During World War II, the Allies developed “dambuster” bombs that skipped repeatedly off the surface of the water before striking their target. The goal was to cleverly bypass their enemies’ defenses both above and below the surface. Although the original dambusters used spinning spheres, the ricochet physics works for many other configurations as well; essentially, the physics are identical to rock-skipping. Conventional bullets can also skip off the water, though the required angle for skipping depends strongly on the shape of the bullet. If the geometry of the bullet impact doesn’t generate enough hydrodynamic lift, there will be no skip. (Image credit: Barnes Wallis Foundation, source; research credit: V. Murali and S. Naik, pdf; submitted by Marc A.)
Search results for: “lift”
Jets from Lasers
Laser-induced forward transfer (LIFT) is an industrial printing technique where a laser pulse aimed at a thin layer of ink creates a tiny jet that deposits the ink on a surface. In practice, the technique is plagued with reproducibility issues, in part because it’s difficult to produce only a single cavitation bubble when aiming a laser at the liquid layer. This is what we see above.
The laser pulse creates its initial bubble just above the middle of the liquid layer. Shock waves expand from that first bubble and quickly reflect off the liquid surface (top) and wall (bottom). When reflected, the shock waves become rarefaction waves, which reduce the pressure rather than increasing it. This helps trigger the clouds of tiny bubbles we see above and below the main bubble.
The effect is worst along the path of the laser pulse because that part of the liquid has been weakened by pre-heating, but impurities and dissolved gases in the liquid layer are also prone to bubble formation, as seen far from the bubble. The trouble with all these unintended bubbles is that they can easily rise to the surface, burst, and cause additional jets of ink that splatter where users don’t intend. (Image and research credit: M. Jalaal et al.; submitted by Maziyar J.)
Guiding Particles with Chladni Patterns
During the 19th century, Ernst Chladni and Michael Faraday independently explored the patterns formed by particles of different sizes placed on a vibrating plate. Faraday found that large particles accumulated at nodes of the plate, where there was no vertical vibration, whereas smaller particles moved toward anti-nodes, where air currents caused by the large vibration amplitude lifted them up.
The situation becomes a little different if you submerge the vibrating plate in water. Then large, heavy particles gather at the anti-nodes. Drag keeps the particles on the plate, while acoustic forces and gravity conspire to move the particles horizontally toward the anti-nodes (top). Because anti-node patterns change with frequency, this actually provides a way to manipulate particle’s trajectories. The researchers demonstrated this by steering a particle through a maze (bottom) as well as by manipulating an entire swarm of beads. (Image and research credit: K. Latifi et al.; via Physics World; submitted by Kam-Yung Soh)
How Rain Can Spread Pathogens
Rainfall can help spread pathogens from an infected plant to healthy ones. This transfer can happen both through droplets and by dry-dispersal of pathogen spores (top). When a raindrop hits a leaf, its initial spread triggers a vortex ring of air that can lift thousands of dry spores into a swirling trajectory (bottom). That boost in height carries spores beyond the slower wind speeds of the plant’s boundary layer and into faster air streams that disperse it toward healthy plants. (Image and research credit: S. Kim et al.)
Tornado from a Drone
One of the challenges in studying tornadoes is being in the right place at the right time. In that regard, storm chaser Brandon Clement hit the jackpot earlier this week when he captured this footage of a tornado near Sulphur, Oklahoma from his drone. He was able to follow the twister for several minutes until it apparently dissipated.
Scientists are still uncertain exactly how tornadoes form, but they’ve learned to recognize the key ingredients. A strong variation of wind speed with altitude can create a horizontally-oriented vortex, which a localized updraft of warm, moist air can lift and rotate to vertical, birthing a tornado. These storms most commonly occur in the central U.S. and Canada during springtime, and researchers are actively pursing new ways to predict and track tornadoes, including microphone arrays capable of locating them before they fully form. (Image and video credit: B. Clement; via Earther)
Freezing Drop Impact
At the altitudes where aircraft fly, it’s often cold enough for water drops to freeze in seconds or less. Once attached to a wing, such frozen drops disrupt the flow, reducing lift and increasing drag. To help understand how such droplets freeze, scientists study droplet impact on cold surfaces. Starting at room temperature (counter-clockwise from upper left), a drop will spread on the surface, then retract. When the temperature is colder, parts of the droplet freeze before retraction completes, leaving a thin sheet with a thicker center. At even colder temperatures, the droplet’s rim destabilizes and freezing occurs before the droplet has time to retract fully. And at the coldest temperatures, the droplet breaks apart into a frozen splash. (Image and video credits: V. Thievenaz et al.)
