Two jets colliding can form a chain-like fluid structure. With increasing flow rate, the rim of the chains becomes wavy and unstable, forming a fishbone structure where droplets extend outward from the fluid sheet via tiny ligaments. Eventually, the droplets break off in a pattern as beautiful as it is consistent. (Photo credits: A. Hasha and J. Bush)
One of the tried and true cooking tips my mother gave me when I was younger was to test the temperature of my griddle before making pancakes by splashing a few drops of water on it. If it was hot enough that the water skittered across the surface before evaporating, then it was ready. Aside from being a way to make great pancakes, this tip demonstrates an everyday application of the Leidenfrost effect. When the surface of the pan is significantly higher than the boiling point of the water, the part of the water drop that hits the pan is vaporized, creating a thin layer of water vapor on which the rest of the droplet rests. The vapor serves as an insulator, protecting the rest of the water drop from the heat of the pan, as well as a lubricant, allowing the drop slip and slide easily across the surface. The same effect lets the brave plunge a hand into liquid nitrogen without damage, but they have to be quick, otherwise their hand will cool to the point that the liquid nitrogen contacts it without a protective layer of nitrogen. (In that case, a nasty case of frostbite may be the least of one’s worries. We do NOT recommend trying this one at home.)
The splashes from droplets impacting jets create truly mesmerizing liquid sculptures. Corrie White is one of the masters of this type of high-speed macro photography. Her work captures the instantaneous battles between viscosity, surface tension, and inertia. The fantastic structure seen here through the falling droplets is created by a series of drops timed so that the later ones strike the Worthington jet produced by the initial drop’s impact. (Photo credit: Corrie White)
The interaction of electric fields and fluids can lead to some unexpected results. Here we see the formation of a water bridge formed between two beakers of demineralized water across which a large voltage difference (~15kV) is applied. The bridge is stable for separation distances up to about 2 cm. In order to achieve this feat, the water is overcoming two destabilizing forces: gravity, which bends the bridge, and capillary action, which makes the liquid bridge thin until it breaks into droplets. According to the authors, both forces are countered by induced polarization forces at interface; in short, the electrical field around the liquid causes the positive and negative charges in the liquid to separate, thereby polarizing the liquid. This separation of charges then creates normal stresses along the surface of the water that oppose the gravitational and capillary forces trying to break the bridge. (Video credit: A. Marin and D. Lohse)
A little polymer goes a long way when it comes to changing a fluid’s behavior. Normally, a falling jet of fluid will develop waviness and be driven by surface tension and the Plateau-Rayleigh instability to break up into a stream of droplets. We see this at our water faucets all the time. But when traces of a polymer are dissolved in water, the behavior is much different. The viscoelasticity of the polymer chains creates a force that opposes the thinning effects caused by surface tension. So, instead of thinning to the point of breaking into droplets, a drop is able to climb back up the jet until it reaches a critical mass where it reverses direction, accelerates downward due to gravity and eventually breaks off the jet. Then the whole process begins again with a new terminal drop. (Video credit: C. Clasen et al)
Ever tried to mix oil and vinegar? Anyone who has ever dealt with salad dressings knows the difficulty of evenly distributing immiscible fluids; the key is to shake them and create an emulsion, where droplets of one fluid are distributed throughout another. In this video, researchers create a double emulsion–oil in water in oil–without touching the two fluids. First they suspend a drop of water on a wire and then coat it with oil. Below, they place a bath of silicone oil, which they vibrate. When the oil-coated droplet falls onto the bath, it bounces on the surface rather than coalescing because a thin layer of air–constantly refreshed due to the vibration of the surface–separates the droplet from the bath. When the amplitude of the vibration is large enough, the oil coating penetrates the water during the bounce, leaving behind a tiny droplet and creating the emulsion. (Video credit: D. Terwange et al; Research paper)
This high-speed video captures the impact of liquid droplets onto a granular surface. While there is some similarity to liquid-solid and liquid-liquid impacts, the permeability of the granular surface helps to “freeze” the splash rather quickly. Energy is dissipated in the initial impact, causing a splash of grains. Then the surface tension, viscosity and inertia of the droplet compete in causing the deformations seen in the video. The deformation appears strongly dependent on the kinetic energy with which the droplet hits the surface (i.e. proportional to the height from which it is dropped). (Video credit: G. Delan et al)
staunchreality-deactivated20120 asks:
Hey there – Love the blog. Most interesting science blog I follow 🙂 This may be a silly question – is propulsion through space purely a function of exit velocity and catching gravity slingshots around planets, or is there enough of anything to push against for rocket propulsion?
