Small sensors powered only by sunlight could float in the stratosphere indefinitely and make weather-related measurements.
There is a known effect for levitating flat objects with two sides called photophoresis. This occurs when one of the sides absorbs lots of light and the other very little, creating a difference in temperature. Just like how temperature differences in the atmosphere cause winds, this temperature gradient makes molecules move in such a way as to create a lifting force on the object.
at Harvard University in Massachusetts and his colleagues designed a device that could use photophoresis to levitate small atmospheric sensors that wouldn’t need motors or batteries to stay aloft.
Advertisement
While photophoretic devices have been tested before, the researchers designed a larger version optimised for carrying sensors. Their device consists of two aluminium oxide discs about 10 centimetres in diameter. Each disc is only 100 nanometres thick – one-thousandth the thickness of sheet of paper – and microscopic holes cover nearly half of the surface, making it very lightweight and easy for heat to flow through it.
To maximise the lifting forces on the device, the team determined that the top disc should be covered in a coating that lets visible but not infrared light pass through it, while a coating on the bottom disc did the opposite. This would generate enough lift to keep the device in the stratosphere, bobbing up and down over the course of a day and night as light switches from mostly visible to mostly infrared, but never falling to Earth. The researchers calculated that the device could carry a 300-milligram payload of equipment like temperature and pressure sensors and instruments for transmitting measurements to Earth.
Schafer and colleagues suggest a weather balloon could deliver hundreds of these devices to the stratosphere, about 25 kilometres above the equator. Here, they could make measurements of stratospheric winds which are useful for predicting weather. Because they are small and light, they could also be practical to transport to Mars where they could collect data about the Martian atmosphere.
at the University of Bristol says that the devices might be helpful for a form of solar geoengineering where aerosol particles would be injected into the stratosphere to reflect sunlight and cool the earth. Floating photophoretic aerosol sensors could continuously monitor that process, she says.
However, Aplin says, the device’s 300-milligram mass allowance may challenge its practicality. Many instruments would have to be powered by solar cells and adding those, together with connective circuits and mounting elements, could limit how many more instruments can be added, she says.
Reference:
