Researchers levitated a small tray using light | Hacker News Reader

Researchers levitated a small tray using light

wired.com

63 pointsBrajeshwar9 comments

For anyone that didn't read the article, the researchers appear to be using light to generate air currents; they're not levitating objects using radiation pressure.

From the paper:

> The CNT layer has a nanostructured surface which tends to trap incoming gas molecules. These traps make the air molecules collide with the surface multiple times on average before leaving, resulting in a higher thermal accommodation coefficient for the CNT-air side compared to the mylar-air side. This difference between the surface properties results in a higher departing velocity for the air molecules on the CNT (bottom) side compared to the mylar (top) side. The net momentum transfer from these gas-surface interactions results in an upward recoil force that levitates the sample.

The light heats up the sample and the sample's construction mean the bottom heats air up faster than the top. This generates the lift. I wouldn't say that it's air currents or a difference in temperature between the top and the bottom. They even mention that it still levitates when the light shines from above.

Am I missing something?

The article directly says

> Researchers have used this physical phenomenon to float invisible aerosols and sort particles in microfluidic devices. But they have never before moved an object big enough to grasp—much less lifted anything that can carry objects itself.

But radiometers have existed for decades... https://en.wikipedia.org/wiki/Crookes_radiometer

Radiometer is a heat engine utilizing air currents... (i.e. EM radiation -> heat -> airflow -> force)... the phenomenon in the article seems to be direct conversion of EM radiation into force.

"Direct photophoresis is caused by the transfer of photon momentum to a particle by refraction and reflection." [1]

[1] https://en.wikipedia.org/wiki/Photophoresis

From the article you link, photophoresis looks similar to the classical explanation of a Crookes radiometer.

The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid medium.

It cites the radiometer:

Just like in Crookes radiometer, light can heat up one side and gas molecules bounce from that surface with greater velocity, hence push the particle to the other side.

Yet the article seems to contradict itself:

This force depends on light intensity and particle size but has nothing to do with the surrounding medium

Actual article.[1] This gives more of a sense of the required conditions. This apparently works in the 1-12 pascal range of pressure. Earth is around 100,000 pascals at sea level, and even Mars is around 600pA. This might be useful for keeping low earth orbit satellites going. Is the effect powerful enough for that? Could a microsat with solar panels generate enough thrust to equal the drag from the near space atmosphere?

[1] https://www.science.org/doi/10.1126/sciadv.abe1127

> Energy from the LEDs heats up the Mylar’s specially-coated underbelly, energizing air particles under the plastic and propelling the plates away with a tiny, but mighty, gust.

> Bargatin envisions researchers one day releasing sensor-laden levitators in the mesosphere and letting them roam, like weather balloons or floating ocean sensors.

But how do you retrieve them to collect the sensory data? I don't expect it has the energy to transmit data.

solar powered signal

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