At your next party, you can break the bottom of a glass bottle with the palm of your hand and the power of fluid dynamics. As shown in the video above, striking the mouth of the bottle accelerates fluid at the bottom, lowering the local pressure below the vapor pressure and causing the formation of cavitation bubbles. When these bubbles collapse, they form very high temperatures and pressures for an instant, and it is this which can break the glass. (Video credit: J. Daily et al., BYU Splash Lab)
Videos
Jets from Hollows
Bubbles rising through a viscous fluid deform and interact. As they collapse into one another, the lower bubble induces a gravity-driven jet that projects upward into the higher bubble. The more elongated the bubble, the faster the jet. The same behavior is seen in the rebound of a cavity at the free surface of a liquid. The authors suggest a universal scaling law for this behavior. (Video credit: T. Seon et al.)
Viscoelastic Jets
Unlike Newtonian fluids, such as air and water, viscoelastic fluids exhibit non-uniform reactions to deformation. In this video, researchers explore the effects of this behavior when a liquid jet falls into another fluid. When fluids move past one another at different speeds in this manner, there is a shearing force which often leads to the wave-like Kelvin-Helmholtz instability between the fluids. Here we see for a variety of wavelengths how the breakdown of a Newtonian and viscoelastic jet differ. The Newtonian jets form clean lines and complicated tulip-like shapes, but the viscoelasticity of the non-Newtonian jets inhibits the growth of these instabilities, surrounding the central jet with wisps of escaping fluid. For more, see Keshavarz and McKinley. (Video credit: B. Keshavarz and G. McKinley)
Dancing Droplet Clusters
When a fluid surface is vibrated, it’s possible to bounce a droplet indefinitely on the surface without the droplet coalescing into the pool. This is because each bounce of the droplet replenishes a thin layer of air that separates the droplet and the pool. If many droplets are added to the surface, as in the video above, a clustering behavior is observed, with many droplets gathering together. There is a limit, however, to the size of the cluster based on the amplitude of vibration. If vibrational amplitudes are pushed to the point of creating Faraday waves–standing waves on the surface of the pool–then large clusters of droplets can be suspended and sustained. (Video credit: P. Cabrera-Garcia and R. Zenit; via io9; submitted by oneheadtoanother)
Detonation in a Bubble
Accidental releases of combustible gases in unconfined spaces can be difficult to recreate in a laboratory environment. Here researchers simulate the conditions using detonation inside a soap film bubble. Combustible gases are pumped inside the soap film and then a spark creates ignition. The resulting flame propagation is visualized using high-speed schlieren photography, making the density gradients in the flame visible. When the mixture of hydrogen fuel to air is balanced, the flame is spherically symmetric with a high flame speed. In contrast, weaker mixtures of fuel/air produce slow flame speeds and mushroom-like flames that leave behind unreacted fuel. This is due to buoyant effects; the time scale associated with buoyancy is smaller than that of the flame speed and chemical reactions when the fuel/air mixture is lean. (Video credit: L. Leblanc et al.)
Accidental Painting
Artist D. A. Siqueiros sometimes used a technique he referred to as “accidental painting” in his work, in which he would pour a layer of one color of paint and then pour a second color over it. The two colors would mix in striking patterns. Here researchers recreate the technique and analyze the fluid dynamics of it. Each paint has a slightly different density thanks to the pigments used to color them. When a denser paint is poured over a less dense one (as in the white on black in the video), this activates the Rayleigh-Taylor instability. The white paint will tend to sink down below the black paint due to gravity. At the same time, the spreading of the two paints also affects the shapes and patterns through mixing and diffusion. (Video credit: S. Zetina and R. Zenit)
Donut-Shaped Bubbles
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
Dribbling Droplets
Ethanol droplets on a hot copper plate bounce under the influence of electrostatic forces from a charged rod. The temperature of the plate is high enough that the droplet is supported by a thin vapor film, which is what keeps it from wetting the plate. Ethanol does not have the strong polarity that water does, but the hydroxyl group on one end does make it susceptible to the electrostatic charge built up on the teflon rod. As a result, the droplets oscillate under electrostatic and gravitational forces, resulting in a dribbling effect. (Video credit: S. Wildeman et al.)
Spray Starch
High speed video of of spray starch from a can. Once the initial transients die down, a cone-shaped annular sheet forms. Disturbances propagate in the sheet, tearing it into filaments that break down into droplets. Beautiful complexity hidden in a simple everyday device. (Video credit: John Savage)
Dynamic Leidenfrost Impact
The Leidenfrost effect occurs when a liquid encounters a solid object much hotter than the liquid’s boiling point, like when water skitters on a hot griddle or someone plunges a hand in liquid nitrogen. A thin layer of vapor forms between the liquid and the solid, thereby (briefly) insulating the remaining liquid. The Leidenfrost effect can be static–like a droplet sitting on a pan–or dynamic, like the video above in which a droplet impacts the hot object. The video shows both a top and a side view of a droplet striking a plate that is over five times hotter than the liquid’s boiling point. On impact, the droplet spreads and flattens, and a spray of even tinier droplets is ejected before rebound. (Video credit: T. Tran and D. Lohse, from a review by D. Quere)