## 2 Experimental Setup  
The experiments were setup in a laboratory that is part of the GFK facility operated in IPMech RAS. This facility has been used before to perform numerous high-resolution tests on water-water and water-oil interactions under isothermal conditions [13; 14], including studies on the material redistribution of the droplet and receiving (target) liquid [16; 17], as well as on the acoustic effects and high-speed streamers that develop due to the droplet impact [25]. In the current work, the facility was modified so that it was able to handle high-temperature materials. The experimental setup is presented in Figure 1. The molten Rose's alloy was used as the working high-temperature fluid; its properties are listed in Table 1. The alloy was melted in a cylindrical vessel of 84 mm diameter and 23 mm height, which was heated electrically from below by a 2 kW heater. The melt was heated to temperatures of 200-300 \({}^{\circ}\)C, which superheated it to 100-200 degrees above its melting point, as well as above the normal boiling point of water (because the latter two temperatures are quite close, see Table 1). Water delivery was performed in two ways. The first was as a droplet falling under gravity from a fixed height of 0.58 m. This height was chosen because previous isothermal tests have shown that the height of the cumulative jet was the highest; the higher initial positions of the droplet were not used in the experiments because the droplet became unstable and fragmented prior to impacting the surface. Smaller heights were also discarded because the impact of the droplet on the heavy melt was too weak to cause any significant perturbations. The droplets were formed by slow water delivery from a thin syringe needle; the droplet was of a nearly-spherical shape due to the effect of surface tension, and grew to a diameter of 4.2 mm, after which it detached naturally from the needle without any initial velocity. The free-fall velocity for the above height was \(\sqrt{2 gh}=3.37\) m/s, and the water volume was 0.0388 mL. The second type of water delivery was from a syringe with the needle removed; the nozzle diameter was 0.3 cm. In this case, water was delivered from a small height (about 100 mm above the surface), thus forming a forced short-duration jet. Delivery from larger heights resulted in significant jet fragmentation, so that water reached the surface almost as a spray rather than a coherent jet. The water velocity prior to the impact was in the range of 3-11 m/s, the volume of water was controlled and set to 1 mL. Video recording was performed using an Optronis CR3000x2 camera at the frame rate of 5000 fps for water droplets and 3000 fps for water jets. The camera was located at a distance of 25 to 75 cm from the impact point, its line of view was inclined at an angle of 5 to 15 degrees with respect to the melt surface. All experiments were recorded at a fixed resolution of 800 \(\times\) 600 pixels in the horizontal and vertical directions. Selected video frames are presented in the following sections; these were taken directly from the video sequence, without any scaling, cropping etc. The length scale for each video sequence was determined by placing a test object (medical syringe needle of 0.8 mm diameter and 40 mm length) above the expected impact point prior to the release of the water. The frames where the test object was visible were used to calibrate the spatial coordinates in Autodesk AutoCAD 2020 software, after which the frames with the interaction were processed. The impact velocity was obtained from the measurements of droplet (or impulse jet) leading point coordinates on the video frames immediately preceding the water-melt contact. The purpose of the experiments performed was to establish the ability of the impact of the water jet or droplet to cause noticeable perturbations and splashes of the heavier hot liquid, as well as to look for the specific effects resulting from the phase transitions and evaporation of a more volatile liquid.