Levitation and collision of droplets

Credits: F. Pacheco-Vázquez, R. Ledesma-Alonso, J. L. Palacio-Rangel, F. Moreau, https: //

If you’ve seen water droplets dance or sway in a hot pot or iron plate, you’ve probably seen the Leidenfrost effect in action.Or you may have seen “MythBusters” episode Adam and Jamie stabbed their wet fingers and hands into the melted lead and pulled it out unharmed.

There is a film of water around the finger because the effect depends on the wetness of the finger. With molten lead, the water film boils to produce steam, which has insufficient heat conduction.That gas, it’s water vaporAbove 328 degrees Celsius (622 degrees Fahrenheit), it insulates the finger long enough to protect it for a short time when immersed in molten lead.

Similarly, the water droplets on the hot plate evaporate at its lower end, creating an insulating cushion that keeps the water droplets floating as a liquid for an amazing amount of time. It was first described in 1751 by the German doctor Johann Gottrov Leidenfrost.

Currently, a group of Mexican and French scientists have announced their experimental results for the first time. This shows that two hot drops of different liquids can also bounce off each other due to the Leidenfrost effect between them. The group calls this the triple Leidenfrost effect. This is because both drops are already on the hotplate and they have their own Leidenfrost effect on the plate, and when they collide with each other and bounce off, they have an additional Leidenfrost effect, which is one-third. To do. vapor Cushion of collision interface between droplets.

In the experiment, the top surface of the hot aluminum plate was slightly recessed, holding the droplets toward the center of the plate.for Water drops At a volume of 0.5 ml (0.5 cc), the droplets were in Leidenfrost at a plate temperature of 210 degrees Celsius. At that point, the droplets lasted about 450 seconds (7.5 minutes) due to the large latent heat of the water (the amount of heat required to change the water from a liquid to a gas at a constant temperature). After that, the droplets completely evaporated and disappeared, turning into water vapor.

Other liquids differed in Leidenfrost temperature and duration. Ethanol droplets were in Leidenfrost at about 150 degrees Celsius and lasted for about 200 seconds, while chloroform lasted for 100 seconds at about 150 degrees Celsius. The survey was conducted in Puebla, Mexico, about 2,200 meters above sea level (7,218 feet, 1.37 miles). For example, the boiling point of water was 93 degrees Celsius (199 degrees Fahrenheit).other Thermodynamic properties There may be similar adjustments.

Small blue ethanol droplets on a hot aluminum plate repeatedly bounced off large, clear water droplets, simultaneously exhibiting three different Leidenfrost effects. Eventually, the blue droplets will shrink in size, become spherical, expel their vapor layer, and coalesce. Credits: F. Pacheco-Vázquez, R. Ledesma-Alonso, J. L. Palacio-Rangel, F. Moreau, https: //

Researchers have determined the Leidenfrost temperature of 11 low-viscosity liquids, each with a different boiling temperature, and then deposited two droplets of different materials on a hot aluminum plate at 250 degrees Celsius (482 degrees Fahrenheit). I did. Each droplet has a unique Leidenfrost effect that has a vapor layer beneath it and rises as it descends towards the center of the plate. Near it, the floating droplets collide.

At that moment, two things happened. The droplets either coalesced or bounced off each other.

If the liquid is the same substance, such as water-water, or has similar properties, such as ethanol-isopropanol, the coalescence occurred in milliseconds.

In a more interesting case, the droplets bounced off each other.This is a liquid with different droplets, such as water-ethanol or water-Acetonitrile. Each emerged from its own Leidenfrost effect. However, the vapor cushion also surrounded each droplet on its side, so when the droplets collided, there was a vapor cushion there, which prevented the droplets from merging. In fact, the pressure of the vapor layer between the droplets was enhanced by the Leidenfrost layer of both droplets, so the repulsion velocity of the droplet could be higher than its collision velocity. This same vapor layer prevented the first coalescence.

The small droplets repeatedly bounced off the large droplets over a few seconds, sometimes minutes (see video above). Eventually, the small droplets changed from pancake shape to spherical when their vapor layer was expelled during the collision time and finally the droplets coalesced. Shooting the process at high speed revealed that the diameter of the small droplets decreased linearly over time before coalescing.

Only two parameters determine the conditions for direct coalescence or bounce: the difference in surface tension between liquids (surface tension is a unique property of liquids, measured by force per unit length) or the difference in boiling temperature. is. If the boiling point difference is large, small droplets, such as glycol-chloroform, can explode violently.

Other dynamics based on the Leidenfrost effect, such as self-propelled droplets, sustained rotation, vibration, and explosion of droplets, have been investigated in recent years, suggesting the possibility of manipulating the Leidenfrost effect. splash For engineering and microfluidics applications. This current task of understanding how Leidenfrost droplets of different liquids interact adds another dimension to potential applications.

“Triple Leidenfrost effect” seen in different drops of hot pot

For more information:
F. Pacheco-Vázquezetal, Triple Leidenfrost effect: Prevention of droplet coalescence on hot plate, Physical review letter (2021). DOI: 10.1103 / PhysRevLett.127.204501

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