[HN Gopher] Researchers levitated a small tray using light
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       Researchers levitated a small tray using light
        
       Author : Brajeshwar
       Score  : 50 points
       Date   : 2022-02-19 12:17 UTC (10 hours ago)
        
 (HTM) web link (www.wired.com)
 (TXT) w3m dump (www.wired.com)
        
       | tromp wrote:
       | > 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.
        
         | Day1 wrote:
         | solar powered signal
        
       | karmakaze wrote:
       | > 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.
        
       | horsawlarway wrote:
       | 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
        
         | [deleted]
        
         | colanderman wrote:
         | 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
        
           | JPLeRouzic wrote:
           | 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_
        
             | Animats wrote:
             | 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
        
       | science4sail wrote:
       | 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.
        
         | Mathnerd314 wrote:
         | 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.
        
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       (page generated 2022-02-19 23:01 UTC)