FYFD

Category: Research

  • Bubble Cleaning

    Bubble Cleaning

    Removing dirt and bacteria from fruits and vegetables is a delicate job; too much force can bruise the produce and hasten spoiling. That’s why fluid mechanicians want to give the job to bubbles. Placing objects in a stream of air bubbles inside a bath is a surprisingly effective method for gently cleaning surfaces. A recent study finds that 22.5 degrees is the optimal angle for sliding bubbles to scrape a surface clean.

    As the bubbles slide past the surface, they exert a shear force that scrapes away debris, just as you might use a loofah in the shower. The angle the bubble makes with the surface determines how long it’s in contact and how much force the bubble exerts. Increasing the angle makes the bubble slide faster, increasing its shear force. But above 22.5 degrees, the bubble’s buoyancy means that it spends less time pressed against the surface, which decreases its cleaning ability.

    The team hopes to use their results to build a “fruit Jacuzzi” device that will direct bubble streams to gently and effectively clean fruits and vegetables in a matter of minutes. (Image and research credit: A. Hooshanginejad et al.; via APS Physics)

  • Draining By Vortex

    Draining By Vortex

    Unstop your bathtub and the draining water will form a tiny tornado-shaped vortex over the outlet. Four centuries ago Torricelli developed a mathematical equation to describe how long it would take to empty the container, based on the height of the fluid in the tank. Now researchers have made a more generalized version of Torricelli’s law, based on experiments with a rotating tank. They found that measuring the water level above the outlet (i.e., taking into account the surface level dip caused by the vortex) gave better agreement. The stronger the vortex, the lower the surface dips and the slower the container drains. (Image and research credit: A. Caquas et al.)

  • How a Leak Can Stop Itself

    How a Leak Can Stop Itself

    Some leaks can actually stop themselves, and a new analysis shows how. When a vertical pipe has a small hole, water initially spouts out of it, then dribbles, and, finally, drips as the water level in the pipe falls, decreasing the driving pressure of the flow. But the pipe doesn’t have to empty to a level below the hole for the leak to stop. Instead, a final droplet can form a cap over the hole, with its shape providing enough pressure to balance the remaining pressure from fluid in the pipe.

    Water leaking from a vertical pipe transitions from continuous flow to discrete drops (left). Dripping continues until the final droplet forms at t = 0 seconds.
    Water leaking from a vertical pipe transitions from continuous flow to discrete drops (left). Dripping continues until the final droplet forms at t = 0 seconds.

    The researchers found that the final drop’s kinetic energy (as well as its potential energy) was critical to determining which drop would stop the flow. The last drop behaves like a lightly-damped harmonic oscillator; it needs enough potential energy to counter the flow and a small enough inertia that it doesn’t slip away down the pipe. (Image credit: top – G. Crofte, experiment – C. Tally et al.; research credit: C. Tally et al.; via APS Physics)

  • Gathering Safely

    Gathering Safely

    One effect of the COVID-19 pandemic is a renewed interest in the physics of disease transmission and what measures can protect us from airborne respiratory illnesses. This recent study looks at how meetings — whether in classrooms, conferences, or care facilities — can transmit infections. Their mathematical model is able to handle many variables — room size, number of people, length of meeting, breaks between sessions, masking, ventilation, and so on. Without prescribing any one policy, the authors aim to inform decision makers so that they can choose what methods (testing, masking, ventilation, etc.) work best for their event.

    That said, they find that ventilation and periodic breaks between meetings are highly effective in reducing a room’s viral load. Leaving enough time between sessions for ventilation to clear the room was as effective (or more effective) than masking and moderate isolation of those infected. Tools like these are vital in enabling gatherings that keep participants safe. (Image credit: Product School; research credit: A. Dixit et al.; submitted by Kam-Yung Soh)

  • Washing By Vortex Ring

    Washing By Vortex Ring

    Spraying a surface clean with a jet of fluid can be an energy-intensive operation. But a recent experiment shows that pulsed flow — which creates vortex rings — could be a viable cleaning alternative. Here we see vortex rings impacting a porous, beaded surface that’s covered in oil. Vortex rings with lots of rotation actually pass through the beads, knocking oil off both the front and back surfaces (Image 1). Even with a lower rotation rate, a vortex ring can still help clean the upper surface (Image 2). (Image and research credit: S. Jain et al.; via APS Physics)

  • Featured Video Play Icon

    Sandgrouse Soak in Water

    Desert-dwelling sandgrouse resemble pigeons or doves, but they have a very different superpower: males can soak in and hold 25 milliliters of water in their feathers, which they carry tens of kilometers back to their chicks. The key to this ability is the microstructure of the bird’s breast feathers. Unlike other species, where feathers have hooks and grooves that “zip” them together, the sandgrouse’s specialized feathers have tiny barbules with varying bending stresses. When dipped in water, their curled shape unwinds, allowing water to soak in through capillary action. Barbules at the tips curl inward, holding the water in place so that the sandgrouse can fly home with it.

