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)
Tag: Faraday waves
The Vibrating Network
We’ve seen the Faraday instability on vibrating fluids (and granular materials) before. Here researchers explore the effect on a a network of fluid-filled cells. Each square is filled with liquid and small holes near the bottom of each cell ensure the liquid levels are the same throughout the array. Then the entire container is vibrated. Above the threshold frequency, standing waves form but do not interact. When the wave amplitudes grow high enough for fluid to get exchanged from cell to cell, patterns begin to form. The waves in adjacent cells synchronize, eventually resulting in a regular pattern across the entire grid. Order out of chaos.(Video credit: G. Delon et al.)
Dancing Sands
Here a collection of dry grains are vertically vibrated, creating a series of standing waves on the surface of the sand. The shapes of these Faraday waves are dependent upon the frequency of the vibration. Despite the solid nature of sand particles, this behavior is much the same as the behavior of a vibrated fluid.
Vibrating Oil
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.
Cornstarch Monsters
[original media no longer available]
Shaking a fluid surface often results in standing waves known as Faraday waves, but with a non-Newtonian fluid like oobleck, at some frequencies it’s possible to incite other behaviors. Oobleck is shear-thickening, meaning that its viscosity increases when force is applied. This is what allows it to develop finger-like protrusions under high frequency vibrations.
The Tibetan Singing Bowl
The vibration caused by rubbing a Tibetan singing bowl excites standing waves in a Faraday instability on the surface of water in the bowl. As the amplitude of excitation increases, jets roil across the surface, creating a spray of droplets, some of which actually bounce on the surface as it vibrates. For more see the BBC and SciAm articles.
Jets from Waves
When vibrated, fluid surfaces can exhibit standing waves known as Faraday waves. In this experiment, increased forcing of these standing waves causes the formation of a jet. Under the right conditions, as the standing wave collapses, a singularity forms on the fluid surface when velocity and surface curvature diverge. The narrow jet column forms as a result of the fluid’s kinetic energy getting focused by the collapse. For more, see this letter to Nature. #
Cornstarch Monsters
The patterns formed when vibrating a liquid on a speaker cone are standing waves known as Faraday waves. With a large enough amplitude, this produces some very cool effects with a shear-thickening non-Newtonian fluid like oobleck. (It would actually be interesting to see what happens when you vibrate a shear-thinning liquid like shampoo…) This video also details how you can set up this demonstration yourself at home.
Vibrating Fluid Interfaces
The Faraday instability forms when a fluid interface is vibrated. This high-speed video shows the differences in the shapes formed by a vibrated fluid interface when the two fluids are miscible–capable of mixing–and when they are immiscible–like oil and water. Note how the miscible interface breaks down quickly into turbulence, but the immiscible interface maintains a complex shape.
Waves on Cornstarch
A thin layer of the non-Newtonian fluid oobleck on a vibrating surface (in this case, a speaker) is a great way to show off nonlinear standing waves known as Faraday waves. The waves form because, under these circumstances, the flat surface of the air/oobleck interface has actually become unstable.