Imagine a droplet sitting on a rigid surface spontaneously bouncing up and then continuing to bounce higher after each impact, as if it were on a trampoline. It sounds impossible, but it’s not. There are two key features to making such a trampolining droplet–one is a superhydrophobic surface covered in an array of tiny micropillars and the other is very low air pressure. The low-pressure, low-humidity air around the droplet causes it to vaporize. Inside the micropillar array, this vapor can get trapped by viscosity instead of draining away. The result is an overpressurization beneath the droplet that, if it overcomes the drop’s adhesion, will cause it to leap upward. For more, check out the original research paper or the coverage at Chemistry World. (Video credit and submission: T. Schutzius et al.)
Videos
The Droplet Slide
One of the joys of science is the sense of discovery that can come even from looking at something seemingly simple. Take, for example, a water droplet sitting on a plate. If you slowly tilt the plate, the droplet’s shape will shift until a critical angle where it starts sliding down the plate. But what happens to two initially different droplets? As this video shows, tilting two droplets of initially different shapes and returning them to horizontal causes the droplets to assume the same shape. There’s a universal behavior at work here–like nature has a kind of reset button that makes gravity and surface tension work together such that a droplet will assume a preferred shape. For an experimentalist, it’s certainly a handy way to create repeatable experiments! (Video credit: M. Musterd et al.)
Cream in Coffee
Pouring cream in coffee produces some of the most mesmerizing displays of fluid dynamics. The density difference between the two fluids sets up Rayleigh-Taylor instabilities that mushroom out and help create the turbulence that eventually mixes the drink. You can learn more about Rayleigh-Taylor instabilities in this FYFD video, and, if you need more awesome caffeine-filled examples of fluids, check out the coffee dynamics blog. (Video credit: S. Geraldine and L. Kang)
Deforming Soap Films
It’s the time of year when new Gallery of Fluid Motion videos start popping up online. We’ve already featured several and no doubt there will be more to come. Today’s post is a submission from Saad Bhamla, who gave this introduction to the work:
Soap bubbles occupy the rare position of delighting and fascinating both young children and scientific minds alike. Sir Isaac Newton, Joseph Plateau, Carlo Marangoni and Pierre-Gilles de Gennes, not to mention countless others, have discovered remarkable results in optics, molecular forces and fluid dynamics from investigating this seemingly simple system.
This video is a compilation of curiosity-driven experiments that systematically investigate the surface flows on a rising soap bubble. From childhood experience, we are familiar with the vibrant colors and mesmerizing display of chaotic flows on the surface of a soap bubble. These flows arise due to surface tension gradients, also known as Marangoni flows or instabilities. In this video, we show the surprising effect of layering multiple instabilities on top of each other, highlighting that unexpected new phenomena are still waiting to be discovered, even in the simple soap bubble.
As illustrated in the video, raising a bubble beneath the soap film moves surfactants in the film, which causes local differences in surface tension. Any time a difference in surface tension exists, fluid will flow from areas of low surface tension to ones with higher surface tension. This is called the Marangoni effect. On a soap bubble, this is visible in the chaotic swirling colors we see. In this system, Bhamla and his co-author found that by raising the bubble in steps, they could “freeze” the Marangoni-induced patterns created by the previous motion. (Video credit and submission: S. Bhamla et al.)
Visualizing Vortices
Flow visualization can be a valuable tool for understanding fluid dynamics. In this video, we see how it can help elucidate the mechanisms of flapping flight. By dyeing vortices from the leading edge in red rhodamine and vortices from the trailing edge in green fluorescein, it’s possible to distinguish their competing effects for wings of different size. The speed and efficiency of a flapping wing depends on the vortices it sheds–these provide its lift and thrust. On a short wing, the leading edge vortex is significant and spins in a counter-clockwise (positive) direction. When it reaches the trailing edge, it meets a vortex spinning clockwise (negative). The interference of the two vortices weakens the shed vortex, thereby slowing the wing. Lengthening the wing weakens the leading edge vortex, which reduces its interference at the trailing edge and makes the longer wings more efficient. (Video credit: T. Mitchel et al.; via @AlbanSauret)
Glow-Stick Ferrofluids
Ferrofluids create all kinds of fascinating shapes when exposed to magnetic fields. In this video, Dianna from Physics Girl shows off what happens when you combine a ferrofluid with glowsticks and explains how ferrofluids get some of their unique properties. Ferrofluids consist of tiny nanoparticles of magnetic material that are surrounded by surfactants and suspended in a carrier fluid. This creates a fluid whose shape depends on gravity, surface tension, and the local magnetic field. By manipulating the relative strength of these forces, you can create everything from spikes to maze-like patterns to whatever this is. (Video credit and submission: Physics Girl)
Bullet-Time Inferno
Remember the bullet time effect from The Matrix? This spectacular video gives you a similar effect with the turbulent flames created by firebreathers. To capture this level of detail, Mitch Martinez uses an array of 50 cameras placed around the performers, allowing him to reconstruct the full, three-dimensional representation of the flames. Similarly, some scientists use arrays of high-speed video cameras to collect 3D, time-resolved data about phenomena like combustion. Because these flows are so complex in terms of their fluid dynamics and chemistry, capturing full 3D data is important to help understand and model the flow better. (Video credit: M. Martinez; via Rakesh R.)
Extinguishing Flames
Putting out fires can be a difficult, water-intensive task. In this video, scientists demonstrate how using a non-Newtonian fluid can make it easier to extinguish and suppress flames. Where water tends to splatter and scatter against an object, a yield-stress fluid can cling and coat to smother the flame. The fluid used here is water with a 0.1% polymer additive, which is enough to significantly change the fluid’s rheological properties. Pre-treating flammable objects with the fluid is also effective at suppressing combustion, raising additional possibilities for using such techniques in fighting the spread of wildfires. (Video credit: B. Blackwell et al.)
Pollock-Style Physics
Here on FYFD, we like to show off the artistic side of fluid dynamics. But some researchers are actively studying how artists use fluid dynamics in their art. In this video, they examine one of Jackson Pollock’s painting techniques, in which filaments of paint were applied by flinging paint off a paintbrush. Getting the technique to work requires a fine balance of forces and effects. Firstly, the paint must be viscous enough to hold together in a filament when flung. Secondly, the centripetal acceleration of the rotation must be high to both form the catenary filament and throw it off the brush. And, finally, the Reynolds number needs to be high enough to add some waviness and instability to the filament so that it looks interesting once it hits the canvas. Also be sure to check out the group’s previous work exploring Siqueiros’s painting techniques. (Video credit: B. Palacios et al.)
Early Rocket Launch
Pre-dawn launches provide some of the most dramatic rocket footage. This video is from an October 2nd Atlas V launch, and the really fun stuff starts at about 0:34. As the rocket climbs to higher altitudes, the atmospheric pressure around it decreases. As a result of this low pressure, the rocket’s exhaust gases balloon outward in a giant plume many times larger than the rocket. This happens in every launch, but it’s visible here because the rocket is at such a high altitude that its exhaust is being lit by sunlight while the observers on the ground are still in the dark. The ice crystals in the exhaust–much of the rocket’s exhaust is water vapor–reflect sunlight down to the earth. Around 0:47, a cascade of shock waves ripples through the plume just before the first-stage’s main engine cuts off. Once the engine stops firing, there’s no more exhaust and the plume ends. (Video credit: Tampa Bay Fox 13 News; submitted by Kyle C)