3-D Animation Shows Complex Geometry of Diesel Particulates

With funding from DOE's Vehicle Technologies Office, researchers at Argonne National Laboratory are studying particulate matter (PM) emitted by diesel engines, in hopes of gaining a better understanding of its form and constituents. What they've learned is that diesel PM has very complex, three-dimensional geometries, which most previous studies have observed only in two-dimensional images.

Three-dimensional animation showing diesel particulate matter produced by a light-duty engine.

This three-dimensional animation shows diesel particulate matter produced by a light-duty engine. As this particulate rotated along a horizontal axis, pictures were taken at every ten-degree viewing angle. This animation revealed that diesel PM forms complex three-dimensional objects, and their shapes are not spherical. Most primary particles within this formation were loosely clustered (aggregated) together. The darker particles represent the overlaid primary particles on the line-of-sight direction.

To expand on previous studies, the Argonne researchers are using a unique combination of thermophoretic sampling techniques and high-resolution transmission electron microscopy (TEM) to observe the detailed three-dimensional geometry of diesel particulates for the first time. These studies have captured detailed information on particulate morphology and fractal geometry, well beyond commercially available particulate size measurements. This work is enhancing scientists' fundamental understanding of the formation and destruction mechanisms for diesel particulates.

A common misconception about diesel particulate matter is that its shape is always spherical. In TEM images of particulates, however, particulates appear to consist of an agglomeration of numerous spherical primary particles. These aggregate particles show a stretched chain-like shape at large, independent of engine speed and load conditions. These particulates, in which tens to hundreds of primary particles were clustered around each other, were distributed in a wide range of sizes from tens of nanometers to a few microns. This finding showed that the spherical particle shape assumed in commercial particulate measurement instruments (such as scattering mobility particle sizers, low-pressure impactors, etc.) does not reflect the true nature of particulate spatial geometry.

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