Biphilic Surfaces with Optimum Hydrophobic Islands on a Superhydrophobic Background for Dropwise Flow Condensation

Drainage during Condensation on Microgrooved Biphilic Surfaces

In this work, we investigate and compare the condensation behavior of hydrophilic, hydrophobic, and biphilic microgrooved silicon samples etched by reactive ion etching. The microgrooves were 25 mm long and 17-19 μm deep with different topologies depending on the etching process. Anisotropically etched samples had 30 μm wide rectangular microgrooves and silicon ridges between them. They were either left hydrophilic or covered with a hydrophobic fluorocarbon or photoresist layer. Anisotropically etched samples consisted of 48 μm wide semicircular shaped microgrooves, 12 μm wide silicon ridges between them, and a 30 μm wide photoresist stripe centered on the ridges. The lateral dimensions were chosen to be much smaller than the capillary length of water to support drainage of droplets by coalescence rather than droplet sliding. Furthermore, to achieve a low thermal resistance of the periodic surface structure consisting of water-filled grooves and silicon ridges, the trench depth was also kept small. The dripped-off total amount of condensate (AoC) was measured for each sample for 12 h under the same boundary conditions (chamber temperature 30 °C, cooling temperature 6 °C, and relative humidity 60%). The maximum increase in AoC of 15.9% (9.6%) against the hydrophilic (hydrophobic) reference sample was obtained for the biphilic samples. In order to elucidate their unique condensation behavior, in situ optical imaging was performed at normal incidence. It shows that the drainage of droplets from the stripe's surface into the microgrooves as well as occasional droplet sliding events are the dominant processes to clear the surface. To rationalize this behavior, the Hough Circle Transform algorithm was implemented for image processing to receive additional information about the transient droplet size and number distribution. Postprocessing of these data allows calculation of the transient water load on the stripe's surface, which shows an oscillatory behavior not previously reported in the literature.

Hierarchically Branched Siloxane Brushes for Efficient Harvesting of Atmospheric Water

Atmospheric water harvesting is considered a viable source of freshwater to alleviate water scarcity in an arid climate. Water condensation tends to be more efficient on superhydrophobic surfaces as the spontaneous coalescence-induced droplet jumping on superhydrophobic surfaces enables faster condensate removal. However, poor water nucleation on these surfaces leads to meager water harvest. A conventional approach to the problem is to fabricate micro- and nanoscale biphilic structures. Nonetheless, the process is complex, expensive, and difficult to scale. Here, the authors present an inexpensive and scalable method based on manipulating the water-repellent coatings of superhydrophobic surfaces. Flexible siloxane can facilitate water nucleation, while a branched structure promotes efficient droplet jumping. Moreover, ToF-SIMS analysis indicated that branched siloxane provides a better water-repellent coating coverage than linear siloxane and the siloxanes comprise hydrophilic and hydrophobic molecular segments. Thus, the as-prepared superhydrophobic surface, TiO2 nanorods coated with branched siloxanes harvested eight times more water than a typical fluoroalkylsilane (FAS)-coated surface under a low 30% relative humidity and performed better than most reported biphasic materials.

Enhancement of Dropwise Condensation Heat Transfer through a Sprayable Superhydrophobic Coating

Improving the shedding rate of condensed droplets has many applications in industries and daily problems, including increasing heat transfer and self-cleaning properties. One way to achieve this goal is by enhancement of the wetting properties of surfaces. In this research, the hierarchical superhydrophobic coating over aluminum has been applied using a relatively cost-effective method, spraying, which is also applicable to any metal surface used as a condenser. According to the results obtained from the experimental tests, the fabricated surface is highly superhydrophobic, with a contact angle of 158° and contact angle hysteresis of less than 5°. The results show that the presented surface increases the heat transfer coefficient by 20.6% at the subcooling temperature of 25.5 °C when the surface temperature and relative humidity are 70 °C and 98%, respectively. In addition, this coated surface showed great potential at lower surface temperatures by increasing the water condensation rate as much as 50.5% at the subcooling temperature of 12 °C, when the surface temperature and relative humidity are 11.25 °C and 70%, respectively. Therefore, it is found that for the fabricated superhydrophobic paint in the present study, the effectiveness of the dropwise condensation mode profoundly depends on surface temperatures besides subcooling temperatures. In other words, a surface with lower temperatures shows better performance for the same subcooling temperatures. In addition, various types of durability tests are carried out. The results reveal that this coating has good durability against high surface temperatures, submerged conditions for 30 days, imposing hot steam for 150 h, corrosion, and organic solvents. Hence, it is suitable for industrial applications.

Design of Nanostructured Surfaces for Efficient Condensation by Controlling Condensation Modes

To meet the different needs of various industrial fields, it is of great application value to find a feasible method for controlling the condensation mode on the surface. Inspired by biological surfaces, tuning the surface structure and wettability is considered as a potential way to control the surface condensation behavior. Herein, the coupling effect of the geometric parameters and wettability distribution of the surface on the condensation process has been investigated systematically at the nanoscale. The results illustrate that the condensation mode is primarily determined by the nanopillar wettability when the nanopillars are densely distributed, while the substrate wettability dominates the condensation mode when the nanopillars are sparsely distributed. Besides, the effective contact area fraction is proposed, which more accurately reflects the influence of geometric parameters on the condensation rate of the nanopillar surface at the nanoscale. The condensation rate of the nanopillar surface increases with the increase of the effective contact area fraction. Furthermore, three surface design methods are summarized, which can control the condensation mode of water vapor on the surface into the dropwise condensation mode that generates Cassie-Baxter droplets, and this condensation process is very attractive for many practical applications.