When liquids hit a surface much hotter than their boiling point, a thin layer of gas can form between the drop and surface, allowing the drop to glide along. This Leidenfrost effect is what makes drops of water skitter across a hot pan. But what happens when the pan isn’t flat? The video above shows a Leidenfrost drop on a ratchet-like surface. Instead of gliding or skittering randomly, the drop self-propels toward the steepest section of the ratchet This behavior allows researchers to design surfaces that guide the drops on an intended path. (Video credit: G. Lagubeau and D. Quéré)
Year: 2013
Spiraling Out of Coils
Anyone who has drizzled honey or another viscous fluid onto a surface is familiar with the rope-like coiling behavior of some liquids. But did you know that same instability can create spirals of bubbles like in this photo? Such behavior is only seen for a narrow range of parameters within the gravitational regime of liquid coiling. As the liquid falls, the center of coiling precesses along its own circle with a frequency much smaller than that of the coiling itself. This means that new coils do not fall exactly on top of old ones, trapping air bubbles between them. As the pile of coils collapses under gravity, the bubbles are carried outward, creating beautiful spiral patterns. (Photo credit: M. Habibi et al.)
Flapping Flags
Sometimes structural forces and aerodynamic forces combine to produce instabilities. One of the most common and familiar examples of this, a flag flapping in the breeze, remains extremely complex to analyze and describe. The flexibility of the flag, and its small but finite resistance to bending, combine with the variability of air flow around the flag to create a fascinating dance of effects. This same aeroelastic flutter can create disastrous results for structures and aircraft. For more on the flapping flag, see Argentina and Mahadevan (2004). (Video credit: S. Morris)
The Red Crown
A drop of red dye falls into a thin layer of milk, forming a crown splash. Notice the pale edges of the droplets at the rim of the crown; this is milk that has been entrained by the original drop. The rim and satellite droplets surrounding the splash are formed due to surface tension effects, chiefly the Plateau-Rayleigh instability–the same effect responsible for breaking a falling column of liquid into droplets like in a leaking faucet. The instability will have a most unstable wavelength that determines the number of satellite droplets formed. (Photo credit: W. van Hoeve et al., University of Twente)
Rock Skipping Tips
Almost everyone has tried skipping rocks across the surface of a pond or lake. Here Professor Tadd Truscott gives a primer on the physics of rock skipping, including some high-speed video of the impact and rebound. In a conventional side-arm-launched skip, the rock’s impact creates a cavity, whose edge the rock rides. This pitches the rock upward, creating a lifting force that launches the rock back up for another skip. Alternatively, you can launch a rock overhand with a strong backspin. The rock will go under the surface, but if there’s enough spin on it, there will be sufficient circulation to create lift that brings the rock back up. This is the same Magnus effect used in many sports to control the behavior of a ball–whether it’s a corner or free kick in soccer or a spike in volleyball or tennis. (Video credit: BYU Splash Lab/Brigham Young University)
Humpback-Inspired Turbine Blades
The bumps–or tubercles–on the edge of a humpback whale’s fins have important hydrodynamic effects on its swimming. Here dye is used to visualize flow over a hydrofoil with tubercle-like protuberances–a sort of artificial whale fin. Dye released from the peaks and troughs of the protuberances flows straight back in a narrow line before breakdown to turbulence. But the dye released from ports on the shoulders of the protuberances twists and spirals into vortices. At angle of attack, these vortices are stronger. They may help keep flow from separating on the upper side of a whale’s fin. (Photo credits: SIDwilliams, H. Johari)
Spin-Up
With the Oscars just over, it seems like a good time for some movie-trailer-style fluid dynamics. This video shows a rotating water tank from the perspective of a camera rotating with the tank at 10 rpm. Initially, the tank and its contents are at rest. When the tank begins spinning, the fluid inside responds. Pink potassium permanganate crystals at the bottom of the tank show fluid motion as they dissolve, and food coloring is spread on the water’s surface to show motion there. Fluid near the edge of the tank reaches the tank’s rotational velocity fastest, due to friction with the wall, while fluid near the center of the tank takes longer to spin up to speed. This creates the spiral-galaxy-like shape in the dye. Eventually viscosity will transmit the effects of the wall’s motion even into the center of the tank. (Video credit: UCLA Spinlab)
Dye Flow
Fluid flow near a surface–inside the boundary layer–can often be unstable. This image shows one possible instability, formed when a cylinder is rotated back and forth about its longitudinal axis. This oscillation and the curvature of the cylinder destabilize flow in the boundary layer, forming vortices that line the cylinder. This particular behavior is called a Görtler instability. To visualize it, threads soaked in fluorescing dye have been embedded into slits in the cylinder. The cylinder is oscillated in a water tank and ultraviolet light is used to fluoresce the dye for the image. (Photo credit: Miguel Canals/University of Hawaii)
Etna’s Eruption
After some rumblings in recent weeks, Italy’s Mt. Etna erupted overnight on February 19th, sending fountains of lava shooting into the dark. This impressive video from Klaus Dorschfeldt, a videographer for Italy’s National Institute for Geophysics and Vulcanology, shows the nighttime eruption, including the dark, turbulent outline of a pyroclastic flow of rock and hot gases escaping down the mountainside. Such flows can be devastating in their effect as they rush and spread down the mountain, flattening, burning, or engulfing everything in their path. When Mt. Vesuvius erupted in 79 A.D., it was the pyroclastic flow that buried the towns of Pompeii and Herculaneum. (Video credit: Klaus Dorschfeldt; via io9)
Fishbones
When two liquid jets collide, they can form an array of shapes ranging from a chain-like stream or a liquid sheet to a fishbone-type structure of periodic droplets. This series of images show the collision of two viscoelastic jets–in which polymer additives give the fluids elasticity properties unlike those of familiar Newtonian fluids like water. The jet velocities increase with each image, changing the behavior from a fluid chain (a and b); to a fishbone structure (c and d); to a smooth liquid sheet (e); to a fluttering sheet (f and g); to a disintegrating ruffled sheet (h), and finally a violently flapping sheet (i and j). The behavior of such jets is of particular interest in problems of atomization, where it can be desirable to break an incoming stream of liquid up into droplets as quickly as possible. (Photo credit: S. Jung et al.)
