This timelapse video shows a particulate suspension as it dries and the pattern formation that results. The mixture of silicon dioxide particles and water is spread over a glass slide. As the water evaporates, capillary action generates a flow toward the edges, but the fluid meniscus pins larger particles to the glass, trapping them. As more and more water evaporates, smaller particles are trapped, causing the formation of uneven stripes in the particulate deposits. You’ve probably seen these patterns before on the side of a muddy car after a rainy day! (See also: how coffee rings form; Video credit: Bjornar Sandnes)
Tag: surface tension
Worthington Jet
A drop of sugar syrup falls into a pool of methylated spirits, producing a Worthington jet and several ejected droplets. Although surface tension holds the jet in a smooth shape, the refractive index of the spirits reveals the turbulent mixing within the jet. (Photo credit: Rebecca Ing)
Viscoelastic Fluids in Space
In honor of astronaut Don Pettit’s launch to the International Space Station (and in the hope that he’ll do more neat microgravity fluids demonstrations while in space!), here’s a look a the behavior of viscoelastic fluids in microgravity. The elasticity of these fluids means that, when strained, the fluid deforms instantaneously and then returns to its initial shape when the strain is removed. Pettit demonstrates both Plateau-Rayleigh instability behavior, where a column of fluid breaks apart due to surface tension variations, and die swell, where a fluid jet expands beyond the diameter of nozzle from which it was extruded. Such swelling is commonly caused by the stretching and relaxation of polymers in the fluid as they react to forces caused by the nozzle opening.
Sound Sculptures
This is another fun and artistic use of non-Newtonian fluids (paint) vibrating on a speaker cone for advertising purposes. The shear-thinning viscous properties of the paint vie with surface tension to create lovely instantaneous sculptures of color. Check out Canon’s Pixma ads for similar artwork.
Surface Tension Instability
Droplets of oleic acid spread across a thin film of glycerol on a silicon wafer. The shapes here are driven by hydrodynamic instabilities, particularly Marangoni effects due to the differences in surface tension between the two fluids. (Photo credit: A. Darhuber, B. Fischer and S. Troian)
Seed-Ejection via Raindrop
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We don’t often think of plants as using fluid dynamics aside from capillary action drawing water from their roots, but many plants also use fluid dynamics to disperse reproductive materials. This high-speed video explores the efficacy of splashing raindrops at ejecting seeds from different blossoms. (Video credit: G. Amador et al)
Freezing Drops
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The physics of droplets freezing is important for understanding applications like ice formation on airplane wings. Here we see how a warm droplet deposited on a cold plate freezes. A freezing front advances through the drop, which expands vertically as it freezes. Ultimately, the expansion of the ice and the surface tension of the water create a pointed singular tip.
High-Speed Droplet Collisions
This high-speed video shows the apparatus often used by photographers for fluid sculptures created from droplet collisions. As amazing as these formations are in still images, seeing their evolution at 5,000 fps is even more lovely.
The Disintegrating Bowl
A viscous fluid droplet impacts a thin layer of ethanol, which has a lower surface tension than the viscous fluid. A spray of tiny ethanol droplets is thrown up while a bowl-shaped crown of the viscous fluid forms. As the ethanol droplets impact the bowl, the lower surface tension of the ethanol causes fluid to flow away from points of contact due to the Marangoni effect. This outflow causes holes to form in the crown, forming a network of thin fluid ligaments. For more, see this paper (PDF) and video. (Photo credit: S.T. Thoroddson et al)
Convection Visualization
Here on Earth a fascinating form of convection occurs every time we put a pot of water on the stove. As the fluid near the burner warms up, its density decreases compared to the cooler fluid above it. This triggers an instability, causing the cold fluid to drift downward due to gravity while the warm fluid rises. Once the positions are reversed, the formerly cold fluid is being heated by the burner while the formerly hot fluid loses its heat to the air. The process continues, causing the formation of convection cells. The shapes these cells take depend on the fluid and its boundary conditions. For the pot of water on the stove and in the video above, the surface tension of the air/water interface can also play a role in modifying the shapes formed. The effects caused by the temperature gradient are called Rayleigh-Benard convection. The surface tension effects are sometimes called Benard-Marangoni convection.
