# Rates of High-Temperature Evaporation of Promising Fire-Extinguishing Liquid Droplets
## 4 Results and Discussion
(i) The evaporation rates of the saline solution are lower than those of water and other studied extinguishing agents. The higher the heating temperature, the greater the difference in evaporation rates is. Even with the addition of 5-10% bischofite, the differences in evaporation rates can reach 20% to 25%. This is due to the influence of a group of factors registered in the experiments. In particular, when a drop of bischofite salt solution (MgCl\({}_{2}\cdot\)6H\({}_{2}\)O+additives) evaporated, a thin crystalline film was formed. Over time, it covered almost the entire drop. Accordingly, the evaporation area decreased. Figure 8: The dependence of the droplet lifetime (complete evaporation) (**a**) and the rate of vaporization (**b**) on the air temperature when heating in a muffle furnace. Data for water without impurities from experiments in Reference [12]. Figure 7: Dependence of droplet lifetimes (complete evaporation) (**a**) and evaporation rates (**b**) on air flow temperature. Data for water without impurities from experiments of Reference [12]. Meanwhile, with an increase in concentration of aqueous solutions of salts, the heat of vaporization is known to grow. Therefore, the amount of energy required for complete evaporation increases. It is also advisable to take into account that the surface tension of the salt solution is higher than that of water. This reduces the area of the droplet contact with the heated substrate at spreading. Therefore, the evaporation times of the bischofite salt solution are at maximal rates (Figure 6). (ii) Minimum lifetimes and maximum evaporation rates were recorded in experiments with droplets of foaming agent emulsions. This is mainly due to significantly (several times) lower surface tension of the foaming agent emulsion compared to other studied compositions. Therefore, the contact angle for emulsion droplets was \(22.6^{\circ}\). As a result, the greatest area of interaction with the substrate and, accordingly, the maximum evaporation rates were achieved. For the EA-5 solution, the surface tension values were higher than for the emulsion, but, in comparison with water, bentonite suspension, and salt solution, this parameter was significantly (almost 2 times) lower. As a consequence, the contact angles and the time of complete evaporation were close to the same characteristics of the emulsion of the foaming agent. (iii) When comparing the times of complete evaporation of water and bentonite suspension, smaller values of \(W_{\rm e}\) can be noted for the latter. This is due to the influence of several mechanisms. The main one is associated with greater thermal conductivity and density, lower heat capacity, and, accordingly, greater temperature diffusivity of the suspension. By the results of video recording, it is also possible to note the pattern of agglomeration of bentonite powder particles in different parts of the droplet. This increased the surface area of water evaporation, since the area of the contact boundary with the heated particles deposited on the substrate surface increased. At the same time, for experiments with suspension droplets on the substrate, the maximum fluctuations of the total evaporation times and the rates of phase transformations were registered. This was due to an important pattern, established in the analysis of video frames. Solid particles in the drop during its evaporation agglomerated chaotically, holding (closing) different volumes of water in-between. The symmetry of the suspension droplet on the substrate was often violated due to these effects. Hence, several larger fluctuations, compared to experiments with other compositions, were observed when averaging the results of radius calculations. In the case of adding 10% bentonite powder to water droplets, the effects of swelling, which are characteristic of moistened bentonite, did not manifest. However, this additive is a natural mineral of clay nature with a high binding capacity, which leads to absorption.