Abstract
Wetting is strongly influenced by adsorbate layers, which are omnipresent on surfaces. The influence of the composition and thickness of adsorbate layers on the water contact angle of sessile drops on different substrates was systematically investigated in the present work. Measurements were carried out for gold-sputtered substrates. These new results are compared to results from a previous study, in which corresponding measurements were carried out for technical steel and titanium substrates. In all experiments, different pretreatments of the samples were used to obtain variations of the adsorbate layer. The samples were either exposed to an oil bath or not, and different cleaning agents were used. The analysis of the adsorbate layer was carried out with X-ray photoelectron spectroscopy (XPS). The results for the different substrates reveal that the water contact angle depends mainly on the composition of the adsorbate layer. The substrate has only an indirect influence, as it influences the composition of the adsorbate layer. The thickness of the adsorbate layers was between 1.4 and 14 nm and was large enough to prevent a direct influence of the substrate on the water contact angle. It is shown that using the information on the adsorbate layer composition from XPS and the results for the water contact angle obtained for the gold samples alone, the water contact angles on the steel and titanium samples can be predicted.
Highlights
Wetting of surfaces plays an important role in many processes
The water contact angles range from 81° to 93° and the adsorbate layer thickness from 1:4 nm to 2:0 nm for all gold sample measurements carried out in the present work
Adsorbate layers on gold-sputtered silicon wafers were analyzed regarding the adsorbate layer thickness and composition using X-ray photoelectron spectroscopy (XPS), and the XPS analysis was combined with water contact angle measurements
Summary
Wetting of surfaces plays an important role in many processes. It is well known that the wetting is strongly influenced by adsorbate layers on the substrate. All reports on a long-range influence of the substrate are for adsorbate layers of long-chain molecules, while the authors who did not observe an influence of the substrate on the contact angle for an adsorbate layer thicker than 1-2 nm studied surfaces contaminated with simpler molecules. Their observations are in good agreement with molecular considerations: nonionic interactions fade out quickly with increasing distance of a particle to a surface. The present study focuses on the influence of adsorbate layers on the wetting of technically clean samples.
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