Contact angle measurements on two (wood and stone) non-ideal surfaces

Surface properties of wood and stones are very important in influencing the bonding and finishing of wood and for road construction. Two important surface properties are wettability and surface free energy. Contact angle measurement is a simple, useful and sensitive tool for quantifying the wettability and the surface energy of different materials in contact with pure water and/or aqueous surfactant solutions. Nevertheless, a sessile drop on a real surface shows different contact angle values due to its lack of symmetry or to the loss of volume by capillary action. For this reason alternative techniques which supply an average contact angle value are required. In view of this fact, we applied axisymmetric drop shape analysis-diameter (ADSA-D) using a top view of a sessile drop and axisymmetric drop shape analysis-profile (ADSA-P) with a side view of a captive bubble. Both techniques calculate the contact angle by solving the Young-Laplace equation although with different algorithms, the former needs the maximum drop diameter while the last makes use of the complete bubble profile. ADSA-D was applied to study two wood species (eucalyptus and pine) and ADSA-P with polished rocks of two different compositions (silicate and calcite). The maximum contour of the drop was fitted to an ellipse after being detected and extracted by means of a semi-automatic procedure of image processing. Thus, two different contact angles were found out. The captive bubble method in conjunction with the current ADSA-P technique allows to obtain comfortable, automatic and reproducible measurements of contact angle on porous stones. (C) 2002 Elsevier Science B.V. All rights reserved.