    Studying nature’s solutions for water-carrying will help engineers design better materials for human use, whether that’s a water bottle that avoids sloshing or a medical swab that’s better at absorbing and releasing fluids. (Image and video credit: Johns Hopkins; research credit: J. Mueller and L. Gibson; via Forbes; submitted by Kam-Yung Soh)

  • Overcoming Turbulence

    Overcoming Turbulence

    Despite their microscopic size, many plankton undertake a daily migration that covers tens of meters in depth. As they journey, they must contend with currents, turbulence, and other flows that could knock them off-course. And, increasingly, research shows that a plankton’s shape makes a big difference in these flows.

    Spherical plankton tend to cluster in areas of flow moving opposite to their direction of travel. But more elongated plankton can resist — or even reverse — this tendency, helping them stay on track. In turbulence, elongated swimmers are also better at keeping their thrust oriented in the desired direction of travel. So both nature and engineers should favor elongated microswimmers when contending with turbulence and potential crossflows. (Image credit: Picturepest/Flickr; research credit: R. Bearon and W. Durham)

  • How Hagfish Slime Clogs

    How Hagfish Slime Clogs

    When attacked, the eel-like hagfish slimes its predator, clogging the fish’s gills so that it can escape. A recent study looks at just what makes the slime so effective. There are two main (non-seawater) components to hagfish slime: mucus and threads. The team’s experiments showed that the slime’s clogging is due almost entirely to the mucus; the clogging power of full slime and mucus-only slime is almost identical.

    So what are the threads for? They make it harder for the mucus to get washed away. Mucus alone isn’t able to clog as effectively after a single rinse, but, with the threads included, the slime hardly budges. That staying power makes it all the harder for a predator to clear its gills once slimed. In fact, it’s still unclear to scientists whether a slimed fish can free itself from the clogging. After all, the attacker can’t use the hagfish’s trick to free itself from slime. (Image credit: dirtsailor2003/Flickr; research credit: L. Taylor et al.)

  • Predicting Heat Waves

    Predicting Heat Waves

    The United States, Europe, and Russia have all seen deadly, record-breaking heat waves in recent years, largely in areas that are ill-equipped for sustained high temperatures. A new paper presents a theory that predicts how hot these heat waves can get and what mechanism ultimately breaks the hot streak.

    Heat waves start when an area of high-pressure air forms over land, with an anticyclone circulating around it. Air at the center of the zone warms and rises, and if the anticylone can’t move, temperatures will just keep rising. Despite the heat, there is still moisture in the rising air of a heat wave. The authors found that if that moist air can reach an altitude where the atmospheric pressure is 500 hPa (a typical altitude of 5-7 km), then the maximum daily temperature will stop rising. At that altitude, the moist air can condense into rain, and, even if that rain evaporates before reaching the ground, it is enough to cool temperatures.

    The key variable in the theory is the atmospheric temperature at 500 hPa, something that meteorological models are able to predict well up to three weeks in advance. That means this theory should enable meteorologists to give advanced warning of high temperatures, helping communities prepare. (Image credit: T. Baginski; research credit: Y. Zhang and W. Boos; via APS Physics)

  • Bending in the Stream

    Bending in the Stream

    Nature is full of cilia, hairs, and similar flexible structures. Unsurprisingly, flows interact with these structures very differently than with smooth surfaces. Here, researchers investigate flow in a channel lined with flexible, hair-like plates. Initially, the channel is filled with oil and dark particles that help visualize the flow. Then, they pump water into the setup.

    As the water intrudes, it forms an interface with the oil. That interface is powerful enough to bend individual hairs in the system. When the hair bends far enough, it can touch its neighbor, sealing the oil inside the gap between them. Along the length of the channel, this behavior leads to trapped pockets of oil that never drain, no matter how much water flows by. (Image and research credit: C. Ushay et al.)