Diesel Particulate Filters (DPFs) are designed to capture exhaust particulates from engines. The filter material collects the particulates deep in the substrate (deep-bed filtration) before transitioning to other filtration methods on the surface. The deep-bed filtration has been characterized by modeling studies but little experimental information about the particulate, or soot, penetration in the filter material is available. Traditional evaluation methods do not provide sufficient contrast between the soot and mounting resins.

Due to the difficulty of locating carbon soot in the pores in these filters, ultra-thin section techniques (under 10-micron-thick sections) were used for the characterization of these samples. Under transmitted light, the carbon soot is black because it blocks the light, and the carbon in the resin material is relatively clear.