DROPLET METHOD FOR INVESTIGATING THE CAPILLARY–POROUS STRUCTURE OF MATERIALS

Abstract

A simple express methodology is proposed for the non-destructive characterization of the capillary–porous structure of building materials, based on monitoring the spreading dynamics of a colored droplet applied to the surface of a specimen. The method relies on recording the temporal evolution of the stain and interpreting the resulting R²(t) dependence within the Lucas–Washburn framework and porous-media theory. This enables, in a single experiment, the evaluation of effective porosity, capillary conductance, permeability, mean effective pore radius, effective diffusion coefficient, as well as wetting parameters (contact angle, surface tension) and indicators of deviation from the ideal capillary regime. The experimental procedure consists of video recording the spreading of a droplet of aqueous dye solution (e.g., methylene-blue), followed by frame extraction and computer processing of the stain contours. From the stain area, an equivalent radius is obtained, the R²(t) relationship is constructed, and its slope is used to extract integral capillary characteristics of the material. The approach provides high informational value at minimal equipment cost, making it suitable for field applications and preliminary screening of restoration zones. Using shell limestone as an example, it is demonstrated that the method allows quantitative identification of structural variations even within a single block or layer-an essential requirement for selecting compatible primers and restoration mortars with controlled rheological and adhesive properties. A consistent data-processing workflow is outlined: linearization of the R²(t) dependence, determination of capillary conductance, evaluation of effective porosity from the absorbed liquid volume, and reconstruction of permeability and contact angle values based on known liquid parameters. Verification criteria for assessing the plausibility of the obtained values are presented, including penetration depth and typical ranges of permeability and mean pore radii for cemented carbonate rocks. The proposed methodology effectively complements diagnostic approaches already used in restoration practice and provides a parametric basis for selecting mixtures adapted to the real geometry of the substrate’s capillary–porous network. Thus, the developed droplet-based analysis toolkit combines operational simplicity with the ability to obtain key structural characteristics required for scientifically grounded design of repair and restoration solutions for shell limestone e heritage objects.