Spontaneous Cyclic Levitation of Glycerol-Water Droplets at Low Pressures.

At low pressures, water often experiences chaotic multiphase transitions that complicate fluid management. Here, we demonstrate that a minimal addition of glycerol to a water droplet can transform these disruptive behaviors into highly regulated dynamics. Instead of typical breakup or rapid self-propulsion, freezing glycerol-water droplets enter a state of gentle, cyclic levitation, repeatedly undergoing "dwelling, liftoff, flight, and impact." We reveal that this phenomenon arises from the dual roles of glycerol. Freeze-concentration preserves interconnected liquid regions and prevents the formation of a closed outer ice shell, thereby limiting the accumulation of expansion-induced stress. In parallel, glycerol lowers the equilibrium freezing temperature, reducing the asymmetric vaporization recoil that drives self-propulsion during recalescence. The suppression of these instabilities enables a cyclic droplet motion driven by self-sustaining thermal oscillation, where basal heating enhances overpressure for liftoff and in-flight vaporization cooling resets the cycle. Our findings unveil a distinct regime of vaporization-driven droplet dynamics that is tunable through composition, offering new strategies for fluid management under extreme conditions, with implications ranging from thermal management to space exploration.