Wetting and Capillary Flow of Surfactant Solutions and Inks

Sammanfattning: This thesis presents results of a series of experimental studies on the spreading dynamics of small drops of complex solutions on solid surfaces. The capillary flow dynamics of complex solutions have also been investigated. In the wetting experiments, water, alcohols, aqueous surfactant solutions and commercial ink-jet inks were used as test liquids, and a number of substrates, including silicate glass, hydrophobised silica, gold-alkanethiolate surfaces with controlled hydrophobicity and several different types of paper were examined with respect to their wettability. The primary goal of these studies was to investigate the dynamic effects of surfactant adsorption and surface tension relaxation on the spreading and capillary flow. Key factors, such as surface chemistry and topography of the substrates and physicochemical properties of the wetting liquids, which have been shown to control the spreading and absorption dynamics, have been identified, assessed and rationalized in the framework of the existing or newly developed theoretical models. The drop spreading and capillary rise experiments point out that the spreading process can be divided into at least two different spreading regimes, dominated by different mechanisms and force contributions. In the first regime, occurring immediately after the drop impact, the inertia forces play a significant role together with the surface forces and gravity. The second regime is characterised by quasi-steady flow. The rate-determining step in this regime is the diffusion transport of surfactant from the bulk to the depleted liquid-vapour interface. This regime is often dominating the overall spreading process of surfactant solutions. The diffusion limitation is for instance mirrored by the fact that the rate of spreading decreases with decreasing critical micelle concentration (cmc) of the surfactant. In the case of hydrophobic substrates, the origin of the effect is the slower diffusion of micelles compared to monomers; sometimes combined with slow micellar dissociation and barriers for direct micellar adsorption. Some of these observed phenomena are also reflected in the results of the more practical studies performed, involving e.g. inks and paper substrates. Further research is required in order to get a more complete picture although this thesis work highlights some central points with respect to paper sizing (hydrophobising) efficiency and ink spreading.