Where is the Water?

Abstract

The imbibition of paper with water is a complex interplay between the transport of liquid water in the fiber network and a swelling of the fibers. While the swelling can be directly associated with the incorporation of water into the fibers, little is known how swelling affects the pore space in paper and the subsequent transport of liquids therein. In this work, the propagation of a water drop supplied to a sheet of paper is monitored in-situ using X-ray microcomputed tomography. This method provides a 3D image sequence that traces the time-dependent progression of swelling and liquid transport over a period between fifteen and forty-five minutes after the application of water. The associated water vapor appears to profoundly swell the fibers even before the liquid water front arrives, i.e., the water invades paper both in its liquid and its vapor state. Incorporation of water into fibers leads to a marked increase in sheet thickness that originates from an increase in fiber volume and, interestingly, from an effective increase of the volume of the pores. Whether the latter change in pore volume has an adverse or boosting effect on liquid transport cannot be established from the data because liquid water does not reside in interfiber pores. Instead, liquid water is found in the lumen of the fibers.