Many situations can generate high-speed liquid jets, including droplet impacts, vibrated fluids, and surface charges. In each of these cases, a concave liquid surface is impulsively accelerated, which causes the flow to focus into a jet. The image above shows snapshots of a microjet generated from a 50 micron capillary tube visible at the right. This jet formed when the meniscus inside the capillary tube was disturbed by a laser pulse that vaporized fluid behind the interface. Incredibly, the microjets generated with this method can reach speeds of 850 m/s, nearly 3 times the speed of sound in air. Researchers have found the method produces consistent results and suggest that it could one day form the basis for needle-free drug injection. You can read more in their freely available paper. (Photo credit: K. Tagawa et al.)
Search results for: “water droplet”
Colliding in Microgravity
On Earth, it’s easy for the effects of surface tension and capillary action to get masked by gravity’s effects. This makes microgravity experiments, like those performed with drop towers or onboard the ISS, excellent proving grounds for exploring fluid dynamics unhindered by gravity. The video above looks at how colliding jets of liquid water behave in microgravity. At low flow rates, opposed jets form droplets that bounce off one another. Increasing the flow rate first causes the droplets to coalesce and then makes the jets themselves coalesce. Similar effects are seen in obliquely positioned jets. Perhaps the most interesting clip, though, is at the end. It shows two jets separated by a very small angle. Under Earth gravity, the jets bounce off one another before breaking up. (The jets are likely separated by a thin film of air that gets entrained along the water surface.) In microgravity, though, the jets display much greater waviness and break down much quicker. This seems to indicate a significant gravitational effect to the Plateau-Rayleigh instability that governs the jet’s breakup into droplets. (Video credit: F. Sunol and R. Gonzalez-Cinca)
Flowing Uphill
Science Friday takes an inside look at self-propelled Leidenfrost droplets like those we’ve featured previously. The Leidenfrost effect takes place when a liquid comes in contact with a surface much, much hotter than its boiling point. Part of the liquid is vaporized, creating a thin gas layer that both insulates the remaining liquid and causes it to move with very little friction. Over a flat surface, this underlying vapor will spread in any direction. But by covering the surface with ratchets, it’s possible to direct the vapor in a particular direction, which propels the droplet in the opposite direction. Check out the video and our previous posts for more! (Video credit: Science Friday; via io9 and submitted by Urs)
When Jets Collide
When two jets of a viscous liquid collide, they can form a chain-like stream or even a fishbone pattern, depending on the flow rate. This video demonstrates the menagerie of shapes that form not only with changing flow rates but by changing how the jets collide – from a glancing impingement to direct collision. When just touching, the viscous jets generate long threads of fluid that tear off and form tiny satellite droplets. At low flow rates, continuing to bring the jets closer causes them to twist around one another, releasing a series of pinched-off droplets. At higher flow rates, bringing the jets closer to each other creates a thin webbing of fluid between the jets that ultimately becomes a full fishbone pattern when the jets fully collide. The surface-tension-driven Plateau-Rayleigh instability helps drive the pinch-off and break-up into droplets. (Video credit: B. Keshavarz and G. McKinley)
Sochi 2014: Making Snow
Much attention ahead of the Sochi Winter Olympics has been dedicated to the question of how this subtropical resort town would provide and maintain adequate snow cover for the Games. Officials promised a combination of natural snow, snow transported from elsewhere, snow stored from the previous year, and, of course, artificial snow. These days many ski resorts rely heavily on snow guns producing artificial snow. There are two main types of snow gun–those which use compressed air and those which have an electrically-driven fan–but the principles behind each are the same. The snow guns provide a continuous spray of air and water, atomizing the water into tiny droplets which freeze rapidly. The effectiveness of snow guns depends on both the temperature and humidity of the surrounding air. With sufficiently dry air, artificial snow can be made even several degrees above freezing. Sochi itself is relatively humid (72% on average for February), but most of the outdoor events are held in Krasnaya Polyana, higher in the mountains where temperatures are typically much lower and artificial snow can be manufactured. That said, temperatures have reached as high as 15 degrees Celsius during the Games so far, and athletes have complained about the changing snow conditions in several events. (Video credit: On The Snow)
FYFD is celebrating #Sochi2014 with a look at the fluid dynamics of the Winter Games. Check out our previous posts, including how lugers slide fast, how wind affects ski jumpers, and why ice is slippery.
