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Slippery liquid-infused porous surfaces are vulnerable to lubricant depletion caused by three major factors encompassing, the evaporation, cloaking, and shearing. Herein, lubricant-infused iron palmitate surfaces (LI-IPSs) are presented addressing the challenges of (1) evaporation-driven lubricant depletion enabled by the immensely clustered micro-/nano-structures of iron palmitate surface (IPS), (2) completely suppressing the wetting ridge even with miscible water-oil combinations enabled by only the handful uptake of oil by IPS. The apparent contact angle θa and sliding angle α of water on LI-IPSs are 98° ± 2° and 3° ± 1°, respectively. Under evaporation-driven lubricant depletion analysis, the shelf-durability of LI-IPSs has been found to last for 10 days in partially open environment at the ambient temperature (25 °C). The heat fluxes and condensate fluxes have been determined. The highest condensate flux on LI-IPSs (14 kgh−1 m−2) is ascribed to the dropwise condensation mode with efficient droplet dynamics. However, the service durability of LI-IPSs has lasted only for 50 ± 5 min under rigorous condensation, which is further addressed by developing a LI-IPS prototype. Based on the excellent wicking capability of IPS, LI-IPS prototype is capable of maintaining the dropwise condensation unless the oil is available in the oil tub. The LI-IPS prototype has been subjected to open environment at ambient temperature for 168 h, and even after that, the stable dropwise condensation has been achieved with the same range of droplet speed as had been observed in the first hour. Meantime, the high condensation heat fluxes at various subcooling temperatures are obtained, justifying the promising ability of LI-IPS. [ABSTRACT FROM AUTHOR]

This paper describes a numerical investigation of the groove-embedded droplet dewetting process, namely the spontaneous transition from the Wenzel state to the Cassie state, using the multiphase lattice Boltzmann method. Numerical simulations are employed to reproduce the dynamic behaviors of extension, squeezing, rupture, and ejection of condensation droplets in a groove, allowing us to examine how the groove geometry and wettability affect the dewetting process. Our results identify three dewetting regimes, namely retention, partial dewetting, and complete dewetting. As the groove aspect ratio and hydrophilicity decrease, the dewetting regime changes from retention to partial dewetting, and then to complete dewetting. The partial dewetting and complete dewetting are two effective ways for droplet removing. In particular, a groove sidewall with enhanced hydrophobicity is desirable to stimulate the dewetting process. [ABSTRACT FROM AUTHOR]

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition. [ABSTRACT FROM AUTHOR]

In this paper, a free energy lattice Boltzmann model of vapor condensation on a surface with a wettability gradient is developed and numerically analyzed to understand the microscopic behaviors of self-propelled droplet condensation. The effect of wettability gradient on droplet self-motion and coalescence as well as vapor condensation is examined and investigated. The condensation rate is presented during the whole droplet condensation process to analyze the role of wettability gradient on droplet condensation. The results indicate that the surface with wettability gradient is preferred for vapor condensation owing to the appearance of self-propelled droplet condensation. Condensate film is initially spread on the high surface energy region, and liquid nucleation sites form, grow and, subsequently, coalesce with other droplets on low surface energy regions. The condensation rate is higher on a surface with a larger wettability gradient due to the more effective removal of condensate. In addition, the condensation rate fluctuates with time at the quasi-steady-state stage. During the condensation process, the droplet coalescence triggers a sudden peak of condensation rate, and the generation of new nucleation results in a rapid increase in the condensation rate. [ABSTRACT FROM AUTHOR]

A model of vapor condensation on a solid surface is developed and numerically analyzed using the free-energy lattice Boltzmann method. Based on the model, the condensation phase change on hydrophobic, hydrophilic and gradient surfaces are simulated with a particular focus on the condensation on a gradient surface. The droplet nucleation, growth, deformation, coalescence and motion during the condensation on a gradient surface are investigated. The present simulation reproduces the self-propelled dropwise condensation on a gradient surface, the film condensation on a hydrophilic surface and the conventional dropwise condensation on a hydrophobic surface. The results indicate that the condensed droplets on a gradient surface can be swept in time to provide a favorable condition for the subsequent condensation. On a smooth gradient surface, owing to the unbalanced wetting force, the vapor condenses into a thin film firstly and then fractures into droplet nucleation as the condensation process goes on. The larger wettability gradient results in a larger amplitude oscillation of condensation rate and a slighter variation of surface coverage. [ABSTRACT FROM AUTHOR]

The condensation flow patterns of ethanol–water mixture in a wide range of ethanol mass concentration inside a hydrophobic microchannel is experimentally studied via a high speed imaging system. The effects of channel surface wettability and ethanol concentration on flow condensation are compared and discussed. The experimental results indicate that the surface hydrophobic modification and ethanol concentration play a significant role in the flow condensation of mixture in a microchannel. The droplet condensation appears almost the whole two-phase flow region when the water steam is the main component, in which the droplet flow, droplet-streak flow, droplet-annular flow, droplet-injection flow and droplet-slug/bubble flow occur sequentially in a hydrophobic microchannel. With increasing ethanol concentration, the droplet-slug/bubble flow after droplet-injection flow disappears and is replaced by the pure slug/bubble flow. When the ethanol vapor is the main component, the droplet condensation almost disappears, and the annular-streak flow, annular flow, injection flow and slug/bubble flow appear sequentially along the flow direction. Both an increase in vapor Reynolds number and a decrease in ethanol concentration cause the injection location move toward the channel outlet. In addition, the surface hydrophobic modification introduces the droplet condensation, which is beneficial for the flow condensation heat transfer. [ABSTRACT FROM AUTHOR]

• A functional surface with periodical layout of multi-layer wettability gradient is developed to improve the condensation heat transfer. • The presence of a larger wettability gradient for the periodic gradient surface induces the efficient droplet removal and the preferable inter-layer and intra-layer actions. • The heat flux on the periodic gradient surface would increase about 20% to the hydrophobic surface when the surface subcooling ∆ T ≤ 12 °C. The functional surface is a crucial approach to enhance the condensation heat transfer. This paper designs and fabricates a functional surface with wettability gradient by the use of etching and chemical vapor deposition methods. The dynamic phase change behaviors and heat transfer characteristics of humid air condensation on vertical surfaces with wettability gradient have been experimentally investigated and compared with the corresponding hydrophilic and hydrophobic surfaces by a visualization system. The results indicate that the heat transfer performance of humid air condensation on traditional gradient surface is superior to hydrophilic case but inferior to hydrophobic case in large-scale application due to small wettability gradient. Considering this, the periodic gradient surface with periodical layout of multi-layer wettability gradient is introduced to improve the condensation heat transfer. The presence of a larger wettability gradient for the periodic gradient surface induces the efficient droplet removal and the preferable inter-layer and intra-layer actions, leading to an improvement of condensation heat transfer. [ABSTRACT FROM AUTHOR]