Viscosity can have a notable effect on droplet impacts. This poster demonstrates with snapshots from three droplet impacts. The blue drops are dyed water, and the red ones are a more viscous water-glycerol mixture. When the two water droplets impact, a skirt forms between them, then spreads outward into a sheet with a thicker, uneven rim before retracting. The second row shows a water droplet impacting a water-glycerol droplet. The less viscous water droplet deforms faster, wrapping around and mixing into the other drop before rebounding in a jet. The last row switches the impacts, with the more viscous drop falling onto the water. As in the previous case, the water deforms faster than the water-glycerol. The two mix during spreading and rebound slower. In the last timestep shown, the droplet is still contracting, but it does rebound as a jet thereafter. (Image credit: T. Fanning et al.)
Search results for: “water droplet”
Supercooling Water
Supercooling is the process of lowering a fluid’s temperature below its freezing point without the fluid becoming solid. Though this may sound bizarre, it’s an effect you can recreate easily in your refrigerator, as detailed in the video above. Supercooling shows up in nature as well, particularly with water droplets at high altitudes. If a plane flies through supercooled water droplets, it can create icing problems on the aircraft’s wings. Alternatively, flying through supercooled water vapor can cause a hole-punch cloud to form when the vapor flash-freezes into snow. (Video credit: SciShow)
Shooting Droplets with Lasers
Last week we saw what happens when a solid projectile hits a water droplet; today’s video shows the impact of a laser pulse on a droplet. Several things happen here, but at very different speeds. When the laser impacts, it vaporizes part of the droplet within nanoseconds. A shock wave spreads from the point of impact and a cloud of mist sprays out. This also generates pressure on the impact face of the droplet, but it takes milliseconds–millions of nanoseconds–for the droplet to start moving and deforming. The subsequent explosion of the drop depends both on the laser energy and focus, which determine the size of the impulse imparted to the droplet. The motivation for the work is extreme ultraviolet lithography–a technique used for manufacturing next-generation semiconductor integrated circuits–which uses lasers to vaporize microscopic droplets during the manufacturing process. (Video credit: A. Klein et al.)
Shooting Droplets
This animation shows high-speed video of a polystyrene particle striking a falling water droplet. Under the right conditions, the particle rips through the droplet, stretching the water into a bell-shaped lamella extending from a thicker rim. When the particle detaches, surface tension rapidly collapses the lamella into a ring which destabilizes. Thin ligaments and droplets fly off the crown-like ring as momentum overcomes surface tension’s ability to hold the droplet together. Be sure to check out the full video on YouTube or later next month at the APS Division of Fluid Dynamics meeting. (Yes, I will be there!) (Image credit: V. Sechenyh et al., source video)
Bead-Infused Droplet
A Leidenfrost droplet impregnated with hydrophilic beads hovers on a thin film of its own vapor. The Leidenfrost effect occurs when a liquid touches a solid surface much, much hotter than its boiling point. Instead of boiling entirely away, part of the liquid vaporizes and the remaining liquid survives for extended periods while the vapor layer insulates it from the hot surface. Hydrophilic beads inserted into Leidenfrost water droplets initially sink and are completely enveloped by the liquid. But, as the drop evaporates, the beads self-organize, forming a monolayer that coats the surface of the drop. The outer surface of the beads drys out, trapping the beads and causing the evaporation rate to slow because less liquid is exposed. (Photo credit: L. Maquet et al.; research paper – pdf)
Droplets Surfing
The Leidenfrost effect can make water droplets skitter across a hot griddle or briefly protect a hand dunked in liquid nitrogen. When a liquid is exposed to a solid surface much, much hotter than its boiling point, the contact vaporizes part of the liquid, and, in the case of a droplet, forms a thin lubricating layer of vapor that the liquid drop can skate around on. Researchers have found that releasing these Leidenfrost droplets on textured surfaces creates self-propelling drops by directing the flow of vapor. In this video, one team demonstrates some of the neat tracks they’ve built for their drops. (Video credit: D. Soto et al.)
Space Balls (of Water!)
The microgravity environment of space is an excellent place to investigate fluid properties. In particular, surface tension and capillary action appear more dramatic in space because gravitational effects are not around to overwhelm them. In this animation, astronaut Don Petit injects a jet of air into a large sphere of water. Some of the water’s reaction is similar to what occurs on Earth when a drop falls into a pool; the jet of air creates a cavity in the water, which quickly inverts into an outward-moving jet of water. In this case, the jet is energetic enough to eject a large droplet. Meanwhile, the momentum, or inertia, from the air jet and subsequent ejection causes a series of waves to jostle the water sphere back and forth. Surface tension is strong enough to keep the water sphere intact, and eventually surface tension and viscosity inside the sphere will damp out the oscillations. You can see the video in full here. (Image credit: Don Petit/Science off the Sphere)
Water and Aerogel
Aerogel is an extremely light porous material formed when the liquid inside a gel is replaced with gas. When combined with water, aerogel powders can have some wild superhydrophobic effects. Here water condensed on a liquid nitrogen cooler has dripped onto a floor scattered with aerogel powder from the nitrogen’s shipping container. The result is that the water gets partially coated in aerogel powder and takes on some neat properties. Its contact angle with the surface increases – in other words, it beads up – which is typical of superhydrophobicity. When disturbed, the water breaks easily into droplets which do not immediately recombine upon contact. With sufficient distortion, they can rejoin. You can see some other neat examples of aerogel-coated water behaviors in this second video as well. (Video credit: ophilcial; submitted by Jason I.)
Shaping and Levitating Droplets
Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)
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)