A ball dropped onto a puddle loses some of its rebound momentum to fluid motion. On impact, a splash curtain and radial jet form as the fluid is displaced by the ball. As the ball rebounds, the splash curtain is drawn inward into a column of fluid drawn up by the ball, reminiscent of the way cats and dogs drink. Eventually, when the gravity’s force on the fluid column overcomes the force of the ball’s inertia, the fluid column pinches off and falls back downwards, leaving the ball free to utilize its remaining kinetic energy as it flies upward. (Photo credit: T. Killian, K. Langley, and T. Truscott)
Category: Research
Rotating or Not-Rotating?
Rotating a fluid often produces different dynamical behavior than for a non-rotating fluid. Here this concept is demonstrated by dropping creamer into a tank of water. Both experiments produce a turbulent plume, but the way the plume spreads and diffuses is much different in the case of the rotating tank, thanks to the Coriolis effect. (Video credit: SPINLab UCLA)
Getting Ketchup to Flow
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)
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)
Martian Lava Coils
NASA’s HiRISE spacecraft has sent back images of lava coils left on the surface of Mars. These features form when lava flows of different speeds move past one another; they’re essentially Kelvin-Helmholtz waves–like the ones often seen in clouds–in the lava flow that have solidified into solid rock! On Earth these coils appear about a foot wide; the Martian versions are 100 feet across. (Photo credit: NASA/JPL/University of Arizona; via Wired; submitted by Brian L)
Why Walking with Coffee is Tough
Almost everyone is familiar with the problem of coffee or tea sloshing over the sides of a mug as one walks, but this may be the first time researchers have systematically studied the problem. The results show that the typical frequency of the human stride closely matches the natural frequency for back-and-forth sloshing of a low-viscosity liquid in a cylindrical container the size of a typical coffee mug. Even though our natural side-to-side motion plays a role in coffee sloshing, its effect is small in comparison. A person’s maximum acceleration, which usually happens early on when walking, sets the initial sloshing amplitude, which is subsequently amplified by the stepping frequency. The researchers did find that the time to spill increased substantially if the subject was focused on not spilling the coffee, though it was unclear if this was due to the subject decreasing their acceleration and step frequency, or whether they were actively damping the oscillations with adjustments in the wrist. If you’re a perpetual coffee spiller, there’s still hope: the authors suggest that flexible cups and/or cups with a series of concentric rings–baffles–could help reduce sloshing in spite of our natural tendency to induce it. (Photo credit: dongga/Flickr; Paper: Mayer and Krechetnikov; submitted by @__pj)
Egg Spinning
Spin a hard boiled egg in a puddle of milk and you get a sprinkler. But how? The science starts at the surface. When the egg spins, the fluid touching its surface is dragged along due to friction, and, because of the fluid’s viscosity, other parts of the fluid will also be spun. Dynamics tells us that the velocity at the surface of the object varies with radius; the velocity at the bottom of a spinning sphere is much smaller than that at its equator because a particle at the equator traverses a larger distance in a single rotation. Likewise, the fluid touching the bottom of the egg is spun slower than the fluid just above it. Bernoulli’s principle tells us that, for an incompressible fluid, the pressure decreases as velocity increases, meaning that a favorable pressure gradient exists along the spinning convex surface. It is this pressure gradient that draws the fluid up the sides of the object. Near the equator, the pressure gradient is weakest and centrifugal force flings the the fluid outward. Surface tension, angular velocity, and viscosity all play a role in the jets and sheets created by the sprinklers. (Video credit: NPR Science Friday with Tadd Truscott et al)
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)
