**3. Results**

#### *3.1. Paper Coating Morphology*

The SEM images of thick and thin SMI/oil nanoparticle coatings on paper are shown in Figure 1 at magnifications of 800× and 2000×. A reference SEM image of the uncoated paper at the same magnifications can be found in the Supplementary Materials (see Figure S1 in Supplementary Materials).

The features of the base paper with cellulose fibers and internal paper fillers remain visible on the surface of thin coatings. The thin coating mainly fills the pores between the fibers and forms only a small layer on top of the fibers. The thick coatings almost fully cover the paper substrate with a more continuous layer. As pure nanoparticle dispersions are used here (no coating binder was used in order to study the intrinsic nanoparticle coating properties), the drying effects of the colloidal suspensions cause some coating defects, resulting in local cracks. The development of cracks in the drying of colloidal suspensions can be attributed to the role of packing and flocculation of the particles [22]. Therefore, the rheological properties of the SMI/oil nanoparticle types have been studied before [20], and the reduction in the surface tension (and consequent influences on capillary forces between nanoparticles) for the SMI/oil nanoparticles in comparison with the SMI nanoparticles is particularly beneficial to reduce the existence of cracks. Also, the smaller nanoparticle sizes of SMI/oil nanoparticles (20–50 nm) compared to pure SMI nanoparticles (100 nm) enhance the packing and reduce the tendency for cracking. The drying of the nanoparticle suspensions on porous substrates may induce stresses that can lead to a fracture of the thin film. However, the presence of oil may improve the coating homogeneity and partially functions as a natural binder. The dry thickness of thick coatings is about 10 ± 2 μm, and the dry thickness of thin coatings is about 7 ± 2 μm. Further detail of the coating structure (e.g., SMI/HCO) and presence of nanoparticles is illustrated in the FEG-SEM images in Figure 2.

**Figure 1.** SEM microscopy of SMI/oil coatings on paper, including thin and thick coatings at different magnifications (indicated scale bars apply to full column).

**Figure 2.** Detailed FEG-SEM microscopy of SMI/oil coating on paper (e.g., SMI/HCO): (**a**) magnification 5000×; (**b**) magnification 50000×.
