On the Shapes of Microliter Alcoholic Sessile Droplets in Normal Atmospheric Conditions*

Alcoholic sessile droplets do not typically behave as pure-liquid (even volatile) ones. This is due to their hygroscopic nature and the ambient moisture present in the atmosphere. In such conditions, the absorbed water concentration near the contact line gets higher in comparison to the bulk of the droplet. This, in turn, induces strong solutal Marangoni stresses from the apex of the droplet toward its contact line, modifying the otherwise expected droplet shapes and spreading laws. Experiments have been conducted by depositing initially pure alcoholic droplets on a sapphire substrate inside a controlled cell. The relative humidity (RH) and temperature inside this cell can be modified, maintained, and monitored for a wide range. The initial volumes of these droplets are in the microliter range. At different RH, once the droplet reaches an apparent quasi-steady condition, the cross-sectional shapes of the droplet have been obtained using an interferometric technique. Finally, the measured profiles are compared with numerical calculations. This study highlights, in particular, the importance of considering the non-ideality of alcohol-water solutions for droplet regime determination.