| AVS 61st International Symposium & Exhibition | |
| Thin Film | Thursday Sessions |
| Session TF-ThA |
| Session: | Thin Film for Permeation Barriers and Membranes |
| Presenter: | Jan Knauf, Advanced Molecular Films GmbH and RWTH Aachen University, Germany |
| Authors: | J. Knauf, Advanced Molecular Films GmbH and RWTH Aachen University, Germany L. Reddemann, Advanced Molecular Films GmbH and Universität zu Köln, Germany K. Cheng, AMF GmbH, Germany A. Böker, DWI-Leibniz-Institute for Interactive Materials, RWTH Aachen University; Lehrstuhl für Makromolekulare Materialien und Oberflächen, RWTH Aachen University, Germany K. Reihs, Advanced Molecular Films GmbH, Germany |
| Correspondent: | Click to Email |
For the first time we observe the effect of scale dependent surface energies on a macroscopic wetting phenomenon. We have been able to produce surfaces structured with sub-nm topographies which exhibit liquid wetting deviating considerably from the behaviour expected from thermodynamic models.
It is known from grazing-incidence X-ray scattering experiments on planar liquid surfaces that surface energy is reduced at very small length scales [1,2]. By deliberately transferring this effect to solid-liquid interfaces it would be possible to create surfaces with distinct wetting characteristics. We have achieved this transfer by preparing defined monolayers of mixed compositions deposited from 1H,1H,2H,2H-perfluoroalkyl thiols of differing chain lengths. As an example, an equimolar binary mixed monolayer from 1H,1H,2H,2H-perfluorodecyl and 1H,1H,2H,2H-perfluorododecyl thiols shows an increase in advancing water contact angle of about 2° compared to the single component monolayers. This increase is considerably less than expected from simple thermodynamic models, as Wenzel´s equation of wetting on rough surfaces predicts an advancing angle difference of 7°. We expected that with suitable contacting liquids and tailored topographies the effect of reduced surface energy could be enhanced or even be inverted towards increased surface energies. Thus, it would be possible to manipulate the apparent wetting behaviour of surfaces by creating well-defined ultimately small topographies.
As model system we prepare monolayers from various mixtures of 1H,1H,2H,2H-perfluoroalkyl thiols (FnH2SH, n = 6,8,10,12,14) of different chain lengths on gold, which have been proven to adsorb randomly on the substrate without forming separated domains. Another crucial feature is the stiff helical conformation that is adopted by fluoroalkyl chains. Thus, sub-nm surface topographies with distinct height differences of 1.2 Å per CF2 group and chain distances of 5.8 Å are created, which have been characterized in detail by static secondary ion mass spectrometry, dynamic contact angle measurements and ellipsometry. Parameters governing the formation of the final monolayers could be obtained and were shown to vary systematically depending on the thiols employed.
[1] S. Mora et al., Phys. Rev. Lett. 90, 216101 (2003)
[2] C. Fradin et al., Nature 403, 871 (2000)