Session: 12-06-02: Condensation and Phase Change Materials

Paper Number: 170648

Enhanced Dropwise Condensation From Atmospheric Water Vapor on a Slippery Superhydrophobic Surface With Wall-Enclosed Micropillars and Multiscale Roughness 

Space cooling using conventional HVAC systems in buildings located in hot and humid regions consumes a significant amount of primary energy. As an alternative, radiant cooling systems installed on walls, ceilings, or floors can offer an energy-efficient solution. However, the cooling capacity of radiant panels is typically limited by condensation from moist air. Incorporating superhydrophobic layers into radiant ceiling panels can enable latent heat transfer below the dew point, thereby significantly enhancing cooling capacity. Similarly, in indirect evaporative coolers coated with superhydrophobic layers, energy recovery can be improved by 10–15%. The key to enhancing the performance of HVAC equipment using superhydrophobic surfaces lies in ensuring that condensed water droplets are easily removed and do not form continuous water films. On a superhydrophobic surface (SHS), water droplets retain a nearly spherical shape with a high apparent contact angle (Cassie-Baxter state), which prevents the transition from dropwise to filmwise condensation by facilitating timely removal of the droplets. However, on microstructured SHSs, droplets may enter a partially or fully wetted state (known as the Wenzel state) as they grow by merging with neighboring droplets, which leads to pinning and drastically reduces the efficiency of the surface.

To mitigate this, numerous studies have explored mechanisms to remove micron-sized droplets via out-of-plane jumping. Various innovative surface geometries have been investigated, including micro/nano hierarchical roughness, scalable nanostructured SHSs, hierarchical nanostructured faceted microcones and microhills, nature-inspired and beetle-mimetic surfaces, and biphilic surfaces. Nevertheless, droplet removal via jumping is inherently inefficient due to the small size of the droplets. In contrast, droplet shedding can be significantly more effective, as it allows for the removal of larger volumes of water. However, gravitational shedding typically occurs only when droplets reach a critical diameter—usually in the millimeter range—depending on surface inclination and intrinsic advancing/receding contact angles. This necessitates prolonged waiting periods, reducing overall efficiency. To address this limitation, the current study introduces a novel SHS featuring wall-enclosed micropillars and multiscale roughness, which facilitates coalescence-induced shedding of microdroplets before they reach the critical diameter required for gravitational shedding.

Five samples with varying pillar depths and wall sizes were fabricated, and condensation experiments were carried out under high humidity conditions (85% RH) with a dew point of 27 °C. The sample surfaces were maintained at 11 °C to achieve a high level of supersaturation. Two distinct condensate removal modes were observed: coalescence-induced jumping on samples with deep micropillars, and coalescence-induced shedding on samples with shallow micropillars. Specifically, samples NW-J, W200-J, and W400-J (with pillar depths ~6 µm) exhibited jumping removal, while samples NW-S and W200-S (with ~1 µm pillar depth) exhibited shedding. Results demonstrated that all samples effectively prevented wetting transitions. Notably, on the W200-S sample, the smallest droplet diameter observed during shedding was approximately 107 µm. The enhanced performance of the three-tier nanotexture on the W200-S sample was found to suppress local pinning of the three-phase contact line and prevent Wenzel neck formation during droplet growth. Consequently, during multi-droplet coalescence, the released surface energy overcame solid-liquid adhesion, resulting in the spontaneous shedding of the merged droplets.

Presenting Author: Jubair Shamim Oak Ridge National Laboratory

Presenting Author Biography: Jubair has been actively conducting research in the field of thermos-fluid engineering since 2013. His research expertise/ interests include but are not limited to transport in porous media, functional materials for energy harvesting, thermal management of HVAC components and surfaces, and interfaces for enhanced dropwise condensation and anti-icing. His previous key research accomplishments are (i) the design and testing of a hybrid compression-adsorption heat pump system, (ii) the design of a multilayer fixed-bed binder-free desiccant dehumidifier using a distinct mesoporous silica gel, and (iii) the design of slippery superhydrophobic surfaces with low depinning force to suppress wetting transition and enhance condensate removal.

Jubair is an active member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Japan Society of Refrigeration and Air-conditioning Engineers (JSRAE). He also serves as the guest editor, review editor, and reviewer for several academic journals. Jubair received the JSRAE academic award in 2023 for his contribution to refrigeration and air-conditioning technology development. Outside of academic pursuits, he loves outdoor and sports activities, including cycling, swimming, tennis, and traveling.

Authors:

Jubair Shamim Oak Ridge National Laboratory
Yukinari Takahashi University of Tokyo
Minhyeok Lee University of Tokyo
Junho Choi Tokyo City University
Wei-Lun Hsu University of Tokyo
Kashif Nawaz Oak Ridge National Laboratory
Nenad Miljkovic University of Illinois at Urbana – Champaign
Hirofumi Daiguji University of Tokyo