Bouncing–pinning criterion for a drop impacting on a superhydrophobic surface

Steam condensation with non-condensable gas exists widely in petro-chemical, power plant and seawater desalination processes. Super-hydrophobic surfaces with high contact angle could be an promising condensing surface to promote DWC and enhance heat transfer. As pure steam condensing droplet completely occupies micro/nano-structures of super-hydrophobic surfaces, super-hydrophobic function becomes “invalid”. For stream-air mixture vapor, the non- condensable gas would occupy the micro/nano-structures instead of water. Droplets form Cassie or transition wetting mode, holding or partly holding super-hydrophobic feature. Inclination angle of the tube can influence the shape, size and dynamic characteristics of the condensing droplets, thus changing the heat transfer performance. Therefore, for getting deeply insights of the effect of the non-condensable gas on condensation heat transfer, it is necessary to study the condensing drops characteristics on super-hydrophobic tube for different inclined angles. According to dropwise condensation model, increasing contact angle hysteresis, departure droplet size, sweeping period would reduce the condensation heat transfer efficiency. In this paper, dropwise condensation is performed on super-hydrophobic copper tube, departure droplet size, sweeping period and the phenomenon of contact angle hysteresis are thoroughly studied and are compared with the case of hydrophobic surface or pure steam to explore the influence of inclined angle and surface wettability on droplet dynamics to achieve condensation heat transfer enhancement. The experimental result shows: (1) On the top of copper tube, for the condensation of steam with 10% non-condensable gas, the contact angle hysteresis on super-hydrophobic surface is smaller than that on hydrophobic surface. Compared with pure steam condensation on super-hydrophobic surface, the contact angle hysteresis of mixed steam is smaller because the non- condensable gas fills the micro/nano-structures. However, on the middle and bottom parts of the tube, the contact angle hysteresis on super-hydrophobic surface is greater than that on hydrophobic surface for the condensation of both pure steam and mixed steam. This phenomenon indicates that the contact angle hysteresis is determined by surface wettability instead of inclination angle. (2) The departure droplet size on super-hydrophobic surface increases with the inclination angle and the opposite trend is found on hydrophobic surface. With the increase of inclination angle, the support force provided by the tube would increase and change direction. So the wettability of the surface and departure droplet size would be changed. (3) The sweeping period of droplet on super-hydrophobic surface is longer because the micro/nano-structures increase the adhesion between the surface and the droplets. With the increase of the inclination angle, the sweeping period decreased slightly, shorter than the sweeping time when the tube is vertical. The sweeping period of vapor with non-condensable gas condensation is greater than that of pure steam. The presence of non-condensable gas increases thermal resistance, thus slowing down the drop growth rate and increasing the sweeping period. To sum up, on the top of copper tube, the contact angle hysteresis on super-hydrophobic surface is smaller than that on the hydrophobic surface. However, the totally opposite trend is found on the middle and bottom parts of the tube. The departure droplet size on super-hydrophobic surface increases with the inclination angle. The sweeping period of droplet on super-hydrophobic surface is longer. This work offers a new avenue to further enhance heat transfer of condensation with non-condensable gas.

A comparative study of mechanical and chemical durability of non-wetting superhydrophobic and lubricant-infused surfaces

Dynamic behavior of water droplets impacting on the superhydrophobic surface: Both experimental study and molecular dynamics simulation study

Controlling the impact of water droplets on nonwetting surfaces is difficult due to their superior liquid repellency. Herein, we experimentally demonstrate that the impact dynamics of water droplets on superhydrophobic surfaces can be effectively altered by adding a very small amount of high molecular weight polymer. The presence of polymer chains in water enhances droplet–surface interactions and liquid viscoelasticity, which couple with the hydrodynamic effect and thus affects the dynamic behaviors of impinging droplets. Whereas various impact phenomena with altered transitional boundaries in the phase diagram have been observed for droplets of aqueous polymer solutions at low concentrations, only deposition was identified for aqueous polymer droplets with high concentrations. Nevertheless, the polymer addditive does not influence droplet spreading but siginificantly slows down droplet retraction and increases the contact time of rebounding droplets on the superhydrophobic surface. Moreover, the induced...

