# A Review of Research on Dropwise Condensation Heat Transfer
## 4 Methods, Problems, and Solutions for Promoting Dropwise Condensation
### Solutions to Main Problems  
#### 4.3.1 Composite Surface Method  
Many scholars have studied the condensation heat transfer of a series of patterned surfaces [77; 78; 79; 80; 81; 82; 83], such as superhydrophobic-hydrophilic, superhydrophilic-hydrophobic, etc. [PERSON] [66] transformed the hydrophilic surface to hydrophobic by the method of light radiation (irradiated by a low-pressure mercury lamp in the air). [PERSON] [60] fabricated a Ni nanocones structure surface on copper substrate by electrochemical deposition. Initially, the surface was hydrophilic, but when exposed to room temperature, it was slowly transformed into a superhydrophobic surface. These studies provide a feasible method of fabricating combination surfaces. [PERSON] [82] obtained the strip patterned surface by partially covering the whole hydrophilic surface and hydrophobic treatment of the exposed surface.  
The hydrophilic and hydrophobic patterned surfaces combine the advantages of hydrophilic surface nucleation and hydrophobic surface droplet departure rates. It will feature filmwise condensation in the center of the surface. However, the heat transfer effect of the patterned surface will deteriorate in some cases. The distribution of hydrophilic and hydrophobic areas on the surface forms the surface tension gradient. Under surface tension gradients, droplets on hydrophobic areas will migrate to hydrophilic areas (droplet migration) [82] and concentrate on the hydrophilic area. The droplet departure rate will increase the additional thermal resistance. The patterned surface will also influence heat transfer performance, e.g., a striped surface; however, too wide stripe belts will cause the overall heat transfer rate to decline. A narrow stripe belt is conducive to enhancing heat transfer. However, if it is too narrow, the hydrophobic area will be merged by the hydrophilic area's droplets and form a continuous liquid film, reducing the heat transfer effect [83]. The angle between the normal patterned surface and the gravity (the main force due to droplet departure) also affects the surface heat transfer performance [80].  
Fabricating micro-nano composite structures of different sizes of the surface is another solution. The advantages of hydrophilic and hydrophobic surfaces are combined by building layered and multi-scale structures that are interlaced with hydrophilic and hydrophobic surfaces. This kind of surface can promote the formation of dropwise condensation better than the pure superhydrophobic surface, and the dropping performance of droplets is better [78; 84]. Compared with the characteristics of random nucleation on the superhydrophobic surface, the nucleation sites on the surface of this composite structure of the hydrophilic surface are controllable [78]. Such surfaces enhance the hydrophobicity of the surface and postpone heat flux deterioration caused by the droplets' forms changing into a Wenzel shape. Meanwhile, this micro-nano composite structure keeps the efficient droplet characteristics of the hydrophobic surface, so there are a large number of small droplets on the surface. Because steam heat is mainly transferred to the surface through small droplets in the condensation process, the heat transfer efficiency can be significantly improved [85; 86]. This kind of surface avoids filmwise condensation as far as possible, but  
Figure 4: The state of droplets on the surface of the micro-nano structure: (**a**) Cassie state; (**b**) Partial Wenzel; (**c**) Wenzel state.  
the droplet departure rate still has a superior limit; with the temperature drop increase, the heat flux will be reduced [87].