Depletion of Lubricant from Nanostructured Oil-Infused Surfaces by Pendant Condensate Droplets

Abstract

Due to recent advances in nanofabrication, phasechange condensation heat transfer has seen a renaissance. Compared to conventional heat transfer surfaces, nanostructured surfaces impregnated with chemically matched lubrication films (hereinafter referred to as "nanostructured lubricated surfaces") have been demonstrated to improve vapor-side phase-change condensation heat transfer by facilitating droplet nucleation, growth, and departure. While the presence of nanoscale roughness improves performance longevity by stabilizing the lubrication film via capillary forces, such enhancement is shortlived due to the eventual loss of lubrication oil by the departing droplets. The objective of this study is to characterize oil depletion caused by pendant droplets during condensation. For our study, we nanostructured, chemically functionalized, and lubricated horizontal copper tubes that are widely used in shell-and-tube heat exchangers in power plants and process industries. Using high-speed fluorescence imaging and thermogravimetric analysis, we show that shedding droplets exert a shear force on the oil in the wetting ridge at the water−oil interface. The viscous shear draws the lubrication film from the nanostructured surface onto the upper portion of the droplet and forms a ring-like oil skirt. Through detailed theoretical analysis, we show that the thickness of this oil skirt scales with the classical Landau−Levich−Derjaguin (LLD) theory for dipcoating. Our results reveal that droplets falling from horizontal tubes break unequally and leave behind small satellite droplets that retain the bulk of the oil in the wetting ridge. This observation is in stark contrast with the earlier description of droplets shedding from tilted flat plates where the entire oil-filled wetting ridge is demonstrated to leave the surface upon droplet departure. By selecting lubrication oils of varying viscosity and spreading coefficient, we provide evidence that the contribution of the wrapping layer to the rate of oil depletion is insignificant. Furthermore, we show that due to the nanoscale features on the tubes, nearly half of the lubrication film remains on the surface after 10 h of continuous steam condensation at ambient pressure, 23 °C, and 60% relative humidity, a 2−3-fold improvement over previous results.The insights gained from this work will provide guidelines for the rational design of long-lasting nanostructured lubricated surfaces for phase-change condensation.

Lubricant Depletion‐Resistant Slippery Liquid‐Infused Porous Surfaces via Capillary Rise Lubrication of Nanowire Array

“…[ 4a ] Furthermore, the removed lubricant presents a potential source of contamination in water harvesting applications. [ 6 ] It is thus critical to minimize or prevent the formation of wetting ridges, leading to stable SLIPS.…”

Enhanced condensation heat transfer using porous silica inverse opal coatings on copper tubes

“…Compared to other working fluids, water has a significantly higher enthalpy of phase change (2.257 MJ/kg at 373.15 K and 1 atm) 14 , 15 . Due to its impact in power and process engineering 16 – 20 , condensation is still an active area of research in spite of its long history that spans over a century.…”

Enhanced Phase-Change Condensation Using Inverse Opals

“…Compared to the classical DWC, prior studies have shown that SLIPS condensation improves the heat transfer rate by reducing the departure radius by nearly 50%. [42][43][44][45][46][47] During experiments, the copper tube was maintained below saturation temperature of the environmental chamber by circulating chilled water whose inlet and outlet temperatures were measured using calibrated thermocouples (Supporting Material Section S3). For reliable heat transfer measurements, a 0.2-2.5 °C temperature difference was maintained between incoming and outgoing cooling water by adjusting the flow rate using a valve.…”

“…We attribute the improvements in heat transfer to smaller droplet departure radius and higher droplet departure frequency on micro/nanostructured lubricated surfaces. [42][43][44]46,49 The solid-liquid composite material design in SLIPS enhances droplet mobility (~1-2° contact angle hysteresis) by providing an atomically smooth and chemically homogeneous liquid-liquid interface. We validated our heat transfer measurements by comparing the heat transfer coefficient for FWC with the classical Nusselt model (Supporting Material S5) for laminar filmwise condensation (red dashed line, Fig.…”