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.
Tag: droplets
Jumping Water Droplets
Superhydrophobic surfaces resist wetting from water, but it turns out they can also trigger interesting behaviors in the tiny droplets condensing on the surface. High-speed video reveals that when two condensate droplets coalesce, the energy released by surface tension causes the new droplet to jump off the surface. The phenomenon is the same as one observed in some types of mushroom–when a condensate droplet touches a wetted spore, the spore is ejected from the mushroom. (Video credit: J. Boreyko)
The Gobbling Drop
A little polymer goes a long way when it comes to changing a fluid’s behavior. Normally, a falling jet of fluid will develop waviness and be driven by surface tension and the Plateau-Rayleigh instability to break up into a stream of droplets. We see this at our water faucets all the time. But when traces of a polymer are dissolved in water, the behavior is much different. The viscoelasticity of the polymer chains creates a force that opposes the thinning effects caused by surface tension. So, instead of thinning to the point of breaking into droplets, a drop is able to climb back up the jet until it reaches a critical mass where it reverses direction, accelerates downward due to gravity and eventually breaks off the jet. Then the whole process begins again with a new terminal drop. (Video credit: C. Clasen et al)
Making Mixed Emulsions
Ever tried to mix oil and vinegar? Anyone who has ever dealt with salad dressings knows the difficulty of evenly distributing immiscible fluids; the key is to shake them and create an emulsion, where droplets of one fluid are distributed throughout another. In this video, researchers create a double emulsion–oil in water in oil–without touching the two fluids. First they suspend a drop of water on a wire and then coat it with oil. Below, they place a bath of silicone oil, which they vibrate. When the oil-coated droplet falls onto the bath, it bounces on the surface rather than coalescing because a thin layer of air–constantly refreshed due to the vibration of the surface–separates the droplet from the bath. When the amplitude of the vibration is large enough, the oil coating penetrates the water during the bounce, leaving behind a tiny droplet and creating the emulsion. (Video credit: D. Terwange et al; Research paper)
Water Drops on Sand
This high-speed video captures the impact of liquid droplets onto a granular surface. While there is some similarity to liquid-solid and liquid-liquid impacts, the permeability of the granular surface helps to “freeze” the splash rather quickly. Energy is dissipated in the initial impact, causing a splash of grains. Then the surface tension, viscosity and inertia of the droplet compete in causing the deformations seen in the video. The deformation appears strongly dependent on the kinetic energy with which the droplet hits the surface (i.e. proportional to the height from which it is dropped). (Video credit: G. Delan et al)
Ejecting Drops
Large droplets ejected from a liquid pool do not coalesce immediately back into the whole. Instead, a thin layer of air gets trapped beneath them, much like the oil lubricating bearings. The weight of the droplet causes the air to drain away, and eventually the droplet comes in contact with the pool. Some of the droplet gets drained away before surface tension snaps the interface back into a low energy state. A new smaller droplet then bounces upward before repeating the process over again. Eventually the droplet becomes small enough that its entire mass gets sucked away by the pool. Researchers call this process the coalescence cascade.
Freezing in a Microchannel
Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devices–sometimes known as labs on a chip–are becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices. In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)
Ultrasonic Levitation of Drops
This video shows an ultrasonically levitated 3 mm drop of propylene glycol changing shape. A couple of things are happening here. Firstly, the drop is suspended due to the acoustic radiation pressure from intense ultrasonic sound waves being produced by a transducer vibrating at 30kHz. Then the power input to the ultrasonic transducer is increased, which strengthens the acoustic field, and this is what causes the drop to flatten. Currently, acoustic levitation is used for containerless processing of very pure materials or chemicals. As with many methods for levitation, it is currently restricted to objects of relatively light weight. (Video credit: J. R. Saylor et al, Clemson University)
Fragmenting Raindrops
This numerical simulation demonstrates the fragmentation of droplets of water falling through a quiescent medium–essentially how a raindrop behaves. As the initial droplet falls, drag forces deform the droplet, contorting it until surface tension causes it to break into smaller droplets, which can themselves be broken up by the same mechanisms.
Atomizing Jets
The breakup of impinging jets into droplets (also called atomization) and the subsequent dynamics of those droplets are important in applications like jet and rocket engines where the mixing of liquid fuel with oxygen is necessary for efficient combustion. This video showcases recent efforts in high fidelity numerical simulation and modeling of such flows. The complexity of the problem requires clever ways of reducing the computational efforts required. One such method uses adaptotive meshing to concentrate grid points in areas where variables are changing quickly while leaving the grid sparse in areas of less interest. Because the flow is constantly evolving, the mesh must be able to adapt as the simulation steps forward in time. Even so, such calculations typically require supercomputers to complete. (Video credit: X. Chen et al)