Holography Helps Uniquely Identify Free-Flowing Particles in the Air

Researchers have used overlapping lasers to create holographic images of free-flowing air particles. A green laser was used to measure light deflection; a red laser to provide a 3D image that could subjectively account for a variety of particle shapes. The work could provide a solution to the inverse problem of measuring free-flowing aerosol particles.

Due in part to the absence of wave-phase information in the measurements of free-flowing particles, no inference is unique — a difficulty known as an inverse problem. In an alternative approach, researchers used digital holography where wave-phase information was encoded in the measurements.

A team from Kansas State University used digital holography and spatial filtering to show that a measured scattering pattern could be uniquely associated with the particle size, shape and orientation producing it. Images of free-flowing particles in the coarse-mode aerosol (CMA) size range and their scattering patterns were obtained simultaneously. This enabled the researchers to correlate a measured pattern to the particle properties of size, shape and orientation, free of assumptions.

Two overlapping lasers are helping Kansas State University researchers create holographic images of free-flowing air particles, which could help climate scientists and biological weapons watchdogs monitor what’s in the air. Courtesy of Kansas State University.

The "after-the-fact" focusing ability provided by the holographic method removed the need to trap or tightly control the flow of particles. If multiple particles were present in the sensing region during the measurement, each particle could be brought into focus at a later time from a single hologram measurement — an ability that could be used to approximate the 3D structure of a single particle.

Previous to this study, researchers could not objectively define free-floating aerosol particles because capturing a particle and looking at it under a microscope could change its physical shape or size.

The green laser can be used to measure the light deflection (left). By using the red laser, researchers also get a 3D image that can subjectively account for a variety of particle shapes (middle). The SEM image on the right is from an electron microscope that is a representative particle like the one providing the left and middle image. Courtesy of Kansas State University.

“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,” professor Matthew 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.

“We get the two properties — size and shape — that we’ve always wanted to get,” Berg said.

There are particle characteristics not revealed in this approach that prevent it from constituting a complete solution to the inverse problem. Foremost is that the refractive index is not provided by the holographic image. What is not tested in the researchers’ work to date is whether the scattered wave-phase information that is available from the hologram could provide information that could be used to determine the refractive index.

The team is working to put the laser setup on an unmanned aircraft to measure free-flowing aerosol particles in the atmosphere.

“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.”

The research was published in Scientific Reports (doi: 10.1038/s41598-017-09957-w).