In this paper, a series of experiments are presented which demonstrate drag reduction for the laminar flow of water through microchannels using superhydrophobic surfaces with random surface microstructure. These superhydrophobic surfaces were fabricated with a simple, inexpensive technique of sanding polytetrafluoroethylene (PTFE) with sandpaper having grit sizes between 120- and 600-grit. A microfluidic device was used to measure the pressure drop as a function of the flow rate to determine the drag reduction and slip length of each surface. A maximum pressure drop reduction of 27 % and a maximum apparent slip length of b = 20 μm were obtained for the superhydrophobic surfaces created by sanding PTFE with a 240-grit sandpaper. The pressure drop reduction and slip length were found to increase with increasing mean particle size of the sandpaper up to 240-grit. Beyond that grit size, increasing the pitch of the surface roughness was found to cause the interface to transition from the Cassie–Baxter state to the Wenzel state. This transition was observed both as an increase in the contact angle hysteresis and simultaneously as a reduction in the pressure drop reduction. For these randomly rough surfaces, a correlation between the slip length and the contact angle hysteresis was found. The surfaces with the smallest contact angle hysteresis were found to also have the largest slip length. Finally, a number of sanding protocols were tested by sanding preferentially along the flow direction, across the flow direction and with a random circular pattern. In all cases, sanding in the flow direction was found to produce the largest pressure drop reduction.

In order to ensure the safety of civil aviation during the freezing weather, the application of superhydrophobic coatings (surfaces) on the aircraft is a useful and supplemental way to mitigate the accumulation of frost, ice or snow. In this work, superhydrophobic surface with hierarchical structure on SAE AMS 4037 aluminum alloy plate is prepared through chemical etching using hydrochloric acid and hydrogen peroxide with lower concentration successfully. Comparing with the bare aluminum substrate (untreated-Al), the superhydrophobic surface (treated-Al-11) possessed an excellent resistance to water, which water contact angle (CA) is as high as 163.6°, combined with minimum roll-off angle (RA) and contact angle hysteresis (CAH). The influences on wettability caused by different molar ratios of hydrochloric acid and hydrogen peroxide are studied by CA, RA, CAH and root mean square roughness (Rq). Hierarchical structure with irregular honeycomb-like protrusions and depressions of treated-Al-11 is found through SEM. Furthermore, treated-Al-11 is characterized by FTIR, EDS and XPS, which are demonstrated the presence of the aluminum stearate (AlSA) molecules on surface of treated-Al-11. Anti-icing tests are revealed that the weight of ice on the surface of treated-Al-11 with superhydrophobicity is significantly less than that of other Al plates at the same test time. The ice weight on treated-Al-11 is 1.7751 g compared to 1.9952 g on untreated-Al surface when test time is 120 min Based on the results of self-cleaning tests, the surface of treated-Al-11 is clean, however solution residues are remained on the untreated-Al surface.

Icephobicity and durability assessment of superhydrophobic surfaces: The role of surface roughness and the ice adhesion measurement technique

Superhydrophobic surfaces are widely used in various industrial and self-cleaning applications. Here, we present a low cost, facile synthesis of UV-resistant copper based superhydrophobic (SH) surface. The copper is electrochemically deposited on an another copper foil at the room temperature. The electrochemical bath consists of an aqueous solution of CuSO4 and H2SO4 acid. The samples were rinsed in ethanol and dried in a vacuum oven at 100 °C. The substrates were dip coated using 1 % Teflon AF solution to lower the surface energy. Also, another batch of copper surfaces was treated in stearic acid solution for hydrophobic coating. Both, the stearate and the Teflon coated copper surfaces shows hierarchical micro-nano surface morphology. The wetting properties on both the surfaces were extensively tested by static contact angle analysis, contact angle hysteresis (CAH) and droplet bouncing and rolling. Both the surfaces show, the average contact angle is about 165° and CAH is about 4°. The stability of the SH coating is tested against exposure to the ultraviolet (UV) A, B and C radiations. The Teflon AF coated surface retains SH property even after 2 hrs of UV exposure, whereas the stearate coating shows degradation in merely 30 min of exposure. Thus, the Teflon coated copper rough surface has potential in outdoor SH applications.

