Measuring the contact angle of individual colloidal particles

The aim of this study was to measure the contact angles of individual colloidal spheres (empty set 4.4 mu m) and compare it to contact angles obtained on similarly prepared planar surfaces. For this purpose the particles were attached to atomic force microscope cantilevers. Then the force between the particle in aqueous medium and an air bubble was measured versus the distance. From the resulting force curves we obtained contact angles and detachment forces of single particles. Contact angles of gold coated silica particles were adjusted between 20 degrees and 100 degrees by self-assembling monolayers from different mixtures of undecanethiols and omega-hydroxy undecanethiols from solution. In parallel, contact angles on flat gold surfaces prepared in the same way were determined by the sessile drop method. A systematic difference between contact angles measured with particles and on planar surfaces was observed. Results are discussed in terms of line tension of the three-phase contact line. In addition, detachment forces were measured. Detachment forces were slightly higher than predicted from flotation theory. This might be caused by a pinning of the three phase contact line. (C) 1998 Academic Press.