Drops of water found to spring from oscillating surface faster than the surface moves

September 11, 2017 by Bob Yirka report
Credit: Physical Review Letters (2017). DOI: 10.1103/PhysRevLett.119.108001

(Phys.org)—A team of researchers with the University of Côte d'Azur in France has found that drops ejected by an oscillating surface can at times travel faster than the surface that ejected them. In their paper published in the journal Physical Review Letters, the team describes experiments they conducted by flinging water from a superhydrophobic surface and what they found.

If you place water on a bendable piece of vibrating plastic, the water droplets will be flung off into the air as the undulates. In this new effort, the found that in some specific cases, some of those can actually travel faster into the air then the plastic base that pushed them. The researchers made this observation as they placed onto a thin piece of plastic made of fluorinated polymers, which they note is similar to Teflon. The team then attached a device that vibrated the plastic at frequencies between 20 and 70Hz. As the oscillator was turned on, the researchers timed the speeds of the drops as they were flung into the air.

The group reports that the highest attained by the drops occurred at the halfway point to its peak, after which it slowed then fell back to the surface. But they also found that some of the droplets left the surface at roughly 1.6 times the speed of the rising surface.

To better understand what was occurring and why, the researchers took a closer look at the drop as it was being pushed off the surface. They found that it was squished slightly, like a tennis ball being hit by a racket. And like a tennis ball, the drop rebounded as it was being pushed off the surface. That rebound added to the release speed. The team describes the effect as superpropulsion. They found that the increase in speed of the drop was dependent on the size of the drop vibration compared to the oscillation frequency—the biggest gains came when the vibration frequency was approximately three times that of the surface's frequency. The researchers also compared the effect to the added lift a person gets on a trampoline when pushing at just the right moment.

Explore further: Fungal spores harness physics to launch themselves

More information: Christophe Raufaste et al. Superpropulsion of Droplets and Soft Elastic Solids, Physical Review Letters (2017). DOI: 10.1103/PhysRevLett.119.108001

ABSTRACT
We investigate the behavior of droplets and soft elastic objects propelled with a catapult. Experiments show that the ejection velocity depends on both the projectile deformation and the catapult acceleration dynamics. With a subtle matching given by a peculiar value of the projectile/catapult frequency ratio, a 250% kinetic energy gain is obtained as compared to the propulsion of a rigid projectile with the same engine. This superpropulsion has strong potentialities: actuation of droplets, sorting of objects according to their elastic properties, and energy saving for propulsion engines.

Related Stories

Water is surprisingly ordered on the nanoscale

May 24, 2017

Researchers from AMOLF and Swiss EPFL have shown that the surface of minuscule water drops surrounded by a hydrophobic substance such as oil is surprisingly ordered. At room temperature, the surface water molecules of these ...

High speed camera study shows why boiling drops take off

July 26, 2012

(Phys.org) -- Everyone knows what happens if you drop water onto a hot pan, it separates into flat bottomed bubbles that appear to float above the bottom of the pan then move around of their own accord until disappearing. ...

Recommended for you

Carefully crafted light pulses control neuron activity

November 17, 2017

Specially tailored, ultrafast pulses of light can trigger neurons to fire and could one day help patients with light-sensitive circadian or mood problems, according to a new study in mice at the University of Illinois.

Strain-free epitaxy of germanium film on mica

November 17, 2017

Germanium, an elemental semiconductor, was the material of choice in the early history of electronic devices, before it was largely replaced by silicon. But due to its high charge carrier mobility—higher than silicon by ...

New imaging technique peers inside living cells

November 16, 2017

To undergo high-resolution imaging, cells often must be sliced and diced, dehydrated, painted with toxic stains, or embedded in resin. For cells, the result is certain death.

6 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Dark_Solar
not rated yet Sep 11, 2017
Ok, so what I'm getting from this article is that the water droplet is ejected from the oscillating surface at a greater speed(magnitude) than is provided by the wavelength of the oscillating surface; is this qualitatively different from the results of dropping a tennis ball/basketball stack? I mean, the article kind of put it's figurative finger on it when it made the tennis ball comparison. \: l
Bongstar420
5 / 5 (1) Sep 11, 2017
Ok, so what I'm getting from this article is that the water droplet is ejected from the oscillating surface at a greater speed(magnitude) than is provided by the wavelength of the oscillating surface; is this qualitatively different from the results of dropping a tennis ball/basketball stack? I mean, the article kind of put it's figurative finger on it when it made the tennis ball comparison. \: l

The droplets are vibrated at a different frequency then the vibrated surface apparently. In the case of the ball, the deformation is the result of forward momentum in the ball transferring to its circumference where the droplets are vibrated on a vibrated surface and are not hitting the surface with forward momentum.

If you vibrated your ball on a vibrating surface, then it would be qualitatively similar.

This phenomena is kind of interesting, but would be more interesting if the kinetic rate was greater than the system input energy.
baudrunner
1 / 5 (1) Sep 11, 2017
The macro scale version of quantum scale Vavilov–Cherenkov radiation.
Jitro
not rated yet Sep 11, 2017
Cherenkov radiation is analogy of Mach shock wave/cone.
Dark_Solar
not rated yet Sep 17, 2017
The droplets are vibrated at a different frequency then the vibrated surface apparently. In the case of the ball, the deformation is the result of forward momentum in the ball transferring to its circumference where the droplets are vibrated on a vibrated surface and are not hitting the surface with forward momentum.

If you vibrated your ball on a vibrating surface, then it would be qualitatively similar.

This phenomena is kind of interesting, but would be more interesting if the kinetic rate was greater than the system input energy.


The thing that makes me point toward the stacked tennis/basketball model is the article's image series indicating the surface is oscillating (undulating) vertically; this seems like it's just a rehashing of a relatively well-understood mechanical system...which brings to mind questions of whether there is 1000fps footage of the tennis/basketball stack drop demonstration. Perhaps the tennis ball behaves similarly to the drop of water?
Dark_Solar
not rated yet Sep 17, 2017
-cont.-

Given that the surface is undulant (according to the photo-plate), it follows that the waveform propagating across the material creates the "basketball" (upward motion of mass as it conforms with the wave) causing the ejection of the "tennis ball" at X velocity relative to the energy imparted by the mass acting under the influence of the wave. I'm quite curious to see if there's a correlation between the behaviors of both systems.

On a sort of side-note, the article indicates the water droplet actually increases in speed (attributed to internal elasticity) after leaving the surface which seems to be a contradiction to standard ballistic calculations --i.e. the projectile leaves the propulsion source and that's the end of it, it's all dissipation of velocity thereafter. Definitely a puzzler....

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.