Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Gravity-driven flows in Evaporating Droplets
Dec 7, 2018
10:15 - 11:15
Dr. David Fairhurst
Nottingham Trent University, UK
Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
In the evaporation of microlitre liquid droplets, the accepted view is that surface tension dominates and the effect of gravity is negligible. This is certainly true in determining droplet shape, as the radius is significantly smaller than the capillary length -- typically several millimetres. However, the same argument is often incorrectly applied to rule out the effect of gravity on the flows within droplets. We image the flow patterns within binary liquid droplets evaporating on tilted substrates and show conclusively that the effects of gravity are important in determining the speed and direction of the observed flow. To image the flow, we developed a novel rotating Optical Coherence Tomography instrument which records rapid vertical slices through the droplet. We also use gas chromatography to determine the time evolution of the concentration of the more volatile component within the droplet and confirm that a simple analysis of volume data provides the same information. We combine the flow and concentration data to establish a phase diagram identifying the three Stages and propose rules for the transitions between them. We have also begun work using the same instrument to investigate the behaviour of multiple droplets, and will present some of this preliminary data.