**3. Surface fabrication for dropwise condensation**

In most thermal systems, liquid condensate typically forms a liquid film on the heat transfer surface because of the high surface energy of common industrial components such as clean metals. To achieve dropwise condensation for highefficient heat transfer, various hydrophobic coatings such as long-chain fatty acid, polymer materials, rare-earth oxide ceramics, and self-assembled monolayers [14, 37, 52–58], are usually applied to increase hydrophobicity for high water repellency. Monolayer coatings, typically a few nanometers in thickness, for example, long-chain fluorocarbons and fatty acids, can increase water repellency of the surface with a negligible additional thermal resistance (**Figure 4a**). However, they are generally not durable due to the low chemical stability and low bonding strength with the substrate [14, 52]. With a similar hydrophobicity, polytetrafluoroethylene (PTFE) coatings have been attempted to increase surface repellency while maintaining low thermal resistance (**Figure 4b**) [59]. Thicker polymer coatings have been shown to maintain robust water repellency during vapor condensation (**Figure 4c**). However, they typically have a large thermal resistance that can even negate the heat transfer enhancement achieved by dropwise condensation [54]. In addition, the initiated chemical vapor deposition (iCVD) and plasma-enhanced CVD techniques have been used to grow ultrathin conformal polymer coatings to achieve hydrophobicity (**Figure 4d**). However, further study is necessary to evaluate the durability of these ultrathin coatings for condensation heat transfer enhancement [55, 58]. Recently, ultrathin graphene (**Figure 4e**) with low thermal

#### **Figure 4.**

*Hydrophobic coatings for achieving dropwise condensation. (a) Self-assembled monolayer coating [52]. (b) Polytetrafluoroethylene (PTFE) coating on carbon nanotubes [53]. (c) Fluorocarbon film from perfluorocyclobutane precursors [54]. (d) Thin film of poly-(1H,1H,2H,2H-perfluorodecyl acrylate)-codivinyl benzene p(PFDA-co-DVB) grafted to a substrate by iCVD [55]. (e) Graphene coatings on copper substrate [56]. (f) Rare-earth oxide ceramics [57].*

### *Advances in Dropwise Condensation: Dancing Droplets DOI: http://dx.doi.org/10.5772/intechopen.92689*

condensation heat transfer model to include more accurate expressions for the heat transfer through an individual droplet [35, 39–48] and droplet size distribution [14, 33, 39, 43, 46, 49–51] are developed, for example, the conduction resistance of the liquid droplet, the thermal resistance of a hydrophobic coating, mass transfer on

In most thermal systems, liquid condensate typically forms a liquid film on the

heat transfer surface because of the high surface energy of common industrial components such as clean metals. To achieve dropwise condensation for highefficient heat transfer, various hydrophobic coatings such as long-chain fatty acid, polymer materials, rare-earth oxide ceramics, and self-assembled monolayers [14, 37, 52–58], are usually applied to increase hydrophobicity for high water repellency. Monolayer coatings, typically a few nanometers in thickness, for example, long-chain fluorocarbons and fatty acids, can increase water repellency of the surface with a negligible additional thermal resistance (**Figure 4a**). However, they are generally not durable due to the low chemical stability and low bonding strength with the substrate [14, 52]. With a similar hydrophobicity, polytetrafluoroethylene

(PTFE) coatings have been attempted to increase surface repellency while maintaining low thermal resistance (**Figure 4b**) [59]. Thicker polymer coatings have been shown to maintain robust water repellency during vapor condensation (**Figure 4c**). However, they typically have a large thermal resistance that can even negate the heat transfer enhancement achieved by dropwise condensation [54]. In addition, the initiated chemical vapor deposition (iCVD) and plasma-enhanced CVD techniques have been used to grow ultrathin conformal polymer coatings to achieve hydrophobicity (**Figure 4d**). However, further study is necessary to evalu-

ate the durability of these ultrathin coatings for condensation heat transfer enhancement [55, 58]. Recently, ultrathin graphene (**Figure 4e**) with low thermal

*Hydrophobic coatings for achieving dropwise condensation. (a) Self-assembled monolayer coating [52]. (b)*

*Polytetrafluoroethylene (PTFE) coating on carbon nanotubes [53]. (c) Fluorocarbon film from perfluorocyclobutane precursors [54]. (d) Thin film of poly-(1H,1H,2H,2H-perfluorodecyl acrylate)-codivinyl benzene p(PFDA-co-DVB) grafted to a substrate by iCVD [55]. (e) Graphene coatings on copper*

**Figure 4.**

**234**

*substrate [56]. (f) Rare-earth oxide ceramics [57].*

the liquid–vapor interface, and the effect of interface curvature.

