Fundamental Aspects of Emersion and Electrowetting Interfaces

Abstract

The process of immersing a solid substrate into a solution and removing it so as to leave a thin layer of solution on the surface is referred to as emersion. The interfacial structure and properties of thin liquid films at bare, organically modified Ag substrates and Ag electrodes are investigated using emersion approach. The effect of viscosity on emersed layer thickness of glycerol/water solutions at Ag is studied and found that the emersed layer thicknesses were sensitive to the viscosity. The molecular basis of the behavior was investigated by elucidating the interfacial structures using emersion PM-IRRAS spectroscopy and found that the composition is not uniform throughout the emersed layer. The behavior of these solutions with shear rate was investigated by measuring the emersed layer thickness as a function of shear rate and found that the emersed layer thickness is sensitive to the shear rate at low viscosities while insensitive at high viscosities. The emersed layer thickness values of a series of NaF solutions were measured in the double layer region encompassing the PZC using ellipsometry at a given velocity. No systematic dependence of emersed layer thickness on potential was observed and were not adequately described by GCS theory. However, dependence of emersed layer thickness on concentration was observed. The average emersed layer thickness for 0.1 M, 0.01 M and 0.05 M NaF solutions are ~1.6 nm and for 0.005 M and 0.001 M NaF are ~ 0.5 nm. This behavior was interpreted using the effective viscosity at the electrode surface.Model electrowetting systems based on Ag surfaces modified with -COOH-terminated alkanethiol SAMs were emersed from solutions of different pH and the thickness of emersed layers using ellipsometry, wetting properties using contact angle measurements, composition and % deprotonation using XPS and interfacial structure using emersion spectroscopies were investigated. This study shows that the subtle change in molecular structure gives very small but observable differences in macroscopic contact angle.