It is common in many industries to use oil as a defoamer to break up existing foams or prevent foams from forming. But with the right surfactants–additives that change the foam’s surface tension–it’s possible to make aqueous foams that are actually stabilized by the presence of oil. This video explores some of the ways that oil can interact with these kinds of foam, beginning with capillary action, which draws the oil up into the junctions between foam films. For more, see Piroird and Lorenceau. (Video credit and submission: K. Piroird)
Category: Research
Pointed Drops
When water droplets sit on a cold substrate, they freeze into a shape with a pointed tip. At first glance, this behavior seems very odd since surface tension usually acts to prevent such sharp protrusions. The shape is, however, a result of water’s expansion as it freezes. The droplet freezes from the substrate upward, with a concave shape to the solidification front. The angle of the point does not depend on the substrate temperature or the wetting angle between the water and surface. Instead, it turns out that this concave front shape and water’s expansion are the key factors that determine the pointed cusp’s angle, and that the final geometry of the cusp is essentially universal. (Video credit: M. Nauenberg; additional research credit: A. Marin et al.)
Granular Jet
Sometimes the similarity between fluid flow and granular flows is quite striking. This video shows a stream of sand falling down a tube and impacting a rod. (Note: the view is rotated 90 degrees counter-clockwise, so down points to the right.) As the sand strikes the rod, it’s deflected into a conical sheet, very much like a water bell. There are even ripple-like instabilities that form in the granular sheet, though they move differently than in a liquid due to the sand’s lack of surface tension. (Video credit: S. Nagel et al.)
Inside a Splash
When a droplet strikes a pool, a thin, fast-moving sheet of liquid expands outward from the region of contact. These ejecta sheets come in many forms depending on surface tension, viscosity, air pressure, and droplet momentum. When the ejecta sheet curls downward to touch the pool, it can spray microdroplets outward or trap a layer of air underneath the droplet. For more, see this video by S. Nagel et al., and the papers Thoroddsen (2002) and Thoroddsen et al. (2008). (Photo credits: S. Thoroddsen et al.; GIF from this video by S. Thoroddsen et al.)
Viscous Fingers
Viscous liquid placed between two plates forms a finger-like instability when the top plate is lifted. The photos above show the evolution of the instability for four initial cases (top row, each column) in which the initial gap between the plates differs. Each row shows a subsequent time during the lifting process. As the plate is pulled up, the viscous liquid adheres to it and air from the surroundings is entrained inward to replace the fluid. This forms patterns similar to the classic Saffman-Taylor instability caused when less viscous fluid is injected into a more viscous one. (Photo credit: J. Nase et al.)
Tiny Fliers
There’s an apocryphal story claiming that, aerodynamically speaking, honeybees should not be able to fly. Obviously, they can, but it’s true that a small, flapping creature and a large, fixed-wing aircraft will not generate lift exactly the same way. NYU professor Leif Ristroph has a lot of projects exploring flapping flight on smaller scales, as seen in this video. His oscillatory fliers and rotating flapping flight simulator have both been featured previously. Part of the beauty of these projects is their size; in a field that’s historically required giant wind tunnels and room-length wave tanks, Ristroph’s work provides insight into long-standing problems using apparatuses that fit on a countertop. (Video credit: Cool Hunting/L. Ristroph et al.)
Coalescence
The coalescence of two liquid droplets takes less than the blink of an eye, but it is the result of an intricate interplay between surface tension, viscosity, and inertia. The high-speed video above was filmed at 16000 frames per second, yet the initial coalescence of the silicone oil drops is still nearly instantaneous. At the very instant the drops meet, an infinitesimally small neck is formed between the droplets. Mathematically speaking, the pressure and curvature of the droplets diverge as a result of this tiny contact area. This is an example of a singularity. Surface tension rapidly expands the neck, sending capillary waves rippling along the drops as they become one. (Video credit: S. Nagel et al.; research credit: J. Paulsen)
Sneezes Vs. Coughs
Sneezing and coughing are major contributors to the spread of many pathogens. Both are multiphase flows, consisting of both liquid droplets and gaseous vapors that interact. The image on the left shows a sneeze cloud as a turbulent plume. The kink in the cloud shows that plume is buoyant, which helps it remain aloft. The right image shows trajectories for some of the larger droplets ejected in a sneeze. Like the sneeze cloud, these droplets persist for significant distances. The buoyancy of the cloud also helps keep aloft some of the smaller pathogen-bearing droplets. Researchers are building models for these multiphase flows and their interactions to better predict and counter the spread of such airborne pathogens. For similar examples of fluid dynamics in public health, see what coughing looks like, how hospital toilets may spread pathogens, and how adjusting viscoelastic properties may counter these effects. For more about this work, see the Bourouiba research group’s website. (Image credit: L. Bourouiba et al.)
Harnessing Ocean Waves
Ocean waves contain substantial amounts of energy, and many projects are underway to harness them as renewable energy sources. Most of these projects use the motion caused by waves to generate electrical energy. In this example, a flexible carpet is attached to hydraulic pumps. As the waves move over the carpet, it oscillates, raising and lowering the piston of the pumps. This adds hydraulic pressure to the discharge lines that run from the wave carpet to the shore. Once on dry land, that hydraulic pressure can be converted to electrical energy. This design addresses one of the major challenges in ocean-wave-energy technologies–namely how to safely transmit power from the wave farm to the shore. (Video credit: University of California Television)
Supernova Core Collapse
A core-collapse, or Type II, supernova occurs in massive stars when they can no longer sustain fusion. For most of their lives, stars produce energy by fusing hydrogen into helium. Eventually, the hydrogen runs out and the core contracts until it reaches temperatures hot enough to cause the helium to fuse into carbon. This process repeats through to heavier elements, producing a pre-collapse star with onion-like layers of elements with the heaviest elements near the center. When the core consists mostly of nickel and iron, fusion will come to an end, and the core’s next collapse will trigger the supernova. When astronomers observed Supernova 1987A, the closest supernova in more than 300 years, models predicted that the onion-like layers of the supernova would persist after the explosion. But observations showed core materials reaching the surface much faster than predicted, suggesting that turbulent mixing might be carrying heavier elements outward. The images above show several time steps of a 2D simulation of this type of supernova. In the wake of the expanding shock wave, the core materials form fingers that race outward, mixing the fusion remnants. Hydrodynamically speaking, this is an example of the Richtmyer-Meshkov instability, in which a shock wave generates mixing between fluid layers of differing densities. (Image credit: K. Kifonidis et al.; see also B. Remington)
