# A Review of Research on Dropwise Condensation Heat Transfer
## 4 Methods, Problems, and Solutions for Promoting Dropwise Condensation
### Solutions to Main Problems
#### 4.3.2 Increased Driving Force Promotes Droplet Shedding  
According to [PERSON]'s heat transfer model [22] mentioned above, the dropwise condensation heat flux is simulated for the contact angles of 90 deg, 120 deg, 150 deg in Figure 4. For the calculation, the following conditions were used: The vapor saturation temperature was 375 K, the nucleation density was \(2.5\times 10^{11}\) m\({}^{-2}\), and thermal conductivity and thickness of the promoter layer were 0.2 W/(m\(\cdot\)K) and 1 \(\upmu\)m, respectively. In Figure 5, with the increase of the sweeping time, the heat flux decreases rapidly, and the decrease rate is faster with the increase of contact angle. It is obvious that increasing the droplet shedding rate can significantly enhance the dropwise condensation heat flux for the contact angles of 90 deg, 120 deg, and 150 deg. Increasing the droplet shedding rate means that the mass transfer rate is increased and will decrease the thermal resistance due to the conduction through the drop itself. To increase the droplet shedding, decreasing the maximum droplet radius is an effective method. For the three curves in Figure 5, when the sweeping period is 5.5 s, The maximum heat flux is found on the condensation surface with a contact angle of 90 deg. When the subcooling is 11 K, the heat flux on the surface of the condensation with a contact angle of 150 deg is the largest. Therefore, for a certain surface, speeding up droplet shedding is the key to enhancing dropwise condensation heat transfer. In general, gravity is the mainly driving force for droplets falling off. How can speed up droplets departure rate by increasing driving force has been studied. [PERSON] [88] found that the droplets can also move spontaneously on the horizontal surface. He prepared radial surface with surface tension gradients on the condensed surface by diffusion-controlled silanization. The treated surface generated droplet migration and promoted droplet shedding, the maximum migration velocity of the droplet on the prepared horizontal surface can be up to 1.5 m/s. Using surface energy gradient as the driving force to accelerate the shedding of droplets is a typical passive method without other energy consumption. Once the technology is mature, it will greatly enhance the heat transfer of dropwise condensation. Figure 5: Dropwise condensation heat flux with respect to the sweeping period and contact angle. For the superhydrophobic surface, [PERSON] and [PERSON] [89] reported the self-propelled jumping during condensation. They believed that this phenomenon happened because the drops coalesce and release excess surface energy, which drives the drops to jump by turning into kinetic energy. Many scholars have researched this phenomenon [7; 65; 90; 91; 92; 93]. For this phenomenon, the larger number of coalesced droplets is, the smaller the droplet radius required [90]. The surface mentioned by [PERSON] and [PERSON] [89] had a drop jumping diameter of only 10 \(\upmu\)m. The surface fabricated by [PERSON] [91] found that the droplet diameter when jumping off reaches 20-45 \(\upmu\)m. He [92] found that the jumping and fall off diameter of droplets was between 14-30.2 \(\upmu\)m. The drop jumping phenomenon of the superhydrophobic surface makes it have a better self-cleaning property, which can also greatly strengthen the dropwise condensation heat transfer. For surfaces with droplet jumping characteristics, it only can be achieved under the condition of low heat flux, but with the increase of heat flux, the phenomenon of droplet jumping cannot be well maintained [65]. [PERSON] [93] found that condensate droplets are electrically charged when they are jumping. Inspired by this phenomenon, [PERSON] [94] applied a tangential alternating current electric field to the condensing surface to increase the droplets shedding rate, and this method could realize the jumping shedding just by a single droplet, which would greatly enhance the heat transfer performance of the surface.