Immersion condensation on oil-infused heterogeneous surfaces for enhanced heat transfer

Abstract

Enhancing condensation heat transfer is important for broad applications from power generation to water harvesting systems. Significant efforts have focused on easy removal of the condensate, yet the other desired properties of low contact angles and high nucleation densities for high heat transfer performance have been typically neglected. In this work, we demonstrate immersion condensation on oil-infused micro and nanostructured surfaces with heterogeneous coatings, where water droplets nucleate immersed within the oil. The combination of surface energy heterogeneity, reduced oil-water interfacial energy, and surface structuring enabled drastically increased nucleation densities while maintaining easy condensate removal and low contact angles. Accordingly, on oil-infused heterogeneous nanostructured copper oxide surfaces, we demonstrated approximately 100% increase in heat transfer coefficient compared to state-of-the-art dropwise condensation surfaces in the presence of non-condensable gases. This work offers a distinct approach utilizing surface chemistry and structuring together with liquid-infusion for enhanced condensation heat transfer.

Figure 1. Parameters affecting condensation heat transfer coefficient.

Model results showing influence of: (a) departure radius with advancing contact angle θ_a = 110° and nucleation density N = 10¹⁰ m⁻², (b) advancing contact angle with nucleation density N = 10¹⁰ m⁻² and departure radius R_max = 800 μm, and (c) nucleation density with θ_a = 110° and R_max = 800 μm. The results assume a vapor pressure of 2700 Pa and surface temperature of 20°C.

Figure 2. Mechanism of immersion condensation.

(a) Schematic showing water vapor diffusing through the thin oil film and forming immersed droplets on the tips of micropillars. (b) Magnified schematic showing the nuclei formation on high-surface-energy sites on micropillar tips in the oil. (c) and (d) Height and phase images of atomic force microscope (AFM) images of TFTS ((Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane) coating. The higher phase angle at the nanoagglomerates indicates local higher surface energy. (e) and (f) Environmental scanning electron microscope (ESEM) images of TFTS-coated micropillar arrays before and after the oil-infusion. (g) and (h) Contact angle hysteresis on a superhydrophobic surface without and with oil-infusion. The hysteresis is ≈3° on the oil-infused surface with a contact angle ≈110°. The microstructure geometries were the same on both surfaces, with diameter of 5 μm, height of 20 μm, and period of 15 μm. (i) and (j) White-light optical microscope images of condensation on micropillar arrays before and after oil-infusion. The micropillar geometries were the same as (g) and (h). The supersaturation in the experiments was S = 1.6. (k) Nucleation rates predicted as a function of contact angle and interfacial energy.

Figure 3. Scalable copper oxide (CuO) surfaces for immersion condensation.

(a) Field emission SEM (FESEM) image of CuO nanostructures. (b) Environmental SEM (ESEM) image of CuO nanostructures infused with Krytox oil. (c) ESEM image of nucleation on TFTS-coated CuO surface. (d) ESEM image of oil-infused TFTS-coated CuO surface. An order of magnitude higher nucleation density was observed compared to (c). (e) Image of dropwise condensation on a hydrophobic copper tube surface. (f) Image of condensation on an oil-infused TFTS-coated CuO surface. Significantly higher droplet density was observed on the oil-infused surface while a low departure radius of 0.98 ± 0.13 mm was maintained.

Figure 4. Experimental immersion condensation heat transfer measurement.

Comparison of overall heat transfer coefficient during condensation on the hydrophobic surface, TFTS-coated superhydrophobic surface, and oil-infused composite surface with an initial chamber pressure of 30 Pa (primarily non-condensable gases). The supersaturation was varied in the range 1 < S < 1.6. The heat transfer coefficient on the oil-infused surface increased by approximately 100% compared to the dropwise and superhydrophobic surfaces.