Small spatial scales in wetting on non-structured and structured surfaces: thin films of liquid crystals & small-scales wetting properties of ionic liquids

Overview & Synopsis

At a time where miniaturization is one of the main challenges in science, the physics of wetting at small-scales is far from being completely understood. The structuring role of the interface(s), the heterogeneities of the substrate or the occurence of line effects over surface effects begin to matter. The aim of the present experimental PhD thesis is then to bring new clues to help the development of theoretical analysis. Two complex small-scale systems are studied: thin films of nematic liquid crystals on liquid substrates as well as droplets and precursors of ionic liquids on solid substrates. Compared to an usual liquid, two ingredients must be added to describe the wetting properties of nematic liquid crystals: elasticity which is the source of long range interactions and anchoring which describes the specific role played by the boundary conditions. On a liquid substrate, thin films with antagonist anchorings exhibit complex structures and thickness coexistence. Different systems were studied under optical microscope both in the nematic rangeand around the nematic / isotropic transition. The results were compared to available models based on continuum theory of nematic elasticity. Ionic liquids are molten salts at ambient temperature. They are now ubiquitous in industry, which explains that their wetting behaviour on small-scales is a hot topic. Small quantities of ionic liquid were deposited on several substrates, smooth or rough. The landscape obtained, droplets and thin films, was then imaged using Atomic Force Microscopy (AFM). The results were compared to available models for thin films and nanoscopic drops.