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)
Tag: science
The Archer Fish’s Arrows
The archer fish hunts by shooting a jet of water at insects in the leaves above and knocking them into the water. How the fish achieve this feat has been a matter of contention. A study of high-speed video of the archer’s shot shows that fluid dynamics are key. The fish releases a pulsed liquid jet, imparting greater velocity to the tail of the jet than the head. As a result, the tail tends to catch up to the head and increase the jet’s mass on impact while decreasing the duration of impact. Simultaneously, the jet tends to break down into droplets via the Rayleigh-Plateau instability caused by surface tension. Surface tension’s power to hold the water in droplets combined with the inertial effects of the pulsed jet create a ball of fluid that strikes the archer’s prey with more than five times the power than vertebrate muscles alone can impart. For more on archer fish, check out this video and the original research paper by A. Vailati et a. (Photo credits: Scott Linstead and BBC; submitted by Stuart R)
Rebounding
A ping pong ball bounces off a puddle, drawing a liquid column upward behind it. This photo shows the instant after the fluid has disconnected from the ball, allowing it to rebound without further loss of momentum to the fluid. The fluid column begins to fall under gravity, the tiny undulations in its radius growing via the Rayleigh-Plateau instability and eventually causing the column to separate from the puddle. You can see the whole process in action in this high-speed video. (Photo credit: BYU Splash Lab)
Following a Breaking Wave
It’s fascinating to sit on the beach and watch the waves roll in and break, but rarely do we get a view like the one in this video. Here researchers have created a breaking wave in a wave tank and recorded the wave as it travels the length of the tank with a high-speed camera moving at the same speed as the wave crest. This perspective, moving alongside the fluid, is a Lagrangian coordinate system; if one instead stood still and watched the wave roll past, it would be an Eulerian measurement. Traveling with the wave, we can see how a lip forms on the wave crest, then rolls down, capturing a tube of air. As water begins to flow over the lip, perturbations grow, causing ripples in the laminar curtain. Then the water strikes the main wave and rebounds turbulently, creating a familiar white cap. In the second half of the video, the process is shown from above, highlighting the entrainment of air and the creation of the bubbles that form the white cap of a breaking wave. (Video credit: R. Liu et al)
Peering Inside the Kettle
Here natural convection is explored experimentally in a quasi-2D environment. The researchers demonstrate how this phenomenon, which is much like that seen in a boiling pot, can be investigated by measuring the refractive distortions caused by the thin heated fluid layer. They also demonstrate types of boiling that can occur. Typically, bubbles nucleate at the heated surface and then rise to pull hot fluid with them. At high enough temperatures above the liquid’s boiling point, however, an unstable layer of vapor can form over the heated surface. This “boiling crisis” or critical heat flux produces a marked reduction in heat transfer due to the insulation provided by the vapor layer. (Video credit: S. Wildeman et al.)
“Surface Tension”
From a series called “Surface Tension,” these ink and water drawings by Marguerite French explore the effects of diffusion, surface tension, and evaporation. The forms left by the thin layer of liquids suggest other natural processes like erosion, weathering, and the rings inside trees. (Photo credits: Marguerite French)
Cavitation in a Bottle
Sudden changes in the pressure or temperature in a liquid can create bubbles in a process known as cavitation. Underwater explosions are just one of the ways to induce cavitation in a liquid. As identified in the above video, the shock waves traveling through the liquid force a change in pressure that creates bubbles. When these bubbles collapse, the container is subjected to an enormous oscillation in pressure, which often results in damage. The same phenomenon is responsible for damage on boat propellers as well as this beer bottle smashing trick. Check out these other high-speed videos of cavitation in a bottle: (Video credit: Destin/Smarter Every Day; submitted by Juan S.)
Those Funny Fins on Airplane Wings
Ever look out an airplane’s window and wondered why a row of little fins runs along the upper side of the wing? These vortex generators help prevent a wing from stalling at high angle of attack by keeping flow attached to the surface. Airflow over the vanes creates a tip vortex that transports the higher-momentum fluid from the freestream closer to the wing’s surface, increasing the momentum in the boundary layer. As a result of this momentum exchange, the boundary layer remains attached over a greater chordwise distance. This also increases the effectiveness of trailing-edge control surfaces, like ailerons, on the wing. (Photo credit: Mark Jones Jr.)
Liquid Mushrooms
The Rayleigh-Taylor instability can form at the interface between two liquids of different density under the influence of gravity, but a similar instability can occur in the absence of gravity. The image sequence above shows the Richtmyer-Meshkov instability, which occurs between two liquids of differing densities (regardless of their orientation) when impulsively accelerated. In this case, the experiment was conducted in a drop tower to simulate microgravity with the apparatus dropped on a spring to provide the impulse. As the instability grows, asymmetries appear. Nonlinear dynamics will amplify these distortions, eventually leading to turbulent breakdown. (Photo credit: C. Niederhaus/NASA Glenn, J. Jacobs/University of Arizona)
Air Entrainment
When a liquid jet falls into a pool, air is often entrained along with the liquid, creating a cavity and, often, bubbles. Shown above is video of a low-speed laminar jet entering a quiescent pool. The jet appears to entrain a thin film of gas, which then breaks up in a three-dimensional fashion, despite the symmetry of the incoming jet. As the speed of the incoming jet is increased and turbulence is introduced, the resulting air entrainment becomes violent and chaotic. For additional information and videos, see Kiger and Duncan 2012 and their supplemental videos. (Video credit: K. Kiger and J. Duncan)
