The efficient internal mixing of colliding non-Newtonian droplets upon coalescence is critical to various technological processes, specifically involving the initiation of the liquid-phase reaction of gelled hypergolic propellants, which are promising fuels for next-generation rocket engines. However, most previous studies on droplet collision used Newtonian fluids, and the non-Newtonian fluids that can be highly nonlinear and even trend reversing are much less understood to date. Motzigemba et al. [1] experimentally found that the deformation of colliding droplets of shear-thinning fluids is substantially larger than that of the Newtonian fluid. In a previous work [2], we numerically studied the initially stationary equal-sized droplet coalescence between a Newtonian and non-Newtonian droplet. Because of the reduced local viscosity and thereby smaller viscous dissipation for shear-thinning fluids, the flow in the non-Newtonian droplet is faster than that in the Newtonian droplet, resulting in unsymmetrical, albeit small, mixing induced by the shear-thinning effect. The above findings are encouraging since the droplet internal motion is driven solely by the surface tension of the initially stationary droplets regardless of the impact inertia. However, as the published references of Newtonian fluid characteristics, internal mixing of non-Newtonian fluid definitely can be substantially augmented because of the correspondingly substantial internal motion generated through the impact inertia. Thus, in terms of the equal-sized head-on colliding droplets, efficient mixing must require breaking the collision symmetry by varying the impact inertia and the rheological properties as well.

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