Binary mixture drops evaporation on micro-structured surfaces*

In many industrial, biological, and everyday practices, the process of binary drops mixtures evaporation represents tremendous value. In this work, water-ethanol binary mixture drops are studied on smooth and on structured surfaces. On structured surfaces, three different evaporative behaviours have been observed; pinning or constant contact radius, stick-slip and mixed mode, which are consistent with literature. In addition, there are two further unexpected evaporation modes, which we report for the first time. We describe this as "increasing contact angle mixed-stick-slip" (ICAMSS) mode due to the preferential evaporation of ethanol and presence of structures and "decreasing contact angle mixed-stick-slip" (DCAMSS) mode due to the presence of structures. The duration of each evaporation mode is then discussed while evaporation regime maps are proposed. Regime maps show that high ethanol concentration drops on short spacing structures tend to evaporate mostly in a pinned mode while the ICAMSS mode ensues on larger spacing structures. High water concentration drops on large spacing structures tend to evaporate mostly in a stick-slip and DCAMSS modes. In conclusion, tailoring evaporation regimes can help in choosing the appropriate liquid and surface structure for particular applications, which may prove beneficial to many patterning and biomedical applications.