3D imaging of the spreading and penetration of aqueous liquids into unsized and sized papers

In a previous paper (Roberts et al. [4]) cryo-scanning electron microscopy (cryo-SEM) was used to identify the mechanisms of fluid flow over short time scales in unsized papers. Using this method one could not image below the surface or below the bulk liquid droplet. We introduce here a cryo-two-photon confocal microscopic technique which enables us to obtain quantitative 3D images of the fluid penetration into paper at arbitrary depth within the sheet and to visualise penetration below the droplet. Using this technique to image fluid penetration we confirm that even under the droplet, the primary mechanism of fluid penetration into unsized papers is through liquid films along channels. In contrast, during experiments performed on sized paper we observe no flow along channels formed by fibre overlaps, crevices or fibre roughness. The only flow path observed is within the fibre wall.