Visualizing pulp fibers using X-ray tomography: Enhancing the contrast by labeling with iron oxide nanoparticles and the use of immersion oil

In this study, we present a protocol to visualize the architecture of tracer fibers in paper using X-ray tomography. We prepared tracer fibers by depositing iron oxide nanoparticles on the surface of select papermaking fibers, through a multicycle labeling technique that achieved 14 wt% of iron. Labeled and unlabeled fibers on their own, as well as laboratory-formed paper containing a small fraction of the tracer fibers, were imaged in air and after immersion in a non-polar oil. We found that labeled fibers could be segmented from the background through simple binarization when in the immersed state whereas segmentation failed when the samples were imaged in air. We propose that the oil served as a mask, created through compositional and density matching of the unlabeled fibers to the saturated void volume. This new labeling and immersion protocol opens avenues to enhance the contrast of tracers for improved characterization of cellulosic materials via X-ray tomographic imaging in an approach that does not require advanced image processing methods for segmentation.