Thanks! Glad you enjoy the blog. And your question is not silly at all.
Whether in the atmosphere or not, rocket engines always operate on the same principle: Newton’s 3rd law. For every force exerted, there is an equal and opposite reaction force. For a rocket, this means that the momentum of the rocket exhaust provides forward momentum–thrust–for the rocket. When acting in an atmosphere, the exhaust doesn’t push against the atmosphere in order to move the rocket–in fact, rockets have to overcome aerodynamic drag when in the atmosphere, which opposes their thrust.
While the operating principle of a rocket remains the same regardless of its surrounding, the ambient pressure (essentially zero in space and non-zero in an atmosphere) does affect the efficiency of the rocket’s nozzle, which can affect the exit velocity of the exhaust, and, thus, the efficiency of the rocket. Under ideal conditions, the exhaust should exit the nozzle at the same pressure as the ambient conditions–whatever they are. If the exhaust pressure is lower than the ambient, the exhaust can separate from the nozzle, causing instabilities in the flow and potentially damaging the nozzle. On the other hand, if the exhaust pressure is too high, then there is exhaust that could be turned into thrust that is going to waste. Unfortunately, matching the exhaust pressure to the ambient pressure is a function of the geometry of the nozzle, which is usually fixed. Engineers of rockets intended to fly from within the atmosphere to space usually have to pick a particular altitude to design around and deal with the inefficiencies while the rocket flies at other ambient conditions.
Outside of the physical mechanics of how thrust is produced, propulsion in space is dominated by the influence of orbital mechanics. Once in an orbit, a spacecraft will stay on that orbital path without expending any thrust. To change between orbits, it is necessary for the spacecraft–rocket or otherwise–to change its velocity–typically referred to as delta-v–by firing an engine or thruster. It’s also possible to change orbits using the gravity of other celestial bodies (Jupiter is a popular one) to change a spacecraft’s delta-v without expending propellant. However, fluid dynamics don’t play a big role in the process aside from the problems of fuel sloshing aboard the spacecraft and the actual mechanism by which thrust is produced.
That said, if anyone is interested in getting a better feel for how orbit mechanics work, I have two recommendations. The first is to watch this video of water droplets “orbiting” a charged knitting needle aboard the ISS. And the second is to play the game Osmos. It is like rocket propulsion and orbit mechanics in action!
(Photo credits: NASA, The Aerospace Corporation, Hemisphere Games)
Two jets of sugar syrup collide and interact to form very different patterns. On the left, the two jets have a low flow rate and create a chain-like wake. The jets on the right have a higher flow rate and produce a liquid sheet that breaks down into filaments and droplets. The result is often likened to fish bones. (Photo credit: Rebecca Ing)
Large droplets ejected from a liquid pool do not coalesce immediately back into the whole. Instead, a thin layer of air gets trapped beneath them, much like the oil lubricating bearings. The weight of the droplet causes the air to drain away, and eventually the droplet comes in contact with the pool. Some of the droplet gets drained away before surface tension snaps the interface back into a low energy state. A new smaller droplet then bounces upward before repeating the process over again. Eventually the droplet becomes small enough that its entire mass gets sucked away by the pool. Researchers call this process the coalescence cascade.