Contact Angle Microscopy on a Carbosilane Dendrimer with Hydroxyl End Groups: Method for Mesoscopic Characterization of the Surface Structure

Abstract

A carbosilane dendrimer containing hydroxyl terminal groups, which showed two types of wetting in dependence of the substrate used, was studied by tapping force microscopy. Due to the preferential adsorption of the hydroxyl groups, the dendrimer displayed autophobic spreading on mica, whereas a substrate which was first coated with a semifluorinated polymer was only partially wetted. In both cases, submicrometer-sized droplets were deposited on the surface. Microscopic contact angles were measured and compared with macroscopic values obtained by a standard sessile drop technique. The comparison showed lower values for the microscopic angle which were explained by deformation of the droplets caused by the tapping tip. The oscillatory motions of the tip intermittently touching a viscoelastic sample were calculated using a simple model of the tapping mode. The indentation of the tip into the sample and the induced phase shift relative to the oscillations in air were determined from the model, showing a good agreement with experimental results. Compared to traditional methods, this approach offers advantages such as (i) three-dimensional visualization of the whole droplet, (ii) submicrometer resolution of the structure near the three-phase boundary, and (iii) accurate determination of small contact angles.