Analogous to the electronic diode, a thermal diode is a device which allows heat to flow to a preferential direction. Compared with other thermal diodes, phase-change thermal diodes yield greater thermal rectification performance due to high latent heat. With coalescing-jumping droplets as a result of dropwise condensation and water evaporation through a vapor space in the forward mode and thermal conduction through a high thermal resistance material in the reverse mode, the thermal rectification of this phase-change thermal diode can be higher than 100. However, due to resistance forces, coalescing-jumping droplets are forced to return to the substrate. While some can coalesce with neighboring droplets and jump again, some adhere to the surface and become larger, leading to progressive...[ Read more ]

Analogous to the electronic diode, a thermal diode is a device which allows heat to flow to a preferential direction. Compared with other thermal diodes, phase-change thermal diodes yield greater thermal rectification performance due to high latent heat. With coalescing-jumping droplets as a result of dropwise condensation and water evaporation through a vapor space in the forward mode and thermal conduction through a high thermal resistance material in the reverse mode, the thermal rectification of this phase-change thermal diode can be higher than 100. However, due to resistance forces, coalescing-jumping droplets are forced to return to the substrate. While some can coalesce with neighboring droplets and jump again, some adhere to the surface and become larger, leading to progressive flooding which degrades the heat transfer performance. To address these issues, an electric field is utilized. In this study, the electrical voltage threshold, the electric field threshold and the droplet charge required to remove a macro-sized droplet from a superhydrophobic surface are investigated. The results show that with an increase in gap width, both the electrical voltage threshold and the electric field threshold increase, while the droplet charge decreases. Additionally, the results of this study also reveal a constant electrostatic force acting on droplets in the air and the maximum electrostatic force acting on droplets on the superhydrophobic surface regardless of the gap width and of applied electric field intensity. Apart from macro-sized droplets, the jumping height, the droplet charge, the jumping angle, the gravitational force, the drag force, the inertia force and the electrostatic force of coalescing-jumping droplets in electric fields are also studied through experiments and mathematical models. The results show that an electric field can enhance the jumping height by over three times due to a significant increase in the electrostatic force. Additionally, this study reports the intersection point at the jumping droplet radius of 35 μm, separating jumping droplet motion into two regimes; the drag-force-dominated regime where the small-sized droplets can jump and reach the top plate, and the gravitational-force-dominated regime where the larger droplets can jump, but return to the substrate. The other intersection point is between the gravitational force and the inertia force showing a decrease in the influence of the inertia force with a greater applied electric field. Moreover, it is also found that the average charge of the droplets is relatively constant in all pressure conditions and applied electric fields. Lastly, a phase-change thermal diode using electrostatic-induced coalescing-jumping droplets is designed and developed, and the effects of applied electrical fields on the effective thermal conductivity and thermal rectification of a phase-change coalescing-jumping-droplet thermal diode are investigated. Similar to the jumping height, the findings show that an electric field potentially enhances the effective thermal conductivity and thermal rectification of the phase-change thermal diode. At the applied electrical voltage of 50 V, the maximum average thermal rectification is 325, showing a 90% greater improvement over thermal rectification in the no-electric-field condition and is considered to be one of the highest performances of all experimental thermal diode studies. This study is the first experimental study showing that an electric field potentially enhances thermal rectification of the thermal diode, and its findings not only can offer new insight into phase-change thermal diodes, but also shed new light on integrating an electric field into a thermal diode. Moreover, the findings of the fundamental macro-sized-drop-jumping and coalescing-jumping-droplet studies can further advance knowledge of the enhancement of heat transfer and can be applied to several applications including self-cleaning, smart windows and condensation heat transfer enhancement.