Measuring liquid penetration in the thickness direction of paper

¹ Department of Physical Sciences, University of Helsinki, PO Box 64, 00014, Helsinki, Finland
² Helsinki Institute of Physics, PO Box 64, 00014, Helsinki, Finland
³ Oy Keskuslaboratorio - Centrallaboratorium Ab, PO Box 70, 02151, Espoo, Finland

Corresponding author: timo.karppinen@helsinki.fi

Received: 3 February 2005
Revised: 25 May 2005
Accepted: 24 June 2005
Published online: 25 October 2005

Abstract

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/m² 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) μl to (1.3±0.1) μl. 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.