**3. Surface fabrication for dropwise condensation**

*21st Century Surface Science - a Handbook*

resistance and good chemical stability has been used to obtain stable dropwise condensation, without obvious heat transfer degradation in a 2-week measurement [56]. Another typical material is the rare-earth oxide (REO), which can be potentially used as a hydrophobic material at scale due to the development of ceramic processing techniques (**Figure 4f**) [57]. Note that the wettability of REO is reported to be intrinsically hydrophilic and the hydrophobicity of REO is due to the adsorbed hydrocarbon species [60–62]. Despite that many new functional coatings mentioned above have shown some potential to achieve dropwise condensation, a costeffective, low-thermal resistance, and robust hydrophobic coating to promote sustainable dropwise condensation has proven to be exceedingly challenging, resulting in ubiquitous filmwise condensation in real industrial applications.

To further improve droplet mobility, various micro/nanostructured surfaces are developed using advanced fabrication technologies. Micro/nanostructured silicon surfaces are fabricated using both wet and dry etching methods [63]. Typically, silicon nanowires synthesized by wet etching methods are vertically aligned [64]. Due to the surface tension of water during the drying process of nanowire synthesis, a large number of micro-defects are naturally formed where the closely aligned silicon nanowires cannot individually stand but form clusters [65]. Compared with the wet etching methods, finer structure geometries can be fabricated by dry etching where the etching rate can be controlled more precisely. In addition to the nanowires with uniform diameters, conical silicon nanowires were also fabricated to promote the formation of high-mobility droplets with the auxiliary Laplace pressure difference (**Figure 5a**) [25]. To meet the need of multiple length scales in manipulating droplet growth, hierarchical silicon nanowires with both microscale and nanoscale features have been fabricated by coupling micro-patterns and nanostructures [66]. **Figure 5b** shows a hierarchical surface with parallel microgrooves that are formed by patterning silicon nanowire arrays with different lengths [67, 68]. **Figure 5c** shows another hierarchical surface, consisting of micropyramids covered by silicon nanowires [69]. Compared to the silicon, metal materials, for example, aluminum, stainless steel, and copper, have better physical and thermal properties, to be exploited for improving heat transfer such as high thermal conductivity, stability, and machinability. Among various surface fabrication

#### **Figure 5.**

*Micro/nanostructured surfaces for condensation enhancement. (a) Conical silicon nanowire arrays [25]. (b) Microgroove silicon nanowires [67]. (c) Micro-pyramids covered by silicon nanowires [69]. (d) Gold nanowires [70]. (e) Closely spaced copper nanowire arrays [65]. (f) 3D copper nanowire networks [71]. (g) Hierarchical surface with micro-patterned copper nanowire arrays [21]. (h) Hierarchical copper meshcovered structure [72].*

methods for metal micro/nanostructures, the electrochemical deposition (or electroplating) method with the assistance of templates is considered to be a convenient and versatile approach [21, 65, 70, 71, 73]. **Figure 5d** shows the gold nanowires fabricated to promote droplet coalescence without direct coalescence [70]. Using a two-step template-assisted electrochemical deposition method, closely spaced copper nanowires were fabricated to promote the formation of mobile droplets in suspended wetting states (**Figure 5e**) [65]. To prevent the formation of microdefects between agglomerated nanowire arrays, the 3D copper nanowire networks consisting of interconnected nanowires with nanoscale bumps were fabricated using 3D porous anodic alumina oxide templates (**Figure 5f**). These nanobumps on the nanowire walls serve as anchors to fix adjacent nanowires [71]. **Figure 5g** shows a hierarchical surface with micro-patterned copper nanowire arrays [21]. In addition, the commercial metal materials, such as copper mesh, foam, and particles, have been applied to develop low-cost superhydrophobic surfaces and to provide a solution for large-scale deployment of enhanced heat transfer surfaces in a diverse range of technologies (**Figure 5h**) [72]. Despite that diverse micro/nanostructured surfaces have been fabricated to meet the multiple length scale need in manipulating droplet growth as mentioned above, there remains a need for advanced surface features to optimize droplet behaviors for future vapor condensation enhancement.
