Holographic Images of Aerosols

Two overlapping lasers are helping researchers to create holographic images of free-flowing air particles, which may help climate scientists and biological weapons watchdogs monitor what’s in the air. (Source: KSU)

Holo­graphic images of free-flowing air particles may help climate change and bio­logical weapons watchdogs better monitor the atmo­sphere, according to a recent Kansas State Univer­sity study. Principle inves­tigator Matthew Berg, associate professor of physics, said his result is a key to under­standing the aerosol compo­sition of Earth’s atmo­sphere.

“We have these small little chunks of particles floating around in the air and people want to know what they are made of, but if we disrupt them, it might change their form,” Berg said. “Until now, there hasn’t been any unique and confident way to confirm particle size and shape pro­perties in their natural form. We have solved the inverse problem.”

Referring to a problem that is worked backward from results to cause, Berg said before this study, the inverse problem with aerosol particles was largely educated guesswork based on mathe­matical calcu­lations. Researchers could not objec­tively define free-floating aerosol particles because merely capturing a particle and looking at it under a micro­scope could change its physical shape or size. Now, they can bounce light waves off the particle and measure the deflec­tion.

The method takes holo­graphic images of particles as they float through the air using two over­lapping lasers: one red and one green. The green laser is the tradi­tional method that can be used to measure the light deflec­tion; by providing the red laser, they also get a 3-D image that can subjec­tively account for a variety of particle shapes. “We get the two pro­perties – size and shape – that we’ve always wanted to get,” Berg said. “We still have all the advan­tages that people had with the last 50 years – light scat­tering, contact free and measure­ments can be done rapidly – and then we can put it on an instru­ment and fly it around in the air.”

Berg is working to put the laser setup on an unmanned aircraft to measure free-flowing aerosol particles in the atmo­sphere. Removing the particles from their natural environ­ment can change the particle form, Berg said. For example, if the particles are frozen in the atmo­sphere and scientists collect them to bring them back to the ground to study, the particles could melt and change their shapes and sizes. “If we think about climate science, they want to know the size and shape of particles floating in the atmo­sphere,” Berg said. “This infor­mation can help climate scientists account for how much sunlight those particles scatter back into space or absorb and if they absorb, by how much will it heat up the sur­rounding atmo­sphere.” (Source: Kansas State U.)

Reference: M. J. Berg et al.: Solving the inverse problem for coarse-mode aerosol particle morphology with digital holography, Sci. Rep. 7, 9400 (2017); DOI: 10.1038/s41598-017-09957-w

Link: Soft Matter Physics, Dept. of Physics, Kansas State University, Manhattan, Kansas, USA

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