Measuring liquid penetration in the thickness direction of paper

We introduce a method to measure liquid absorption into paper based on measuring white light reflected from it. The method was used with 49.8-51.7 g/m(2) pilot papers, whose hydrophobicity was tuned by their fiber content and furnish. In addition, one of the samples was internally sized. Impact wetting was used where a droplet of mineral oil, isopropanol or deionized water was applied to the paper surface opposite to the one facing the monitor. The volume of liquid per droplet was (0.9+/-0.2) mu l to (1.3+/-0.1) mu l. The sample was illuminated with an ordinary 20 W light bulb from above. The light intensity reflected off the sample was recorded with a fast charge-coupled camera during the liquid penetration. Optical changes in the paper sample were studied by calculating the average intensity of the reflected light from a selected area. The results showed that the speed of pore wetting, mainly in the z-direction, could be measured with each liquid-sample combination. Hence a digital camera based light reflection measurement can provide information about thickness direction liquid transport in paper.