Aerosol particles larger than approximately one micrometer in size constitute a category of natural and manmade particulates known as the coarse-mode aerosol (CMA). Common types of CMA particles include mineral dust (MD) and biologic particles, like pollen. The ability to characterize CMA particles is important for agriculture, climate science, and manufacturing processes. For example, MD represents the largest component, by mass, of aerosols in continental regions and is an important mechanism for the transport of nutrients to land and marine ecosystems. Dust also reduces visibility, which impacts aviation. To best understand the impact of CMA particles in most applications, basic information such as particle size, shape, and number concentrations are needed.
In this project, we demonstrated how flowing MD aerosol particles can be imaged in-air within a sensing volume of approximately one cubic centimeter in a time dependent manner. This is done with digital in-line holography (DIH) to render quasi-3D representations of individual particles in a 3D sensing volume in time steps of approximately 10 ms to create a video (below) of the flowing particles.
Figure 1: Diagram (left) of the simple optical arrangement used to create videos of flowing MD aerosols in 3D. Particles are aerosolized by a sieve above a test box where they then flow through a laser beam as they settle. Pulses of light illuminate the moving particles, giving a series of digital holograms that yield particle images following later computational processing. On the right is a photograph of the experiment with Dr. Florian Gaudfrin, the scientist who built it.
Figure 2: Approximate 3D shapes of flowing MD aerosol particles using the arrangement of Fig. 1. The Fresnel diffraction integral is used with a recorded hologram to generate particle-image reconstructions in 121 image layers stacked along the z-axis producing the streak-like structures in (a). In (b) is the appearance of the reconstructions in several layers for the particle circled in (a). In (c) is the particle hull obtained from all perimeters, which is re-scaled along the z-axis by alpha to give the contracted hull in (d). A 3D surface is then obtained from the contracted hull in (e). Repeating the process for all streaks yields the final 3D distribution of MD particles in (f). Further explanation for how this process is done can be found in this [paper].
This video shows an example of the MD flow that is obtained from the experiment. Here the particles are imaged in 3D in as the flow in air from a series of multiple holograpms recorded every 10 ms. After reconstructing the particle images from each hologram, the images are stitched together to produce the video. Here, the particles seen are approximately 50 micrometers in size.