# Characterizations of Interfacial Solar Water Evaporation
## 1 Introduction  
Solar water evaporation directly uses solar energy to generate vapor at sub-boiling temperatures or steam above the boiling temperature [1]. The resulting thermal energy can then be used in heating and energy storage applications. Solar water evaporation is a fundamental thermal process that has played a significant global role since prehistoric times in widespread applications, including clean water. In the modern era, solar water evaporation has been employed extensively in industry, such as for steam sterilization and electricity generation [2]. However, solar-thermal energy remains underutilized owing to the complexity and cost of increasing the optical concentration to the required levels. Furthermore, even though solar water evaporation readily occurs in the natural environment, capturing and converting energy from the water evaporation appearance has seldom been investigated. For solar water evaporation systems without optical concentration (e.g., solar stills), heat is generated at the receiver surface, whereas vapor is generated elsewhere in the system. The separation of heat generation from vapor generation inevitably leads to heat losses that contribute to solar stills having a poor evaporation efficiency of 30-45%, large thermal masses, and needless temperature dips from the heat generation to evaporation surfaces. Volumetric solar absorption techniques employ optical nanofluids to shift heat generation inside the fluid, which has been shown to minimize surface heat losses. However, this approach was not created expressly for applications involving evaporation at high surface temperatures, so the evaporation efficiency is only somewhat improved. Furthermore, dispersing and pumping nanofluids under prolonged and strong sunlight is difficult.  
Interfacial solar water evaporation is a significant approach that localizes heat at the liquid surface to achieve an evaporation efficiency of about 90% at low optical concentrations. Instead of heating the entire water body, interfacial solar water evaporation only heats the evaporative section, which eliminates volumetric heating, minimizes the amount of photothermal material required, and offers various options to dynamically adjust theevaporation performance, such as modifying the evaporation vapor flow and vapor temperature [3]. These benefits make interfacial solar water evaporation a promising approach to expanding the use of solar-thermal technology in small, standalone, and portable systems. An extensive body of literature is readily accessible on the inherent characteristics, performance assessment, and practical demonstration of photothermal materials potentially applicable to interfacial solar water evaporation. However, there has yet to be a systematic review of the most important properties of various photothermal materials. The characterization of these performances is important for understanding the photothermal materials in specific applications [4]. Thus, this review presents a critical appraisal of characterization techniques that have been applied in the literature to evaluate the intrinsic properties of photothermal materials and their performance in different metrics.  
A demanding evaluation of the characterization methods might be very helpful because it is crucial to characterize various features, including chemical composition and physical properties, in order to comprehend the photothermal performance of photothermal materials for particular applications. The major components are then broken down into a number of sub-sections. This discussion's organization results from taking into account the underlying connection between these two key qualities. In general, this review article's main purpose is to familiarize materials researchers, especially those who are just starting out in their careers, with methods that are frequently employed for the characterization of the most crucial features of the diverse range of photothermal materials.