High speed picoliter droplet top-view analysis for advancing and receding contact angles, boiling regimes and droplet-droplet interaction

Abstract

Free falling picoliter and sub-picoliter droplets generated by an ultrasonic atomizer are studied from a top-view perspective with a high-speed camera while falling on a heated surface. Due to the small droplet size processes are dominated by surface tension (Re = 0.5, We = 10−4, Oh = 2 × 10−2) resulting in almost perfect spherical shapes. Our presented model analyzes the droplets top-view appearance and allows the extraction of static and dynamic advancing and receding contact angles. In addition, the extracted evaporation rate allows to determine the critical heat flux (CHF) and Leidenfrost point (LFP). The evaporation regimes: nucleate boiling, transition boiling and film-boiling can be clearly defined by CHF and LFP. The presented model assumes the droplet evaporation starts with a shape corresponding to the static advancing contact angle Θa and continues to the static receding contact angle Θr with pinned contact line, i.e. with a constant visible droplet area. Afterward, it evaporates with a dynamic receding contact angle Θdr until the end of its lifetime. The initial advancing contact angle is calculated from the volume of the arriving droplet in air and the measured contact area directly after landing on the surface. The analysis yields Θa = 37.2 ± 3.3° and Θdr = 9.1 ± 1.6° for DI-water on silicon with natural oxide layer at T = 110°C. Furthermore, different complex scenarios of interactions between droplets are observed for sessile droplets formation and during the evaporation process.