Influence of different chemical surface patterns on the dynamic wetting behaviour on flat and silanized silicon wafers during inclining-plate measurements: An experimental investigation with the high-precision drop shape analysis approach

Abstract

Static and dynamic wetting behaviours of liquid droplets on functionalized solid surfaces are strongly influenced by the physical and chemical properties of the surface. The characterization and the correlation of these influence quantities to the wettability of a surface by means of contact angle measurements and drop shape analysis still remain a challenging task. Generally, contact angle measurements are performed by measuring the experimentally available advancing θa and receding θr contact angle. In this regard, water and ethylene glycol contact angles were measured on four well prepared model surfaces, which were hydrophobic modified by straightforward silanization with 1H,1H,2H,2H-perfluorooctyltrichlorosilane. The preparation procedure for the surfaces differed in terms of different pretreatments and applied coating procedures, leading to nearly flat surfaces without significant differences in surface roughness but different chemical surface textures. Atomic force microscopy and spectroscopic ellipsometry were applied to determine the surface roughness and layer thickness of the coatings, respectively, and the contact angle calculation was performed using the high-precision drop shape analysis (HPDSA) approach. Therefore, sessile-drops during the inclination of the sample surface were video recorded, which resulted in a sequence of non-axisymmetric drops. The resulting set of contact angle data (inclinationangle φ, contact angle θ) were further analysed by a recently developed contact angle analysis approach, which is based on the fitting of a modified sigmoid function onto the course of the contact angles relative to the inclination angle. This procedure has a fine local resolution and is very useful to characterize the overall wetting properties of a sample surface from a huge amount of contact angle data (about 74000 drop images=contact angles in this study) by only four fitting parameters. In this contribution, we illustrate the potential of this procedure to analyse the wetting and motion behaviours of sessile drops in dependence on different chemically patterned surface structures with a high local and temporal resolution, whereas even the naming of a more or less homogeneous surface coating was possible.