# Dropwise Condensation Mechanism on Smooth Hydrophilic Surfaces
## 1 Introduction  
Dropwise condensation has the potential to improve efficiency and reduce maintenance costs of heat transfer systems. However, efforts to design and fabricate surfaces to sustain long-term dropwise condensation have not been successful [1, 2]. The main reason for this lack of success is that the physics behind dropwise condensation is not fully understood [3]. Currently, there are two competing theories on the mechanism of dropwise condensation, specifically, the classical nucleation and film rupture theories [4]. Nucleation theory, first introduced by [PERSON] and [PERSON] in 1935 [5], claims that droplets nucleate heterogeneously on a surface. Then, the droplets grow and coalesce to make larger droplets. Heat transfer is dominant through the droplets (defined as the active area) and the area between the droplets is considered inactive. [PERSON] and [PERSON] [6] experimentally showed that nucleation theory is the mechanism of dropwise condensation. They studied the static dropwise condensation of steam on a gold surface and showed that no film larger than a monolayer exists between droplets. Therefore, their finding supports that the area between droplets is not active in heat transfer. The majority of researchers support nucleation theory as the governing mechanism of dropwise condensation [7]. Numerical models thathave been developed to predict dropwise condensation are mostly based on nucleation physics [3; 7]. Monte Carlo simulations [8] and the molecular dynamics simulations [9; 10] have been used to investigate the nucleation process. However, a few studies focused on how the initial droplets form on the surface [10]. On the other hand, film rupture theory, first introduced in 1937 by [PERSON] et al. [11], hypothesized that vapor condenses as a thin film on a surface. At a critical thickness, the thin film breaks up, and subsequently forms small droplets on the surface. According to this theory, a thin film (e.g., a film with a thickness larger than a monolayer) exists between droplets. [PERSON] et al. [12] argued that dropwise condensation is a dynamic process. Therefore, the static dropwise condensation experiment conducted by [PERSON] and [PERSON] [6] could not represent the actual dropwise condensation process. [PERSON] et al. experimentally showed the presence of a thin film between droplets during the dynamic dropwise condensation of steam on a steel surface. The work of [PERSON] and [PERSON] [6] is one of the main experimental studies that support nucleation theory, while the research of [PERSON] et al. [12] is one of the main studies that shows support for film rupture theory. These conflicting experimental studies are one of the main reasons for the existence of two competing theories on the mechanism of dropwise condensation. In 2017, [PERSON] et al. [10] used the molecular dynamics simulation to investigate the onset of dropwise condensation. Their study showed that initial droplets could form according to the nucleation or film rupture theory, depending on relative surface free energy. This means the two classical hypotheses cannot independently describe the formation of nanoscales droplets at the onset of dropwise condensation. In this work, we study the dropwise condensation mechanism by conducting experiments similar to the works of [PERSON] and [PERSON] [13] and [PERSON] et al. [12]. Due to the need for visualization of interfacial phenomena with high-frequency features, such as ultrathin film instabilities, we developed an automated SPRi instrument, the first-ever reported instrument capable of imaging a sub-nanometer film at high-speed (up to 10,000 FPS) in ambient conditions [14]. This newly developed near-field imaging system was applied [14; 15; 16; 17] to understand the physics of dropwise condensation, which have been under debate for the past nine decades [4].