Tag: fluid dynamics

Squeeze or Splatter?
Many a white shirt has met the disaster of a nearly-empty condiment bottle. One moment, you’re carefully squeezing out ketchup, and the next — sppplltlttt — you’re covered in red splatters. This messy phenomenon of gas displacing a liquid is widespread, showing up in condiments, some volcanic eruptions, and even the reinflation of a collapsed lung. Researchers have now constructed a mathematical model to fully capture and explain the process.
When you squeeze a container with both air and a liquid — like ketchup — in it, the air is easily compressed but the liquid is not. The extra pressure of the air creates a driving force that pushes the liquid out, despite its viscous resistance. Most of the time, these two forces are balanced, and the ketchup flows smoothly out of the container. But when the volume of ketchup is small compared to the air, squeezing can overpressurize the air, driving the ketchup out in an uncontrolled burst.
Luckily, the mathematics also suggest a solution to this problem: squeeze more slowly and double the size of the nozzle. You can also, they note, simply remove the top to avoid splatter. (Image credit: Rodnae Productions; research credit: C. Cuttle and C. MacMinn; via Ars Technica; submitted by Kam-Yung Soh)
December 27, 2022
Self-Propelled Droplets
Drops of ethanol on a heated surface contract and self-propel as they evaporate. My first thought upon seeing this was of Leidenfrost drops, but the surface is not nearly hot enough for that effect. Instead, it’s significantly below ethanol’s boiling point. Looking at the drops in infrared reveals beautiful, shifting patterns of convection cells on the drop. The patterns are driven by the temperature difference along the drop; at the bottom, the drop is warmest, and at its apex, it is coldest. Those differences in temperature create differences in surface tension, which drives a surface flow that breaks the drop’s symmetry. The asymmetry, the authors suggest, is responsible for the drop’s propulsion. (Image and video credit: N. Kim et al.)
December 26, 2022
“A Sense of Scale – Reminiscence”
In so much of fluid dynamics, size does not matter. We see the same patterns mirrored across nature from a fuel injection nozzle to galactic clusters. And no one plays with that sense of scale better than artist Roman De Giuli, whose microscale practical effects give the impression of flying above glittering alien coastlines. Ink and paint squeeze around craggy islands, leaving perfect streamlines to mark their passage. Fractal fingers expand like river deltas seeking the path to the sea. Enjoy more of De Giuli’s work on his website and Instagram. (Image and video credit: R. De Giuli; via Colossal)
December 23, 2022
Nanoconfined Water
Water is a decidedly weird substance. It’s densest above its freezing point; it has a slippery liquid-like layer on its solid form; and, in the right form, it can bend like a wire. So it’s not surprising that water demonstrates some odd behaviors when it’s confined inside a space so narrow it’s only one molecule thick.
A new, simulation-based study finds that this nanoscale-confined water flows with a wide variety of behaviors, depending on the temperature and pressure. In some conditions, the water ceases to act molecularly, with hydrogen atoms flowing through a lattice of oxygen atoms. These superionic forms were thought only to exist in the extreme conditions of a gas giant’s interior, but these simulations suggest we can find them under far milder circumstances. (Image and research credit: V. Kapil et al.; via Physics World; submitted by Kam-Yung Soh)
December 22, 2022
Simulating Schools
In nature, fish school for many reasons: protection from predators, increased sensing, and hydrodynamic advantages. To capture this complex behavior, researchers are building their own digital fish, governed by known rules. Here, scientists give each fish social rules — based on vision range and preferred distance from a neighbor — and hydrodynamic rules — based on a fish’s wake. With the rules in place, they can then observe the schooling behaviors of their digital fish. Like their real counterparts, these schools show different flocking based on apparent “moods”. (Image and video credit: J. Zhou et al.)
December 21, 2022
Soapy Solutions
When a drop of soap falls into a pool of water, its surface-loving molecules spread out on the water’s surface. Exactly how the soap spreads depends on the local concentration of its surfactant molecules, which create areas with different surface tensions that cause flow. All in all, it’s a tough process to predict because it varies in time at every point on the pool. But a recent paper offers a new class of exact solutions for the problem.
The paper considers a surfactant-laden droplet spreading over a (relatively speaking) deep pool. Other researchers showed recently that this situation can be described with a complex version of the Burgers’s equation, which was originally developed to describe turbulent flows. The authors solved the equation for a variety of initial conditions and found that the time-dependent spread of the surfactants was sensitive to the initial surface distribution. The higher the initial surface concentration, the faster the surfactants spread. (Image credit: T. Despeyroux; research credit: T. Bickel and F. Detcheverry; via APS Physics; submitted by Kam-Yung Soh)
December 20, 2022
Draining a Bottle
Turn a bottle upside-down to empty it, and you’ll hear a loud glug-glug-glug as the liquid in the bottle empties and air rushes in. In this video, researchers aim a high-speed camera at the very first bubble that forms during the process. Once the bubble reaches the wider area of the bottle, it tends to pinch off in the bottle’s neck. That creates a narrow jet that pierces the bubble and flies all the way to the other side, leaving a column of liquid inside the rising bubble. Increasing the fluid’s viscosity has remarkably little effect, at least until the liquid is extremely viscous. (Image and video credit: H. Mayer et al.)
December 19, 2022
Reflections of the Storm
Fall and winter storms rip Lake Erie with violent waves. Photographer Trevor Pottelberg of Ontario captures the dramatic eruptions of mist and spray from these massive, turbulent waves. It’s amazing how many different characters a wave can take on. Just compare Pottelberg’s waves with those caught by Lloyd Meudell or Ray Collins. It’s almost hard to imagine all of these waves growing from the same wind-driven start. See more from Pottelberg on his website and Instagram. (Image credit: T. Pottelberg; via Colossal)
December 16, 2022
Waves in Liquid Crystals
Liquid crystals are now ubiquitous in displays, but scientists are still discovering new properties for this state of matter. Here, a team explores nematic liquid crystals, whose rod-like shape rotates in three dimensions as they apply a voltage. The layer of liquid crystals is held between polarizing filters, creating regions of light and dark that depend on the liquid crystals’ orientations.
Traveling waves and other wave patterns form in this liquid crystal as the voltage applied to it increases.
As the researchers increase the voltage, traveling waves form. With higher voltages, the waves appear to slow a stop. The slowing waves result from the molecules tilting far from a vertical orientation, which makes it harder for individual molecules to rotate since they experience greater resistance from their neighbors. (Image, video, and research credit: V. Panov et al.; via APS Physics)
December 15, 2022
Classifying Waves
In a lab, researchers create their waves in a long, clear-sided tank, where they can observe how the waves form, travel, and interact. To generate the wave, they use a plate, attached to a piston. Push the water at one end, and a wave forms. The type of wave that forms depends on both the velocity and the stroke length of the piston, as shown in this video. By mapping out these two variables, researchers can observe all different sorts of waves, from peaceful solitary waves to wild, plunging breakers. (Image and video credit: W. Sarlin et al.)
December 14, 2022