Contact Angles for Liquid Drops at a Model Heterogeneous Surface Consisting of Alternating and Parallel Hydrophobic/Hydrophilic Strips

Model heterogeneous surfaces consisting of alternating and parallel 2.55 μm hydrophobic and 2.45 μm hydrophilic strips were prepared on a gold film by patterning self-assembled monolayers of hexadecanethiol and mercaptohexadecanoic acid using an elastomer stamp. The advancing and receding contact angles were measured for liquid drops (distilled water, buffer solutions with pH = 8.0, 10.0, and 11.0, ethylene glycol, glycerol, and formamide) placed on this specially prepared surface. Contortion of the three-phase contact line is a significant property of these systems. Both contact angles, advancing and receding, were 2−10° lower when measured with the strips normal to the three-phase contact line than those measured with the strips tangential to the three-phase contact line. For most of the systems examined, experimental contact angles, when measured for the liquid drop edge situated along the strips of the model heterogeneous surface (noncontorted three-phase contact line), were in an agreement with theoretical values calculated from the Cassie equation. Also, for most of the systems examined, there was an agreement of experimental contact angles, as measured for the liquid drop edge located normal to the strips (i.e., when the three-phase contact line was contorted), with theory calculated from the modified Cassie equation, including the line-tension term. Only in selected cases could the theory as expressed by the Cassie equation or the modified Cassie equation not predict the experimental contact angles. These were the systems in which the liquid phase interacted strongly with COOH groups of the self-assembled monolayer and completely spread over this hydrophilic portion of the surface.