A new type of Leidenfrost effect that causes droplets of different liquids to bounce off each other when placed on a hot surface has been discovered by researchers in Mexico and France. Led by Felipe Pacheco-Vázquez at the Autonomous University of Puebla, the team found that this behaviour stems from the three simultaneous points of contact between the two droplets and the surface – creating what the team has called a triple Leidenfrost effect.
When a liquid droplet is placed on a sufficiently hot surface, its evaporation creates a cushion of vapour that separates it from the surface – causing the droplet to levitate. Known as the Leidenfrost effect, this levitation both hinders the evaporation of the droplet and allows it to glide across the surface with little friction. Although this effect has been known for centuries, physicists are still making surprising discoveries about how levitating droplets behave in certain scenarios.
In their study, Pacheco-Vázquez’s team looked at what happened when a series of droplets are placed on a hot, slightly concave surface, where the droplets naturally gravitate towards the lowest point in the centre. When water droplets were used, the researchers saw that they coalesced almost immediately on contact with each other: quickly merging into a single, large droplet. But when the water droplets and ethanol droplets were used, the team found that different types of droplets bounced off each other several times, before eventually coalescing.
Three vapour layers
To explain this behaviour, the researchers considered the three separate vapour layers involved in the process. These layers are found beneath each droplet, and between the two droplets as they make contact. This third layer forms due to the difference in boiling temperatures between the two droplets: causing the hotter droplet (water in this case) to act as a surface on which ethanol, with its lower boiling point, will evaporate. This generates a second layer of ethanol vapour which prevents the two liquids from coalescing – and the droplets will bounce away from each other.
Eventually, the faster-evaporating ethanol droplet reaches a size similar to the capillary length, which is related to the density difference between the two liquids, and the surface tension at their interface. At this point, the ethanol can no longer produce enough vapour to withstand its attraction to water, and the liquids finally merge together.
Fizzy water droplets levitate at room temperature
The team repeated this experiment using 10 different types of liquid. As predicted by their new theory, they observed the same bouncing behaviour for droplets with widely differing boiling points. In cases where boiling points were more closely matched, the droplets coalesced without any bouncing.
In reference to the three vapour layers involved in the process, the researchers have dubbed it the triple Leidenfrost effect. They now hope that their theories could open up new applications in microfluidics: including a better understanding of how fuel droplets interact with each other in overheated engines.
The research is described in Physical Review Letters.