# A Review of Research on Dropwise Condensation Heat Transfer
## 4 Methods, Problems, and Solutions for Promoting Dropwise Condensation
### The Main Problem of Enhancing the Heat Transfer of Dropwise Condensation  
There are two main problems with dropwise condensation enhancing heat transfer; the first one is hydrophilic or hydrophobic. Hydrophilic and hydrophobic surfaces play different and equally important roles in the condensation process. The hydrophilic surface is the key to promote the nucleation rate of a droplet, and the hydrophobic surface is the necessary condition to obtain the dropwise condensation and larger heat transfer coefficient. The second one is how to speed up the droplet departure rate and maintain a stable dropwise condensation. For a certain surface, the droplet departure rate has a limit. With the increase in surface subcooling temperature \(\Delta\)T, the steam condensation rate will exceed the droplet departure rate. This will cause droplet deposition on the surface and change the condensation formed dropwise condensation into filmwise condensation. For example, in the process of condensation, filmwise condensation is more likely to occur at the geometric center of the hydrophobic surface [71; 72], which is mainly caused by the fact that droplets at the center cannot fall off in time. The superhydrophobic surface can accelerate droplet departure, which can alleviate the above phenomenon. However, the superhydrophobic surface also has an adverse effect on droplet nucleation. According to the experimental study, compared with the hydrophobic surface, the superhydrophobic surface does not have a good heat transfer effect of the initial stage of condensation, but with time, the superhydrophobic surface has a higher heat transfer coefficient [73].  
The droplets have two morphology states on the hydrophobic surface of the micro-nano structure, namely Cassie state and Wenzel state. The Cassie state (Figure 4a) occurs when the droplet is statically suspended on the structures, in which the droplets easily fall off the surface, but the thermal resistance between the droplets and the surface is larger. The Wenzel state (Figure 4c) forms when the structures underneath the droplet are completely immersed by the liquid, so the contact thermal resistance between the droplets and the surface is smaller, but the droplets do not easily fall off the surface. [PERSON] [74] believed that partial Wenzel (Figure 4b) was the most favorable for heat transfer, which is the intermediate state between Cassie and Wenzel. In the condensation process, droplets will gradually deposit in the pores of microstructures, and the flooding phenomenon will occur. This phenomenon will make the droplet morphology states from Cassie become Wenzel, leading to the superhydrophobic surface losing its surface function. The properties of superhydrophobic surfaces can be regained by drying surfaces [75] or infiltrating oily substances [76] on the micro-nano structures surface, but all of them are palliative methods.  
The two problems mentioned above of dropwise condensation affect the droplet nucleation rate and departure rate. Suppose the droplets cannot fall off in time and coalescence on the condensation surface. In that case, the height of the droplets will be much higher than that of the filmwise condensation liquid film, and the thermal resistance of the droplets will also be much higher than that of the liquid film, thus reducing the heat transfer coefficient, impacting the effect of enhancing heat transfer, and even making the heat transfer efficiency of the dropwise condensation lower than filmwise condensation.