# A Review of Research on Dropwise Condensation Heat Transfer
## 4 Methods, Problems, and Solutions for Promoting Dropwise Condensation
### Methods for Promoting Dropwise Condensation
#### 4.1.1 Low Free Energy Surfaces  
Metallization is an effective method of reducing surface free energy; it can be achieved by plating some precious metals, such as gold, silver, palladium, rhodium, and so on. Some scholars believe that the surface of gold and other noble metals is a natural dropwise condensation surface [35; 36]. [PERSON] [37] and [PERSON] [38] believed that the adsorption of organic substances in the vapor was the cause of the dropwise condensation property of the gold surface, and for the pure vapor condition, the gold surface formed filmwise condensation. Dropwise condensation can also be achieved by chromium plating on the metal surface [39], which saves costs to some extent. At present, the popular metallization methods are electroplating and electrolessing plating; the latter is generally considered to be quicker and with a longer life. The surface metallization method is longer than the other coating, and the thermal resistance are small, but the cost of the noble metal is too high, and the life of the chromium plating cannot be guaranteed. At the same time, the electroplating chromium plating can also form chrome, which easily causes environmental pollution.  
[PERSON] [40; 41] proposed the method of reducing surface free energy by ion implantation. He injected nitrogen, argon, helium, hydrogen, and other elements into the condensed surface and promoted dropwise condensation. For ion implantation, firstly, the specific elements are ionized into ions in the ion source. The ions selected by the magnetic mass analyzer are accelerated in the high voltage electric field and bombard the metal surface. The surface is bombarded to form an amorphous state, and then surface energy is reduced. Recently, [PERSON] [42] successfully injected chromium ions into the aluminum substrate and promoted dropwise condensation. For this surface, the lifetime can exceed half a year. This method has an obvious strengthening effect and can realize large-scale production, but it will destroy the substrate.  
Dropwise condensation can also be achieved using organic polymer coating, which is considered the most effective method for achieving long-term stable dropwise condensation [43]. Commonly, polymer coating materials are polymer of fluorocarbon polymer, Silica gel polymer, Hexamethyldisiloxane polymer [44], Benzotriazoles [45] et al. Fluorocarbon polymer such as polytetrafluoroethylene (PTFE) [38], Fluorinated propylene polymer [38], Poly (p-xylene) polymers, and polyhexafluoropropylene [44; 46] can promote dropwise condensation on the surface. Fluorocarbons have low thermal conductivity and will produce a high thermal resistance when used as coatings. Hence, the thickness of the coating should be as thin as possible. However, thin coating means low service life. Compared with other fluorocarbon polymers, PTFE has a longer service life with a smaller thickness (1 \(\upmu\)m versus about 10 \(\upmu\)m for others), making it more promising. Polymer coating ings can be fixed on surfaces by using the splash technology, electro-deposition, vacuum deposition, self-assembled coating [47], plasma polymerization [48], initiated chemical vapor deposition (iCVD) [49], and deposition [50]. In order to reduce the coating thickness under the premise of ensuring the lifetime, the coated surface can be further treated by ion implantation technology, bombarded with a high-energy ion beam to enhance the stability further.  
The main methods for reducing surface free energy and their advantages and disadvantages are shown in Table 1.