The Richtmyer-Meshkov (RM) instability occurs when the interface between two fluids of different density is impulsively accelerated – usually by the passage of a shock wave. The image above shows a thin layer of gaseous sulfur hexafluoride embedded in air. Each vertical line, from left to right, shows the distortion of the two fluids at subsequent time steps after a Mach 1.2 shock wave passes through the gases. The interface’s initial waviness grows into mushroom-like shapes that mix the two gases together, ultimately leading to turbulence. Scenarios involving the RM instability include supersonic combustion ramjet engines, supernovas, and inertial confinement fusion. The RM instability is closely related to Rayleigh-Taylor instability and shares a similar morphology. (Photo credit: D. Ranjan et al.)
Category: Research
Evaporating Drops
When still drops evaporate from a surface, they do so in several phases, as illustrated in the video above. Initially, the drop forms a spherical cap. At this point the velocity within the droplet is so small that it is difficult to resolve, but particles within the drop move outward toward the contact line. As the drop evaporates, they form a circle of sediment – the familiar coffee ring. As the drop flattens, radial velocity increases, drawing more and more particles to the coffee ring. Eventually the drop pulls away from the ring, leaving surface tension and evaporation to compete in driving the internal flow. During this phase, some parts of the contact line try to re-establish the flow pattern that made the first ring; this leaves behind circular segments broken up by the increasing instabilities in the contact line. In the final stage, surface tension smooths some of the irregularities and drives an inward velocity that leaves behind radial sediment segments. (Video credit: G. Hernandez-Cruz et al.)
Vibrating Droplets
When still, water drops sitting on a surface are roughly hemispherical, drawn into that shape by surface tension. But on a vibrating surface, the same water drop displays many different shapes, like those in the video above. Researchers have observed more than 30 different mode shapes by varying the driving frequency. The metal mesh placed beneath the glass on which the drops sit helps the researchers determine the drop’s shape. As the drop deforms, the mesh appears to distort due to the refraction of light through the changing shape of the drop’s water-air interface. The distortion allows observers to visualize (and in some experiments even reconstruct) the shape of the drop’s surface. Understanding this kind of droplet behavior is valuable for many applications, including ink-jet printing and microfluidic devices. (Video credit: C. Chang et al.; via Science)
Elastic Walls and Viscous Fingers
The Saffman-Taylor instability, characterized by the branchlike fingers formed when a less viscous fluid is injected into a more viscous one, is typically demonstrated between two rigid walls, as in part (a) of the figure above. But what happens if one of the rigid walls forming the Hele-Shaw cell is replaced with an elastic wall? This is the case for (b) and (c) in the figure. The flexibility of the wall causes the expansion of the air-fluid interface to slow down relative to the rigid wall case and causes the interface to move toward a narrowing fluid-filled gap (as opposed to a constant thickness one). Both of these effects reduce the viscous instability mechanism that drives the fingering instability. With a high enough mass flow rate as in ©, there is still some instability in the interface, but it is dramatically reduced. (Photo credit: D. Pihler-Puzovic et al.)
Rebounding Off Dry Ice
Droplet rebound is frequently associated with superhydrophobic surfaces but can also be generated by very large temperature differences. For very hot substrates, a thin layer of the drop vaporizes on contact via the Leidenfrost effect and helps a drop rebound by preventing it from wetting the surface. This video shows almost the opposite: a water droplet hitting solid carbon dioxide (-79 degrees C). Upon contact, the solid carbon dioxide sublimates, creating a thin layer of gas that separates the droplet from the surface. You can also see the vortex ring that accompanies the drop’s impact. Water vapor near the carbon dioxide surface has condensed into tiny airborne droplets that act as tracer particles that reveal the vortex’s formation and the rebounding droplet’s wake. (Video credit: C. Antonini et al.; Research paper)
Navigating the Interface
Walking on water may be the stuff of legend at human scales, but it’s a fact of everyday life for many smaller species. Waxy, hydrophobic coatings typically make such insects’ points of contact (feet, legs, etc.) water-repellent, and their light weight can be supported by surface tension. Navigating the interface between air and water is more complicated, though, and these creatures have evolved several mechanisms to help. Some, like water striders, use appendages they insert below the surface for propulsion. At 0:49 in the montage above, you can see flow visualization of the vortices generated by a stroke. Other insects release a chemical in their wake that lowers the local surface tension and drives them away via the Marangoni effect. For more, see here and especially this Physics Today article. (Video credit: D. Hu and J. Bush)
Contaminants Flowing Uphill
Here’s an example of some baffling fluid dynamics. Researchers have found that, when pouring a fluid from one container into a lower one containing a fluid with floating particulates, it’s possible for the floating particles to travel upstream against gravity and the flow. The phenomenon is driven by surface tension. The particulates floating in the lower container decrease the fluid’s surface tension relative to the pure fluid pouring in from above. This creates a gradient in surface tension that, via the Marangoni effect, drives a small flow upstream, in the direction of the greater surface tension. In the video above, this flow takes the form of two recirculating vortices in the pouring channel, oriented such that their upstream velocities run along the outside of the channel. Occasionally this flow draws particulates up the waterfall and into the recirculating zones, creating upstream contamination. We reported this previously, but the researchers have now released videos demonstrating the effect, including in pipettes and a water flume. Usually it’s taken for granted that matter cannot move upstream, so this could be a game-changer, especially at small scales where surface tension already dominates. For more, see their paper. (Video credit: S. Bianchini et al.)
Convection on the Sun
New photographs showing ultra-fine structure in the sun’s chromosphere and photosphere have been released. They offer a fascinating view into the magnetohydrodynamics of the sun, where the fluid behaviors of plasma are constantly modified by the sun’s magnetic field. The left image shows fine-scale magnetic loops rooted in the photosphere, while the right image shows our clearest photo yet of a sunspot. The dark central portion is the umbra, where magnetic field lines are almost vertical; it’s surrounded by the penumbra, where field lines are more inclined. Further out, we see the regular convective cell structure of the sun. (Photo credit: Big Bear Solar Observatory/NJIT; via io9 and cnet)
Bubbles With Tails
In water and other Newtonian fluids, a rising bubble is typically spherical, but for non-Newtonian fluids things are a different story. In non-Newtonian fluids the viscosity–the fluid’s resistance to deformation–is dependent on the shear rate and history–how and how much deformation is being applied. For rising bubbles, this can mean a teardrop shape or even a long tail that breaks up into fishbone-like ligaments. The patterns shown here vary with the bubble’s volume, which affects the velocity at which it rises (due to buoyancy) and thus the shear force the bubble and surrounding non-Newtonian fluid experience. (Video credit: E. Soto, R. Zenit, and O. Manero)
Flow Over a Delta Wing
Fluorescent dye illuminated by laser light shows the formation and structure of vortices on a delta wing. A vortex rolls up along each leading edge, helping to generate lift on the triangular wing. As the vortices leave the wing, their structure becomes even more complicated, full of lacy wisps of vorticity that interact. Note how, by the right side of the photo, the vortices are beginning to draw closer together. This is an early part of the large-wavelength Crow instability. Much further downstream, the two vortices will reconnect and break down into a series of large rings. (Photo credit: G. Miller and C. Williamson)
