Teaching practice of experimental techniques for droplet impact on and evaporation from hot surfaces

Salt fog deposition and corrosion are critical problems that affect the safe, efficient, and long-term operation of marine gas turbines, aeroengines, and other power equipment operating in harsh ocean environments. In actual marine service conditions, salt-containing droplets may enter the flow passages with the intake air, evaporate, collide with hot walls, and finally form salt deposits on solid surfaces. These coupled processes involve fluid mechanics, heat and mass transfer, phase change, crystallization, surface deposition, and engineering failure mechanisms. However, in conventional undergraduate and graduate experimental teaching, these physical processes are often taught separately in courses such as engineering thermodynamics, heat transfer, and fluid mechanics. Therefore, students lack an intuitive experimental platform for observing the dynamic coupling among processes such as droplet impact, evaporation, boiling transition, and salt deposition. To address this problem, this study proposes an experimental teaching technique for investigating droplet impact and evaporation on hot surfaces. The purpose is to construct a teaching-oriented experimental system that connects frontier engineering problems with fundamental course knowledge, helps students understand the mechanisms of salt fog evaporation and deposition, and supports experimental teaching reform against the background of emerging engineering education.

An experimental system encompassing droplet impact, evaporation, and deposition was designed and built. The main components of the system were a droplet generation unit, a hot-wall impact platform, a controllable electric heating system, a high-speed camera, an illumination system, and a data acquisition system. The experimental platform could generate droplets with millimeter-scale diameters, control the impact velocity by adjusting the needle height, regulate wall temperature via a closed-loop heating system, and record the transient morphology of droplets via high-speed imaging. Pure water droplets as well as saline droplets were tested. Saline droplets with different mass concentrations were prepared to simulate salt-containing droplets in marine environments. Wall temperature, droplet impact velocity, and salinity were selected as the main control variables. During the experiment, the droplet impact process, maximum spreading state, evaporation mode, rebound behavior, splashing behavior, and final salt deposition pattern were recorded. The equivalent droplet diameter and dimensionless spreading factor were calculated from image measurements. After evaporation, the deposited salt patterns were photographed, and representative deposition regions were characterized by scanning electron microscopy (SEM) to reveal the local crystal morphology and deposition structure. In the teaching design, the experiment was organized into three modules to separately address the effects of impact velocity, wall temperature, and salinity. Students were required to complete solution preparation, equipment adjustment, high-speed image acquisition, temperature control, image processing, data calculation, and mechanism analysis.

The experimental results showed that wall temperature was the dominant factor determining droplet evaporation mode and salt deposition morphology. At relatively low wall temperatures, the saline droplet maintained contact with the heated surface for a longer period, and the final deposit exhibited a typical coffee-ring-like pattern. This pattern was mainly related to contact-line behavior, outward capillary flow, and nonuniform solute transport during evaporation. With increasing wall temperature, the droplet evaporation mode changed gradually, and nucleate boiling became more intense. Under these conditions, bubble generation, local disturbance, and secondary droplet splashing increased remarkably, resulting in a less regular deposition morphology and more salt particles scattered around the main deposition region. With further increase in temperature, a stable vapor layer formed between the droplet and the hot wall, and the Leidenfrost effect became obvious. The droplet rebounded rapidly from the surface, and no clear salt deposition pattern remained on the wall. In addition to wall temperature, impact velocity considerably influenced spreading, splashing, and deposition behaviors. At a low impact velocity, droplets spread without severe breakup. As velocity increased, splashing became more intense, and some secondary droplets contributed to scattered salt deposition beyond the main deposit. At very high impact velocities, horizontal splashing and secondary droplet coalescence were observed. Salinity also affected the boiling and impact behavior of droplets. Compared with pure water droplets, saline droplets under identical conditions exhibited stronger boiling disturbance and a more obvious central jet at a lower wall temperature. This phenomenon was attributed to the presence of salt ions and microcrystals, which provided heterogeneous nucleation sites and enhanced vapor generation inside the droplet. SEM observations showed that the salt deposit comprised porous regions and stacked cubic salt crystals, indicating that the final deposition morphology was jointly controlled by droplet spreading, evaporation mode, local crystallization, and splashing.

The proposed experimental technique provides a visual, operable, and engineering-oriented platform for teaching droplet impact, evaporation, and salt deposition on hot surfaces. It transforms the abstract knowledge of thermodynamics, heat transfer, and fluid mechanics into observable physical phenomena, such as droplet spreading, nucleate boiling, film boiling, Leidenfrost rebound, Marangoni flow, coffee-ring deposition, and salt crystallization. The experiment also enables students to practice modern testing methods, such as high-speed photography, temperature measurement, closed-loop heating control, image analysis, and microstructure characterization. From the perspective of teaching reform, this platform integrates scientific research problems with experimental education and supports the cultivation of students’ ability for engineering practice, data processing, mechanism analysis, and innovative thinking. The results demonstrate that surface temperature, impact velocity, and salinity jointly determine the evaporation mode and deposition pattern of saline droplets. Wall temperature plays the most decisive role among these factors. In future teaching practice, this experimental system can be further integrated with digital image processing, database construction, virtual simulation, artificial intelligence, and digital twin technologies to develop an intelligent, shareable experimental teaching resource. Such advances will strongly support the cultivation of high-level engineering skills to solve complex problems in marine power equipment, thermal protection, spray cooling, and multiphase flow systems.