A surface with photocontrollable wetting behavior is introduced. A monolayer of a polymeric material
containing 4‘-[trifluormethoxy-4,4‘-dibenzoazo] dyes in the side chains has been transferred on quartz
slides and silicon wafer. The azobenzene chromophore possesses two distinct isomers, cis and trans.
Transition between these states can be triggered by illumination with light of two different wavelengths.
It will be demonstrated that with the use of light and a mask, fine cis−trans patterns on the order of
micrometers can be written in the monolayer. The corresponding interface exhibits different wetting
behavior. This is visualized by a surface decoration with water droplets. The formation of water
microdroplets on the patterned monolayer can be controlled by light. Writing and erasing of patterns is
completely reversible. The system has potential for studying wetting behavior on microstructured surfaces.
A versatile modular setup is described which incorporates ellipsometry, surface plasmon spectroscopy, waveguide modes, their corresponding imaging techniques and Brewster angle microscopy in a single instrument. The important design criteria are discussed with special emphasis given to the requirements imposed by imaging under an oblique angle of incidence. Several experimental examples demonstrate the power of the instrument. Imaging nullellipsometry of a patterned monolayer on a highly reflecting support demonstrates a lateral resolution of approximately 1 μm and an accuracy in the thickness determination in the sub-nm region. The localization of the evanescent field of a surface plasmon was exploited to characterize adsorption layers in turbid and thus highly scattering solutions. An example of how an anisotropic sample can be characterized with the aid of waveguide modes is provided.
The triblock organosiloxane liquid crystalline 4-͑pentamethyl-disiloxanyl-n-pentyloxy͒-4Ј-cyanodiphenyl, which forms a smectic S A phase in the bulk, was investigated at the air-water interface. Imaging null ellipsometry was employed for visualization of morphology and thickness determination. The surface pressure-area (-A) isotherm exhibits several plateaus, each corresponding to the formation of an interdigitated bilayer. The system is ideally suited for assessment of the influence of the interface on the order and structure of the smectic bulk phase. By further compression all subsequently formed bilayers are of identical thickness and the value matches the layer spacing of the S A phase. Despite these apparent similarities, deviations between bulk and interfacial regions were revealed by optical second-harmonic generation. In contrast to the bulk, the first and second bilayers contain an excess of dipoles as well as molecules with tilt. The transition region between the bulk and interface is composed of at least two bilayers. The findings were further verified by surface potential measurements. ͓S1063-651X͑98͒13002-7͔PACS number͑s͒: 61.30.Ϫv
The adsorption isotherm of a soluble anionic surfactant was
measured and discussed within the framework
of established thermodynamical models. The adsorbed amount derived
by these models was compared
with the results obtained by second harmonic generation, SHG. SHG
can be used as a highly surface
specific probe which allows the determination of orientation and number
density. Since no thermodynamic
quantities are involved in the interpretation of SHG measurements, the
data can be used to assess the
validity and underlying assumptions of thermodynamic adsorption models.
The SHG experiments reveal
a good agreement with the Frumkin model. No indication of a
subsurface layer can be found.
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