Every child learns about the water cycle in school, but an academic description of the process often lacks nature’s grandeur. In “Monsoon II” photographer Mike Olbinski captures the majesty of cloud formation and rainfall in a way that rekindles awe for the scale of the process. It begins with bright clouds popping up, the result of warm moist air rising from the ground and cooling at altitude. As more water vapor evaporates, rises, and condenses, water droplets collide in these clouds, coalescing and growing until they grow too large and heavy to stay aloft. These are the droplets that fall in sheets of rain, blurring the air beneath them. There’s an incredible beauty to watching rain fall from a distance; it looks calm and localized in a way that’s utterly at odds with the experience from inside the storm. (Video credit: M. Olbinski; submitted by jshoer)
Search results for: “water droplet”
Healing Soap Films
As fragile as a soap bubble seems, these films have remarkable powers of self-healing. The animation above shows a falling water droplet passing through a soap film without bursting it. An important factor here is that the water droplet is wet–passing a dry object through a soap film is a quick way to burst it, as those who have played with bubbles know. The droplet’s inertia deforms the soap film, creating a cavity. If the drop’s momentum were smaller, the film could actually bounce the droplet back like a trampoline, but here the droplet wins out. The film breaks enough to let the drop through, but its cavity quickly pinches off and the film heals thanks to the stabilizing effect of its soapy surfactants. (Image credit: H. Kim, source)
Acoustic Levitation
Destin from Smarter Every Day has a great new video exploring acoustic levitation. With carefully placed speakers, you can create a standing wave with sound that’s capable of levitating lightweight objects against the force of gravity. Around 4:00, Destin demonstrates this with colored water droplets, which is where the real fireworks start. As he turns up the volume on the speakers, the big droplets explode. This happens when surface tension can no longer hold the drop together. But the high-speed footage offers other clues about what’s going on. Notice how the drops flatten out as the sound volume increases. If you look back to the standing wave animation at 1:33, you’ll notice that just to either side of the nodes (the spots that don’t move), the wave is still oscillating back and forth a little bit. As you increase the sound volume, that standing wave gets stretched to a larger amplitude, which means that those little oscillations just to either side of the node get stronger (and steeper), too. This change in acoustic pressure squishes the drops into pancakes as the fluid tries to stay right at the node. Eventually the droplet is just too flattened for surface tension to keep it together and it bursts into smaller droplets. (Video credit: Smarter Every Day; submitted by Matthew P.)
Cloud Formation
Clouds are so ubiquitous here on Earth that it’s easy to take them for granted. But there’s remarkable complexity in the mechanics of their formation. This great video from Minute Earth steps through the processes of evaporation and condensation that drive basic cloud formation. After evaporation, buoyancy lifts warm, moist air upward. That warm air expands and cools until it reaches an altitude where water droplets can condense onto dust particles in the atmosphere. These droplets form the wispy cloud we see. Turbulence mixes these droplets and helps them collide and grow. Interestingly, although we understand the basic process of cloud formation, relatively little is understood about the details, and the subject is still very much an area of active research. (Video credit: Minute Earth; via io9)
Encapsulating Drops
Building and manipulating drops containing multiple chemicals is useful in pharmaceutical applications. But it can be a challenge to encapsulate multiple fluids without mixing them immediately. The research poster above describes a clever and simple method of building these compound drops. It uses a crosswise array of fibers, as seen in the top image. Dyed water droplets are placed at each intersection, pinning them in place. Then a larger drop of oil is added to the vertical fiber. As it runs down the fiber, it collects and encapsulates the individual droplets, creating the compound drop seen in the bottom photo. (Photo credit: F. Weyer et al.)
Grow Your Own Snowflakes
If your Christmas holiday was a little too green (like mine was), Science Friday has just the activity for you – grow your own snowflakes! With a few materials you probably already have and some dry ice from the store, you can grow and observe ice crystals at home. Although these crystals form from water vapor instead of water droplets like proper snowflakes, they do exhibit different structures depending on temperature and humidity, just the way natural snowflakes do. (Video credit: Science Friday/F. Lichtman)
Raindrops on Sand
Here is a high-speed look at the impact of a raindrop on a sandy beach. In this case, a water droplet is falling on a bed of uniform glass beads, but the situation is effectively the same. Depending on the speed of the drop at impact, many types of craters are possible. The higher the impact velocity, the greater the momentum of the drop at impact and the more likely the drop is to tear apart when surface tension can no longer hold it together. Interestingly, there is remarkable similarity between the shape and behavior of these liquid drop impacts and those of a catastrophic asteroid impact. (Video credit: R. Zhao et al.)
Iridescent Clouds
Look up at the clouds on the right day and you may catch a glimpse of a rainbow-like phenomenon known as cloud iridescence. These colors occur when sunlight is diffracted through small water droplets or ice crystals. For the effect to be apparent, the cloud must be optically thin, meaning that most of the rays of sunlight must pass through only a single droplet or ice crystal. This means the effect is usually visible only near the edges of clouds or as new clouds are forming. You can see more photos of the phenomenon here, and there’s a great video where cloud iridescence makes an appearance during a rocket launch in this previous entry. (Photo credit and submission: C. Havlin)
Bounce or Freeze?
Icing is a major problem for aircraft. When ice builds up on the leading edge of a wing it creates major disruptions in flow around the wing and can lead to a loss of flight control. One of the important factors in predicting and controlling ice building up is knowing when and where water droplets will freeze. The video above shows how surface conditions on the wing affect how an impacting droplet freezes. On a subzero hydrophilic surface, a falling droplet spreads and freezes over a wide area, which would hasten ice buildup. A hydrophobic surface is slightly better, with the droplet freezing over a smaller area, whereas a superhydrophobic surface shows no ice buildup. Unfortunately, at present superhydrophobic surfaces and surface treatments are extremely delicate, making them unsuitable for use on aircraft leading edges. (Video credit: G. Finlay)
Hydrophobicity and Viscous Flow
Hydrophobic surfaces are great for creating some wild behaviors with water droplets, but they make neat effects with other liquids, too. The viscous honey in the first segment of this Chemical Bouillon video is a great example. Because the honey doesn’t adhere to the hydrophobic surface, the viscoelastic fluid does not maintain the form it had when drizzled on the surface. Instead, the honey contracts, with surface tension driving Plateau-Rayleigh-like instabilities that break the contracting ligaments apart to form nearly spherical droplets of honey on the surface. (Video credit: Chemical Bouillon)
