When birds come in for a landing, they pitch back and heave their wings as they come to a stop in a perching maneuver. Some birds, researchers noticed, partially fold their wings during the move, creating what’s known as a swept wing. Curious as to the effect of this sweep, the team recreated the wing motion of a perching bird using two flat plates — one rectangular and one swept — and measured the flow around them during the maneuver. They found that the swept wing had greater lift, thanks to a spanwise flow inherent to swept wings that helped stabilize the leading-edge vortex. (Image credit: D. George; research credit: D. Adhikari et al.; via APS Physics)
Category: Research
Measuring Drag
After a noticeable rise in the prevalence of home runs beginning in 2015, Major League Baseball commissioned a report that found the increase was caused by a small 3% reduction in drag on the league’s baseballs. When such small differences have a big effect on the game, it’s important to be able to measure a baseball’s drag in flight accurately.
In the past, that measurement has often been done in a wind tunnel, but the mounting mechanisms used there result in drag measurements that are a little higher than what’s seen from video tracking in actual games. Now researchers have developed a new free-flight method for measuring a baseball’s drag. The drag measurements from their new method are lower than those for wind-tunnel-mounted baseballs and in better agreement with video-based methods. The authors’ method should be adaptable to other sports like cricket and tennis, which will hopefully provide new insight into the subtleties of their aerodynamics. (Image credit: T. Park; research credit: L. Smith and A. Sciacchitano; via Ars Technica; submitted by Kam-Yung Soh)
Acidic Aerosols
As ocean waves crash, they generate aerosols — tiny liquid and solid particulates — that interact with the atmosphere. Curious about the chemistry of these tiny drops, researchers set out to measure their acidity. That’s easier said than done. Over time, aerosol droplets acidify as they interact with acidic gases in the atmosphere and capturing fresh aerosols in the field is next to impossible.
To tackle these challenges, researchers instead moved the aerosols to the laboratory, filling a wave channel with seawater and agitating it to generate aerosols they could then measure. They found that the smallest aerosols become a million times more acidic than the bulk ocean in only two minutes! Find out more about their experiment and its implications over at Physics Today. (Image credit: E. Jepsen; research credit: K. Angle et al.)
Dripping Impact
How does water drip, drip, dripping onto stones erode a crater? Water is so much more deformable that it seems impossible for it to wear harder materials away, even over thousands of impacts. To investigate this, a team of researchers developed a new measurement technique: high-speed stress microscopy. In the process, they found that water owes its incredible erosive power to three factors: 1) The drop’s impact creates surface shock waves along the material, which helps increase erosive power; 2) After the shock wave passes, a decompression wave in the material helps loosen surface matter; and 3) The spreading drop sends a non-uniform wave of stress across the material that simultaneously presses and scrubs at the surface. Together, these factors enable simple, repetitive droplet impacts to wear away at hard surfaces. (Image credit: cottonbro; research credit: T. Sun et al.; via Cosmos; submitted by Kam-Yung Soh)
Coalescence Symmetry
When droplets coalesce, they perform a wiggly dance, gyrating as the capillary waves on their surface interfere. When the droplets have matching surface tensions, like the two water droplets in the animation on the lower left, the coalescence dance is symmetric. But for differing droplets, like the water and ethanol droplets merging on the lower right, coalescence is decidedly asymmetric.
Two water droplets merge symmetrically. A water droplet and an ethanol droplet merge asymmetrically. The asymmetry arises from the droplets’ different surface tensions. The size and speed of the capillary waves that form on a droplet depend on surface tension, so droplets of different liquids have inherently different capillary waves. During merger, the interference of these capillary waves causes the asymmetry we see. (Image credit: top – enfantnocta, coalescence – M. Hack et al.; research credit: M. Hack et al.)
Featherwings in Flight
The featherwing beetle is tiny, less than half a millimeter in length. At that scale, flying is a challenge, with air’s viscosity dominating the forces the insect must overcome. The featherwing beetle, as its name suggests, has feather-like wings rather than the membranes larger beetles use. But a new study shows that these odd wings work surprisingly well.
The beetle’s bristled wings flap with an exaggerated figure-8 motion, with the wings clapping together in front of and behind the insect. The beetle expends less energy moving its feathery wings than it would if they were solid, and it moves its wing covers at the same time to counter each stroke and keep its body steady. (Image and research credit: S. Farisenkov et al.; video credit: Nature; submitted by Kam-Yung Soh)
Raindrops on the Windshield
When I was a child, I was fascinated by the raindrops that shimmied along the windshield of our car. Some would slide up the glass. Some would run down. And some just seemed to wiggle in place, until the car’s speed changed. As common as this sight is, the physics of these droplets is quite complicated and not completely understood.
Each droplet has a host of forces on it: gravity flattening it or pulling it down an incline; a drag force from the wind flowing over it; and friction between the drop and the surface trying to pin it in place. Recently, scientists have developed a new mathematical model that captures some of the behaviors behind these drops. The work describes the wind speed necessary to move a drop of a given size sitting on a flat surface. The authors also explored how that critical wind speed changes when a drop sits on a tilted surface aligned or against the wind. (Image credit: P. Gupta; research credit: A. Hooshanginejad and S. Lee; via Science News; submitted by Kam-Yung Soh)
Using Turbulence in Flight
When small, heavy particles are in a turbulent flow, they settle faster than in a quiescent one. Their interactions with turbulent eddies sweep them along, extracting energy that lengthens their overall path but reduces the time necessary for them to fall. Using the same principles, researchers are finding ways for rotorcraft and other vehicles to extract energy from turbulence for more efficient flight.
The technique forces a vehicle to behave like a heavy particle by sensing turbulent gusts from its own accelerations and adding forcing to those accelerations when they are in the desired direction of flight. In essence, the vehicle uses the turbulence of its surroundings to find helpful tailwinds. (Image credit: A. Soggetti; research and submission credit: S. Bollt and G. Bewley)
How Fabric Dries
How do damp clothes dry in air? Such a seemingly simple question has vexed physicists for years because it’s extremely difficult to observe what happens inside the cloth fibers. Now researchers have used magnetic resonance techniques to track the material’s drying process.
Inside wet fabric, water exists in one of two states: it can be bound to the fabric fibers through hydrogen bonds or it circulates as a vapor in the voids between. Before this study, scientists had no way of confirming the relationship between these two states. Models simply assumed that most of the drying took place as water vapor left the fabric.
In their measurements, the team watched textiles dry in open-topped containers exposed to dry air. With their magnetic resonance technique, they could track the bound water in the textile over time. They found that the model that fit their data the best is one in which the bound water and water vapor reach equilibrium instantaneously. (Image credit: K. Cao; research credit: X. Ma et al.; via APS Physics; submitted by Kam-Yung Soh)
Squishy Actuators
Hard materials don’t always work well in robotics. Here, researchers build soft actuators that can bend, curl, and tighten in order to manipulate objects. They begin by injecting liquid elastomer into a tube (Image 1), followed by a bubble of air. Buoyancy makes the air bubble rise within the tube, creating an asymmetric cross-section where the solidified elastomer has a thin shell along one side and a thicker wall along the other (Image 2). When high-pressure air is pumped into the soft tubes, their asymmetric cross-section makes them bend and twist (Image 3). The team found that they can tune the elastomer tubes to form complex shapes good for gripping and flexing — perfect for a soft robot! (Video and image credit: T. Jones et al.; research credit: T. Jones et al.)
