When water droplets sit on a cold substrate, they freeze into a shape with a pointed tip. At first glance, this behavior seems very odd since surface tension usually acts to prevent such sharp protrusions. The shape is, however, a result of water’s expansion as it freezes. The droplet freezes from the substrate upward, with a concave shape to the solidification front. The angle of the point does not depend on the substrate temperature or the wetting angle between the water and surface. Instead, it turns out that this concave front shape and water’s expansion are the key factors that determine the pointed cusp’s angle, and that the final geometry of the cusp is essentially universal. (Video credit: M. Nauenberg; additional research credit: A. Marin et al.)
Tag: water droplets
Vibrating Droplets
When still, water drops sitting on a surface are roughly hemispherical, drawn into that shape by surface tension. But on a vibrating surface, the same water drop displays many different shapes, like those in the video above. Researchers have observed more than 30 different mode shapes by varying the driving frequency. The metal mesh placed beneath the glass on which the drops sit helps the researchers determine the drop’s shape. As the drop deforms, the mesh appears to distort due to the refraction of light through the changing shape of the drop’s water-air interface. The distortion allows observers to visualize (and in some experiments even reconstruct) the shape of the drop’s surface. Understanding this kind of droplet behavior is valuable for many applications, including ink-jet printing and microfluidic devices. (Video credit: C. Chang et al.; via Science)
Making Metal Water-Repellent
Chemical treatments can be used to render metals hydrophobic, causing water to bead on the surface rather than spreading to wet it. Treating the surface by immersing it in boiling water before applying the chemicals creates a nanoscale texture that accentuates the hydrophobicity. Even on a common metal like aluminum, this combination of texturing and chemical treatment leads to superhydrophobic behavior. Here the technique is demonstrated by spraying water droplets on a piece of treated aluminum. (Video credit: B. Rosenberg et al.; submitted by D. Quinn)
Frozen Powder Drops
Droplet impacts on granular surfaces and water interactions with superhydrophobic surfaces are not unfamiliar topics for FYFD. But this behavior of water droplets falling on a superhydrophobic powder is unusual, to say the least. When the droplets impact in powder, they rebound with a partial coating of powder. In the case of the superhydrophobic powder, the shape of the droplet is “frozen” by the powder. A satellite droplet is ejected from the region not coated in powder and the resultant main drop falls back to the surface and comes to rest with little to no deformation. The researchers report a critical velocity at which the behavior is observed. (Video credit: J. Marston et al.; via Physics Buzz)
Bouncing Off
A water droplet falling onto a superhydrophobic surface will rebound and bounce without wetting the surface. Capillary and internal waves reflect in the drop until it comes to rest at a high contact angle, formed at the boundary where the liquid, solid, and air meet. Such surfaces can have interesting interactions with water, as when two droplets coalesce on a surface and then begin bouncing or when superhydrophobic objects are dropped into a bath. (Video credit: Gangopadhyay Group, University of Missouri)
Breaking Water with Sound
Previously we saw how vibration could atomize a water droplet, breaking it into a spray of finer droplets. Here astronaut Don Pettit shows us what the process looks like in microgravity using some speakers and large water droplets. At low frequencies the water displays large wavelength capillary waves and vertical vibrations. Higher frequencies–like the earthbound experiment on much smaller droplets–cause fine droplets to eject from the main drop when surface tension can no longer overcome their kinetic energy. (submitted by aggieastronaut, jshoer and Jason C)
(Source: /)Jumping Water Droplets
Superhydrophobic surfaces resist wetting from water, but it turns out they can also trigger interesting behaviors in the tiny droplets condensing on the surface. High-speed video reveals that when two condensate droplets coalesce, the energy released by surface tension causes the new droplet to jump off the surface. The phenomenon is the same as one observed in some types of mushroom–when a condensate droplet touches a wetted spore, the spore is ejected from the mushroom. (Video credit: J. Boreyko)
Liquid Nitrogen and the Leidenfrost Effect
One of the tried and true cooking tips my mother gave me when I was younger was to test the temperature of my griddle before making pancakes by splashing a few drops of water on it. If it was hot enough that the water skittered across the surface before evaporating, then it was ready. Aside from being a way to make great pancakes, this tip demonstrates an everyday application of the Leidenfrost effect. When the surface of the pan is significantly higher than the boiling point of the water, the part of the water drop that hits the pan is vaporized, creating a thin layer of water vapor on which the rest of the droplet rests. The vapor serves as an insulator, protecting the rest of the water drop from the heat of the pan, as well as a lubricant, allowing the drop slip and slide easily across the surface. The same effect lets the brave plunge a hand into liquid nitrogen without damage, but they have to be quick, otherwise their hand will cool to the point that the liquid nitrogen contacts it without a protective layer of nitrogen. (In that case, a nasty case of frostbite may be the least of one’s worries. We do NOT recommend trying this one at home.)
Playing Pac-Man with Water Droplets
The vibrations of a plate in the horizontal and vertical directions can be used to control the motion of a drop placed on the surface. Here a droplet of water on a superhydrophobic surface is controlled by joystick a la Pacman. For more, see papers here and here.
Vibration-Induced Atomization
Atomization–breaking a liquid into a fine spay of droplets–is common in engines, printers, and in the shower. Here a droplet of water is placed on a thin metal diaphragm that is vibrated at 1 kHz with increasing vibrational amplitude. Capillary waves form on the droplet, and once a critical vibrational amplitude is achieved, tiny droplets are ejected. Full atomization of the original droplet is achieved in about 0.3 seconds real-time. #