If you’ve seen water drops dance and jitter on a hot pan or griddle, you’ve seen the Leidenfrost effect in action. Or you may have seen the “Mythbusters” episode where Adam and Jamie thrust their wet fingers and hands into molten lead and pulled them out unharmed.
The effect relies on the finger being wet, so it has a film of water on and around it. The water film becomes steam when molten lead boils. This is a poor heat conductor. That gas, which is water vapor, insulates the finger long enough to protect it for a short period of time when dipped into the molten lead, at 328 degrees Celsius (622 degrees Fahrenheit) or higher.
Likewise, a water drop on a hot plate evaporates at its bottom edge, creating a insulating cushion that keeps the drop levitating as a liquid for a surprising amount of time. It was first described by German doctor Johann Gottlob Leidenfrost in 1751.
Now, scientists from France and Mexico have published for the first-time the results of experiments that showed two hot liquid drops can bounce off each other due to the Leidenfrost effects between them. The group calls this a triple Leidenfrost effect, since both drops are already on a hot plate experiencing their own Leidenfrost effect with respect to the plate, and an additional Leidenfrost effect when they collide with and bounce off one another, developing a third vapor cushion at the collision interface between the drops.
The hot aluminum plate was slightly concave on the top to help keep droplets in the center. For water droplets 0.5 ml in volume (0.5 cc), the droplets entered a Leidenfrost state at a plate temperature of 210 degrees Celsius. At that point, the droplet lasted about 450 seconds (7.5 minutes) due to water’s large latent heat (the amount of heat required to change water from a liquid to a gas at constant temperature). The droplet was then completely evaporated and turned into water vapor.
Other liquids had different Leidenfrost temperatures and duration times: Ethanol droplets entered the Leidenfrost state at about 150 degrees Celsius and lasted about 200 seconds, and chloroform at about 150 degrees Celsius for 100 seconds. The research was conducted in Puebla, Mexico, at about 2,200 meters (7,218 feet, 1. 37 miles) above the sea level, where, for example, the boiling point of water was only 93 degrees Celsius (199 degrees Fahrenheit). Other thermodynamic properties might have similar adjustments.
After the researchers determined the Leidenfrost temperatures for 11 low viscosity liquids, each with different boiling temperatures, they deposited two droplets of different materials on the hot aluminum plate with a temperature of 250 degrees Celsius (482 degrees Fahrenheit). Each droplet experienced its own Leidenfrost effect, with a vapor layer beneath it. This allowed it to levitate as it moved towards the center of the plate. The levitating droplets would collide near this point.
In that instant, one thing happened: The droplets either collided or bounced off each other.
Coalescence happened in milliseconds if the liquids were of the same substance, such as water-water, or if they had similar properties, for example, ethanol-isopropanol.
In some cases, droplets bounce off each other. This happened when the droplets were of different liquids, for example, water-ethanol or water-acetonitrile. Each droplet levitated from its own Leidenfrost effect. Each droplet was protected by a vapor cushion on its sides. This prevented droplets from colliding. The rebounding velocity of a droplet can sometimes exceed its impacting velocity because of the pressure difference between the droplets. This is due to the Leidenfrost layer in both droplets. This vapor layer was what stopped initial coalescence.
The smaller droplets bounced off the larger droplet repeatedly over several seconds or minutes (see video below). The smaller droplet eventually changed from a flat pancake to a spherical form, as its vapor layer was removed during collision time. Finally, the drops coalesced. The process was filmed at high speed and revealed that the droplet’s diameter decreased linearly with the time it took to coalesce.
Only 2 parameters could determine the conditions for direct cohesion or bouncing. These were the surface tensions of the liquids (surface tension refers to an inherent property in a liquid and is measured in force per unit length). If the boiling points were large, smaller droplets could explode explosively, such as in glycol-chloroform.
Other dynamics based on the Leidenfrost effect have been explored in recent years, such as self-propulsion of droplets, sustained rotations, oscillations and exploding droplets, suggesting the possibility of manipulating the Leidenfrost effect on droplets for applications in engineering and microfluidics. The current research into the interaction of Leidenfrost droplets of different liquids adds another dimension of potential applications.
F. Pacheco-Vazquez et al, Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.127. 204501
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Levitating and colliding liquid drops (2022, January 14)
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