Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "splashes"

Everyone has seen drops of liquid falling onto a dry surface, yet the process is still being unraveled by researchers. We have learned, for example, that lowering the ambient air pressure can completely suppress splashing. Viscosity of the fluid also clearly plays a role, but the relationship between these and other variables is unclear. The images above show two droplet impacts in which the viscosity differs. The top image shows a low viscosity fluid, which almost immediately after impact forms a thin expanding sheet of fluid that lifts off the surface to create a crownlike splash. In contrast, the higher viscosity fluid in the bottom image spreads as a thick lamella with a thinner outer sheet that breaks down at the rim. Researchers found that both the high- and low-viscosity fluids have splashes featuring these thin liquid sheets, but the time scales on which the sheet develops differ. Moreover, lowering the ambient pressure increases the time required for the sheet to develop regardless of the fluid’s viscosity. (Image credit: C. Stevens et al.; submitted by @ASoutIglesias)

When a drop falls on a dry surface, our intuition tells us it will splash, breaking up into many smaller droplets. Yet this is not always the case. The splashing of a droplet depends on many factors, including surface roughness, viscosity, drop size, and—strangely enough—air pressure. It turns out there is a threshold air pressure below which splashing is suppressed. Instead, a drop will spread and flatten without breaking up, as shown in the video above. For contrast, here is the same fluid splashing at atmospheric pressure. This splash suppression at low pressures is observed for both low and high viscosity fluids. Although the mechanism by which gases affect splashing is still under investigation, measurements show that no significant air layer exists under the spreading droplet except near the very edges. This suggests that the splash mechanism depends on how the spreading liquid encroaches on the surrounding gas. (Video credit: S. Nagel et al.; research credit: M. Driscoll et al.)

Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)

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.

Artist Fabian Oefner enjoys capturing both art and science in his work. In his latest series, “Orchid”, the blossom-like images are the result of splashes. He layered multiple colors of paint, ending with a top layer of black or white, then dropped a sphere into the paint. The images show how the colors mix and rebound, a delicate splash crown seen from above. The liquid sheet thickens at the rim and breaks up into ligaments from the instability of the crown’s edge. It makes for a remarkable demonstration of the effects of momentum and surface tension. Several of Oefner’s previous collections have appeared on FYFD (1, 2, 3). (Photo credit: F. Oefner)

A liquid’s surface tension can have a big effect on its splashes. In this video, a 5-mm droplet hits a surface covered in a thin layer of a liquid with lower viscosity and surface tension. The result is a dramatic effect on the spreading splash. As the initial curtain grows and expands, the lower surface tension of the impacted fluid thins the splash curtain. Fluid flows away from these areas due to the Marangoni effect, causing holes to grow. The sheet breaks up into a network of liquid filaments and ejected droplets before gravity can even bring it all to rest. For more, see this previous post and review paper. (Video credit: S. Thoroddsen et al.)

We take for granted that drops which impact a solid surface will splash, but, in fact, drops only splash when the surrounding air pressure is high enough. When the air pressure is low enough, drops simply impact and spread, regardless of the fluid, drop height, or surface roughness. Why this is and what role the surrounding air plays remains unclear. Here researchers visualize the air flow around a droplet impact. In (a) we see the approaching drop and the air it pulls with it. Upon impact in (b) and (c) the drop spreads and flattens while a crown of air rises in its wake. The drop’s spread initiates a vortex ring that is pinned to the drop’s edge. In later times (d)-(f) the vortex ring detaches from the drop and rolls up. (Photo credit: I. Bischofberger et al.)

In the image above we see two spheres of the same size, shape, and material being dropped into water. The left sphere has almost no splash, whereas the one on the right has a spectacular curtain-like splash. Why the big difference? It all comes down to the surface treatments. The glass sphere on the left is hydrophilic, but the right one has been treated to be hydrophobic. As a result, the water-fearing molecules of that sphere push the water away, allowing air to be entrained below the water’s surface instead. This creates a big splash that’s absent when the water moves smoothly around the hydrophilic sphere. (Photo credit: L. Bocquet et al.)

Artist Corrie White uses dyes and droplets to capture fantastical liquid sculptures at high-speed. The mushroom-like upper half of this photo is formed when the rebounding jet from one droplet’s impact on the water is hit by a well-timed second droplet, creating the splash’s umbrella. In the lower half of the picture, we see the remains of previous droplets, mixing and diffusing into the water via the Rayleigh-Taylor instability caused by their slight difference in density relative to the water. There’s also a hint of a vortex ring, likely from the droplet that caused the rebounding jet. (Photo credit: Corrie White)

Everyone knows the familiar plonk of a stone falling into a pond but few realize the complexity of the physics.  When a solid object falls into a pool, a sheet of liquid, the crown splash, is sent upward.  Simultaneously, the object pulls a cavity of air down with it. As the water moves inward, this cavity is pinched, creating an hourglass-like shape reminiscent of the shape of a rocket’s nozzle. As the diameter of that pinched cavity shrinks, the velocity of the upward escaping air increases, resulting in the formation of an air jet moving faster than the speed of sound. This air jet is followed by a slower liquid jet that may rebound to a height higher than then original height of the dropped object. So next time you throw a stone into a pond, enjoy the knowledge that you’ve broken the sound barrier. (Photo credit: D. van der Meer; see also Physics World)