Simultaneous Crystallization Effects in Multiple Levitated Plasma-Functionalized Graphene Nanoflake Nanofluid Droplets

Acoustic levitation is gaining traction as a container-free method for examining crystallization effects, though liquid-to-solid phase change has seen little investigation for levitated nanofluids. Recent developments allow evaluations of the morphological and thermal evolution of multiple levitated nanofluid droplets freezing simultaneously. The fundamental effect of adding nanoparticles to a levitated crystallization system is crystal growth rate enhancement from improved mass transfer at the solid front. While nucleation times are unaffected as freezing is initiated by secondary ice nucleation particles (INPs), enhancement produces higher instantaneous nucleation pressures and more cracking in the primary ice shell. In turn, more INPs are ejected, resulting in faster protrusion formation on the droplet surface (hastened further in systems containing adjacent droplets). The crystal matrix also includes more defects, resulting in liquid bead formation for up to 67% of droplets. During crystal decomposition, thermal gradients create convective currents (Ma values in the order of 103), though these were dampened by the same transport phenomena that enhance crystal growth. Suspension loss after a crystallization-decomposition cycle reduced opacity and light absorbance: the droplets were 62% closer in appearance to water. However, the nonisobaric cooling process resulted in smaller particle clusters compared to droplets frozen on a solid surface.