# Conditions for and Characteristics of the Dispersion of Gel Fuel Droplets during Ignition
## 1 Introduction
An analysis of studies [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20] devoted to the dispersion of fuels showed that its intensity depends on a large number of factors, such as the heat supply mechanism (conductive, convective, radiation), the heating medium's temperature, the concentration of components in the composition of gel fuels, suspensions, emulsions, and solutions, the properties of the components and their ratios in the mixture (solid particles, combustible liquid, water, surfactants), droplet sizes, and the conditions for their formation, which determine the droplet structure (a combustible liquid is the shell, water is the center, or vice versa), as well as the location of the droplet in space (sitting on a surface, on a holder, flying in a gas flow). In real conditions, the dispersion characteristics of multicomponent fuels are affected by a combination of the factors listed above. Therefore, when the conditions of ignition and combustion of such fuels change, individual factors from the above, with their combined effect on the dispersion process, can have a positive effect or weaken this effect. The known results make it possible to establish the dispersion mechanism, which, in general, is reduced to the destruction of vapor bubbles, but they do not allow us to determine the causes, features, and scales of this phenomenon during the ignition and combustion of fuel mixtures. It is also worth noting that the component base of the studied fuels is quite limited and is represented mainly by coals (hard, brown), combustible liquids (kerosene, fuel oil, various types of oils), and one non-combustible liquid (water), which are independent technological energy resources that are widely used on practice. The main concepts of the theory that are applicable in practice for predicting the guaranteed course of droplet dispersion, including fuel mixtures of arbitrary component composition, have not yet been developed. The purpose of this work was to develop recommendations for the composition of multicomponent droplets (including components that are in composite fuels) for the guaranteed implementation of evaporation, boiling, puffing, or microexplosion under high-temperature heating that is sufficient for the stable ignition of a composite fuel. In this paper, for the first time, the causes of the onset of secondary atomization (puffing and microexplosion) of droplets of multicomponent liquids (including composite fuels) are established based on the concepts of wetting thermodynamics and the theory of the two-component surface energy of substances and materials. In this work, we will prove a formulated hypothesis that explains the causes of the onset of dispersion (puffing and microexplosion) of droplets under high-temperature heating that is sufficient for the stable ignition of multicomponent fuels. The hypothesis assumes that the main factor affecting the onset of dispersion (puffing or microexplosion) under high-temperature heating of multicomponent droplets is the mutual solubility/insolubility of the components. The solubility of multicomponent mixtures was estimated based on the well-known approach of \"like dissolves like\" [24]. This is based on a comparison of the dispersive and polar components of the surface tension of the components that make up the fuels. Surface tension is related to the interaction energy of molecules in the bulk of the condensed phase, the measure of which is the adhesion energy. The work of adhesion is the work that must be done under isothermal conditions to separate the molecules of the condensed phase, i.e., the rupture of the interfacial surface of the components that make up the multicomponent mixture. Weak bonds (van der Waals interactions) are characterized by a dispersive component. The polar component of surface tension characterizes strong bonds between molecules (hydrogen, ionic, covalent, dipole-dipole). Almost all known studies on puffing and microexplosion are carried out on mixtures containing water, which is the most common highly polar substance.