# Secondary Atomization of a Biodiesel Micro-Emulsion Fuel Droplet Colliding with a Heated Wall
## 1 Introduction
A fuel droplet interacted with the stainless steel surface heated to a temperature of 25-350 \({}^{\circ}\)C. The study shows that the Leidenfrost point, i.e., the boundary between nucleate and film boiling, has a weak dependence on _We_. Temporal variations of normalized droplet contact diameter are studied at various surface temperatures. The paper identifies the spreading peculiarities for high-_We_ droplets of the studied alternative fuel in the film boiling regime. In particular, the average spreading diameter was increasing. At the same time, in the cases of non-heated surface, film evaporation or nucleate boiling, it was slightly shrinking. It was established that the maximum spreading diameter at \(We>28\) increases with the surface temperature growth. [PERSON] et al. [11] relate this effect to a decrease in surface tension and viscosity at growing \(T_{0}\) when the \(T_{0}\) is lower than the \(T_{\rm b}\) of biofuel. In the case of \(T_{0}>T_{\rm b}\), i.e., the film boiling regime, it is explained by the vapor flow generated between the wall and the droplet, which forces the spreading. [PERSON] et al. [12] proposed the law of biofuel droplet spreading at high _We_, relating the spreading factor \(\beta\) and nondimensionlized time \(\tau\)--\(\beta\sim\tau^{0.5}\). A good agreement was achieved between the experimentally obtained maximum spread factor \(\beta_{max}\) of a camelina biofuel droplet and the one theoretically predicted by the Ukiwe-Kwok model. In terms of secondary atomization of isolated fuel droplets, the authors of [13; 14; 15] aimed at developing the empirical models to predict the size, velocity, and amount of secondary droplets resulting from the collision of parent drops with non-heated dry surfaces. In the case of homogeneous single-component fuel droplets, the studies are quite detailed on the wall impact regimes, mechanisms, and disintegration criteria accounting for the heat exchange mode [16], unlike multi-component droplets containing a dispersed phase of another liquid, i.e., emulsion [17; 18], micro-emulsion [19; 20; 21; 22; 23], or nano-emulsion [24]. Moreover, the studied processes become even more complicated because the saturation temperatures of the liquids in emulsions, micro-emulsions, and nano-emulsions may differ drastically. This results in some unique aspects of droplet interaction with the heated wall (even those described in [25; 26]) that should be studied in detail for each high-potential two- or multicomponent fuel composition, just like it was done for urea water solution droplets [27]. Therefore, here we aim to study the interaction of multicomponent emulsion droplets with a heated surface with an emphasis on the secondary atomization as the most interesting regime for practical application. We are going to take into account the specific aspects outlined in this section concerning the outcomes of interaction between the droplets of conventional hydrocarbon fuels or alternative fuels, and their properties (surface tension, viscosity, and density). The purpose of this study is to research the hydrodynamic regimes of interaction between biodiesel micro-emulsion fuel droplets and a heated wall, as well as to evaluate the integral characteristics of atomization by analyzing the number and size of the resulting child droplets.