Hydrophobia
On a recent trip to G.E., the Slow Mo Guys used their high-speed camera to capture some great footage of dyed water on a superhydrophobic surface. Upon impact, the water streams spread outward, flat except for a crownlike rim around the edges. Then, because air trapped between the liquid and the superhydrophobic solid prevents the liquid from wetting the surface, surface tension pulls the water back together. If this were a droplet rather than a stream, it would rebound off the surface at this point. Instead, the jet breaks up into droplets that scatter and skitter across the surface. There’s footage of smaller droplets bouncing and rebounding, too. Superhydrophobic surfaces aren’t the only way to generate this behavior, though; the same rebounding is found for very hot substrates due to the Leidenfrost effect and very cold substrates due to sublimation. As a bonus, the video includes ferrofluids at high-speed, too. (Video credit: The Slow Mo Guys/G.E.)
Frozen Bubbles
Snowflakes aren’t the only frozen crystals to play with outside in the winter. Photographer Angela Kelly recently posted a series of frozen soap bubbles made by her and her son. In temperatures well below freezing, the thin film of the soap bubble does not survive long before it begins to freeze. The bubbles do not freeze all at once; instead the frost creeps gradually across it. For bubbles sitting on a surface, the ice front expands upward, much the same as with a freezing water drop. Once frozen, the bubbles crack or rip when touched instead of melting and popping. (Photo credit: A. Kelly; via BoredPanda; submitted by jshoer)
Liquid Umbrella
When a water drop strikes a pool, it can form a cavity in the free surface that will rebound into a jet. If a well-timed second drop hits that jet at the height of its rebound, the impact creates an umbrella-like sheet like the one seen here. The thin liquid sheet expands outward from the point of impact, its rim thickening and ejecting tiny filaments and droplets as surface tension causes a Plateau-Rayleigh-type instability. Tiny capillary waves–ripples–gather near the rim, an echo of the impact between the jet and the second drop. All of this occurs in less than the blink of an eye, but with high-speed video and perfectly-timed photography, we can capture the beauty of these everyday phenomena. (Photo credit: H. Westum)
Avoiding Splashback
Here’s a likely Ig Nobel Prize candidate from the BYU SplashLab: a study of splashing caused by a stream of fluid entering a horizontal body of water or hitting a solid vertical surface. In other words, urinal dynamics. The researchers simulated this activity using a stream of water released from a given height and angle and observed the resulting splash with high-speed video. They found a stream falls only 15-20 centimeters before the Plateau-Rayleigh instability breaks it into a series of droplets, and that this is the worst-case scenario for splash-back. The video above shows how a stream of droplets hits the pool, creating a complex cavity driven deeper with each droplet impact. Not only does each impact create a splash, the cavity’s collapse does as well. Similarly, when it comes to solid surfaces, they found that a continuous stream splashes less. They’ve also put together a helpful primer on the best ways to avoid splash-back. (Video credit: R. Hurd and T. Truscott; submitted by Ian N., bewuethr, John C. and possibly others)
For readers attending the APS DFD meeting, you can catch their talk, “Urinal Dynamics,” Sunday afternoon in Session E9 before you come to E18 for my FYFD talk.
Beads-on-a-string
Viscoelastic fluids are a type of non-Newtonian fluid in which the stress-strain relationship is time-dependent. They are often capable of generating normal stresses within the fluid that resist deformation, and this can lead to interesting behaviors like the bead-on-a-string instability shown above. In this phenomenon, a uniform filament of fluid develops into a series of large drops connected by thin filaments. Most fluids would simply break into droplets, but the normal stresses generated by the viscoelastic fluid prevent break-up. For this particular photo, the stresses are generated by clumps of surfactant molecules within the wormlike micellar fluid. Similar effects are observed in polymer-laced fluids. (Photo credit: M. Sostarecz and A. Belmonte)
