Binary Mixture Droplet Evaporation on Microstructured Decorated Surfaces and the Mixed Stick-Slip Modes

The interactions between liquid droplets and solid surfacesduringwetting and phase change are important to many applications and arerelated to the physicochemical properties of the substrate and thefluid. In this work, we investigate experimentally the evaporationof pure water, pure ethanol, and their binary mixture droplets, accessinga wide range of surface tensions, on hydrophobic micro-pillared surfacesvarying the spacing between the pillars. Results show that on structuredsurfaces, droplets evaporate following three classical evaporativebehaviors: constant contact radius/pinning, stick-slip, ormixed mode. In addition, we report two further droplet evaporationmodes, which are a mixed stick-slip mode where the contactangle increases while the contact radius decreases in a stick-slipfashion and a mixed stick-slip mode where both the contactangle and the contact radius decrease in a stick-slip fashion.We name these evaporation modes not yet reported in the literatureas the increasing and decreasing contact angle mixed stick-slipmodes, respectively. The former ensues because the fluid surface tensionincreases as the most volatile fluid evaporates coupled to the presenceof structures, whereas the latter is due to the presence of structuresfor either fluid. The duration of each evaporation mode is dissimilarand depends on the surface tension and on the spacing between structures.Pure water yields longer initial pinning times on all surfaces beforestick-slip ensues, whereas for binary mixtures and pure ethanol,initial pinning ensues mainly on short spacing structures due to thedifferent wetting regimes displayed. Meanwhile, mixed stick-slipmodes ensue mainly for high ethanol concentrations and/or pure ethanolindependent of the solid fraction and for low ethanol concentrationson large spacing. Contact line jumps, changes in contact angle andpinning forces are also presented and discussed. This investigationprovides guidelines for tailoring the evaporation of a wide rangeof surface tension fluids on structured surfaces for inkjet printing,DNA patterning, or microfluidics applications.