Evaporation of Liquid Marbles on a Heated Surface: Coating-Assisted Leidenfrost Phenomenon

The Leidenfrost effect is a well-known heat transfer phenomenon, which predicts that liquid droplets will show prolonged evaporation time when they are placed on a hot surface with a temperature higher than a critical value. This effect is due to film boiling, where a vapor film helps insulate the drop from the hot surface. In this paper, we show that specially engineered droplets — liquid marbles — can exhibit Leifenfrost effect at any temperature above the boiling point without experiencing any transition. Liquid marbles are spheres with a liquid core that are coated with hydrophobic particles. When brought into contact with a solid surface, liquid marbles are completely nonwetting due to the fact that the hydrophobic powder is in between the liquid and solid surface. Liquid marbles may be used as excellent microreservoirs for biosample handling and chemical reagent manipulation. In our study, liquid marbles are synthesized by coating water droplets with graphite particles. We investigate the thermal evaporation of the fabricated graphite liquid marbles on a hot substrate at prescribed temperatures, and compare the results with pure water droplets. The evaporation time of both liquid marbles and water droplets are recorded at various temperatures. If the temperature is above the Leidenfrost point, the evaporation of both liquid marbles and water droplets are prolonged with similar amount of time (about 100s), which indicates that similar physics might at play in both cases: heat transfer is impeded by a thin layer of vapor. If the temperature is below the Leidenfrost point, water droplets evaporate a hundred times faster. This is because the vapor film cannot self-sustain and levitate the droplet anymore. On the other hand, liquid marbles still evaporate slowly with the same level of time as Leidenfrost evaporation times, which indicates that the Leidenfrost effect still takes effect for liquid marbles even below the critical temperature. This might be due to the fact that the coating of the liquid marble helps levitate the liquid core, maintaining a layer of insulating vapor. In the end, we report detailed deformation of liquid marbles during evaporation. This coating-assisted Leidenfrost phenomenon could be useful in many applications where film boiling is desired. The strong thermal robustness of graphite liquid marbles over a wide temperature range, together with the inert reactivity, electrical conductivity and superior lubrication properties of graphite, make graphite liquid marbles potentially useful in a wealth of applications in microfluidics and lab on a chip devices.