Levitating and colliding liquid drops

by David Appell

F. Pacheco-Vzquez, R. Ledesma-Alonso, J. L. Palacio-Rangel, and F.

If you've seen water drops dance on a hot pan or griddle, you've seen the Leidenfrost effect in action. Adam and Jamie pulled their wet fingers and hands out of molten lead in the "Mythbusters" episode.

The effect relies on the finger being wet, so it has a film of water around it. A poor conductor of heat is created when molten lead creates steam. When dipped into the molten lead, the water vapor insulates the finger for a short time so that it doesn't get damaged.

A water drop on a hot plate can evaporate at its bottom edge, creating a cushion that keeps it from levitating as a liquid for a long time. It was first described by a German doctor.

A group of scientists from Mexico and France have published the results of experiments showing that two hot drops of different liquids can bounce off one another. The group calls it a triple Leidenfrost effect, since both drops are already on a hot plate, and there is an additional Leidenfrost effect when they collide with and bounce off one another.

In the experiments, the hot aluminum plate had a slightly concave top surface to keep the droplets from hitting the center of the plate. The droplets entered a state of Leidenfrost at a plate temperature of over 200 degrees Celsius. The droplets lasted about 450 seconds due to the large amount of heat required to change water from a liquid to a gas at a constant temperature. After that, the droplet evaporated and was no longer there.

The duration times of other liquids were different, with chloroform lasting about 100 seconds and the Ethanol droplets lasting about 200 seconds. The boiling point of water in Mexico was only 93 degrees Celsius, which is 199 degrees Fahrenheit, where the research was conducted. Similar adjustments might be made to other thermodynamic properties.

A small blue droplet of alcohol on a hot aluminum plate bounced off a larger, clear droplet of water, displaying three different Leidenfrost effects at the same time. The blue droplets become spherical and can become evacuated when they coalesce. F. Pacheco-Vzquez, R. Ledesma-Alonso, J. L. Palacio-Rangel, and F.

The researchers deposited two droplets of different materials on a hot aluminum plate after they determined the Leidenfrost temperatures for 11 low viscosity liquids. The droplets were levitating as they made their way down toward the center of the plate. The levitating droplets would collide.

The droplets either coalesced or bounced off one another.

If the liquids were of the same substance, or if they had similar properties, coalescence would happen in a second.

The droplets bounced off each other. This happened when the droplets were of different liquids. Each levitated from its own effect. There was a vapor cushion on each droplet that prevented a merger of the droplets. The pressure in the vapor layer between the droplets was enhanced by both droplets, so the rebound velocity of a droplet could sometimes be larger than its impact velocity. Initial coalescence was prevented by the same vapor layer.

The smaller droplets bounced off of the larger droplets. The smaller droplet changed from a pancake shape to a spherical shape when the vapor layer was evacuated during the collision time. The diameter of the smaller droplets decreased linearly with time before coalescing.

The conditions for direct coalescence or bouncing were determined by the difference in surface tensions between the liquids and the difference in boiling temperatures. The smaller droplets could explode if the difference in boiling points was large.

The possibility of manipulating the Leidenfrost effect on droplets for applications in engineering and microfluidics has been explored in recent years. The current work of understanding how leefrost drops interact with other liquids adds another aspect to potential applications.

The Triplefrost Effect: preventing coalescence of drops on a hot plate is a paper by F. Pacheco-Vzquez. There's a DOI for "PhysRevLett. 127. 204501."

Journal information: Physical Review Letters.

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The news aboutitating and colliding liquid drops was retrieved fromphys.org on January 14, 2022.

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