Surface engineered nanoparticles dispersed in kerosene: The effect of oleophobicity on droplet combustion

Liquid propellants benefit from solid particle additives that can optimize combustion performance by promoting phase change heat transfer. In this study, aluminum (Al) nanoparticles with and without self-assembled monolayer surface functionalization were combined with kerosene to examine the changes in droplet regression behavior associated with manipulating particle surface chemistry. Aluminum nanoparticles were coated using a long-chain perfluorinated carboxylic acid as the surface binding moiety to induce an oleophobic surface. The resulting particles are thus comprised of self-assembled monolayers of perfluorohexadecanoic acid (PFHD) (C15F31COOH) around the alumina (Al2O3) shell encapsulating the Al core. The PFHD serves many functions including altering particle wettability and acting as a surfactant that facilitates a stabilized dispersion of particles in kerosene. The Al-PFHD particles are more oleophobic compared with the more oleophilic surface associated with the amorphous alumina shell on aluminum particles. Mixtures with Al-PFHD exhibit a two stage burning behavior with average initial burn rate constant increased by about 121% when compared to pure kerosene. Further analysis using a thermogravimetric analyzer (TGA) showed that Al-PFHD particles in kerosene exhibit evaporation at reduced temperatures that may enhance energy transport during droplet combustion by phase change and convection. These results provide new insight bridging the gap between materials and heat transfer toward achieving the goal of designing particle additives that improve liquid propellant combustion.