Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion–in which immiscible fluids are encapsulated into a multi-layer droplet–to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams and D. A. Weitz)
Videos
Simulating a Curveball
Spinning an object in motion through a fluid produces a lift force perpendicular to the spin axis. Known as the Magnus effect, this physics is behind the non-intuitive behavior of football’s corner kick, volleyball’s spike, golf’s slice, and baseball’s curveball. The simulation above shows a curveball during flight, with pressure distributions across the ball’s surface shown with colors. Red corresponds to high pressure and blue to low pressure. Because the ball is spinning forward, pressure forces are unequal between the top and bottom of the ball, with the bottom part of the baseball experiencing lower pressure. As with a wing in flight, this pressure difference between surfaces creates a force – for the curveball, downward. (Video credit: Tetra Research)
Geyser Physics
Three basic components are necessary for a geyser: water, an intense geothermal heat source, and an appropriate plumbing system. In order to achieve an explosive eruption, the plumbing of a geyser includes both a reservoir in which water can gather as well as some constrictions that encourage the build-up of pressure. A cycle begins with geothermally heated water and groundwater filling the reservoir. As the water level increases, the pressure at the bottom of the reservoir increases. This allows the water to become superheated–hotter than its boiling point at standard pressure. Eventually, the water will boil even at high pressure. When this happens, steam bubbles rise to the surface and burst through the vent, spilling some of the water and thereby reducing the pressure on the water underneath. With the sudden drop in pressure, the superheated water will flash into steam, erupting into a violent boil and ejecting a huge jet of steam and water. For more on the process, check out this animation by Brian Davis, or to see what a geyser looks like on the inside, check out Eric King’s video. (Video credit: Valmurec; idea via Eric K.)
Supercell Thunderstorm
Photographer Mike Olbinski has captured a spectacular timelapse of a supercell thunderstorm over the plains of Texas. Supercells are characterized by a strong, rotating updraft known as a mesocyclone, seen clearly in the video. These storms are commonly isolated occurrences, forming when horizontal vorticity in the form of wind shear is redirected upwards by an updraft. Such a strong updraft is typically created by a capping inversion, a situation where a layer of warmer air traps the colder air beneath it. (This is why one sees a distinctive cut-off at the top of some clouds.) As warm air rises from the surface, either the air above the cap will cool or the air below the cap will warm. Either situation results in an instability with cooler air on top of warmer air, providing a catalyst for the kind of dramatic weather seen here. (Video credit: M. Olbinski; via io9)
Visualization Via Temperature
One downside to many flow visualization techniques, like those using dye, smoke, or particles, is the difficulty of dealing with their aftermath. You can only introduce so much of them into a wind or water tunnel before it’s necessary to shutdown and clean everything. One alternative is to use temperature, as shown in the video above. By simply introducing a warmer fluid and using an IR camera, it’s possible to accomplish many of the same effects without the mess. (Video credit: A. Khandekar and J. Jacob; submitted by J. Jacob)
Droplets Within Droplets
This video shows a multi-layered droplet, in which several droplets are formed one inside the other as an initial drop falls through a layer of oil sitting atop another liquid. When the drop falls, its potential energy gets transformed into interface energy, creating a fascinating interplay of surface tension, deformation, and miscibility between the fluids. Such self-contained multi-layered droplets, similar to multiple emulsions, could be helpful in pharmaceutical development. (Video credit: E. Lorenceau and S. Dorbolo 2004)
“Levitating Water”
Al Seckel, a cognitive neuroscientist and expert on illusions, created this “Levitating Water” installation, in which multiple streams of water appear as a series of levitating droplets thanks to a strobing light. The well-timed strobe lighting tricks the brain into seeing many different falling droplets as the same, nearly stationary droplet. The effect is similar to the one created by vibrating a stream of falling water. (Video credit: wunhanglo)
Droplet Bounce
This high-speed video shows the remarkable resilience of a water droplet upon impact against as a solid surface. The droplet deforms into a pancake-shape, with its center depressing almost flat before rebounding upward. The rest of the drop follows, splitting into several droplets as capillary waves dance across its surface. When one satellite drop almost escapes, the main droplet just barely comes in contact with it, the coalescence enough to tip surface tension into pulling them together instead of breaking them apart. (Video credit: K. Suh/ChemistryWorldUK)
The Kelvin-Helmholtz Instability in the Lab
Though often spotted in water waves or clouds, the Kelvin-Helmholtz instability is easily demonstrated in the lab as well. Here a tank with two layers of liquid – fresh water on top and denser blue-dyed saltwater on the bottom – is used to generate the instability. When level, the two layers are stationary and stable due to their stratification. Upon tilting, the denser blue liquid sinks to the lower end of the tank while the freshwater shifts upward. When the relative velocity of these two fluids reaches a critical point, their interface becomes unstable, forming the distinctive wave crests that tumble over to mix the two layers. (Video credit: M. Stuart)
Turning Sound into Light
Sonoluminescence – the creation of light from sound – was discovered in the 1930s, and, due to the difficulty of obtaining direct measurements, the exact mechanism remains highly debated even today. The phenomenon typically takes place within a tiny cavitation bubble inside a liquid. When bombarded with ultrasonic sound, such a bubble will repeatedly expand and collapse. Once a bubble is established, the cycle can be kicked off by increasing the driving acoustic pressure. This will collapse the bubble, drastically increasing its pressure and temperature (up to thousands of degrees Kelvin) and causing the bubble to emit a pulse of light before the pressure imbalance causes it to expand again. Several theories exist as to how the light is generated, the leading one being that the high temperatures in the bubble ionize the noble gases within and that those free electrons emit light via thermal bremstrahlung radiation. Sonoluminescence happens outside the lab, too. Both the previously discussed pistol shrimp and the mantis shrimp generate such light-emitting bubbles when hunting. (Video credit: The Point Studios; suggested by Bobby E.)