Inside a Wind Tunnel
When I was in graduate school, I worked in a facility known as the High-Speed Wind Tunnel Lab. We were located next door to the Low-Speed Wind Tunnel, and every few months we’d receive a phone call asking whether we could film someone in the high-speed wind tunnel. This was impossible for several reasons – the size of human beings and the necessity of drawing the hypersonic tunnels down to vacuum-like pressures before initiating flow being only two of them – but what it really did was highlight the difference in definitions.
What these (usually) weather forecasters wanted was to simulate hurricane force winds on a human being. And to an aerodynamicist, that hundred mile-an-hour flow is still low-speed. Because we’re comparing it to the speed of sound, not the normal range of wind speeds a human experiences. That said, watching humans struggle inside a wind tunnel is always entertaining.
As you can see from the Slow Mo Guys here, counteracting the lift and drag forces from these wind speeds is tough. On the bottom left, Dan has managed to balance his weight and the drag forces to hold himself in a virtual chair. Meanwhile, Gav’s attempt to jump forward against the wind just pushes him backward as his lab coat parachutes behind him. (Image and video credit: The Slow Mo Guys)
Dip Coating
Imagine dipping a rod into a liquid mixture filled with particles. When you pull the rod out, do particles stick to it? The answer depends on the relative importance of two sets of forces: the viscous drag as you lift the rod and adhesive power of surface tension. Scientists express this as a dimensionless ratio known as the capillary number.
When the capillary number is small, viscous drag dominates, and any particles that try to stick to the rod get pulled away (upper left). But as you increase the capillary number, surface tension helps particles clump together and stick to the rod (lower left and right). If the surface tension forces are strong enough – meaning that the capillary number is high – you can actually get multiple layers of particles adhering to the dipped surface. (Image and research credit: E. Dressaire et al.)
“Winter’s Magic”
Don Komarechka’s beautiful short film, “Winter’s Magic,” captures the beauty of soap bubbles as they freeze. It’s a delicate process and one difficult to capture in video. The bubble freezes first at the bottom, where it touches the cold surface – in this case, snow. That freezing releases latent heat and creates a temperature gradient along the thin liquid film. With that temperature gradient comes a variation in surface tension, and it’s this that creates the flow that lifts the ice crystals from the surface and turns the bubble into a snow globe. Eventually, as the frozen crystals continue growing, flow in the bubble walls comes to halt as the film solidifies.
For more on the physics of freezing bubbles, check out this interview with the researchers, or, to learn more on how to film freezing bubbles, check out Komarechka’s description. (Video and image credit: D. Komarechka; via Laughing Squid; h/t to Jennifer O.)
Water-Walking Geckos
Many animals can run on water. The tiniest creatures, like water striders, use surface tension to keep themselves atop the water. Larger creatures like the basilisk lizard or the grebe slap the water’s surface to generate a vertical impulse that keeps them aloft. Geckos, it turns out, can run on water, too, but they’re too big to stay up with surface tension and too small to support their weight by slapping. So they’ve developed their own method.
As you see in the top image, geckos use the slapping method for part of their support. Their slaps generate a little less than half of the force needed to keep them out of the water.
Surface tension is an important component, too. Geckos are extremely water repellent, which helps boost the lift they get from surface tension. In the bottom image, you see a gecko attempting to run on soapy water, which has a lower surface tension. The gecko is mostly submerged and more swimming than running – a clear demonstration that surface tension is important to its water-walking.
Finally, the gecko undulates its body as it runs, much the way an alligator swims. The researchers suspect this helps the gecko generate forward thrust. Altogether, it creates a water-walking gait that, for now, is unique among observed mechanisms. (Image and research credit: J. Nirody et al.; via Ars Technica; submitted by Kam-Yung Soh)