Transp Porous Med (2010) 84:587–603 DOI 10.1007/s11242-009-9523-x Relating Liquid Location as a Function of Contact Time Within a Porous Coating Structure to Optical Reflectance P. A. C. Gane · K. Koivunen Received: 20 September 2009 / Accepted: 23 December 2009 / Published online: 23 January 2010 © Springer Science+Business Media B.V. 2010 Abstract Refractive index differences between a first and a second fluid can be utilised to obtain information about the location and amount of the fluids in a porous medium in the case where the light absorption coefficient of the skeletal material is small and the light scattering coefficient high using optical measurement methods. An example of such a medium is an air-filled paper coating, and the fluid that of a printing ink liquid phase absorbing into the coating during printing. We examined capillary absorption of mineral oil, used in offset print- ing ink, into model coatings compressed from dispersed mineral pigments with a range of latex binder levels, and established a porosity-normalised relationship for light reflectance change as a function of absorbed mass of the liquid established at a given time after initial contact with the liquid. The results suggest a significant change in reflectance due to the absorption, and progressive absorption behaviour of the liquid in the coatings can be mon- itored by the change in reflectance following a newly established relationship derived from the observational data. The findings support the concept of a preferred pathway flow for the wetting front, defined by differential pore size and connectivity, and a longer time satura- tion front flow lagging behind the wetting front, which theoretically at the limit of infinite time coincides with the wetting front, the time constant of the approach being related to the permeability of the porous network. Keywords Printing · Ink setting · Liquid absorption into porous media · Liquid distribution in porous media · Light scattering from liquid-filled pores · Pore filling · Polymer network–liquid interactions P. A. C. Gane · K. Koivunen (B ) Faculty of Chemistry and Materials Sciences, Department of Forest Products Technology, School of Science and Technology, Aalto University, 00076 Aalto, Finland e-mail: kimmo.koivunen@tkk.fi P. A. C. Gane Omya Development AG, Baslerstrasse 42, 4665 Oftringen, Switzerland 123