Superhydrophobic surface has wide application in self-cleaning, anti-pollution, anti-adhesion, drag reduction, but the stable and efficient manufacture of bionic superhydrophobic surfaces is a challenge. We have reproduced the microscopic structure of natural plants and achieved superhydrophobic surface preparation without chemical modification by mechanical process. This program need many experiments to generate processing parameters based on the bionic surface structure. To solve this problem, based on the natural Green Bristlegrass superhydrophobic surface, an optimized bio-inspired striped configuration was presented and the computational fluid dynamics approach was integrated into the fabrication, which was estimated the superhydrophobicity of bio-inspired surface. The volume of fluid (VOF) model and the Level Set function were used to determine the position of the interface during the liquid impact process by solving the change of the volume fraction of each phase in the mixed fluid, thus tracking the gas-liquid phase interface. According to the simulation of striped surface structure, we have processed the bionic samples successfully and they showed an excellent superhydrophobic property, the contact angle was 154° ± 2° and the sliding angle was <5°. The contact time of bionic sample was 12.8 ms. This article used the VOF simulation method to evaluate the design and parameters of the structure, so that the selection of processing parameters were more optimized. Furthermore, the study was essential to access the bio-inspired design and fabrication procedure of superhydrophobic surface.

Energy conservation and environmental protection have become pivotal components of the green economy. In recent years, underwater drag reduction technology has garnered significant interest. This study discusses a novel composite surface that combines divergent riblets with a superhydrophobic coating (D-rib&amp;SHS) to enhance the drag reduction rate. Alongside this new surface, a riblet surface with a superhydrophobic coating (rib&amp;SHS, without yaw angles) and a smooth surface are used as comparison groups. The turbulent boundary layer flow of these three surfaces is measured using a two-dimensional particle image velocimetry system. The results indicated that the maximum drag reduction rate of D-rib&amp;SHS is approximately 27% higher than that of rib&amp;SHS, and the drag reduction range is increased to Reθ≈4100 compared to rib&amp;SHS (Reθ≈2200). Using correlation algorithms, it observed that the spacing between low-speed streaks over D-rib&amp;SHS is larger than that of rib&amp;SHS and the smooth surface. This finding suggested that the spacing between the hairpin vortex legs of D-rib&amp;SHS is wider. The increased spacing between the hairpin vortex legs reduces the likelihood of vortex head formation between the two quasi-streamwise vortices, ultimately suppressing the auto-generation of hairpin vortices. Consequently, the development of hairpin vortex packets over D-rib&amp;SHS is also inhibited. These phenomena observed over D-rib&amp;SHS can be attributed to the combined effects of velocity slip on the superhydrophobic coating and the secondary flow over the divergent riblets near the wall. In addition, unlike divergent riblets that are not covered with a superhydrophobic coating, where the drag reduction effect is more pronounced with a yaw angle of 30° compared to 10°, D-rib&amp;SHS with a 10° yaw angle demonstrated a superior drag reduction effect compared to D-rib&amp;SHS with a 30° yaw angle. This innovative composite surface enhances the drag reduction effect and expands the drag reduction Reynolds number range, offering a new approach to mitigating drag in turbulent boundary layer flows.

A fluorine-free cerium palmitate superhydrophobic surface on an aluminum plate was fabricated via a two-step electrodeposition method. The mechanical durability, anti-corrosion performance, water repellency and drag reduction properties were tested. The results indicate that a superhydrophobic surface with a densely packed convex island shape with micro-pores and nano-scale strips was formed on the aluminum plate. Furthermore, the as-prepared surface exhibits excellent water-repelling ability with a water contact angle of 162.3° and a sliding angle of 1.5°. Owing to the protective effect of the convex island structure on the surface, the surface also shows superb mechanical durability, which is a shortcoming of ordinary electrodeposited surfaces. Moreover, compared with bare aluminum, the corrosion inhibition efficiency of the superhydrophobic surface is 99.55% and the surface drag decreases by 65.3% at a lower flow rate. Therefore, it is believed that the environmentally friendly fluorine-free superhydrophobic surface has promising potential applications in marine engineering.

