# A Review of Research on Dropwise Condensation Heat Transfer
## 4 Methods, Problems, and Solutions for Promoting Dropwise Condensation
### Methods for Promoting Dropwise Condensation
#### 4.1.2 Micro-Nano Structure Surface  
Scholars have discovered that lotus leaves and cicada wings are natural hydrophobic surfaces in recent years, so they have conducted in-depth research on these surfaces' structure characteristics and found that these surfaces are composed of some tiny needle-like structures. Other natural superhydrophobic surfaces also have many micro-nano structures of their surfaces. Inspired by this discovery, researchers have used different methods (such as etching, chemical deposition, etc.) to produce a series of micro-nano structural surfaces. [PERSON] [51], [PERSON] [52], and [PERSON] [53] used nanosecond laser processing technology (laser etching) to fabricate micro-nano structure on the surface, transforming a hydrophilic titanium surface into a hydrophobic one. [PERSON] [54] fabricated a needle-like micro-nano structure by directly oxidizing a copper surface. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Methods**} & \multirow{2}{*}{**Materials Used**} & \multicolumn{2}{c}{**Manufacturing Process**} & \multirow{2}{*}{**Advantages**} & \multirow{2}{*}{**Disadvantages**} \\ \hline \hline \multirow{8}{*}{Metallization} & & & & Difficult to maintain \\  & Gold. & & & No additional & continuous \\  & Silver. & Electroless & thermal & & dropwise \\  & Palladium. & plating, & resistance. & & condensation under \\  & Rhodium & Electroplate & Simple & & pure steam. \\  & (Noble metals) & & processing. & & Difficult to \\  & & & No pollution & & industrialize. \\  & & & & Effect decreases \\  & & & & & after oxidation \\ \hline \multirow{8}{*}{In implantation} & \multirow{2}{*}{Nitrogen. Argon, Helium. Hydrogen. Chromium et al.} & \multirow{2}{*}{Ions bombard surface} & \multirow{2}{*}{Ions bombard surface} & \multirow{2}{*}{Ions bombard surface} & \multirow{2}{*}{Ions bombard surface} \\  & & & & & \\  & & & & & Easy to \\  & & & & industrialize & \\ \hline \multirow{8}{*}{Organic polymer} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} & \multirow{2}{*}{Fluorocarbon. Sciling gel.} \\  & & & & & \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main methods for reducing surface free energy. In addition to the above techniques of direct surface modification, the micro-nano surface can be made by depositing other materials on the surface. The methods, deposition materials, and contact angles of various micro-nano structure surfaces are shown in Table 2. Depositing other materials on the surface can fabricate a variety of micro-nano structure surfaces. Currently, the main structures include nanowires (nanoneedles) [54; 55; 56; 57; 58; 59], nanocones [60; 61], nanorods [62; 63], nanosheets (nanoflowers) [64; 65; 66], nanoblocks [67; 68; 69], and so on. Different micro-nano structure surfaces have different effects on hydrophobicity. The larger the specific surface area of the surface and the stronger the air-trapping ability is, the easier it is to achieve a larger contact angle. This is also why nanowires, nanocones, and nanorods can more easily achieve larger contact angles than nanosheets and nanoblocks. Compared with nanowire structures, nanosheets are more inclined to wire and surface contact with droplets. This phenomenon will cause the droplets to adhere to the micro-nano structure after coalescence so that the droplets are difficult to fall off and the surface has a poor self-cleaning performance. The nanosheet structure becomes rough by silanizing, which can effectively solve the problem of poor self-cleaning performance. For this method of fabricating micro-nano structure surfaces, different experimental environments, substrate, and operation methods may produce different effects.