# Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface
## 1 Introduction  
Condensation is a ubiquitous phenomenon in nature and is of interest in several technical applications, including water harvesting, seawater desalination, power generation, and so on [1; 2; 3; 4]. The condensation heat transfer enhancement benefits to improving the energy utilization efficiency and saving energy in the industrial process. It is documented that condensation can be divided into filmwise condensation and dropwise condensation [5]. Filmwise condensation mainly occurs on the hydrophilic surface or under high subcooling conditions, where liquid film hinders the direct contact between the condensation surface and steam and hence leads to an increase in thermal resistance. Thus, rapid removal of the condensate and thinning of liquid film thickness can be utilized to enhance filmwise condensation heat transfer. Differing from filmwise condensation, dropwise condensation mainly occurs on the hydrophobic surface, which goes through the process of droplet nucleation, growth, coalescence, and departure. Owing to the rapid removal of droplets and constant emergence of nucleation sites, the heat transfer coefficient of dropwise condensation is one order of magnitude higher than that of filmwise condensation [6; 7]. However, at high subcooling, the nucleation rate of droplets increases rapidly, and the occurrence of flooding increases the heat transfer resistance since the condensate cannot be shed in time. As a result, the dropwise condensation would transform into filmwise condensation, deteriorating the heat transfer performance [8; 9]. In this context, it is urgent to developan efficient way to remove the condensate droplet on a cold wall and hence enhance dropwise condensation. Condensate on the hydrophobic surface can shed rapidly, while the high energy barrier makes droplet nucleation difficult [10; 11]. In recent years, researchers have developed a series of hybrid surfaces containing both hydrophilic and hydrophobic parts to make comprehensive use of both benefits of each surface [12; 13; 14; 15; 16]. For example, the liquid film on the hydrophilic area can suck and transport the droplets on the hydrophobic area quickly through striped hydrophilic modification on the hydrophobic surface [17; 18; 19], which reduces the droplet detachment diameter and accelerates the renewal frequency of droplets on the hydrophobic area. Although sacrificing part of the hydrophobic area with high heat transfer performance, the rapid condensate departure effect due to the liquid film transportation on the hydrophilic area will enhance the heat transfer on the rest hydrophobic area. In this way, a better comprehensive condensation heat transfer performance can be obtained. In the 1990s, [PERSON] et al. [20] first explored the condensation heat transfer enhancement of the hybrid surface with the coexistence of dropwise and filmwise condensations. The surface was divided into finer patterns, such as horizontal or vertical stripes. However, in their works, the improvement of condensation heat transfer performance on the hybrid surface was limited, the heat flux of which was weaker than that of a full hydrophobic surface. This is because they failed to balance the benefits and losses of introducing the hydrophilic part into the hydrophobic surface, which needs further investigation. In 2015, [PERSON] et al. [15] carried out a similar experimental study in pure steam conditions, mainly exploring the influence of the size of hydrophilic and hydrophobic stripe structures on condensation heat transfer performance. It is found that with the increase in the hydrophobic region width, the condensation heat transfer coefficient increased at first and then decreased, that is, there was an optimal hydrophobic region width. The heat flux of the optimal hybrid surface was 23% higher than that of complete dropwise condensation on the hydrophobic surface at subcooling 2 K. Recently, [PERSON] et al.