Lava is rather fascinating as a fluid. Lava flow regimes range from extremely viscous creeping flows all the way to moderately turbulent channel flow. Lava itself also has a widely varying rheology, with its bulk properties like viscosity and its response to deformation changing strongly with temperature and composition. As lava cools, instabilities form in the fluid, causing the folding, coiling, branching, swirling, and fracturing associated with different types and classes of lava. (Image credit: E. Guddman, via Mirror)
Tag: instability
Water-Based Tractor Beam
Researchers in Australia have demonstrated a “tractor beam” capable of manipulating floating objects from a distance using surface waves on water. And, unlike some research, you can try to replicate this result right in the comfort of your own bathtub! When a wave generator oscillates up and down, it creates surface waves that move objects and particles on the water’s surface. When the wave amplitudes are small, the outgoing wave fronts tend to be planar, as in part (a) of the figure above. These planar waves push surface flow away from the wave generator in a central outward jet, and new fluid is entrained from the sides to replace it. This creates the kind of flowfield shown in the streaklines of part (b).Increasing the amplitude of the surface waves drastically changes the surface flow’s behavior. Larger wave amplitudes are more susceptible to instabilities due to the nonlinear nature of the surface waves. This means that the planar wave fronts seen in part (a) break down into a three-dimensional wavefield, like the one shown in part (c). Near the wave-maker, the surface waves now behave chaotically. This pulsating motion ejects surface flow parallel to the wave-maker, which in turn draws fluid and any floating object toward the wave-maker. The corresponding surface flowfield is shown in part (d). The researchers are refining the process, but they hope the physics will one day be useful in applications oil spill clean-up. (Video credit: Australia National University; image and research credit: H. Punzmann et al. 1, 2; via phys.org; submitted by Tracy M)
Supernova Explosion
Type 1a supernovae occur in binary star systems where a dense white dwarf star accretes matter from its companion star. As the dwarf star gains mass, it approaches the limit where electron degeneracy pressure can no longer oppose the gravitational force of its mass. Carbon fusion in the white dwarf ignites a flame front, creating isolated bubbles of burning fluid inside the star. As these bubbles burn, they rise due to buoyancy and are sheared and deformed by the neighboring matter. The animation above is a visualization of temperature from a simulation of one of these burning buoyant bubbles. After the initial ignition, instabilities form rapidly on the expanding flame front and it quickly becomes turbulent. (Image credit: A. Aspden and J. Bell; GIF credit: fruitsoftheweb, source video; via freshphotons)
The Real Shape of Raindrops
We often think of raindrops as spherical or tear-shaped, but, in reality, a falling droplet’s shape can be much more complicated. Large drops are likely to break up into smaller droplets before reaching the ground. This process is shown in the collage above. The initially spherical drops on the left are exposed to a continuous horizontal jet of air, similar to the situation they would experience if falling at terminal velocity. The drops first flatten into a pancake, then billow into a shape called a bag. The bags consists of a thin liquid sheet with a thicker rim of fluid around the edge. Like a soap bubble, a bag’s surface sheet ruptures quickly, producing a spray of fine droplets as surface tension pulls the damaged sheet apart. The thicker rim survives slightly longer until the Plateau-Rayleigh instability breaks it into droplets as well. (Image credit: V. Kulkarni and P. Sojka)
Fireworks Taking Off
Aerial fireworks are essentially semi-controlled exploding rockets. Here Discovery Channel shares high-speed video of fireworks taking off. The turbulent billowing exhaust on the ground is reminiscent of other rocket launches. The tube-launched firework clip is a great example of an underexpanded nozzle. The pressure of the gases in the tube is higher than the ambient air, so when the gases escape, the exhaust fans out to equalize the pressure. And, finally, the explosion that propels the colorful chemicals outward forms jets that can affect the final form of the display. To my American readers: Happy 4th of July! And be safe! (Video credit: Discovery Slow-Down)
“Clourant”
Photographers Cassandra Warner and Jeremy Floto produced the “Clourant” series of high-speed photographs of colorful liquid splashes. The artists took special care to disguise the origin of splashes, making them appear like frozen sculptures. The photos are beautiful examples of making fluid effects and instabilities. Many of them feature thin liquid sheets with thicker rims just developing ligaments. In other spots, surface tension has been wholly overcome by momentum’s effects and what was once ligaments has exploded into a spray of droplets. (Photo credit: C. Warner and J. Floto; submitted by jshoer; via Colossal)
Distorted Rings
The Marangoni effect is generated by variations in surface tension at an interface. Such variations can be temperature-driven, concentration-driven, or simply due to the mixing between fluids of differing surface tensions as is the case here. The pattern in the image above formed after a dyed water droplet impacted a layer of glycerin. The initial impact of the drop formed an inner circle and outer ring. This image is from 30 seconds or so after impact, after the Marangoni instability has taken over. The higher surface tension of the water pulls the glycerin toward it, resulting in a flower-like pattern. (Photo credit: E. Tan and S. Thoroddsen)
“Chromatic Mushrooms”
Chemical Bouillon’s art often mixes chemistry and fluid dynamics. Here dense UV dyes falling through a less dense fluid form long strings with mushroom-like caps or tree-like branches. (For reference, gravity is pointing up relative to the video frame in most clips.) This behavior is related to the Rayleigh-Taylor instability that deforms interfaces and causes mixing between unstably stratified fluids. (Video credit: Chemical Bouillon)
Kelvin-Helmholtz in the Lab
The Kelvin-Helmholtz instability looks like a series of overturning ocean waves and occurs between layers of fluids undergoing shear. This video has a great lab demo of the phenomenon, including the set-up prior to execution. When the tank is tilted, the denser dyed salt water flows left while the fresh water flows to the right. These opposing flow directions shear the interface between the two fluids, which, once a certain velocity is surpassed, generates an instability in the interface. Initially, this disturbance is much too small to be seen, but it grows at an exponential rate. This is why nothing appears to happen for many seconds after the tilt before the interface suddenly deforms, overturns, and mixes. In actuality, the unstable perturbation is present almost immediately after the tilt, but it takes time for the tiny disturbance to grow. The Kelvin-Helmholtz instability is often seen in clouds, both on Earth and on other planets, and it is also responsible for the shape of ocean waves. (Video credit: M. Hallworth and G. Worster)
The March of Drops
I love science with a sense of humor. This video features a series of clips showing the behavior of droplets on what appears to be a superhydrophobic surface. In particular, there are some excellent examples of drops bouncing on an incline and droplets rebounding after impact. For droplets with enough momentum, impact flattens them like a pancake, with the rim sometimes forming a halo of droplets. If the momentum is high enough, these droplets can escape as satellite drops, but other times the rebound of the drop off the superhydrophobic surface is forceful enough to overcome the instability and draw the entire drop back off the surface. (Video credit: C. Antonini et al.)
Researchers in Australia have 
