Nanostructured jumping-droplet thermal rectifier

Analogous to an electrical rectifier, a thermal rectifier (TR) can ensure that heat flows in a preferential direction. In this paper, thermal transport nonlinearity is achieved through the development of a phase-change based TR comprising an enclosed vapor chamber having separated nanostructured copper oxide superhydrophobic and superhydrophilic functional surfaces. In the forward direction, heat transfer is facilitated through evaporation on the superhydrophilic surface and self-propelled jumping-droplet condensation on the superhydrophobic surface. In the reverse direction, heat transfer is minimized due to condensate film formation within the superhydrophilic condenser and inability to return the condensed liquid to the superhydrophobic evaporator. We examine the coupled effects of gap size, coolant mass, heat transfer rate, and applied electric field on the thermal performance of the TR. A maximum thermal diodicity, defined as the ratio of forward to reverse heat transfer, of 39 is achieved.

Figure 1

(a) Schematic of the experimental setup with (b) thermal-node network demonstrating all thermal resistances, and (c) isometric photographic view of the experiment. Schematic in (a) is not to scale. The thermal interface material [TIM, gray color in (a)] was applied to all interfaces between components.

Figure 2

Photographic view of the vapor chamber (a) prior to assembly showing the spacer (left) and top Cu surface (right), and (b) after assembly and fastening.

Figure 3

Top-view SEM images of the adopted nanostructured surfaces. (a) Top-view SEM image of the CuO superhydrophilic surface and contact angle results; (b) dotted orange region in (a) and regions with multiple magnifications; (c) top-view SEM image of the CuO-based superhydrophobic surface with a focused ion-beam milling and contact angle results; (d) dotted blue region in (c).

Figure 4

Pressure leak test showing the transient pressure for the assembled vapor chamber.

Figure 5

Thermal performance of the two chambers having either smooth Cu surfaces (labeled as Cu vapor chamber) or superhydrophilic-superhydrophobic surfaces (labeled as CuO vapor chamber) with a gap size of 1.27 cm and a coolant mass of 8 g for each chamber. (a) Heat flux as a function of bottom and top temperature. (b) Chamber pressure as a function of average chamber temperature as calculated by Eq. (2) [Fig. 1]. (c) Thermal resistance and effective thermal conductivity as a function of chamber average temperature. (d) Specific effective thermal conductivity as a function of average chamber temperature.

Figure 6

Thermal performance of the CuO nanostructured vapor chamber for various coolant masses at a gap spacing of $H=1.27\mathrm{cm}$ (inset schematic). Measured (a) heat flux, (b) thermal resistance, and (c) effective thermal conductivity as a function of average chamber temperature. Inset in (a): Schematic representation of the chamber cross section with a superhydrophobic top plate, and superhydrophilic bottom plate. Gravity points downward in the schematic. Schematic not to scale.

Figure 7

Thermal performance of vapor chamber prototypes having (a)–(c) $H=1.27$ and 2.54 cm, with 2 g of coolant (DI water), and (d)–(f) $H=1.27$, 1.90, and 2.54 cm with 8 g of coolant. The experimental measurements report (a), (d) heat flux, (b), (e) thermal resistance, and (c), (f) effective thermal conductivity, as a function of average chamber temperature.

Figure 8

Transient temperature response with and without electric field for a chamber prototype having $H=1.27\mathrm{cm}$ with 4 g coolant, at (a) heat flux of $4\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$, and (b) heat flux of $8.8\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$.

Figure 9

Thermal-conductivity based TR characterization as a function of temperature difference for chambers having (a) coolant mass of 2 g and gap size of $H=1.27\mathrm{cm}$, (b) coolant mass of 2 g and $H=2.54\mathrm{cm}$, (c) coolant mass of 4 g and $H=1.27\mathrm{cm}$, (d) coolant mass of 4 g and $H=2.54\mathrm{cm}$, (e) coolant mass of 8 g and $H=1.27\mathrm{cm}$, and (f) coolant mass of 8 g and $H=2.54\mathrm{cm}$. Red circles and black squares represent forward and reverse mode, respectively.

Figure 10

Thermal-resistance based TR characterization and a function of temperature difference for chambers having (a) coolant mass of 2 g and gap size of $H=1.27\mathrm{cm}$, (b) coolant mass of 2 g and $H=2.54\mathrm{cm}$, (c) coolant mass of 4 g and $H=1.27\mathrm{cm}$, (d) coolant mass of 4 g and $H=2.54\mathrm{cm}$, (e) coolant mass of 8 g and $H=1.27\mathrm{cm}$, and (f) coolant mass of 8 g and $H=2.54\mathrm{cm}$. Red circles and black squares represent forward and reverse mode, respectively.