Most everyone is familiar with the difficulty of getting ketchup out of its bottle. Part of the trouble is that ketchup is a shear-thinning fluid, meaning that its viscosity decreases with an increasing rate of shear. Thus, a shear-thinning fluid flows better once it starts moving. This is why the ketchup moves much faster once it is initially disturbed. LiquiGlide, a new coating material demonstrated above, has gained a lot of popular attention in the press recently for solving the difficulty of the stuck condiments. It appears that the coating reduces the static coefficient of friction between the food and the bottle, meaning that the ketchup starts sliding down the wall even before an increase in shear stress starts the flow. (submitted by @szescstopni)
Month: May 2012
Floral Still Life
Fluid motion is captured as a floral still life in these high-speed photos by Jack Long. The artist keeps mum about his set-up but notes that these are single capture events, not constructed composites. It looks as if the blossoms are created from the impact of a falling fluid with the upward jet that forms the stem. The leaves and vase appear to be created from upward splashes, but whether those are generated by vibration or dropping an object is unclear. See Long’s Flickr page for more. (Photo credit: Jack Long via Gizmodo)
Hydrophobic Water Entry
Many factors can affect the size and shape of the splash when an object impacts water and wettability–the ability of a liquid to maintain contact with a solid–is one of them. Here a sphere coated in a hydrophobic (water-repellent) nano-layer impacts water, creating a large air, streaky air cavity and a substantial splash. Contrast this with the behavior of a hydrophilic sphere entering the water, and you can imagine divers might want to invest in some hydrophilic coatings prior to the London Olympics. (Video credit: L. Bocquet et al)
Simulated Turbulence
This image, taken from a direct numerical simulation, shows turbulence in a stably stratified flow in which lighter fluid sits atop a denser fluid. In the image lighter colors represent denser fluid. Turbulence is created by the shear forces caused when the lighter fluid on top moves faster than the denser fluid on the bottom; however the stable stratification will tend to counteract or stabilize the turbulence. Note the vast variety and detail of the scales involved in turbulence; this is what makes it such a difficult process to simulate and model. (Image credit: G. Matheou and D. Chung, NASA/JPL-Caltech)
Viscous Fingers
When less viscous fluids are injected into a more viscous medium, the low-viscosity fluid forms finger-like protrusions into the background fluid. This is known as the Saffman-Taylor instability. The video above shows this effect but in a more dynamic setting. Blue-dyed water and a clear solution of water and glycerol fifty times more viscous than the water are injected in alternating fashion to a microfluidic channel. The blue water spreads into the clear glycerol solution via fingers that quickly diffuse, creating a homogeneous–or uniform–mixture. (Video credit: Juanes Research Group)
Microgravity Cornstarch
We’ve seen the effects of vibration on shear-thickening non-Newtonian fluids here on Earth before in the form of “oobleck fingers” and “cornstarch monsters”, but, to my knowledge, this is the first such video looking at the behavior in space. The vibrations of the speaker cause shear forces on the cornstarch mixture, which causes the viscosity of the fluid to increase. This is what makes it react like a solid to sudden impacts while still flowing like a liquid when left unperturbed. In microgravity there is one less force working against the rise of the cornstarch fingers, so the formations we see in this video are subtly different from those on Earth.
Jet Collisions
When two jets of liquid collide, they form a sheet of fluid. As the speeds of the jets change, the sheet can become unstable, forming a set of liquid ligaments and droplets that look like a fish’s bones. This is shown in the video above. For purposes of orienting yourself, flow in the video is moving right to left and the video has been rotated 90-degrees clockwise (i.e. the two out-of-frame jets forming the flow seen are falling due to gravity). (Video credit: Sungjune Jung, University of Cambridge)
Soap Bubbles Bursting
To the human eye, the burst of a soap bubble appears complete and instantaneous, but high-speed video reveals the directionality of the process. Surface tension is responsible for the spherical shape of the bubble, and, when the bubble is pierced, surface tension is broken, causing the soap film that was the bubble to contract like elastic that’s been stretched and released. Droplets of liquid fly out from the edges of the sheet until it atomizes completely.
Stalling a Wing
At small angles of attack, air flows smoothly around an airfoil, providing lifting force through the difference in pressure across the top and bottom of the airfoil. As the angle of attack increases, the lift produced by the airfoil increases as well but only to a point. Increasing the angle of attack also increases the adverse pressure gradient on the latter half of the top surface, visible here as an increasingly thick bright area. Over this part of the surface, the pressure is increasing from low to high–the opposite of the direction a fluid prefers to flow. Eventually, this pressure gradient grows strong enough that the flow separates from the airfoil, creating a recirculating bubble of air along much of the top surface. When this happens, the lift produced by the airfoil drops dramatically; this is known as stall.
