Wetting of complex liquids
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
Previous worked showed that adding surfactants to water, even at concentration below the critical micelle concentration (CMC) results in changes in the dynamic contact angles1-4. These previous works were focused on surfactants with a molecular weight between 100 - 400 g/mol. Less focus has been given to surfactants with molecular weight 10 to 50 times higher. In this range of molecular weights there are still many unanswered question regarding the influence of the molecular weight of the surfactants on the wetting behavior. In this thesis, I look into the dynamic dewetting behavior of large triblock polymeric surfactants, Pluronic, and compare it with small molecular weight surfactants. In comparison to them, the larger Pluronic changes the surface tension already at nanomolar concentration. The dynamic contact angles were measured using a home built rotating-drum set-up. Just like small molecular weight surfactants, addition of Pluronic leads to a decrease in the dynamic contact angles. Different is the much lower surfactant concentration needed. Already at concentrations below 0.5% CMC there is a significant difference in the dynamic contact angle. The combination of dewetting experiments, surface rheology and bulk measurements of aggregates concentration shows that the relevant concentration scale is the concentration of full surface coverage (CFSC). For small molecular weight surfactants the CMC and CFSC are equal. For Pluronic those two concentrations differ. The dynamic contact angles of Pluronic solutions were measured for velocities up to 200 mm/s at room temperature.
Dynamic wetting can also happen at much higher velocities and temperatures below 0oC, e.g. when suporcooled rain drops impact an airplane flying with speed over 100 m/s .Even though there were some efforts in the last years to study drop impact at different conditions of speed and temperature5-13. Little is known about impacts at speeds higher than 50m/s and temperatures below 0oC. Second part of the thesis concerns drop impact on different surfaces with speed of the impacting droplets was 50 and 90 m/s and the temperature varied between -15oC and 0oC. A special experimental set-up was developed to observe the drop impact at those conditions. This side project was done in collaboration with Dr. E. Bonaccurso from the Surface Technology & Advanced Materials Airbus Group Innovations, Munich, Germany. I show that the impact, wetting and freezing of supercooled drops at velocities above 50 m/s and temperatures below 0oC, depends on different parameters, such as hydrophobicity and softness of the surface, surrounding air temperature and impact speed.