The durability of superhydrophobic surface is a major problem to restrict industrial application of superhydrophobic materials from laboratory research, which can be attributed to a more general issue of mechanical stability for superhydrophobic coatings. Therefore, in order to handle this issue, we have fabricated a mechanically stable drag-reducing coating composed of elastic polydimethylsiloxane (PDMS) and hydrophobic copper particles on model ships, which can resist mechanical abrasion and has displayed a durable drag-reducing effect. In comparison with normal Au superhydrophobic coatings, the as-prepared PDMS/copper coatings showed durable drag reduction performance with a similar drag-reducing rate before (26%) and after (24%) mechanical abrasion. The mechanism for the enhanced mechanical stability and maintained drag reduction of the superhydrophobic surfaces was investigated through characterizations of surface morphology, surface wettability, and water adhesive force evaluation before and after abrasion. This is the first demonstration to realize the application of durable drag reduction by improving the mechanical stability of superhydrophobic coatings. We do believe that superhydrophobic surfaces with good resistance to mechanical abrasion or scratching may draw wide attention and gain significant applications with durable drag-reducing properties.

Aircraft icing refers to ice formation and accumulation on the windward surface of aircrafts. It is mainly caused by the striking of unstable supercooled water droplets suspended in clouds onto a solid surface. Aircraft icing poses an increasing threat to the safety of flight due to the damage of aerodynamic shape. This review article provides a comprehensive understanding of the preparation and anti-icing applications of the superhydrophobic coatings applied on the surface of aircrafts. The first section introduces the hazards of aircraft icing and the underlying formation mechanisms of ice on the surface of aircrafts. Although some current anti-icing and de-icing strategies have been confirmed to be effective, they consume higher energy and lead to some fatigue damages to the substrate materials. Considering the icing process, the functional coatings similar to lotus leaf with extreme water repellency and unusual self-cleaning properties have been proposed and are expected to reduce the relied degree on traditional de-icing approaches and even to replace them in near future. The following sections mainly discuss the current research progress on the wetting theories of superhydrophobicity and main methods to prepare superhydrophobic coatings. Furthermore, based on the bouncing capacity of impact droplets, the dynamic water repellency of superhydrophobic coatings is discussed as the third evaluated parameter. It is crucial to anti-icing applications because it describes the ability of droplets to rapidly bounce off before freezing. Subsequently, current studies on the application of anti-icing superhydrophobic coatings including the anti-icing mechanisms and application status are introduced in detail. Finally, some limitations and issues related to the anti-icing applications are proposed to provide a future outlook on investigations of the superhydrophobic anti-icing coatings.

Facile and effective construction of superhydrophobic, multi-functional and durable coatings on steel structure

The amazing water repellency of many biological surfaces, exemplified by lotus leaves, has recently received a great deal of interest. These surfaces, called superhydrophobic surfaces, exhibit water contact angles larger than 150 degrees and a low contact angle hysteresis because of both their low surface energy and heterogeneously rough structures. In this paper, we suggest a biomimetic method, "biosilicification", for generating heterogeneously rough structures and fabricating superhydrophobic surfaces. The superhydrophobic surface was prepared by a combination of the formation of heterogeneously rough, nanosphere-like silica structures through biosilicification and the formation of self-assembled monolayers of fluorosilane on the surface. The resulting surface exhibited the water contact angle of 160.1 degrees and the very low water contact angle hysteresis of only 2.3 degrees, which are definite characteristics of superhydrophobic surfaces. The superhydrophobic property of our system probably resulted from the air trapped in the rough surface. The wetting behavior on the surface was in the heterogeneous regime, which was totally supported by Cassie-Baxter equation.