# A Review of Research on Dropwise Condensation Heat Transfer
## 2 The Mechanism of Dropwise Condensation
### Research on The Mechanism of Dropwise Condensation  
There are three different hypotheses about the mechanism of dropwise condensation. Firstly, [PERSON] [11] put forward the theory of liquid film breaks. He believed that the condensate always condenses in the form of a film in the previous condensation process, and the heat transfer can only pass through the liquid film at this time. As the condensation process proceeds, the thickness of the liquid film gradually thickens. When the thickness of the liquid film reaches a critical value, it will break. This phenomenon is due to surface tension, and the liquid film turns into many small droplets. At this time, some parts of the steam directly condense on the surface; otherwise, the steam will condense onto other droplets, causing the droplets to grow until departure. The route of droplets departure will form a bare belt. The above process repeats on a bare surface. However, this theory cannot explain why droplets are more likely to occur in some places. Using a microscope, this theory has been further questioned, and a nucleation sites theory has been suggested.  
The nucleation sites theory holds that dropwise condensation, like boiling, is a nucleation phenomenon. There are many condensation cores randomly distributed among the surface. Steam initially condenses at the condensation core to form initial droplets. The diameter of these initial droplets is smaller than the distance between the condensation cores, so the initial droplets are independent of each other. With the further development of the condensation process, steam is condensed on the initial droplets, making the initial droplets grow up. Then heat is mainly transferred to the surface through the thin area of the  
Figure 1: The contact angle of the droplets on different surfaces: (**a**) surface contact angle; (**b**) advancing contact angle and receding contact angle.  
droplet edge. When the droplets grow large enough, they will begin to coalesce and depart. [PERSON] [12], [PERSON] [13], and [PERSON] [14] observed the condensation phenomenon via a microscope, combined with theoretical analysis, and then provided strong support for the nucleation sites theory. However, with only a few molecular thicknesses on the surface, the liquid film cannot be observed by microscope, so it is impossible to determine whether there is a liquid film between the droplets.  
With the further development of technology, reflection spectroscopy has been applied to the study of dropwise condensation mechanism. [PERSON] [15] first put forward the adsorption theory by reflection spectroscopy, which is, both the liquid film and droplets exist on the condensation surface. This theory is a combination and improvement based on the first two theories. According to the adsorption theory, there is a thin liquid film and many condensation cores of the condensation process. The heat transfer rate is mainly carried out by the liquid film, the droplets on the condensation core absorb the condensate from the liquid film continuously and simultaneously as per the adsorption theory, and the droplets on the condensation core also grow until departure. The route of droplets departure will from a film belt. [PERSON] [16; 17] also observed the condensation process of the experiment and agreed with this theory. More and more scholars have recognized the adsorption theory because of its more scientifically based observations.