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(Color online) Schematic of the planar jumping-drop thermal diode (not to scale). (a) Forward mode with self-propelled jumping drops returning the working fluid from the superhydrophobic condenser to the superhydrophilic evaporator for continuous phase-change heat transfer and (b) reverse mode with liquid trapped by the colder superhydrophilic surface.
(Color online) Scanning electron images of (a) the superhydrophobic surface with galvanically deposited micro/nano structures and (b) the superhydrophilic surface with sintered copper wick. Insets: the corresponding copper plates.
(Color online) Orientation-independence of the diode performance. (a) All three orientations were tested on the same chamber with the heat sink held at 25 °C. (b) and (c) Infrared imaging of the diode at steady-state in the reverse and forward modes, respectively. The diode (outlined by a dotted box) had an overall dimension of 76 × 76 × 18 mm3. For both images, the cold side was attached to a heat sink at T 0 = 25 °C, while the other side was heated by a resistive film with Q = 15 W. Note that the insulating spacer slightly protruded from the sides. The injection and vacuum ports were connected to the superhydrophobic plate, located at the bottom in both cases.
(Color online) Semi-log plot of the forward thermal conductivity (kf ) versus the average vapor temperature of the thermal diode (Tv ). For the corresponding conductivity ratio (kf /kr ), the reverse conductivity is assumed constant at kr = 0.29 W/m·K. Solid symbols correspond to measurements obtained during a week-long continuous run while open symbols represent additional tests. When Tv < 50 °C, the forward conductivity was approximately independent of orientation for heater powers (Q) ranging from 30 W to 90 W. When Tv > 50 °C, however, the forward conductivity had a strong orientation dependence, where the “with gravity” orientation yielded much higher values than the other two for a low power of 30 W chosen to delay dryout. The orientation dependence beyond a vapor temperature of 50 °C indicated the onset of boiling, where the removal of vapor bubbles was strongly dependent on the gravitational orientation. The dashed line corresponds to the maximum forward conductivity predicted by Eq. (2), assuming no enhancement by boiling.
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