Leidenfrost Effect-Induced Chaotic Vortex Flow for Efficient Mixing of Highly Viscous Droplets

Efficiently mixing highly viscous liquids in microfluidic systems is appealing for green chemistry such as chemical synthesis and catalysis, but it is a long-standing challenge owing to the unfavorable diffusion kinetics. In this work, a new strategy is explored for mixing viscous droplets by harnessing a peculiar Leidenfrost state, where the substrate temperature is above the boiling point of the liquid without apparent liquid evaporation. Compared to the control experiment where the droplet stays at a similar temperature but in the contact boiling regime, the mixing time can be reduced significantly. Moreover, it is demonstrated that the liquid mixing originates from the chaotic convection flow in the Leidenfrost droplet, characterized by the internal vortex motion evidenced by the microscale visualization. A correlation between mixing time and droplet volume is also proposed, showing a good agreement with experimental results. It is further shown that Leidenfrost droplets can be used to synthesize nanoparticles of the desired morphology, and it is anticipated that this simple and scalable fabrication approach will find applications in the biological, pharmaceutical, and chemical industries. High-efficiency mixing of viscous liquids in microfluidics manipulation is a long-standing challenge because of unfavorable diffusion kinetics. Here, a peculiar Leidenfrost phenomenon, which can achieve mixing of viscous droplets within several hundred ms due to internal vortex motion, is resorted to. This new mixing mechanism can be used to synthesize nanoparticles of the desired morphology. image