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What Are the Physics of Droplets?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

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Droplets would seem to many to be the least exciting topic for a physics article. Yet, as a frequent investigator of physics will tell you, it is those topics that can offer the most fascinating results. Hopefully, by the end of this article you too shall feel that way and maybe look at the rain a little different.

Leidenfrost Secrets

Liquids that come into contact with a hot surface sizzle and seem to hover above it, moving in a seemingly chaotic nature. This phenomena, known as the Leidenfrost effect, was eventually shown to be a result of a thin layer of the liquid evaporating and creating a cushion that permits the droplet movement. Conventional thought had the actual path of the droplet dictated by the surface it was moving on but scientists were surprised to find that the droplets instead are self-propelled! Cameras above and to the side of the surface were used over many trials and various surfaces to record the paths droplets took. The research showed that large droplets did tend to go to the same location but mainly because of gravity and not because of the surface details. Smaller droplets, however, had no common path they took and instead followed any path, regardless of the gravitational center of the plate. Internal mechanisms within the droplet must be overcoming gravitational effects, therefore, but how?

That is where the side view caught something interesting: the droplets were spinning! In fact, whatever direction the droplet spun in was the direction the droplet took off in, with a slight off-center tilt toward that direction. The asymmetry allows for the necessary acceleration required with the spin for the droplet to control its destiny, rolling like a wheel around the pan (Lee).

But where does the sound of sizzling come from? Using that high speed camera set up from before along with an array of microphones, scientists were able to find that size was a big role in determining the sound. For small droplets, they simple evaporated too quickly, but for larger ones they move about and partially evaporate. Larger droplets will have a larger amount of contaminants in it, and the evaporation only removes the liquid from the mix. As the droplet evaporates, the concentration of impurities grows until the surface has a high enough level of them to form a shell of sorts that interferes with the evaporation process. Without that, the droplet cannot move because it is denied its vapor cushion with the pan and so the droplet falls, exploding and releasing an accompanying sound (Ouellette).

Flying Droplets

Rain is the most common droplet experience we encounter outside of the shower. Yet when it hits a surface, it will either spread out or will seemingly explode, flying back into the air as much smaller droplet pieces. What is truly going on here? Turns out, it’s all about its surrounding medium, the air. This was revealed when Sidney Nagel (University of Chicago) and team studied droplets in a vacuum and found they never splashed – ever. In a separate study done by the French National Centre for Scientific Research, eight different liquids were dropped onto a glass plate and studied under high speed cameras. They revealed that as a droplet makes contact, momentum pushes the liquid outward. But surface tension wants to keep the droplet intact. If moving slow enough and with the right density, the droplet holds together and just spreads out. But if moving fast enough, a layer of air will be trapped underneath the leading edge and actually generate lift just like a flying machine. It will cause the droplet to lose cohesion and literally fly apart! (Waldron)

Pulled Apart Into Orbit

Placing a droplet into an electrical field does…what? It seems like a difficult proposition to contemplate because it is, with scientists as far back as the 16th century wondering what happens. Most scientists came to the consensus that the droplet would be warped in shape or gain some spin. It turns out to be way cooler than that, with the “electrically conductive” droplet having microdrops bead off from it and form rings that look very much like planetary ones. It is partially because of a phenomena known as “electrohyrdodynamic tip streaming,” in which the charged droplet seems to deform into a funnel, with the top pushing down on the bottom until a breakthrough releases microdrops. This, however, will only occur when the droplet exists in a fluid of lower conductance.

What if the reversal was true and the droplet was the lower one? Well, the droplet spins and the tip streaming instead occurs along the direction of rotation, releasing the drops that then fell into an orbit of sorts around the main droplet. The microdrops themselves are fairly consistent in sizing (in the micrometer range), are electrically neutral, and can have their size tailored based on the viscosity of the droplet (Lucy).

Works Cited

  • Lee, Chris. “Free-wheeling water droplets plot their own path off a hot plate.” Arstechnica.com. Conte Nast., 14 Sept. 2018. Web. 08 Nov. 2019.
  • Lucy, Michael. “Like little rings of Saturn: How electricity pulls a drop of liquid apart.” Cosmosmagazine.com. Cosmos. Web. 11 Nov. 2019.
  • Ouellette, Jennifer. “Study finds ultimate fate of Leidenfrost droplets depends on their size.” Arstechnica.com. Conte Nast., 12 May 2019. Web. 12 Nov. 2019.
  • Waldron, Patricia. “Splashing Droplets Can Take Off Like Airplanes.” Insidescience.org. AIP, 28 Jul. 2014. Web. 11 Nov. 2019.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Leonard Kelley

Comments

Leonard Kelley (author) on December 07, 2020:

It is amazing how nature can scale such similar structures, even if the mechanics are different.

Doug West from Missouri on December 05, 2020:

Good article. I particularly like the pics that look like Saturn. That is amazing.

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