A water droplet falling onto a superhydrophobic surface will rebound and bounce without wetting the surface. Capillary and internal waves reflect in the drop until it comes to rest at a high contact angle, formed at the boundary where the liquid, solid, and air meet. Such surfaces can have interesting interactions with water, as when two droplets coalesce on a surface and then begin bouncing or when superhydrophobic objects are dropped into a bath. (Video credit: Gangopadhyay Group, University of Missouri)
Tag: droplets
Falling Oil
A drop of silicone oil falling through a liquid with lower surface tension distorts into multiple vortex rings connected by thin films. This behavior is caused by the interaction between viscous and capillary forces and is observable for only a narrow range of oil viscosities. (Photo credit: A. Felce and T. Cubaud)
Fireball in Slow Motion
The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter. It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines. Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion. The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.
Astro Puffs
Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops. Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)
Pinch-Off
This high-speed video reveals a fascinating bit of kitchen sink physics. When a water droplet pinches off from the nozzle, the thin filament of fluid that connected the droplet to the water on the nozzle often breaks off as well. Surface tension snaps the filament together into a sphere, causing wild oscillations and even ejection of microjets in the tiny satellite droplet. (Video from S. Thoroddsen et al. 2008’s Annual Review)
Oil in Alcohol
In this video two droplets of oil fall through a bath of isopropyl alcohol. The oil is denser than alcohol, and the two fluids are miscible. The velocity and density gradients where the two fluids meet generate hydrodynamic instabilities that create the distinctive patterns seen in the falling drops. (Video credit: BYU Splash Lab)
Liquid Pearls
Researchers create liquid pearls–a liquid droplet surrounded by a gel-like exterior–by dropping the fluid through a special bath. The initial droplet contains a mixture of the liquid core and an alginate solution. When the drop falls through a bath containing calcium ions, the alginate turns into a hydrogel shell around the liquid core. In order to prevent mixing during the droplet impact, researchers use a surfactant that helps the thin alginate layer persist while gelling takes place. The resulting liquid pearl is permeable to chemicals; researchers hope this may allow them to be used to contain microorganisms or cells in a three-dimensional environment during testing. (Video credit: New Scientist, N. Bremond et al.; see also Gallery of Fluid Motion)
The Chaos of a Bouncing Droplet
This video explores chaos in a bouncing droplet. A drop of silicon oil bounces on a vibrating bath of oil; the thin layer of air injected with each bounce between the droplet and bath keeps them from coalescing. Initially, the droplet behaves like a bouncing ball, jumping once per oscillation. As the vibration amplitude increases, the droplet begins making a small jump, then a large jump, then a small jump, and so on. This is called period doubling since the droplet now jumps in a pattern with twice the period of the original and is a hallmark of nonlinear dynamical systems. Further increase in the vibration amplitude leads to chaotic bouncing and occasional ejecta. (Video credit: D. Terwagne et al.)
How Mosquitoes Fly in the Rain
One might think that rainfall would keep the mosquitoes away, but it turns out that rain strikes don’t bother these little pests much. Because the insect is so small and light compared to a falling raindrop, the water bounces off instead of splashing. This results in a relatively small transfer of momentum, although the mosquito does get deflected quite strongly. Researchers estimate that the insects endure accelerations up to 300 times that of gravity, which is more than 10 times what a human can withstand. (Video credit: A. Dickerson et al; submitted by Phillipe M.)
Moving Droplets with Electric Fields
Many microfluidic devices employ techniques that manipulate droplet motion for applications like sorting, manufacturing, or precisely controlling chemical reactions at a small scale. The video above shows the oscillations of a droplet on an inclined surface as it is perturbed with an electric field. (Video credit and submission: K. Nichols)
