Artist Skye Kelly’s “Creep (strain)” sculpture shown above is made from toffee. The viscous fluid deforms under the force of gravity, resulting in elongated drips and slow jets that buckle and coil upon reaching the floor. (Photo credits: Skye Kelly; via freshphotons)
A ball dropped onto a puddle loses some of its rebound momentum to fluid motion. On impact, a splash curtain and radial jet form as the fluid is displaced by the ball. As the ball rebounds, the splash curtain is drawn inward into a column of fluid drawn up by the ball, reminiscent of the way cats and dogs drink. Eventually, when the gravity’s force on the fluid column overcomes the force of the ball’s inertia, the fluid column pinches off and falls back downwards, leaving the ball free to utilize its remaining kinetic energy as it flies upward. (Photo credit: T. Killian, K. Langley, and T. Truscott)
The liquid rope coiling effect occurs in viscous fluids like oil, honey, shampoo, or even lava when they fall from a height. The exact behavior of the coil depends on factors like the fluid viscosity, the height from which the fluid falls, the mass flow rate, and the radius of the falling jet. Here Destin of the Smarter Every Day series outlines the four regimes of liquid coiling behavior commonly observed. As with many problems in fluid dynamics the regimes are described in terms of limits, which can help simplify the mathematics. The viscous regime (2:34 in the video) exists in the limit of a small drop height, whereas the inertial regime (3:15) exists in the limit of large drop height. Many complicated physical problems, including those with nonlinear dynamics, are treated in this fashion. For more on the mathematics of the coiling effect, check out Ribe 2004 and Ribe et al. 2006. (Video credit: Destin/Smarter Every Day; submitted by inigox5)
Fluid motion is captured as a floral still life in these high-speed photos by Jack Long. The artist keeps mum about his set-up but notes that these are single capture events, not constructed composites. It looks as if the blossoms are created from the impact of a falling fluid with the upward jet that forms the stem. The leaves and vase appear to be created from upward splashes, but whether those are generated by vibration or dropping an object is unclear. See Long’s Flickr page for more. (Photo credit: Jack Long via Gizmodo)
Spin a hard boiled egg in a puddle of milk and you get a sprinkler. But how? The science starts at the surface. When the egg spins, the fluid touching its surface is dragged along due to friction, and, because of the fluid’s viscosity, other parts of the fluid will also be spun. Dynamics tells us that the velocity at the surface of the object varies with radius; the velocity at the bottom of a spinning sphere is much smaller than that at its equator because a particle at the equator traverses a larger distance in a single rotation. Likewise, the fluid touching the bottom of the egg is spun slower than the fluid just above it. Bernoulli’s principle tells us that, for an incompressible fluid, the pressure decreases as velocity increases, meaning that a favorable pressure gradient exists along the spinning convex surface. It is this pressure gradient that draws the fluid up the sides of the object. Near the equator, the pressure gradient is weakest and centrifugal force flings the the fluid outward. Surface tension, angular velocity, and viscosity all play a role in the jets and sheets created by the sprinklers. (Video credit: NPR Science Friday with Tadd Truscott et al)
Reader David L asks:
I understand that fire is a form of energy rather than a fluid in the physical/tangible sense. However, is it possible for fire to exhibit fluid-like behaviours to a certain extent.
In other words, could the dynamic properties of fire be described with pseudo-variables analogical to variables that describe a physical fluid (i.e. viscosity, density, Re, etc.)?
Actually, combustion is a major topic of research among fluid dynamicists. Since the part of fire that we identify as visible flame is a reacting mixture of gas and some solid particles, it moves according to the same equations of motion as any other gas. However, when studying combustion thermodynamical equations and chemical reactions must also be tracked in addition to mass and momentum, which makes modeling fire very difficult. Combustion plays a major role in internal flows like those in car, jet, and rocket engines. (Photo credit: master.blitzy)
Sometimes bursting one bubble just leads to more bubbles. This high-speed video shows how popping a bubble sitting on a fluid surface can lead to a ring of daughter bubbles. When the surface of the bubble is ruptured, filaments of the liquid that made up the surface are drawn back toward the pool by surface tension, trapping small pockets of the air that had been inside the bubble. A dimple forms on the surface and rebounds as a jet that lacks the kinetic energy to eject droplets. Watch as the jet returns to the interface, and you will notice the tiny bubbles around it. At 56 ms, one of the daughter bubbles on the left bursts. See Nature for more. (Video credit: J. Bird et al)
This high-speed video shows the behavior of oil on a vibrating surface. As the amplitude of the vibration is altered various behaviors can be observed. Initially small waves appear on the surface of the oil, then the surface erupts into a mass of jets and ejected droplets, reminiscent of a vibrated interfaces within a prism or vibration-induced atomization. When the amplitude is reduced after about half a minute, we see Faraday waves across the surface, as well as tiny droplets that bounce and skitter across the surface. They are kept from coalescing by a thin layer of air trapped between the droplet and the oil pool below. Because of the vibration, the air layer is continuously refreshed, keeping the droplet aloft until its kinetic energy is large enough that it impacts the surface of the oil and gets swallowed up.
This video creates the illusion of a jet of water frozen in mid-air. The effect is achieved by vibrating the water at the frequency of the speaker, then filming at a frame rate identical to the vibrational frequency. Thus the water pulses at the exact rate that the camera captures images, making the water appear stationary even though it is moving. (submitted by Simon H)
