This video shows a multi-layered droplet, in which several droplets are formed one inside the other as an initial drop falls through a layer of oil sitting atop another liquid. When the drop falls, its potential energy gets transformed into interface energy, creating a fascinating interplay of surface tension, deformation, and miscibility between the fluids. Such self-contained multi-layered droplets, similar to multiple emulsions, could be helpful in pharmaceutical development. (Video credit: E. Lorenceau and S. Dorbolo 2004)
Tag: surface tension
Droplet Bounce
This high-speed video shows the remarkable resilience of a water droplet upon impact against as a solid surface. The droplet deforms into a pancake-shape, with its center depressing almost flat before rebounding upward. The rest of the drop follows, splitting into several droplets as capillary waves dance across its surface. When one satellite drop almost escapes, the main droplet just barely comes in contact with it, the coalescence enough to tip surface tension into pulling them together instead of breaking them apart. (Video credit: K. Suh/ChemistryWorldUK)
Bubble Lenses
In this video, artist Jesse Zanzinger experiments with the lens-like refractive properties of bubbles. Though focused on the bending of light, there’s plenty here in terms of coalescence, surface tension, and miscibility. He has a similar video that includes a shot of his set-up here. (Video credit: J. Zanzinger)
Breaking into Droplets
A falling column of liquid, like the water from your faucet, will tend to break up into a series of droplets due to the Plateau-Rayleigh instability. This instability is driven by surface tension. Small variations in the radius of the column occur naturally. Where the radius shrinks, the pressure due to surface tension increases, causing liquid to flow away, which shrinks the column’s radius even further. Eventually the column pinches off and breaks into droplets. What’s especially neat is that the size of the final droplets can be predicted based on the column’s initial radius and the wavelength of its disturbances. (Video credit: BYU Splash Lab)
Electrowetting
The electrowetting effect can change the shape of a liquid droplet on a surface by applying a voltage across the surface and droplet. Surface tension is a kind of measure of the energy required to maintain a certain drop shape, and that energy can be both chemical and electrical. In the video above, the droplet maintains a small contact area naturally (with no voltage). It expands and flattens under an electrical charge. Varying the voltage will change the degree to which the droplet flattens, but only to a point. Electrowetting is used to control variable lenses and some types of electronic displays. The technology may be used to replace current generation LCDs. (Video credit: V. Arya/Duke University)
Self-Assembly via Evaporation
When working at the microscale, engineering structures like those used for drug delivery systems requires ingenuity. Since it isn’t possible to manipulate particles manually, researchers harness physical effects to do the work for them. Here a droplet filled with millions of polystyrene microparticles sits on a hydrophobic surface, which helps keep the drop’s spherical shape. As the drop evaporates, surface tension and internal flow in the drop help the microparticles self-assemble into a microscopic soccer-ball-like shape. (Video credit: A. Marin et al.; submission by A. Marin)
Tears of Wine
Wine drinkers may be familiar with the “tears of wine” often seen on the wall’s of a glass. The effect is a combination of evaporation and surface tension. As the low-surface-tension alcohol evaporates from the wine film left by swirling the glass, the higher local surface tension draws wine up the walls of the glass. Eventually enough wine gathers that droplets form and slide back down. This timelapse video shows how the beads form and move, almost dancing around the glass. The video’s author, Dan Quinn, has a second video with an awesome visual explanation of the behavior that’s well worth watching, too! (Video credit and submission: D. Quinn)
Washing Your Face in Space
What happens to a wet washcloth when wrung out in space? Astronaut Chris Hadfield answers this question from students with a demonstration. Without gravity to pull the water downward, surface tension effects dominate and the wrung cloth forms a tube of water around it. Surface tension and capillary action draw the fluid up and onto Hadfield’s hands as long as he holds the cloth. After he lets go, we see that the water remaining around the cloth soaks back in (again due to capillary action) and the wet, twisted washcloth simply floats without releasing water or relaxing its shape. While pretty much what I would have expected, this was a very cool result to see! (Video credit: C. Hadfield/CSA; submitted by Bobby E)
Liquid Sculptures
Water droplet art celebrates the infinite forms created from the impact of drops with a pool and rebounding jets. It’s a still life captured from split second interactions between inertia, momentum, and surface tension. These examples from photographer Markus Reugels are among some of the most complex shapes I’ve seen captured. Be sure to check out his website for more beautiful examples of liquids frozen in time. (Photo credits: Markus Reugels; via Photigy)
Stopping Jet Break-Up
When a stream of liquid falls, a surface tension effect called the Plateau-Rayleigh instability causes small variations in the jet’s radius to grow until the liquid breaks into droplets. For a kitchen faucet, this instability acts quickly, breaking the stream into drops within a few centimeters. But for more viscous fluids, like honey, jets can reach as many as ten meters in length before breaking up. New research shows that, while viscosity does not play a role in stretching and shaping the jet as it falls–that’s primarily gravity’s doing–it plays a key role in the way perturbations to the jet grow. Viscosity can delay or inhibit those small variations in the jet’s diameter, preventing their growth due to the Plateau-Rayleigh instability. In this respect, viscosity is a stabilizing influence on the flow. (Photo credit: Harsha K R; via Flow Visualization)
