Richard A Lovett
Nils Tack, a postdoctoral researcher at Brown University (US) is fascinated by shrimp. Not because he’s looking to create the perfect shrimp cocktail, but because he’s captivated by how they swim.
Somehow, they manage to paddle 10 legs in an effective and powerful manner, scurrying around and changing directions so quickly that (as anyone who’s tried, knows) they are very difficult to catch – at least by hand – in tide pools.
How do they do it?
To find out, Tack’s team filmed their swimming motion and designed tiny probes to measure the stiffness of their legs during each part of their paddling cycle.
What they found, he reported last month at a meeting of American Physical Society’s Division of Fluid Dynamics in Indiana, was that as their legs push backward against the water, they are stiff and strong – much like canoe paddles. But as they swing forward, they are softer and more flexible, curling up to reduce drag.
Even more interestingly, they move in groups, fanning out on the power stroke, but coming together on the return to draft each other, like cyclists in a peloton.
Nor are shrimp the only animals to swim in this manner.
In fact, Tack says, it appears to be the case for many others, ranging from zooplankton to lobsters. “They work the same way across the scale,” he says.
That’s important, he adds, because it might help space scientists develop robotic swimmers for the subsurface oceans of worlds like Jupiter’s icy moon Europa or Saturn’s moon Enceladus. Maybe the most efficient robotic explorer swims like a shrimp?
Not that shrimp are the only animal from which engineers might learn important tips.
Inspired by nature
Chengyu Li of Villanova University (US) is studying fruit flies, trying to figure out how they zero in on that banana peel in your rubbish can – and what we might learn from them to create robotic fliers to seek out sources of air pollution or toxic wastes via their vapour plumes.
Fruit flies start by flying zigzagging patterns during which they use scent detectors in their antennae to seek their targets. But, Li says, the beating of their wings plays a role by drawing scents toward their antennae – kind of like a human waving a hand over a bottle of delicate perfume.
Figuring out how fruit flies simultaneously do this while still knowing what direction the scent comes from, he says, is exactly the type of thing that might someday be adapted for odour-sniffing drones.
Meanwhile, Bardia Hejazi of the Max Planck Institute for Dynamics and Selforganization (Germany) is turning to honeybees for additional help creating better robotic fliers.
The flight and foraging behaviour of honeybees has been extensively studied, he says, but nobody knew how these tiny insects cope with difficult flying conditions. So, his team used a fan to blow air through a grid of airways that could be rapidly opened or closed in order to create challenging turbulence. A hive of bees was placed downrange, and the bees’ flights recorded via GoPro cameras filming from multiple directions.
“There are miniature robots in the same size as these insects,” Hejazi says, explaining that learning how bees cope with gusty winds and air turbulence might be useful for making even-better miniature robotic fliers.
Powering your own sensors
Other scientists at the meeting were looking for ways to do everything from designing implantable medical sensors powered by your own body to finding improved ways to fight climate change.
Implantable sensors, of course, already exist.
“[They] can offer physicians and patients real-time health information from inside the human body,” says Lucy Fitzgerald, a PhD candidate at the University of Virginia. But there’s a problem: “They’re very difficult to power, often requiring a battery to be implanted with the sensor, inside the body.”
And when the battery dies? Time for more surgery.
The human body, however, is actually a good power plant, reliably generating about 100 watts of energy: about the same as a bright household light bulb.
“This is more than enough power to supply implantable sensors,” Fitzgerald says. For example, she says, there is an implantable blood-pressure sensor that runs on 150 microwatts, and an atrial fibrillation detector that runs on 19 microwatts. “This is a really tiny proportion of the power available.”
Harvesting that power could be done via tiny piezoelectric devices that act as parts of the sensor. These work by generating electricity when bent or stressed. “[They] convert mechanical strain into voltage,” Fitzgerald says. Stress them one way, they create positive voltage, stress them the other, they create negative voltage.
That means that if such a device is put into a patient’s airway (or blood stream), it will flex back and forth with each breath (or heartbeat) creating an oscillating signal that a tiny chip can transmit in real time.
Lab tests and animal tests prove this works, she says, though it still has to be scaled down to something that can safely be implanted in a patient’s body. “We were able to put a version of our sensor inside a living rabbit.”
“It’s really tiny,” she adds. “Very difficult to see with the human eye.”
As for climate change, Nathan Blanc of Technion – Israel Institute of Technology notes that, until a few years ago, people were hoping for “one magic technology” that would save the world.
“Nowadays, I think it’s more like finding a combination of many different technologies,” he says.
His slice of this pie focuses on air conditioning. “Global air conditioning is responsible for about two billion tonnes equivalent of carbon dioxide per year,” he says. “That’s four percent of global emissions, and it is expected to double, and more, by 2050.”
To offset this, Blanc’s team is looking to take advantage of something known as the thermoacoustic effect.
It’s based on the chilling effect of expanding gas. “We’ve all seen this when we spray deodorant on our body,” he says. “It feels cold.”
To create it without giant spray bottles, his team beamed soundwaves through a gas chamber. Do it at the right frequency, and one end of the chamber is cooler than the other—exactly what you need for an air conditioner.
“It feels like magic when you see it the first time,” he says.
It’s not a new technology. NASA is using it on the James Webb Space Telescope, and scientific labs have long used it for cryogenic studies. But historically, it’s required super-high pressures that could turn it into a bomb if something broke—“a big no-no for domestic air conditioning,” Blanc says.
His team, however, has discovered that all it takes to make it work at lower pressures is to add a bit of humidity to the system. The reason involves the way water absorbs and releases heat when it vaporises at one end of the tube and condenses at the other.
“This greatly increases the energy density,” Blanc says.
It may even be possible to use solar heat to power create the needed sound waves. If so, “you can place this kind of system on your roof and have a solar-driven air conditioner.”