects on hydrophobic rough surfaces. Additionally, the influence of surface patterning and electrowetting-induced spreading on the heat transfer and contact line dynamics of narrow water ribbons on a heated surface is reported. Chemical and structural patterning of the surface constrains water to a ribbon shape in the absence of electrical actuation. Liquid ribbons spread under electrical actuation are able to sustain higher evaporation rates and provide a more pronounced cooling effect than ribbons without actuation. The spatial selectivity offered by the concept may prove useful for future on-chip thermal management systems